Field Crops Research

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In this paper we review both the methods for estimating N2 fixation and the quantity of N2 fixed by annual crop legumes. Available data on the N difference method for estimating N2 fixation reveal it to be unreliable. Although we advocate the use of isotope-based techniques their limitations necessitate the simultaneous collection of supporting data in the form of nodulation status, soil mineral N availability, and N yields of non-N2 fixers. To improve confidence in isotopic techniques for estimating N2 fixation differences in the form and function of plant roots are highlighted as critical areas for future research, as is a greater understanding of the total N contained in legume roots. A survey of the quantities of N2 fixed reveals the principal crop legumes to be ranked in the following descending order: soybean, lupin, field pea, faba bean, common bean, lentil, and chickpea.
 
Higher harvest index, widely reported as a main trait supporting major improvements in rice yield potential, also supports the higher performance of hybrids over inbreds. Although higher sink size is generally claimed for being the driver of higher grain filling in hybrid rice, it is relevant to question whether efficient sink regulation, that is timing and magnitude of the successive sink activities over crop growth from early stage, plays a role in supporting higher harvest index and is a key feature ensuring higher performance of hybrid rice. The partitioning coefficient (PC), as the increase in dry matter of one single organ over that of shoot, was calculated over short and successive periods to quantify the dynamics of dry matter partitioning. Four hybrid and four inbred genotypes of similar crop duration were grown under the same cropping management on the IRRI farm during a wet and a dry seasons. While PC to blade and sheath was similar for both plant types in both seasons at the early stage, PC to culm of hybrids was higher than that of inbreds, and PC to blade was lower, in the late vegetative and early reproductive phases. During the late reproductive phase, PC to panicle of hybrids was higher than that of inbreds, and PC to culm was lower whereas culm elongation was similar for both plant types. During grain filling, PC to culm was significantly more negative with hybrids which indicated stronger remobilization with hybrids. Specific culm length before anthesis, as low as 40 cm g−1, accounted for storage ability, and at maturity, as high as 130 cm g−1, for remobilization ability. Sink strength index, as a better indicator of dry matter partitioning efficiency than harvest index, and unfilled grain size, as an indicator of assimilate wastage, were suggested as key indices to account for the better sink regulation in hybrid rice and to improve screening protocols for increasing yield potential and tolerance to lodging.
 
Vernalization requirement, photoperiod response and earliness per se (EPS) of bread wheat cultivars are often determined using controlled environments. However, use of non-field conditions may reduce the applicability of results for predicting field performance as well as increase the cost of evaluations. This research was undertaken, therefore, to determine whether field experiments could replace controlled environment studies and provide accurate characterization of these three traits among winter wheat cultivars. Twenty-six cultivars were evaluated under field conditions using two natural photoperiod regimes (from different transplanting dates) and vernalization pre-treatments. Relative responses to vernalization (RRVGDD) and photoperiod (RRPGDD) were quantified using the reciprocal of thermal time to end of ear emergence, whereas earliness per se was estimated by calculating thermal time from seedling emergence until end of ear emergence for fully vernalized and lately planted material. An additional index based on final leaf numbers was also calculated to characterize response to vernalization (RRVFLN). To test whether the obtained indices have predictive power, results were compared with cultivar parameters estimated for the CSM-Cropsim-CERES-Wheat model Version 4.0.2.0. For vernalization requirement, RRVGDD was compared with the vernalization parameter P1V, for photoperiod (RRPGDD), with P1D, and for earliness per se, EPS was compared with the sum of the component phase durations. Allowing for variation in EPS in the calibration improved the relation between observed versus simulated data substantially: correlations of RRPGDD with P1D increased from r2 = .34 (p < .01), to .82 (p < .001), and of RRVGDD with P1V, from r2 = .88 (p < .001), to .94 (p < .001). In comparisons of observed versus simulated anthesis dates for independent field experiments, the estimated model coefficients resulted in an r2 of .98 (p < .001) and root mean square error of 1d. Overall, the results indicated that combining planting dates with vernalization pre-treatments can permit reliable, quantitative characterization of vernalization requirement, photoperiod response and EPS of wheat cultivars. Furthermore, emphasize the need for further study to clarify aspects that determine EPS, including whether measured EPS varies with temperature or other factors.
 
Central composite design of cultivar, plant density, date of sowing, and dose of nitrogen.
ANOVA of specific leaf area (SLA, cm 2 g À1 ) of plants sown on different dates in 2002.
ANOVA of specific leaf area (SLA, cm 2 g À1 ) in the sowing-date experiment.
Specific leaf area (SLA) is an important parameter in evaluating leaf function, yield potential, and ecological adaptability. Data from our field experiments from 2000 to 2005 were used to evaluate the values of SLA and related agronomic factors. SLA remained stable during winter, differed significantly among the dates of observation, and declined from seedlings to maturation in all the cultivars tested. ANOVA analysis showed that SLA differed significantly with sowing date but not with the cultivar, and was correlated positively with population density but negatively with the weight of leaves, roots, and the stem and with potassium application. A parabolic relationship was found between SLA and application of phosphorus. SLA at different observation dates before anthesis was negatively correlated with shoot weight at anthesis. An appropriate combination of population density and application of phosphorus and potassium could be helpful in securing desirable values of SLA (high at the seedling stage and low from the middle growth stage to maturity) and optimal population morphogenesis and, in turn, high yields.
 
The sustainability of cropping systems can be increased by introducing a cover crop, provided that the cover crop does not reduce the cash crop yield through competition. The cover crop may be sown at the same time as a cash crop in the crop rotation. We carried out an experiment in 1999–2000 and 2000–2001 in the Paris Basin, to analyze the effects of simultaneously sowing winter wheat (Triticum aestivum L.) and red fescue (Festuca rubra L.), a turf grass. Competition between wheat and fescue was analyzed with one variety of red fescue, Sunset, and two varieties of wheat, Isengrain and Scipion, each sown at a density of 150 plants m−2. In this study, we evaluated the effect of undersown fescue on wheat yield and analyzed the competition between the two species in detail. The undersown red fescue decreased wheat yield by about 12% for Isengrain (8.7 t ha−1 for undersown Isengrain versus 9.8 t ha−1 for Isengrain alone) and 7% for Scipion (7.4 t ha−1 for undersown Scipion versus 8.0 t ha−1 for Scipion alone). During the early stages of wheat growth (up to the ‘1 cm ear’ stage, corresponding to stage 30 on Zadoks’ scale), undersown fescue and fescue sown alone grew similarly. However, fescue biomass levels were much lower (5.6 and 4.7 g m−2 for fescue grown alone and undersown fescue) than wheat biomass levels on the undersown plots (120 g m−2 for Isengrain and 111 g m−2 for Scipion). From the e1 stage onwards, the wheat canopy rapidly extended, whereas that of red fescue remained sparse. The time lag between the beginning of the rapid increase in LAI and PAR interception by wheat grown alone and that for fescue grown alone was 590 dd in the second year. This resulted in much slower growth rates for undersown fescue than for undersown wheat. Biomass production rate was therefore low for undersown fescue (12% those of fescue grown alone, on average, at the time of wheat harvest), as were levels of water and nitrogen use. Neither the water deficit that occurred during the second experiment nor the nitrogen nutrition status of the wheat on plots with undersown fescue significantly affected wheat biomass production after anthesis.The global interception efficiency index IGɛi indicated that the fraction of the PARo intercepted by the wheat during its growth (255 days) was 0.35.
 
Barley is one of the most salt tolerant crop species, and differences between barley genotypes for salinity tolerance have been previously documented. Greenhouse experiments were conducted with barley seedlings (up to fourth leaf) from 14 genetic lines grown in control and saline (EC = 20 dS m−1) conditions. Some of these barley genotypes are parental lines to diverse mapping populations. Others have been bred and released for their tolerance to salinity. Gas exchange, chlorophyll fluorescence parameters, above ground dry matter and carbon isotope discrimination were measured to determine salinity tolerance. Two-week exposure to saline conditions decreased above-ground dry mass (AGDM), net photosynthesis (A), stomatal conductance (gs), internal CO2 concentration (Ci), efficiency of light harvesting of photosystem II (F′v/F′m), photochemical quenching (qP), and carbon isotope discrimination (Δ) relative to control plants. Measurement of gs provided the best information to assess genetic differences in barley for absolute performance when subjected to salinity stress. Lines with the highest gs values under control conditions also showed some of the highest absolute values for A and F′v/F′m under saline conditions. All lines were enriched in 13C (lower Δ) with salinity, but Δ was of limited value to assess differences between lines. Salinity susceptibility indexes (SSI) were used to estimate the relative tolerance of lines to salinity. They varied considerably between parameters and provide only relative information that can be difficult to reconcile with above absolute values of performance under saline conditions.
 
Changes in photosynthate source-sink relationships during the development of rape (Brassica campestris L. cv. ‘Span’) growing under field conditions were investigated using 14CO2. At flowering, leaves and stems were the principal sites of assimilation taking up 46% and 41% of the 14CO2, while pods took up only about 5%. Roots and pods, and later, seeds were sinks for photosynthates produced at flowering. At the onset of pod fill, leaves, stems and pods took up 19, 43 and 32%, respectively, while at the onset of seed ripening, they assimilated 17, 29 and 46%, respectively. Seeds were principal sinks during pod fill and ripening. These findings explain many of the apparent inconsistencies in the literature regarding the role of shoot parts in carbon nourishment of the seeds. As rape plants develop, the leaves, stems and pods are the principal sites of CO2 assimilation. Photosynthates produced from all three parts are translocated to the developing seeds. The amount of 14C-labelled assimilates found in the seed was linearly related to the dry weight of the seed.
 
The meaning of variation in 15N/14N isotope ratio in plants grown in the field is better known when variation is due to environment than when it is due to plant genotype. To study the physiological and genetic meaning of variation of such a ratio, a set of 99 recombinant inbred lines of maize were evaluated at low and high N-input and organ 15N abundances were correlated to agronomic and physiological traits. At the level of means, at high N-input there appeared no difference in 15N partitioning according to plant organs, with the same abundances for blades, stalks + sheaths and kernels. However, at low N-input blades and kernels were 15N-enriched, whereas stalks were significantly 15N-depleted with an abundance close to that observed for high N-input. 15N abundance of whole-plant and organs showed significant genotypic effects and genotype by nitrogen input interaction, varying according to the organ and the stage, silking and grain maturity. Genetic variation for 15N abundance and correlations involving 15N abundance were always lower at high N-input than at low N-input. 15N abundances of blades and stalks + sheaths were negatively related to silking date whatever the stage (silking or maturity) and N-fertilization whereas kernel 15N abundance was not affected by silking date. At low N-input, whole-plant 15N abundance at maturity was positively correlated to whole-plant and kernel protein content whereas at high N-input such correlation disappeared. Whole-plant 15N abundance at silking was negatively related to root fresh weight and to glutamine synthetase activity measured in young plants grown in hydroponics. Twelve QTLs for 15N abundance were detected, mainly at high N-input; among them, 10 coincided with QTLs involved in nitrogen use efficiency (grain yield, N-uptake and N remobilization) and the root system. Interpretation of all results leads to the conclusion that two mechanisms could explain genetic variation in 15N discrimination ability: morpho-physiological differences, in particular in the root system, and activities of the first enzymes of nitrogen metabolism, with a positive relationship between enzyme activity and discrimination abilities.
 
The accumulation and distribution of ¹⁵N-labelled fertilizer by wheat grown on a red-brown earth in the Goulburn-Murray Irrigation region was investigated in 1984 and 1986. ¹⁵N fertilizer was applied to microplots in a larger field experiment involving the factorial combination of irrigation and N. Irrigation treatments included a rainfed control (Rf) and irrigation on a weekly interval (Iw) commencing in the spring. Nitrogen was applied at 0 or 150 kg N ha⁻¹ at sowing and ¹⁵N fertilizer (20–22 kg N ha⁻¹) was applied at tillering, early-boot and heading. Microplots were sampled at maturity in 1984 and at anthesis and maturity in 1986.
 
The effect of soil mineral N (NO3+NH4) on the proportion (P) of N2 fixed by lupin (Lupinus angustifolius cv. Illyarrie) grown on red-earth over two seasons was measured in field crops by 15N isotope dilution, using either the natural abundance of 15N or enrichment of soil N with K15NO3. The reference plants used were wheat or linseed. Differences in concentrations of soil mineral N were induced by pretreatment with either cereal straw (3, 6 or 15 t ha−1) or ammonium nitrate (60, 90, 100, 120, or 150 kg N ha−1).The two 15N method produced similar estimates of P in untreated soil, but for some treatments there were substantial discrepancies between the natural abundance and the enrichment methods. In addition, although the effect of fertilizer N was evident regardless of the reference plant, linseed gave higher estimates of P than did wheat. The possible reasons for these differences are discussed.From values determined after enrichment of soil N with 15N and with wheat as a reference, P declined by up to 0.23 units with additions of ammonium nitrate up to 150 kg N ha−1. In the N fertilizer treatments, mineral N substituted for fixed N2 since the total N of lupin was unaffected by the level of soil mineral N. Pretreatment with N fertilizer (90 or 150 kg N ha−1) diminished the potential benefit to soil N from N2 fixation by lupin, producing an average net loss of 33 kg N ha−1. At sowing, cereal straw had reduced soil mineral N. However, although this soil treatment did not significantly reduced total lupin N, no significant increase in N2 fixation was detected.
 
The effects of three levels of phosphorus (5, 40, 75 kg P ha−1) and potassium (0, 30, 60 kg K ha−1) on growth, N2 fixation, nodulation and acetylene reduction in Centrosema acutifolium and C. macrocarpum were studied during establishment on an Oxisol in the Eastern Plains of Colombia. Nitrogen fixation was estimated by the 15N dilution technique, using Melinis minutiflora as the reference plant.Phosphorus limited growth and N2 fixation to a greater extent than did potassium. Phosphorus supply increased dry-matter production, on average, by 193%, N concentration in shoot tissue by 10% and percentage of nitrogen derived from the atmosphere (%Ndfa) by 15% at 14 weeks after sowing. These increases resulted in 259% more N2 being fixed at 75 kg P ha−1 than at 5 kg P ha−1. In contrast, K supply enhanced dry-matter production by 85% and the amount of N2 fixed by the same proportion. Severe P deficiency quickly led to a strong reduction in nodule weight, whereas K deficiency decreased nodule weight per unit root weight only at later stages of growth.With a supply of 75 kg P ha−1, with or without K, the legumes derived over 87% of their N from the atmosphere. The percentage N derived from the atmosphere of C. macrocarpum was more sensitive to P deficiency (63%Ndfa with 5 kg P ha−1) than that of C. acutifolium (76%Ndfa) at 10 weeks. Phosphorus was considered to have a direct effect on N2 fixation, which would make it necessary to take into account its effect on the symbiosis when screening legumes for tolerance to low soil fertility.
 
The effects of two fertilizer treatments on growth and symbiotic nitrogen fixation of eight pre-established tropical forage-legumes (Centrosema acutifolium, C. macrocarpum, Zornia glabra, Pueraria phaseoloides, Desmodium ovalifolium, Stylosanthes macrocephala, S. guianensis and S. capitata were evaluated under a cutting regime by using the 15N dilution technique. Rooting pattern and nodulation were also evaluated. The relative N accumulation curves of the legumes and control (savanna grasses) were studied in a separate experiment. The legumes were established in furrows separated by native savanna in an Oxisol of the Eastern Plains of Colombia to give a density of 5–6 plants m−2. The two fertilizer treatments, (a) a basal fertilizer of micro- and macro-elements including 80 kg P ha−1 and 70 kg K ha−1, and (b) the same fertilizer without P and K, were applied to the legumes at the start of the rainy season one year after establishment.With P:K fertilizer, all legumes derived at least 70% of their nitrogen from the symbiosis (% Ndfa), whereas without P:K both lower values and larger differences in the %Ndfa (44–84 %Ndfa) between species were observed. The greatest effect of P:K on %Ndfa was observed in D. ovalifolium (70 and 44 %Ndfa with and without P:K respectively), but the effect on its yield was relatively small. In contrast, S. macrocephala responded with a large increase in yield (380% with P:K), although it was the only species in which P:K did not have a significant effect on %Ndfa. Total shoot N derived from fixation with P:K fertilizer ranged from 25 kg N ha −1 for D. ovalifolium to 115 kg N ha−1 for P. phaseoloides, with three periods of regrowth over a total of 17 weeks. Without P:K fertilizer, 11–48 kg N ha−1 were derived from fixation. Legume ranking for total N derived from fixation mostly reflected yield differences. However, D. ovalifolium ranked clearly lower for total N from fixation than for yield due to its low %Ndfa and low N concentration.
 
The availability of a large number of expressed sequence tags (ESTs) has facilitated the development of molecular markers in members of the grass family. As these markers are derived from coding sequences, cross-species amplification and transferability is higher than for markers designed from genomic DNA sequences. In this study, 919 EST-based primers developed from seven grass species were assessed for their amplification across a diverse panel of 16 grass species including cereal, turf and forage crops. Out of the 919 primers tested, 89 successfully amplified DNA from one or more species and 340 primers generated PCR amplicons from at least half of the species in the panel. Only 5.2% of the primers tested produced clear amplicons in all 16 species. The majority of the primers (66.9%) were developed from tall fescue and rice and these two species showed amplification rate of 41.6% and 19.0% across the panel, respectively. The highest amplification rate was found for conserved-intron scanning primers (CISP) developed from pearl millet (91%) and sorghum (75%) EST sequences that aligned to rice sequences. The primers with successful amplification identified in this study showed promise in other grass species as demonstrated in differentiating a set of 13 clones of reed canary grass, a species for which very little genomic research has been done. Sequences from the amplified PCR fragments indicated the potential for the transferable CISP markers for comparative mapping purposes. These primer sets can be immediately used for within and across species mapping and will be especially useful for minor grass species with few or no available molecular markers.
 
(a) Stem, (b) leaf and (c) head biomass as a fraction of shoot biomass at anthesis in sun¯ower cultivars released in Argentina between 1930 and 1995. Symbols: open-pollinated cultivarsˆ(&,&), hybridsˆ(S,!), open symbolsˆExp. 1, closed symbolsˆExp. 2. Error bars are 2 SEM and are drawn when larger than symbols.
(a) Harvest index of sun¯ower cultivars released in Argentina between 1930 and 1995. The regression line describes the association between the variables only for the open-pollinated cultivars. (b) Relationship between harvest index and the proportion of the growing cycle corresponding to grain growth, i.e. (time from anthesis (A) to physiological maturity (FM))/(time from emergence (E) to FM). Symbols as in Fig. 1.
Relationship between grain oil concentration and (a) kernel-to-grain ratio; (b) concentration of oil in kernel. Symbols as in Fig. 1. Data from Exp. 2.
Concentration of protein in the grain for sun¯ower cultivars released in Argentina between 1930 and 1995. Symbols as in Fig. 1. Data from Exp. 2.
We compared the partitioning of dry matter and the oil and protein content in the grain of sunflower (Helianthus annuus L.) cultivars released in Argentina between 1930 and 1995. At anthesis, the fraction of plant dry matter partitioned to leaf and head increased, whereas the fraction in the stem decreased with year of cultivar release. Improved oil yield was related to the increase in: (i) harvest index (grain mass/shoot mass) from about 0.3 to 0.5; (ii) kernel-to-grain ratio from 0.6 to 0.8; and (iii) kernel oil concentration from 58 to 70%. The duration of the period of grain growth as a fraction of season length accounted for half of the variation in harvest index. Kernel oil concentration was negatively associated with protein concentration which dropped from 23 to 13%. Further improvement in partitioning may be possible, including increase in harvest index resulting from enhanced contribution of stored assimilate to grain growth. Owing to the dramatic improvement in partitioning achieved in the last seven decades, however, further increase in yield may need to focus on increasing biomass production.
 
Historic sets of sunflower (Helianthus annuus L.) cultivars released in Argentina between 1930 and 1995 were compared in two field experiments. Breeding and selection shortened cycle length: most of the reduction came from a reduction in time to anthesis, which together with a stable time from anthesis to maturity increased the proportion of the growing cycle given to grain growth. Shoot biomass at maturity was unrelated to year of cultivar release but a greater portion was produced after anthesis. The ratio between leaf area index duration from anthesis to maturity and grain number was taken as a measure of source-to-sink ratio; this ratio was smaller in hybrids than in open-pollinated cultivars. Variation in both the amount of oil per grain and grain mass were small in comparison with the variation in source-to-sink ratio indicating that additional carbon may have attenuated the effects of less leaf area duration per grain. Owing to the relative stability of the amount of oil per grain and the reduction in grain mass, the grain oil concentration increased despite the drop in source relative to sink. Further yield improvement will mostly depend upon the increase in the supply of substrates to fill the grain provided grain size is not restricted by events occurring before anthesis.
 
Bread wheat has been frequently characterised as sink-limited during grain filling but the degree of sink-limitation could vary with the contribution of breeding in increasing the number of grains per unit land area. This hypothetical change in the level of sink-limitation due to breeding has been insufficiently documented. Two source–sink manipulation experiments under field conditions with three released cultivars and an advanced breeding line representing important steps in wheat breeding in the Mediterranean area of Spain were conducted in order to quantify whether genetic improvement of grain yield in Mediterranean wheat modified the source–sink balance during grain filling, and how it affected grain weight and post-anthesis photosynthetically active radiation intercepted by the crop (IPAR) and radiation use efficiency (RUE). Average grain weight of control and trimmed spikes during grain filling was not significantly affected by halving the number of growing grains in the two oldest cultivars, but it was significantly increased in the most modern line, and had an intermediate response in the intermediate cultivar. In those cases in which a certain degree of source-limitation during grain growth occurred the magnitude of the response reflected a co-rather than a source-limitation. Considering grains from different positions within the spikes the smaller (distal) grains responded more markedly than the larger (proximal) grains. No differences in post-anthesis IPAR were found between the trimmed and control sub-plots for any of the genotypes. However, trimming the spikes reduced post-anthesis RUE, a fact corroborated by a simultaneous reduction in leaf net photosynthetic rate at noon. It seems that bread wheat breeding has tended to reduce the strength of the sink-limitation during post-anthesis even under Mediterranean conditions, and consequently the most modern line presented a sort of co-limitation.
 
Increasing yield is a high priority in most breeding programs. Approximately 600 soybean cultivars had been released by the end of the last century in Northeast China. Understanding the agronomic and physiological changes is essential for planning further plant breeding strategies in soybean. In this study, 45 representative soybean cultivars, from maturity groups 00 and 0, released from 1950 to 2006 in Northeast China were compared in field conditions for 3 consecutive years. A positive correlation between seed yield and year of cultivar release was indicated with a 0.58% average annual increase. Seed number per plant was the most important contributor to yield gain, with a 0.41% increase per year. Pod number per plant and seed size varied slightly with the year of cultivar release. Although variation in protein was from 37.0% to 45.5%, and oil concentration was from 16.7% to 22.0%, their concentrations were not consistently related to year of cultivar release. A 33% increase in the photosynthetic rate, 10.6% increase in plant dry weight and 19.0% increase in harvest index (HI) were found, while leaf area index (LAI) decreased by 17.3%. Modern cultivars have higher photosynthetic rates than their predecessors. The reduced plant height gave increased resistance to lodging, with the lodging score dropping from 3.2 in 1951 to 1.0 in 2006. Seed resistances to disease and pest infestation were also improved. Yield stability was enhanced over years, which could be attributed to the stable pod production across different environments. A flow diagram to explain the contributors to genetic improvement of soybeans in Northeast China was developed.
 
This paper describes results of a three-year field experiment and a simulation study to quantify the magnitude of improvement in yield potential at different levels of management and climatic variability. In two crop seasons for crops planted on time, there was a significant increase of 1.0–1.3% per year in yield of cultivars but it was negligible in the third season when March was relatively warmer. In late plantings done in one season, the rate of improvement in yield varied between −0.16 and 0.57% per year. This indicates that in warm conditions, particularly during grain-filling period, there was only a negligible difference among the cultivars. It suggests that new cultivars yield more than the older ones only when the temperatures are favorable for growth and development. The trends in simulated yields were similar in direction but lower in magnitude compared to the trends in observed yields in field experiments. The simulated rate of change in yield was always greater in the potential production conditions compared to the modest level of management as is practiced today. Simulation always showed a negative trend in yielding ability of cultivars in seasons when grain-filling duration was less than 29 days. When the grain-filling duration exceeded 29 days, the trend in most cases was positive varying up to +0.67% per year. The late-released cultivars used in this study have long vegetative duration that extends grain-filling to high temperatures. Crop seasons that favored long vegetative duration or had high March temperatures that shorten grain-filling showed a negative trend in yield improvement whereas seasons with cool March showed a positive improvement. Conclusions based on limited experimentation in field studies, therefore, can be biased depending upon the agro-environment experienced by the cultivars. Simulation studies can supplement such field studies and assist in reducing this bias.
 
Harvest index (HI) and its stability are key determinants of crop yield. HI of Argentinean maize hybrids released at different decades was compared using as a framework the relationship between yield and final shoot biomass on a per plant basis (YP and SP, respectively). The objectives of this work were (i) to determine differences among hybrids in the parameters of the YP–SP relationship and in HI stability and (ii) to explain these differences by studying the response of yield components (i.e. kernel number per ear and weight per kernel) to resource availability. Two experiments were carried out at Balcarce, Argentina (37°45′S, 58°18′W; elevation 130 m) during two growing seasons, 1998–1999 and 1999–2000. A wide range of YP and SP was generated by manipulation of plant density. Shoot dry matter per plant, grain yield per plant and its components were measured at physiological maturity. Hybrids differed in their YP response to resource per plant availability and in HI stability. The greater HI stability of non-prolific plants of modern hybrids was associated with lower biomass thresholds for yield at low SP and with greater reproductive plasticity at high SP. Prolificacy contributed to HI stability in all hybrids to a similar extent. All yield components contributed to explain yield and HI responses to variation in SP. At low SP, YP increments were mainly related to KNP increments. At intermediate SP, increases in YP were related to both yield components to a similar extent. However, a plateau for KW was found in the two oldest hybrids. At large SP, YP response to SP was related to prolificacy.
 
We investigated the response of maize kernel number to plant density in four hybrids released in Argentina between 1965 and 1993. Assuming kernel number is the main yield component, and using as a framework the relationship between kernel number per plant (KNP) and plant growth rate bracketing silking (PGRs), we tested the alternative hypotheses that modern hybrids produce more kernels because they have (a) greater PGRs or (b) more kernels per unit PGRs than their older counterparts. Three experiments were carried out including a range of plant densities from 3–5 to 15–18 plants m−2. PGRs was calculated from shoot dry matter measured 10 days before and 20 days after silking. Shoot dry matter, grain yield and its components were measured at physiological maturity. Grain yield of the oldest hybrid averaged 7.7 t ha−1, and increased with year of release at a rate of 173 kg ha−1 per year. The response of grain yield to plant density was curvilinear. Kernel number per square meter accounted for most of the variation in yield with both year of release and plant density. For both sources of variation, there was a trade-off between kernel number and mass. Both PGRs and KNP decreased with increasing plant density in all four hybrids. Whereas variation in PGRs among hybrids was small, the oldest hybrid set 93 (low density) and 113 (high density) kernels per unit PGRs in comparison to the newest that set 167 and 193. We conclude that more kernels per unit PGRs, rather than greater PGRs, accounted for the genetic improvement of yield potential in the hybrids investigated.
 
The increases in crop yield that played an important role in maintaining adequate food supplies in the past may not continue in the future. Soybean (Glycine max L. Merrill) county yield trends (1972–2003) were examined for evidence of plateaus using data (National Agricultural Statistics Service) for 162 counties (215 data sets) in six production systems [Iowa, Nebraska (irrigated and non-irrigated), Kentucky and Arkansas (irrigated and non-irrigated)] representing a range in yield potential. Average yield (1999–2003) was highest in irrigated production in Nebraska (3403 kg ha−1) and lowest in non-irrigated areas in Arkansas (1482 kg ha−1). Average yield in the highest yielding county in each system was 31–88% higher than the lowest. Linear regression of yield versus time was significant (P = 0.05) in 169 data sets and a linear-plateau model reached convergence (with the intersection point in the mid-1990s) in 35 of these data sets, but it was significantly (P = 0.10) better in only three data sets (<2% of the total). Absolute (kg ha−1 year−1) growth rates were associated with productivity, but relative rates were not with the mean relative rates ranging from 1.0 to 1.3% over the six systems. There was, however, a two- to threefold range in relative rate among counties within systems in Nebraska, Iowa, Kentucky and Arkansas (irrigated). Yield did not change (linear regression not significant, P = 0.05) between 1972 and 2003 in 41 counties in non-irrigated areas of Arkansas and Nebraska and in six Kentucky counties of which four had high levels of double-cropping soybean after wheat (Triticum aestivum L.). I found no convincing evidence that soybean yields are reaching plateaus but the technology responsible for this yield growth was apparently completely ineffective in low-yield, high-stress environments.
 
The average maize grain yield of Jilin Province increased from 2835 kg/ha in 1974 to 6329 kg/ha in 1993, an average increase of 6.2% per year. Comparative analysis and regression of historical statistical data revealed that genetic improvement of cultivars, application of chemical fertilisers and adoption of improved cultural practices contributed 62.0, 50.9 and 37.0%, respectively, to total yield gain over these 20 years. Decreased application of organic manure, incidence of diseases and pests, and continuous cropping reduced yield by 27.1%, while unidentified factors accounted for a further loss of 22.8%.
 
Knowledge of the changes in agronomic and photosynthetic traits associated with genetic gains in grain yield potential is essential for an improved understanding of yield-limiting factors and for determining future breeding strategies. The objectives of this study were to identify agronomic and photosynthetic traits associated with genetic gains in grain yield of facultative wheat (Triticum aestivum L.) between 1981 and 2008 in Henan Province, the most important wheat producing area in China. During the 2006–2007 and 2007–2008 crop seasons, a yield potential trial comprising 18 leading and new cultivars released between 1981 and 2008 was conducted at two locations, using a completely randomised block design of three replicates. Results showed that average annual genetic gain in grain yield was 0.60% or 51.30 kg ha−1 yr−1, and the significant genetic improvement in grain yield was directly attributed to increased thousand grain weight which also contributed to the significant increase in harvest index. The genetic gains in rates of net photosynthesis at 10, 20 and 30 days after anthesis were 1.10% (R2 = 0.46, P < 0.01), 0.68% (R2 = 0.31, P < 0.05) and 6.77% (R2 = 0.34, P < 0.05), respectively. The rates of net photosynthesis at 10 (r = 0.58, P < 0.05), 20 (r = 0.59, P < 0.05) and 30 (r = 0.65, P < 0.01) days after anthesis were closely and positively correlated with grain yield. A slight decrease in leaf temperature and an increase in stomatal conductance after anthesis were also observed. Grain yield was closely and positively associated with stomatal conductance (r = 0.69, P < 0.01) and transpiration rate (r = 0.63, P < 0.01) at 30 days after anthesis. Therefore, improvement of those traits was the likely basis of increasing grain yield in Henan Province between 1981 and 2008. The genetic improvement in yield was primarily attributed to the utilization of two elite parents Yumai 2 and Zhou 8425B. The future challenge of wheat breeding in this region is to maintain the genetic gain in grain yield and to improve grain quality, without increasing inputs for the wheat–maize double cropping system.Highlights► Average annual genetic gain in grain yield between 1981 and 2008 in Henan Province, of China was 0.60% or 51.30 kg ha−1 yr−1, and the significant genetic improvement in grain yield was directly attributed to increased thousand grain weight which also contributed to the significant increase in harvest index. ► The genetic gains in rates of net photosynthesis at 10, 20 and 30 days after anthesis were 1.10% (R2 = 0.46, P < 0.01), 0.68% (R2 = 0.31, P < 0.05) and 6.77% (R2 = 0.34, P < 0.05), respectively. ► The rates of net photosynthesis at 10 (r = 0.58, P < 0.05), 20 (r = 0.59, P < 0.05) and 30 (r = 0.65, P < 0.01) days after anthesis were closely and positively correlated with grain yield. ► A slight decrease in leaf temperature and increased stomatal conductance after anthesis were also observed. ► Grain yield was closely and positively associated with stomatal conductance (r = 0.69, P < 0.01) and transpiration rate (r = 0.63, P < 0.01) at 30 days after anthesis. ► Therefore, improvement of those traits was the likely basis of increasing grain yield in Henan Province between 1981 and 2008. ► The genetic improvement in yield was primarily attributed to the utilization of two elite parents Yumai 2 and Zhou 8425B.
 
Although the narrow-leafed lupin was 1st domesticated in Central Europe in the early 20th century, it remained a minor crop until further domestication in Western Australia (WA) in the 1950–1970s, and Australia has dominated world production since the mid-1980s. In order to investigate varietal adaptation and changes over time in Australian breeding, seed yield and a range of related traits were measured in multi-environment historical variety trials. Cultivar differences in most traits were well explained by vernalization response, accounting for 38–94% of the genotype main effect. Vernalization responsive (VR) cultivars were much more daylength responsive than vernalization unresponsive (VU) varieties, but much less responsive to ambient temperature, tending toward 0 as vernalization induction decreased. Phenology had a strong influence on yield and related traits, all of which were measured exclusively in WA, under warm Mediterranean climates with weak, variable vernalization induction. The early phenology VU cultivars were more yield-responsive to WA environments than VR types, and were characterized by rapid growth rates, high seed and biological yield, harvest index, fecundity, large seeds and a lower proportion of total biomass invested in the main stem. Cultivar productivity, yield responsiveness, harvest index and fecundity have increased over time in both vernalization response types, while flowering has become earlier. Seed yield increases are associated with increased main stem productivity, with higher seed numbers due to both increased pod set and seed numbers per pod over time. Lupin breeders have selected strongly for drought escape, an appropriate strategy for the strongly Mediterranean climate of the northern WA grainbelt, but which is likely to limit yield potential in longer season environments. Because ambient temperature and vernalization responses are confounded, and there is little variation within each cultivar pool, it is currently not possible to develop longer season cultivars for warm Mediterranean climates. To produce specifically adapted cultivars with phenology appropriate for short- and long-season environments, as is the case in widely adapted legumes such as chickpea or lentil, a broader range of temperature responses in VU backgrounds is required. This will require careful parental selection, based on an understanding of flowering responses to temperature, photoperiod and vernalization.Highlights► We demonstrate how Australian narrow-leafed lupin breeding has changed over time, using multi-environment historical variety trials. ► Productivity, yield responsiveness, harvest index and fecundity have increased over time. ► Breeders have selected strongly for drought escape via temperature responsive phenology in the short season Mediterranean WA. ► Late phenology is confounded with vernalization, and effective in cool, long season eastern environments, but not in warm WA. ► Lupin potential, particularly in warm long season climates, is limited by low variation within each vernalization group.
 
Average commercial maize yield in the US has increased from about 1 Mg/ha in the 1930s to about 7 Mg/ha in the 1990s. Although the increase has been the result of both genetic and agronomic-management improvements, we contend that most of this improvement is the result of the genotype×management interaction. The genetic improvement in maize yield is associated neither with yield potential per se, nor with heterosis per se, but it is associated with increased stress tolerance, which is consistent with the improvement in the genotype×management interaction. The potential for future yield improvement through increased stress tolerance of maize in the US is large, as yield potential is approximately three times greater than current commercial maize yields. The mechanism by which maize breeders have improved stress tolerance is not known, but we speculate that increased stress tolerance may have resulted from the selection for yield stability. Stability analyses were performed on a number of high-yielding maize hybrids, including three hybrids that have been involved in some of the highest maize yields recorded in producers’ fields, to examine the relationship between yield and yield stability. Results of the stability analyses showed that high-yielding maize hybrids can differ in yield stability, but results do not support the contention that yield stability and high grain yield are mutually exclusive.
 
By 2025 the global population will exceed 7 billion. In the interim per capita availability of arable land and irrigation water will go down from year to year while biotic and abiotic stresses expand. Food security, best defined as economic, physical and social access to a balanced diet and safe drinking water will be threatened, with a holistic approach to nutritional and non-nutritional factors needed to achieve success in the eradication of hunger. Science and technology can play a very important role in stimulating and sustaining an Evergreen Revolution leading to long-term increases in productivity without associated ecological harm.
 
Carbon dioxide (CO2), along with light, water and nutrients, represents an essential resource needed for plant growth and reproduction. Projected and recent increases in atmospheric carbon dioxide may allow breeders and agronomists to begin intra-specific selection for yield traits associated with CO2 sensitivity. However, selection for maximum yield, particularly for cereals, is continuous, and it is possible that modern cereal cultivars are, in fact, the most CO2 sensitive. To test CO2 responsiveness, we examined two contrasting spring wheat cultivars, Marquis and Oxen, over a 3-year period under field conditions at two different planting densities. Marquis was introduced into North America in 1903, and is taller, with greater tiller plasticity (i.e. greater variation in tiller production), smaller seed and lower harvest index relative to modern wheat cultivars. Oxen, a modern cultivar released in 1996, produces fewer tillers, and has larger seed with a higher harvest index relative to Marquis. As would be expected, under ambient CO2 conditions, Oxen produced more seed than Marquis for all 3 years. However, at a CO2 concentration 250 μmol mol−1 above ambient (a concentration anticipated in the next 50–100 years), no differences were observed in seed yield between the two cultivars, and vegetative above ground biomass (e.g. tillers), was significantly higher for Marquis relative to Oxen in 2006 and 2007. Significant CO2 by cultivar interaction was observed as a result of greater tiller production and an increased percentage of tillers bearing panicles for the Marquis relative to the Oxen cultivar at elevated carbon dioxide. This greater increase in tiller bearing panicles also resulted in a significant increase in harvest index for the Marquis cultivar as CO2 increased. While preliminary, these results intimate that newer cultivars are not intrinsically more CO2 responsive; rather, that yield sensitivity may be dependent on the availability of reproductive sinks to assimilate additional carbon. Overall, understanding and characterizing vegetative vs. reproductive sink capacity between cultivars may offer new opportunities for breeders to exploit and adapt varieties of wheat to projected increases in atmospheric carbon dioxide concentration.
 
Characterization of germplasm by DNA-markers provides a tool for precise germplasm identification and a quantitative assessment of genetic diversity. The genetic structure of the durum wheat germplasm grown in the Mediterranean basin varies largely from traditional landraces and cultivars characterized by a high versatility, to the modern varieties characterized by high yield potential, wide adaptation, and commercial end-use quality. The objective of this study was to assess the phylogenetic relationships among 24 durum wheat cultivars selected from relevant germplasm obtained at different periods in Italy and Spain, and to quantify the genetic erosion caused in durum wheat by breeding activities during the last century in these two countries.Genetic similarity between cultivars was studied by AFLP markers through the calculation of the Dice’s coefficient. The results showed a high degree of genetic similarity between the old Spanish cultivars and the collection of Italian cultivars, suggesting that wheat could have been introduced in the Iberian Peninsula via Italy. Genetic diversity estimates based on AFLP data confirmed the maintenance of genetic diversity with time since the values of Polymorphic Information Content were 0.27 for old cultivars (released before 1945), 0.28 for intermediate cultivars (released between 1950 and 1985) and 0.29 for modern cultivars (released between 1988 and 2000). These results indicate that genetic variability in Italian and Spanish durum wheat seems to have been maintained quite constant throughout the breeding process over the last century.
 
An analysis of trends in yield and yield stability throughout the century was made for 21 countries (Algeria, Argentina, Australia, Canada, Chile, Egypt, France, Germany, India, Italy, Japan, Mexico, New Zealand, South Africa, Spain, Sweden, Tunisia, UK, Uruguay, USA and the former USSR). Regressions (linear, bi-linear or tri-linear fitted with an optimisation technique) were used to evaluate the trends in yield during the century. Residuals and relative residuals of these regressions were used to evaluate in absolute and relative terms, respectively, trends in yield stability. Countries varied greatly in their yields and yield gains as well as in changes in harvested area. But almost all of them showed a remarkable lack of yield gain during the initial 3 to 5 decades of this century, followed by noticeable increases in yield. Yield trends for relatively young agricultural wheat-exporting countries, such as Argentina, Australia, Canada and USA, reveal an important breakpoint ca. 2 decades earlier than European countries with longer tradition in wheat production. In addition, yield gains in many countries have apparently been levelling off during the last decade. Trends in yield residuals during the present century revealed a decrease in yield stability in 14 of the 21 countries analysed, but the increase in yield residuals was relatively small (≤0.3 Mg ha−1) compared with increases in yield. Therefore, relative yield residuals indicated that yield stability, as a percentage of yield, increased or at least did not change for most of the analysed countries. Moreover, it is suggested that wheat production systems have been, in general, highly successful in increasing yield while maintaining or increasing relative yield stability with respect to that existing at the beginning of the century. Finally, no relationship was found between variations in yield stability, both in absolute and relative terms, and the increase in yield comparing the present values and those at the beginning of the century.
 
Twenty-one years ago, Donald proposed the ideotype concept as an analytical and physiological basis eventually to replace more empirical breeding approaches, i.e., ‘defect elimination’ and ‘selection for yield’. An appraisal of the Donald ideotype leads to the view that the ideotype approach continues to offer a basis for increasing the efficiency in attaining plant breeding objectives. It offers: •- a structure for objectively defining plant breeding aims in terms of plant characters and their purposes, i.e. quality or market characteristics, resistance to environmental stress, resistance to disease, and ability to perform efficiently under the competitive pressures of the crop environment;•- a theory that explains the relationships between harvest index, biological yield and morphological characters in the main types of plant environments met with in the culture and breeding of annual seed crops, i.e. widely spaced plants, plants in mixed communities (e.g. varietal mixtures and segregating populations), and the cultivar in dense crop; and•- a theory with the conclusion that, without deliberate counter-selection towards the form of the communal ideotype (e.g. uniculm habit, short stem, narrow erect leaves), characters of the competitive ideotype (e.g. freely tillering and branching, tall stature and large leaves with lax posture) will tend to assert themselves under the pressure of natural selection through competition and be selected in plant breeders' plots, with negative effects on yield.Evaluations of the ideotype concept, usually in the form of tests of the uniculm concept for cereals, generally support the hypothesis of the weak competitor as the successful crop ideotype, or where inconclusive, as in the case of wheat in optimal environments, indicate that further, more critical experiments are needed.The ideotype approach is seen to have potential for defining the characters of high-yielding cultivars of new crops in new environments.
 
A field experiment was conducted on an Andosol to evaluate wheat (Triticum aestivum L.) yield, P and N uptake and soil P fraction after long-term fertilization (no fertilizer, NPK, NP, NK and PK treatments). Application rates of N, P and K fertilizers were 100, 65 and 83 kg ha−1 year−1 by ammonium sulfate, superphosphate, and potassium chloride, respectively. Phosphorus fertilization was critical for grain yield since the NK treatment did not increase yield compared with no fertilizer treatment. Agronomic efficiency of P was greater than agronomic efficiency of N, although apparent recovery of P and N were 17 and 53%, respectively. Combination application of fertilizer P and N resulted in the greatest grain yield over 23-year cultivation. Interaction impact on grain yield between P and N ranged from 71 to 109%, and was greater than the values for cereals in the earlier works. The N/P ratios of wheat decreased by P application and increased by N application. The N/P ratios in NPK and NP treatments were higher than the values attaining maximum yield for cereal crops reported by other works.
 
In many Asian countries the future will demand a higher rate of rice production increase than that obtained in the 1970's. However, low rice prices discourage production and lead to rice shortages. There is a strong interest in identifying rice genotypes which effectively utilize resources on the land, and in developing low-cost production technology for tropical Asia. A method proposed earlier to evaluate N utilization efficiency of 24 rice genotypes, and initially tested on a soil with high N fertility, was tested on a soil with low N fertility in order to evaluate the performance of the genotypes under contrasting conditions. In six consecutive seasons on an infertile Vertic Tropaquept soil in Maligaya, Nueva Ecija, The Philippines, field performance was ranked on the basis of 12 parameters considered in the initial study. Significant differences among genotypes were observed in 68 out of 72 instances during the six seasons. Five parameters were selected for ranking in unfertilized plots and seven for ranking in plots receiving N fertilizer. Principal-component analysis showed that yield was the most significant single parameter in most instances. A two-parameter combination involving yield and ratio of panicle weight to either total N uptake or soil N uptake, however, correlated much better with rankings based on five or seven parameters. In contrast to the IRRI data on a fertile Andaqueptic Haplaquoll soil, significant differences among genotype means for soil N uptake in fertilized plots and total N uptake in unfertilized plots were found in all six seasons at Maligaya. Genotypes 11 and 18 ranked consistently high with all combinations of parameters, whether fertilized or unfertilized. For example, genotype 11 ranked first in all cases at IRRI, and either first or second in all cases at Maligaya. Others such as genotypes 5 and 12 ranked consistently low. The results indicate consistency in rice-genotype performance on soils deficient in N supply as well as those adequately supplied, in both wet and dry seasons, thus extending the applicability of the methodology.
 
The response of 49 pea cultivars with different drought tolerance was studied. The tolerance to stress was determined according to the grain yield or the harvest index in rainfed farming. In these conditions variability among the genotypes in turgor maintenance, measured as the slope of the turgor potential (ψp) function against water potential (ψw), was observed. The cultivars, which best maintained turgor, were those which were more drought-tolerant. Turgor maintenance was significantly related to osmotic adjustment (OA). However, OA does not explain all the variability observed in dψp/dψw. Therefore, other factors such as tissue elasticity may also be influential. Soluble carbohydrate concentration increased (from 1.5 to 7 times) when the studied cultivars were subjected to water stress. The lines with a conventional leaf-type, showed a greater sugar content than semileafless lines when watered as well as when subjected to desiccation. The stimulation of sugar levels induced by drought was proportional to OA. During stress, the average soluble sugar content of all cultivars would be equivalent to 17.3% or 8.6% of ψs100, if all carbohydrates were present in the tissue as monosaccharides or disaccharides, respectively. This suggests that sugars play an important role in OA in peas. The free proline level also increased (from 4 to 40 times) in response to water stress. However, the contribution of this amino acid to ψs100 was small (approximately 1%) and no significant relationship was observed between proline content and OA. The cultivars which accumulated more proline had lower water contents upon turgor loss. This seems to indicate that proline may play a role in minimizing the damage caused by dehydration.
 
Hybrid indica rice (Oryza sativa L.) cultivars play an important role in rice production system due to its heterosis, resistance to environmental stress, large panicle and high yield potential. However, no attention has been given to its yield responses to rising atmospheric [CO2] in conjunction with nitrogen (N) availability. Therefore we conducted a free air CO2 enrichment (FACE) experiment at Yangzhou, Jiangsu, China (119°42′0′′E, 32°35′5′′N), in 2004–2006. A three-line hybrid indica rice cv. Shanyou 63 was grown at ambient and elevated (ca. 570 μmol mol−1) [CO2] under two levels of supplemental N (12.5 g Nm−2 and 25 g Nm−2). Elevated [CO2] had no effect on phenology, but substantially enhanced grain yield (+34%). The magnitude of yield response to [CO2] was independent of N fertilization, but varied among different years. On average, elevated [CO2] increased the panicle number per square meter by 10%, due to an increase in maximum tiller number under enrich [CO2], while productive tiller ratio remained unaffected. Spikelet number per panicle also showed an average increase of 10% due to elevated [CO2], which was supported by increased plant height and stem dry weight per tiller. Meanwhile, elevated [CO2] caused a significant enhancement in both filled spikelet percentage (+5%) and individual grain weight (+4%). Compared with the two prior FACE studies on rice, hybrid indica rice cultivar appears to profit much more from elevated [CO2] than japonica rice cultivar (ca. +13%), not only due to its stronger sink generation, but also enhanced capacity to utilize the carbon sources in a high [CO2] environment. The above data has significant implication with respect to N strategies and cultivar selection under projected future [CO2] levels.
 
A new large-grain cultivar, Akita 63, of japonica-type rice exhibited high yields with high physiological nitrogen (N)-use efficiency for grain production. Akita 63 and three reference cultivars, Yukigesyou, Toyonishiki and/or Alitakomachi were grown in a field with different levels of N supply for three years. The grain yield of Akita 63 was 22–58% greater than that of the reference cultivars. The highest yield was 9.83 t ha−1 of brown (hulled) rice (approximately 12.3 t ha−1 of rough (unhulled) rice). The dry weight of the aboveground part and the number of spikelets at harvest, and the total leaf area (LAI) at the full-heading stage for a given amount of N accumulated in the aboveground part (plant N) did not differ between Akita 63 and the reference cultivars. The grain yield or panicle dry weight for a given amount of plant N at harvest and for a given unit of LAI were, however, greater in Akita 63 (superior in physiological N-use efficiency) than in the reference cultivars because of a higher proportion of dry matter partitioning into panicles in Akita 63 than in the reference cultivars. As the grain size of Akita 63 was about 35% larger than that of the reference cultivars, the sink capacity (the number of spikelets per unit land area × grain size) for a given amount of plant N was much larger in Akita 63. Reaccumulation of starch in the culms and leaf sheaths was very limited in Akita 63, but remarkable in the reference cultivars in the late stage of grain filling. When pot-grown Akita 63 and the reference cultivar, Toyonishiki, at different stages of ripening were fed with 13CO2 and 13C partitioning into constituent organs was examined at harvest, the proportion of 13C partitioned into panicles was much higher in Akita 63 throughout the ripening period.These results indicate that Akita 63 is a new type of high yielding cultivar of japonica-type rice, superior in physiological N-use efficiency.
 
The relationships between the rate of starch accumulation and the activities of enzymes involving starch biosynthesis in developing grains of field grown rice (Oryza sativa) cultivar Tainung 67 and its sodium azide-induced mutant SA419 were investigated and compared throughout the grain filling period. The results indicated that the activities of most of the grain enzymes involving sugar–starch conversion rose and reached their maximum between 7 and 14 days after anthesis (DAA), but declined rapidly after 21 DAA. The changes in the rate of starch accumulation correlated well with the changes in the activities of sucrose synthase (SUS), invertase, hexokinase, AGPglucose pyrophosphorylase, UDPglucose pyrophosphorylase, phosphoglucoisomerase, phosphoglucomutase, soluble starch synthase (SSS), granule bound starch synthase (GBSS), starch-branching enzyme (SBE) and starch debranching enzyme (SDBE), during the grain filling period. The rapid grain-fill and shorter period of grain filling in mutant SA419, as compared with Tainung 67, were associated with its higher activity of starch synthesizing at the early phase of grain growth. The lower amylose content of SA419 grains (8%) in comparison with Tainung 67 grains (20%) was possibly due to their inferiority to synthesize amylose through GBSS and SDBE.
 
Officially published data for the Czech Republic (CR) from 1920 to 2000 and for selected European countries (mostly from 1960 to 2000) were analysed. In the last 40 years, the yield of the five main crops was comparable with European Union (EU) for wheat, barley and rape, but lower for potato and sugar beet. The fastest yield growth was found for flax (2.15% per year), maize and wheat (1.61 and 1.53%), while growth was slower for hops and root crops and slowest for grassland hay (0.22%). The highest yield variation caused by individual years was for wine grapes (32.5%), poppy, edible legumes and flax (18.5–18.3%), while the lowest level of variation was for cereals, i.e. oats, barley, wheat, rye and hay from arable land (9.7–12.0%). For many crops, yield variation decreased over time. The most adaptable crops, whose yield increased most in fertile years, were flax, wheat, edible legumes, maize, rape and barley (regression coefficient of 1.76–1.24), while the lowest level of adaptability was shown by hops, sugar beet, hay from grassland and poppy (0.68–0.14). The higher the level of adaptability the higher the yield growth over the 75 years analysed. The differences in yield of the most commonly grown crops between the EU and the CR can be explained by the different levels of adaptability of the crops in the two regions.
 
Poor yields of East African highland bananas (Musa spp., AAA-EAHB) on smallholder farms have often been attributed to problems of poor soil fertility. We measured the effects of mineral fertilizers on crop performance at two sites over two to three crop cycles; Kawanda in central Uganda and Ntungamo in southwest Uganda. Fertilizers were applied at rates of 0N–50P–600K, 150N–50P–600K, 400N–0P–600K, 400N–50P–0K, 400N–50P–250K and 400N–50P–600K kg ha−1 yr−1. In addition 60Mg–6Zn–0.5Mo–1B kg ha−1 yr−1 was applied to all treatments, with the exception of the control plots which received no fertilizer. Fresh bunch mass and yield increased with successive cycles. Yield increases above the control ranged from 3.1 to 6.2 kg bunch−1 (average bunch weight for all treatments 11.5 kg bunch−1) and 2.2–11.2 Mg ha−1 yr−1 (average yield for all treatments 15.8 Mg ha−1 yr−1) at Kawanda, compared with 12.4–16.0 kg bunch−1 (average bunch weight for all treatments 14.7 kg bunch−1) and 7.0–29.5 Mg ha−1 yr−1 (average yield for all treatments 17.9 Mg ha−1 yr−1) at Ntungamo. The limiting nutrients at both sites were in the order K > P > N. Potassium, N and P foliar nutrient mass fractions were below previously established Diagnosis and Recommendation Integrated System (DRIS) norms, with the smallest K mass fractions observed in the best yielding plots at Ntungamo. Total nutrient uptakes (K > N > P) were higher at Ntungamo as compared with Kawanda, probably due to better soil moisture availability and root exploration of the soil. Average N, P and K conversion efficiencies for two crop cycles at both sites amounted to 49.2 kg finger DM kg−1 N, 587 kg finger DM kg−1 P and 10.8 kg finger DM kg−1 K. Calibration results of the model QUEFTS using data from Ntungamo were reasonable (R2 = 0.57, RMSE = 648 kg ha−1). Using the measured soil chemical properties and yield data from an experiment at Mbarara in southwest Uganda, the calibrated QUEFTS model predicted yields well (R2 = 0.68, RMSE = 562 kg ha−1). We conclude that banana yields can be increased by use of mineral fertilizers, but fertilizer recovery efficiencies need to improve substantially before promoting wide-scale adoption.
 
Brassica napus L. and × Triticosecale Wittmack surrounding Vicia faba L. plots were tested as isolation mechanisms to reduce inter-plot outcrossing rates at ICARDA, Tel Hadya, Syria. Pollinators identified were Apis mellifera L. and three species of solitary bee, Anthophora canescens Br., Synhalonia sp. aff. hungaria (Fr.) and Eucera cincta Fr. Brassica and, to a lesser extent, triticale were efficient in reducing bee activity in faba bean plots. However, the reduction in bee activity did not result in a corresponding reduction in inter-plot outcrossing rates. Suggestions are made for investigation of the use of Brassica to reduce intra-plot outcrossing rates.
 
Field experiments were conducted during the early cropping seasons of 1982 and 1983 to investigate the response of intercropped maize and okra (Abelmoschus esculentus) grown under five spatial arrangements. In okra, plants grown in alternate hills with maize had the least number of branches, the tallest stems, the least dry matter yields and lowest fruit yields. Okra planted in three rows alternating with three rows of maize had the highest dry matter and fresh fruit yields. Similarly, in maize, plants grown in three alternating rows with okra had the highest yields among the intercropped systems but there were no significant differences in the dry matter and grain yields of maize planted in alternate hills with or between the rows of okra. Percent light transmission and yields of both crops followed similar trends under the various intercropping systems. The spatial arrangement that allowed the lowest light transmission also produced the lowest yields.
 
Traditional breeding efforts are expected to be greatly enhanced through collaborative approaches incorporating functional, comparative and structural genomics. Potential benefits of combining genomic tools with traditional breeding have been a source of widespread interest and resulted in numerous efforts to achieve the desired synergy among disciplines. The International Center for Tropical Agriculture (CIAT) is applying functional genomics by focusing on characterizing genetic diversity for crop improvement in common bean (Phaseolus vulgaris L.), cassava (Manihot esculenta Crantz), tropical grasses, and upland rice (Oriza sativa L.). This article reviews how CIAT combines genomic approaches, plant breeding, and physiology to understand and exploit underlying genetic mechanisms of abiotic stress adaptation for crop improvement. The overall CIAT strategy combines both bottom-up (gene to phenotype) and top-down (phenotype to gene) approaches by using gene pools as sources for breeding tools. The strategy offers broad benefits by combining not only in-house crop knowledge, but publicly available knowledge from well-studied model plants such as arabidopsis [Arabidopsis thaliana (L.) Heynh.]. Successfully applying functional genomics in trait gene discovery requires diverse genetic resources, crop phenotyping, genomics tools integrated with bioinformatics and proof of gene function in planta (proof of concept). In applying genomic approaches to crop improvement, two major gaps remain. The first gap lies in understanding the desired phenotypic trait of crops in the field and enhancing that knowledge through genomics. The second gap concerns mechanisms for applying genomic information to obtain improved crop phenotypes. A further challenge is to effectively combine different genomic approaches, integrating information to maximize crop improvement efforts. Research at CIAT on drought tolerance in common bean and aluminum resistance in tropical forage grasses (Brachiaria spp.) is used to illustrate the opportunities and constraints in breeding for adaptation to abiotic stresses.
 
Irrigated crops are increasingly facing water scarcity and other forms of abiotic stress, including the presence of salts and other pollutants in soil and irrigation water, waterlogging and flooding of soils, low pH in acid sulfate soils, and anaerobic and toxic conditions in the rootzone. More progress has been made with the alleviation of some of these stresses than with others. This paper examines why salinity of soil and irrigation water is common and nearly inevitable in the irrigated lands of the semi-arid tropics and subtropics. It considers opportunities for and constraints in making management changes that would increase the productivity of water (i.e., the yield per unit of water diverted from the source or consumed in evapotranspiration) at system and basin level. Success of water management interventions in reducing the impact of abiotic stresses on rice production under saline conditions and on acid soils depends on improved control over the components of the water and ion balances at field and basin level. One of these measures is the installation of drainage systems. The benefits and costs of sub-surface drainage are illustrated by an example from Egypt's Nile Delta. An integrated approach to water resource management is an essential but not sufficient condition to prevent conflicts between different users and consumers of the scare resource.
 
Low and unstable rice productivity in many areas of Asia is associated with many abiotic and biotic stresses such as drought, salinity, anaerobic conditions during germination, submergence, phosphorus and zinc deficiency, etc. To develop rice varieties with tolerance to these stresses, we undertook a large backcross (BC) breeding effort for the last 6 years, using three recurrent elite rice lines and 203 diverse donors, which represent a significant portion of the genetic diversity in the primary gene pool of rice. Significant progress has been made in the BC breeding program, which resulted in development of large numbers of introgression lines with improved tolerance to these stresses. Promising lines have been developed with excellent tolerances (extreme phenotypes) to salinity, submergence and zinc deficiency; resistance to brown plant hopper, ability to germinate under the anaerobic condition and low temperature. Our results indicated that there exist tremendous amounts of ‘hidden’ diversity for abiotic and biotic stress tolerances in the primary gene pool of rice. Furthermore, we demonstrated that despite the complex genetics and diverse physiological mechanisms underlying the abiotic stress tolerances, introgression of genes from a diverse source of donors into elite genetic backgrounds through BC breeding and efficient selection (careful screening under severe stress) is a powerful way to exploit this hidden diversity for improving abiotic stress tolerances of rice. We have developed three large sets of introgression lines, which not only provide an unique platform of breeding materials for developing new rice cultivars with superior yield and stability by trait/gene pyramiding, but also represent unique genetic stocks for a large-scale discovery of genes/alleles underlying the abiotic and biotic stress tolerances of rice using genomic tools.
 
Saline soils are characterized by an array of properties that are adverse to rice cultivation. The problem of salinity is compounded by mineral deficiencies (Zn, P) and toxicities (Fe, Al, organic acids), submergence, deep water and drought. These soil stresses vary in magnitude and interactions over time and place, making long-term adaptability of a cultivar dependent on its level of tolerance to all the stresses that occur in its growing environment. Thus, in breeding rice for saline environments, multiple stress tolerance traits must be considered. Some degree of cultivar tolerance for these stresses is in rice germplasm. However, understanding the physiological mechanisms of these traits, their biochemical basis, inheritance and efficient screening techniques are needed to hasten breeding progress.
 
Grain yield components: tillers [no. m -2 ], productive tillers (P-tiller) [%], straw weight (straw) [g m -2 ], filled-grain weight (Grain) [g m -2 ], spikelets (Spike) [no. m -2 ], filled grain (F-grain) [%], and HI for Apo (V1) in the main plot and varieties V2 to V4 in sub-plots.
Grading of general root health on a scale of 1-9 in Biocide, Control, TP, and Fallow for sequential root samples in DAE at Tarlac in DS 2006 and 2007; treatments are explained in the text.
Total soil mineral N and percentage of NH 4 + -N in Control (C), Biocide (B), TP, and Fallow (F) at the Tarlac field site; treatments are explained in the text.
Aerobic rice is a new production system for water-short environments. Adapted varieties are usually direct dry seeded and the crop grown under aerobic soil conditions with supplementary irrigation as necessary. Occasionally, yield failures occur which may be related to soil health problems. In the dry season of 2006 and 2007, we conducted a field experiment in the Philippines, to identify the major causes of such yield failure. Four treatments were implemented: (i) Control of direct dry-seeded rice (improved upland variety Apo), (ii) Biocide application, (iii) transplanting into aerobic soil, and (iv) 1 year fallow. Yield in the Biocide treatment was 2 t ha−1 in both years. In all other treatments, yield was 0–0.3 t ha−1. Plants grown in the Biocide treatment showed a reduced degree of galling of roots caused by root-knot nematodes (RKN) and better general root health than the Control treatment. Potentially pathogenic fungi were isolated from root samples (Pythium sp., Fusarium sp., and a Rhizoctonia-like species). Abiotic effects of the Biocide treatment were an increase in KCl-extractable N (initial season 2007) and a decrease in initial soil pH. In the Control treatment, soil pH increased from 6.5 to 8.0 over the two seasons. In 2007, plant tissue analysis indicated Mn deficiency in the Control treatment. Plants that were subjected to foliar micronutrient sprays reacted positively to Fe and Mn sprays in the Control treatment, and Mn spray in the Biocide treatment. We concluded that the Biocide application led to favorable soil conditions by reducing biotic stresses such as RKN and improving nutrient availability. In the other treatments, an interaction of RKN and micronutrient deficiencies with increasing soil pH led to yield failure.
 
Banana is the primary food crop in Uganda, but yields are low due to a complex of abiotic and biotic constraints. However, quantitative information on the importance, interactions, and geographic distribution of yields and constraints is scanty. We monitored yields, biotic and abiotic constraints in 159 plots in Central, South and Southwest Uganda in 2006–2007. About half the plots were on-farm demonstrations that received fertilizer (average 71N, 8P, 32 K kg ha−1 year−1) through a development project, the rest were ordinary farmer fields (i.e. controls). Fresh banana yields in controls were significantly (P ≤ 0.05) higher in Southwest (20 t ha−1 year−1) compared with Central (12 t ha−1 year−1) and South (10 t ha−1 year−1). Demonstrations yielded 3–10 t ha−1 year−1 more than controls. Yield losses were calculated using the boundary line approach. In Central, yield losses, expressed as percentage of attainable yield, were mainly attributed to pests (nematodes 10% loss, weevils – 6%) and suboptimal crop management (mulch 25%). In South, poor soil quality (pH – 21%, SOM – 13%, N-total – 13%, and Clay – 11%) and suboptimal crop management (weeds – 20%) were the main constraints. In Southwest, suboptimal crop management (mulch 16%), poor soil quality (K/(Ca + Mg) − 11%) and low rainfall (5%) were the primary constraints. The study revealed that biotic stresses (i.e. pests, weeds) are particularly important in Central, whereas abiotic stresses (i.e. nutrient deficiencies, drought) dominate in South and Southwest. This study concludes that (i) technologies currently available allow farmers to double yields and (ii) past research efforts have mistakenly neglected abiotic constraints.
 
Maize (Zea mays L.) grain yield is particularly sensitive to water deficits that coincide with the tasseling-silking period, causing marked reductions in grain number. More knowledge about crop responses to water supply is required, however, to explain the causes of kernel number reductions under the mild stresses characteristic of humid regions. The objectives of this study were to: (i) quantify crop evapotranspiration, Ec, and its relationship with shoot biomass production, grain yield, and kernel number; and (ii) determine the impact on final kernel number of supplying fresh pollen to silks whose appearance is delayed by water deficits at silking. Field experiments were conducted at Balcarce (37°45′S, 130 m) during 1988/89 and 1989/90 with two sowing dates (6 weeks apart) to provide differences in evaporative demand. Plastic covers were placed on the ground of water-deficit plots to generate a 40-day period of lowered water supply bracketing silking. Control plots received rain plus additional furrow irrigation in order to keep the ratio between crop (c) and potential (Ep) Penman evapotranspiration greater than 0.9. Plant water status indicators revealed differences between treatments, but failed to reflect soil water status. Water deficit reduced plant height, maximum leaf area index, and shoot biomass. Shoot biomass accumulation was correlated with Ec, but higher water-use efficiencies (WUE) were found for the water-stress treatments. Grain yield was correlated to kernels m−2 (r = 0.88; 6 d.f.), and both grain yield and kernels m−2 were related to Ec during the treatment period, resulting in reductions of 4.7 grains m−2 and 17.7 kg ha−1 for each mm reduction in Ec. The number of kernels per ear did not improve when fresh pollen was applied to late appearing silks, suggesting that ovaries which failed to expose their silks synchronously with pollen shedding were deleteriously affected by water stress.
 
Cereal crop species and varieties differ in competitive ability against weeds mainly as influenced by differences in canopy architecture. The FASSET crop model was used to separate the effects of a number of crop traits on the suppressive ability of winter wheat varieties and the ability to tolerate weeds. The model simulated the competition between different varieties of winter wheat and a sown grass mixture for light, water and nitrogen. Crop physiological parameters of eight varieties and one variety mixture were estimated from measurements in a 3-year field experiment. The parameters estimated were thermal time from emergence to flag leaf appearance, thermal time from flag leaf appearance to anthesis, thermal time from anthesis to yellow ripeness, height development rate and final plant height, specific leaf area, leaf area per N uptake, vertical displacement of leaf area, and extinction coefficient for light.
 
Foliar content of free abscisic acid (ABA) and proline, leaf water potential (ψ1), and osmotic potential (ψ0) were determined in seven diverse rice (Oryza saliva L.) cultivars after these were subjected to a 10-day period of withholding irrigation or to well-watered conditions. For three of the cultivars, periodic determination of leaf ABA was performed during differential irrigation and recovery. Diurnal kinetics of leaf ABA content, along with observations on leaf bulk turgor (ψt) and gas exchange, were established in two cultivars under well-watered conditions. Water-stress-induced proline and ABA accumulation differed among cultivars. Across cultivars, stress-induced proline accumulation was correlated negatively with midday ψ1 and positively with osmotic adjustment, but ABA accumulation was not. Proline accumulation therefore gave a similar discrimination among cultivars as did predawn turgid osmotic potential and ψ1. Diurnal oscillations of ψ0 differed among cultivars but were not related to ABA accumulation or ψ1. Interpretation of stress-induced ABA accumulation kinetics was difficult due to large diurnal oscillations. The relative amplitude of diurnal ABA oscillation reflected diurnal changes in ψ1 and ψt across cultivars. Diurnal ABA response to ψt of a drought-resistant cultivar was less sensitive than that of a susceptible cultivar under well-watered conditions. Highest ABA concentrations were observed at 16:00 h. Diurnal ABA accumulation lagged 4 hours behind changes in leaf bulk turgor. The results suggest that high- and low-ABA-accumulating rice cultivars exist, but no clear linkage of this trait to behavior under water stress was observed.
 
A field experiment was conducted to test the hypothesis that young cotton (Gossypium hirsutum L.) fruits (bolls) serve as sinks for ABA, or that they are a source of IAA for subtending leaves. Cotton was grown in eight four-row plots in Phoenix during the summer of 1989 and irrigated about every two weeks. In one test, flowers at anthesis were removed on 22 and 29 June and on 7 and 14 July from the first node of fruiting branches of one center row, but not the other row, in each of four replications. Subtending leaves were harvested seven days after deflowering for ABA and IAA analyses. Removal of one flower per plant had no consistent effect on the concentrations of ABA and IAA in the subtending leaf. In another test, all flowers and bolls were removed from all plants of one center row, but not the other row, of four replications on 30 June and 5 and 13 July. Uppermost fully expanded mainstem leaves were harvested from the same plots on 20, 25, and 27 July, and on 9 August (7, 12, 14, and 27 days after final defruiting) for ABA and IAA analyses. Removal of all fruits apparently caused a slight increase in ABA concentration in mainstem leaves, but the effect disappeared with time after defruiting. The ability of leaves to accumulate ABA apparently decreased slightly as boll load increased. Complete fruit removal did not, however, affect the IAA content of leaves.
 
Top-cited authors
Kenneth Cassman
  • University of Nebraska at Lincoln
Victor O. Sadras
  • South Australian Research and Development Institute
Gustavo A Slafer
  • Universitat de Lleida
Ken Giller
  • Wageningen University & Research
P.A. Tittonell
  • Instituto Nacional de Tecnología Agropecuaria