Plant growth, water relations and transpiration of two species ofAfrican nightshade (Solanum villosum Mill. ssp. miniatum(Bernh. ex Willd.) Edmonds and S. sarrachoides Sendtn.)under water-limited conditions
ABSTRACT The adaptation to drought stress of two African nightshade species, Solanum villosum and S. sarrachoides was investigated in pot and field experiments between 2000 and 2002. Two genotypes of S. villosum (landrace and commercial) and one accession of S. sarrachoides were grown under droughted, moderate stress and well-watered conditions. Leaf expansion, stem elongation and transpiration began to decline early in the drying cycle with fraction of transpirable soil water (FTSW) thresholds of 0.46–0.64. Osmotic adjustment (OA) of both species was in the range of 0.16–0.19 MPa and could not maintain positive turgor below water potentials of À1.80 to À2.04 MPa. The responses evaluated were similar in the three genotypes suggesting similar strategies of adaptation to drought stress. Under field conditions, the S. sarrachoides accession showed a higher leaf area than the S. villosum commercial genotype. It is concluded that the three African nightshade genotypes have limited OA capacity and adapt to drought mainly by regulating transpiration. This was achieved by reduction of leaf area. In general, it is necessary to maintain FTSW above 0.5– 0.6 to prevent decline in leaf expansion, stem elongation, and transpiration.
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ABSTRACT: Typical responses of leaf expansion and gas exchange rate to plant available soil water (PAW) can be described with two straight lines that intersect at PAWt, i.e., the PAW threshold for which the rate of the process in stressed plants starts to diverge from a reference value. PAWt is a parameter widely used in modelling plant responses to water deficits. It also reflects some apparent physiological mechanism because plants appear to be able to sense soil water status or related variable(s). In this paper comparisons are made between PAWt for various species (monocots and dicots) and plant processes (leaf expansion and gas exchange) in order to: (i) point out methodological sources of variation in published values of PAWt; and (ii) analyse variations in PAWt in relation to plant and environmental factors. Reported values of PAWt vary over almost the whole possible range of PAW (i.e., 0 to 1). Average thresholds reflect the greater responsiveness to water deficits of tissue expansion (average PAWt = 0.56) relative to gas exchange (0.40). Average PAWt for leaf water potential (0.61) and stomatal conductance (0.37) are very close to the average thresholds for expansion and gas exchange, respectively. Soil water thresholds for leaf expansion are also shown to discriminate between plant types (0.50 for monocots vs. 0.66 for dicots) and soils (0.72 for coarse- vs. 0.43 for fine-textured soils). The simplicity of characterising plant responses to water stress in terms of PAWt is attractive. In agreement with known physiological relationships, however, our analysis highlights how, for given processes and species, the measured value of PAWt can be affected by evaporative demand, root distribution, soil texture and soil bulk density, among other factors, thus making explicit some of the assumptions underlying the use of fixed soil-water thresholds in simulation models.Field Crops Research. 01/1996;
- Advances in Agronomy, Vol 71. 01/2001; 71:193-231.
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ABSTRACT: Several mechanisms have been proposed to explain the high tolerance of sorghum (Sorghum bicolor (L.) Moench) to drought. This paper reports a field study on the effects of soil moisture stress on the rooting habits, transpiration rate, and xylem anatomy of two sorghum cultivars, Tabat (drought susceptible) and Gadambalia (drought tolerant). Two levels of water stress, -0.02 MPa (wet) and -0.75 MPa (dry), were applied. Tabat had a higher root length density (RLD), higher late metaxylem (LMX) vessels per nodal root, higher leaf area, and higher transpiration rate than Gadambalia. In Tabat, soil moisture stress reduced RLD by 30%, nodal roots by 31%, number of LMX vessels in the root by 42%, leaf area by 13%, and transpiration rate by 11%. In Gadambalia soil moisture stress did not affect RLD at depths gtoreq 0.2 m, number of nodal roots, or number of LMX vessels per nodal root. However, leaf area and transpiration rate were reduced by 3 and 11%, respectively. Under dry conditions, Gadambalia displayed a higher water extraction efficiency than Tabat throughout the profile (0-0.9 m). In Gadambalia, unlike Tabat, the stem was highly sclerified. A 1- to 3-cell-thick layer of schlerenchyma was observed beneath the epidermis. The peripheral vascular bundles were surrounded with a 3- to 6-cell-thick schlerenchyma sheath. However, in roots anatomical differences were less prominent. Drought tolerance fewer nodal roots per plant, fewer LMX vessels per nodal root, a smaller leaf area, and a well developed sclerenchyma.01/1999; 39:168-173.
Plant growth, water relations and transpiration of two species of
African nightshade (Solanum villosum Mill. ssp. miniatum
(Bernh. ex Willd.) Edmonds and S. sarrachoides Sendtn.)
under water-limited conditions
P.W. Masindea,b, H. Stu ¨tzela,*, S.G. Agongb, A. Frickea
aInstitute of Vegetable and Fruit Science, University of Hannover, Herrenha ¨user Str. 2, D-30419 Hannover, Germany
bDepartment of Horticulture, Jomo Kenyatta University of Agriculture and Technology, P.O. Box 62000, 00200 Nairobi, Kenya
Received 31 August 2005; received in revised form 10 May 2006; accepted 6 June 2006
The adaptation to drought stress of two African nightshade species, Solanum villosum and S. sarrachoides was investigated in pot and field
experiments between 2000 and 2002. Two genotypes of S. villosum (landrace and commercial) and one accession of S. sarrachoides were grown
under droughted, moderate stress and well-watered conditions. Leaf expansion, stem elongation and transpiration began to decline early in the
drying cyclewith fraction of transpirable soil water (FTSW) thresholds of 0.46–0.64. Osmotic adjustment (OA) of both species was in the range of
0.16–0.19 MPa and could not maintain positive turgor below water potentials of ?1.80 to ?2.04 MPa. The responses evaluated were similar in the
threegenotypes suggesting similar strategies of adaptation to drought stress. Under field conditions, the S. sarrachoides accession showed a higher
leafareathanthe S.villosumcommercial genotype.Itisconcludedthatthe threeAfricannightshadegenotypeshavelimitedOAcapacityandadapt
to drought mainly by regulating transpiration. This was achieved by reduction of leaf area. In general, it is necessary to maintain FTSWabove 0.5–
0.6 to prevent decline in leaf expansion, stem elongation, and transpiration.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Fraction of transpirable soil water; Leaf expansion; Normalized transpiration ratio; Osmotic adjustment; Relative water content; Water potential
African nightshade refers to a loose grouping of Solanum
Kenya and various parts of Africa (Chweya and Eyaguirre,
1999). The species of African nightshade utilized by some rural
communities in Kenya include Solanum villosum Mill. subsp.
miniatum (Bernh. ex Willd.) Edmonds and S. sarrachoides
Sendtn. S. villosum has erect plants with rhombic to ovate-
lanceolate leaves and red or orange berries when ripe. On the
other hand, S. sarrachoides plants are erect and bushy with
pale green stems, ovate-lanceolate to lanceolate leaves and
pale green berries when ripe (Edmonds and Chweya, 1997;
Agronomic studies to develop optimal cultivation practices
for improved yield and nutritive quality of these crops have
concentrated mainly on fertilizer use (Murage, 1990; Khan
et al., 1995, 2000). There is only scattered information
concerning water management of African nightshade. The
general assumption is that it is intolerant to water stress
(Edmonds and Chweya, 1997). Related species, Solanum
nigrum L. and S. ptycanthum Dun. have been shown to be
highly sensitive to water stress, experiencing more than 50%
reduction of the height, leaf area and biomass when watering to
water holding capacity was done biweekly instead of weekly
(McGiffen et al., 1992). Furthermore, these species shifted
sarrachoides has been shown to utilize soil moisture less
quickly as compared to S. ptycanthum, hence it is considered to
be more drought tolerant (Tan and Weaver, 1997).
Plants respond to water deficits by decreasing leaf area,
which reduces transpiration and hence conserves water during
periods of drought (Jones, 1992). This is achieved by reduction
Scientia Horticulturae 110 (2006) 7–15
* Corresponding author. Tel.: +49 511 762 2634; fax: +49 511 762 3606.
E-mail address: email@example.com (H. Stu ¨tzel).
0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
of leaf expansion due to decreased turgor (Nagel et al., 1994;
Serpe and Mathews, 2000), retarded leaf initiation (Belaygue
et al., 1996) and enhanced leaf senescence (Pic, 2002). Plant
water use is closely correlated with leaf area development rate.
Plant species orgenotypes with high leaf area development rate
also have a high water use rate (Salih et al., 1999). Rajendrudu
et al. (1996) have reported that a high leaf area development
rate in spiderplant (Cleome gynandra) was associated with a
high transpiration rate and that this was detrimental to the
plant’s leaf water status. Thus, while a high water use may be
desirable under well-watered conditions, it can lead to lack of
tolerance to water deficits in the event of a drought. Reduction
of plant leaf area is important for the plant’s survival under
drought (Sinclair and Muchow, 2001).
Leaf expansion and transpiration for various crops show
different sensitivities to declining plant available soil water.
However, in general terms these processes reduce when about a
third of the transpirable soil water remains in the soil (Sadras
and Milroy, 1996; Turner, 2001). Leaf expansion and
transpiration have been shown to decline when 0.3–0.4 of
the transpirable soil water remains in the soil for crops such as
field pea (Pisum sativum L.) (Lecoeur and Sinclair, 1996),
maize (Zea mays L.) (Muchow and Sinclair, 1991; Ray et al.,
2002) and soybean (Glycine max (L.) Merr) (Ray and Sinclair,
1998; Sinclair et al., 1998). In chickpea (Cicer arietinum L.),
leaf expansion declined when about 0.6 of the transpirable soil
water was remaining in the soil (Soltani et al., 1999). Leaf
expansion in a drying soil can be reduced before any
measurable decline in leaf water status in some crops (Saab
and Sharp, 1989; Dodd et al., 2002). This has been attributed to
a non-hydraulic signal produced when roots are growing in a
drying or compacted soil, which act to inhibit leaf expansion
(Saab and Sharp, 1989; Roberts et al., 2002). The hormone
ABA is known to play a major role in this signal, but other
hormones, ions and growth inhibitors have been shown to be
involved (Munns, 1992; Davies et al., 2002). This root signal
provides the shoot with a measure of the water availability and
in this way plants growing in dry soil can reduce their
transpiration even with only small changes in soil water
potential and before changes in leaf water status (Davies et al.,
In a drying soil, the soil hydraulic conductivity declines as
the volumetric water content decreases and the rate of water
uptake by plants from the soil may be lower than the potential
declines, and this reduces transpiration (Sinclair and Ludlow,
1986). In vegetable amaranth (Amaranthus sp.), transpiration
declines in plants exposed to drying soil mainly due to
reduction in stomatal conductance (Liu and Stu ¨tzel, 2002). In
various crops, stomatal conductance begins to decline when
about 0.4 of the transpirable soil water is remaining in the soil
(Sadras and Milroy, 1996).
Relative water content (RWC), osmotic potential at full
turgor (OP) and water potential (WP) of leaves are measures of
plant water status, which are useful in monitoring the
development of stress in plants growing under droughted
conditions (Jones, 1992). Plants respond to declining water
potential under drought through osmotic adjustment (OA) as a
result of accumulation of solutes within cells to help maintain
turgor of shoots and roots (Turner et al., 2000). This allows
turgor dependent processes such as stomatal opening and
expansive growth to continue at reduced rates under declining
water potentials (Jones, 1992). Drought affects transpiration of
plants partly through inhibition of leaf area development
(Jones, 1992). For optimisation of water management these
relationships have to be known quantitatively.
The objective of this study therefore was to evaluate the
adaptive responses of selected African nightshades to drought
in terms of leaf area, relative water content, water potential,
osmotic potential and transpiration. The study aimed at
establishing the thresholds of plant available water at which
In addition, the study also aimed at evaluating differences inthe
adaptation between genotypes. The hypotheses tested are that
leaf expansion, leaf water status and transpiration of African
nightshade decline when the fraction of transpirable soil water
(FTSW) falls below specific thresholds and that these thresh-
olds are genotype dependant.
2. Materials and methods
2.1. Pot experiments
2.1.1. Experimental design
Two pot experiments were conducted in 2000 and 2002 at
the Institute of Vegetable and Fruit Science, University of
Hannover, Germany (latitude 528140240N, longitude 98430480
E). Each was a factorial experiment laid out in a completely
randomized design with three replications. Two genotypes of S.
villosum, a landrace cultivated in Western Kenya (landrace,
land) and a commercial genotype (Kenya Seed Co. Ltd.,
Nairobi, Kenya) (commercial, com), and one accession of S.
sarrachoides (Genebank of Kenya, GBK 028726 and IPK
Genebank Gatersleben, Germany, Sol 262/97) (acc) originating
from Eastern Kenya were grown at two water levels, droughted
and well-watered. The landrace and commercial genotypes of
S. villosum were used in 2000, while the S. sarrachoides
accession was included in the 2002 experiment.
2.1.2. Sowing and cultivation details
Pots made from PVC pipes, 1 m in length and 0.20 m in
diameter were filled with loess soil obtained from the Ruthe
research station, south of Hannover, to soil bulk densities of
1.25 and 1.18 Mg/m3in 2000 and 2002, respectively. The pot
water holding capacities were 0.28 and 0.27 w/w in 2000 and
2002, respectively. The experiments were conducted in a
glasshouse with set temperatures of 26 8C day and 20 8C night.
During the experimental period, the average daily temperatures
were 25.0 8C (17.0–37.7 8C) and 25.4 8C (16.9–36.30 8C) in
2000 and 2002, respectively. The average relative humidity in
the glasshouse was 63.4% (50.6–87.8%) in 2002, and 83.7%
(79.9–86.9%) in 2002. Seeds of the selected genotypes were
sown directly into the pots. 1.5 and 1.0 g N/plant was applied 3
weeks after seedling emergence in 2000 and 2002, respectively,
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–158
through irrigation using fertilizer Flory 9 (15% N:7% P2O5:6%
MgO). The soil surface was then covered to a depth of about
3 cm with quartz gravel to minimize soil evaporation. Plants
were well-watered daily after emergence. The drought
treatments began 31 and 30 days after sowing in 2000 and
2002, respectively. At the onset of the treatments, soil moisture
was raised to 84% and 90% water holding capacity (WHC) in
2000 and 2002, respectively. Droughted pots received no more
water thereafter, while well-watered pots were irrigated daily
by hand to maintain the respective water holding capacities in
the 2 years.
The stress level for each plant was expressed as a function of
soil water content. The fraction of transpirable soil water
(FTSW) left in each pot on any given day i was calculated as:
FTSW ¼pot weight at dayi ? final pot weight
initial pot weight ? final pot weight
where initial pot weight refers to the weight of a pot at 100%
WHC, while final pot weight refers to the weight of the same
pot when the transpiration of stressed plants was less than 10%
of that of well-watered plants.
2.1.3. Plant growth and water status determination
Five to 7 plants of each genotype and water level were
marked for daily measurements of leaf length and width, and
plantheightusing ameter rule.Leaveswere numbered fromthe
base of the stem and measurements of the length (L) and the
width (W) at the widest part of the leaf were done on leaves 14
and 15 in 2000, and 11, 12 and 13 in 2002. The measurements
were stopped when there were no more increases in leaf
dimensions and height of plants in the droughted treatments.
relationship was established between LA and the actual leaf
area measured by leaf area meter (model 3100; LICOR,
Lincoln, NE, USA) on individual leaves (n = 184 and 90 for S.
villosum and S. sarrachoides, respectively) using linear
regression procedure (SAS, 1999). The differences in LA
and plant height between two consecutive days were taken as
the leaf expansion and stem elongation rates, respectively.
These rates for droughted plants were divided by those of well-
watered plants to give relative leaf expansion rate (RLER) and
relative main stem elongation rate (RSER). RLER and RSER
were then expressed as functions of FTSW to show the
relationship between leaf expansion and stem elongation and
the fraction of transpirable soil water.
Sampling for water relations measurements was carried out
at the onset of drought treatments, at 60% and 40% WHC in
2000, and 70%, 60% and 40% WHC in 2002. The last sampling
in both years was performed when the transpiration of
droughted plants was below 10% of that of well-watered
plants. Water relations measurements were done on the
youngest fully expanded leaves between 11.00 and 15.00 h.
The relative water content of a leaf (RWC) was calculated as:
fresh weight ? dry weight
full turgid weight ? dry weight
where fresh weight was obtained immediately after cutting, full
turgid weight after placing the leaf in deionised water in a petri
dish at 20 ? 2 8C in dim illumination for 24 h and dried at
100 8C to a constant weight before weighing.
Osmotic potential (OP) was measured on leaf discs of 1 cm
diameter that had been dipped in liquid nitrogen, using a
psychrometer (C52-chamber, Wescor Inc., Logan, USA). To
quantify osmotic adjustment, Ludlow’s full-turgor adjustment
method was used (Ludlow et al., 1983). In this method, bound
water is neglected and the leaf osmotic potential at full turgor
(OP100) of both droughted and well-watered plants is given by:
OP100T¼ OPT? RWCT
where subscript T refers to treatments, droughted and well-
The osmotic adjustment (OA) was determined as the
difference between osmotic potential at full turgor of well-
watered and droughted plants. Leaf water potential (WP) was
measured by the pressure chamber method (Scholander et al.,
respectivevaluesofthewell-wateredplants togive ratios,which
were then expressed as a function of FTSW. Transpiration was
data were analysed using the double normalization procedure
outlined by Ray and Sinclair (1998). Firstly, transpiration ratio
relative to the average transpiration of well-watered plants:
transpiration of droughted plants
average transpiration of well-watered plants
A second normalization was done so that the TR of each plant
pot was high (FTSW > 0.50). First, a mean TR was calculated
the daily TR for each pot was divided by this mean TR to give a
daily normalized transpiration ratio (NTR).
2.2. Field experiment
2.2.1. Experimental design
A field experiment was carried out at the Jomo Kenyatta
Kenya (1525 m above sea level, latitude 18100480S, longitude
378070120E) between August and October 2001. The soils are
well drained, moderately deep to deep dark brown coloured,
friable and gravely clay over petroplinthite, classified as eutric
cambisols (FAO/UNSECO, 1974; Muchena et al., 1978).
However, amendments have been done including adding new
soils to horizon A. Two genotypes, a commercial (S. villosum)
and an accession (S. sarrachoides) were used. The seed source
The experiment was carried out as a split plot design with three
replications. The main plots consisted of three water levels,
consistedoftwogenotypes.Plastic sheets wereplacedvertically
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–159
in the soil at a depth of 1.0 m all round the main plots to limit
lateral water movement. At the onset of drought treatments, all
plots were thoroughly irrigated. Soil samples were taken 48 h
later using a soil auger at depths of 0–20 and 20–40 cm, and the
the soil samples before and after drying at 105 8C for 48 h.
Estimation of the amount of water necessary to maintain soil
wasbasedonthe soilwaterbalanceconcept(Kramer andBoyer,
1995). The following expressions were used:
W ¼ SWC1? SWC2
SWC ¼ GW? soil bulk density ? d ? a
whereWrefers tothe amountofwater tobe applied (kg),SWC1
(kg) the soil water content for the 0–20 cm soil layer at 80% FC
for the well-watered or 60% FC for the moderate stress treat-
ments, SCW2(kg) the measured soil water content, GWthe
gravimetric soil water content (%) in the soil layer, d (m) the
thickness of the soil layer and a is the area (m2) of the plot.
During watering, the estimated amount of water was applied
carefully by hand to avoid runoff and deep drainage. The soil
water status was determined by measuring the gravimetric soil
water content at 4–8 days intervals for the droughted and
moderate stress treatments (before rewatering) and 2–3 days
intervals for the well-watered and moderate stress treatments
(after rewatering). Soil sampling for gravimetric water
determination on the well-watered plots was always conducted
just before watering. The same was applied for the moderate
stress plots once rewatering had began. No irrigation was done
on the droughted plots after onset of treatments.
2.2.2. Sowing and cultivation details
Seedlings were grown in small pots (10 cm diameter) for 1
monthandthentransplantedonpreparedplotsof2 m ? 2.5 m,at
a spacing of 40 cm between and within rows, resulting in 30
days after transplanting when the treatments began. At the onset
of the treatments, 90 kg N/ha and 40.2 kg P/ha were applied
superphosphate (46% P2O5). At the same time a rainout shelter
film. The film material allowed transmission of 43% of incident
rolling the plastic film up during the day, but was fully closed in
the event of rain and at night. The total solar radiation received
from the time of transplanting to the end of the experimental
period was 1263.7 MJ m?2with a mean of 19.4 MJ m?2day?1.
Theaveragedailytemperaturewas19.5 8C(7.1–29.2 8C),witha
relative humidity of 61.1% (49.5–85.0%). The rainout shelter
was 1–2 m above the crop canopy.
2.2.3. Harvesting and leaf area measurements
Seven harvests, based on the soil water status, were carried
out. Due to the amount of work involved in separating plant
parts and leaf area measurement, only one plant in a central row
from each plot was cut at the base and divided into lamina,
petioles and stems. Plant leaf area was measured using a leaf
area meter (model AAM-8, Hayashi Denko Co. Ltd., Japan).
2.3. Data analyses
The relationships between relative parameters, i.e. RLER,
RSER and NTR, and FTSW were developed through linear
relative parameter ¼ 1
relative parameter ¼ 1 þ A ? ðFTSW ? FTSWtÞ
where A is the slope of the linear decline and FTSWtis the
FTSW threshold at which the relative parameter began to
decline. The coefficient of determination, R2, was calculated
as the sum of squares of the residual divided by the total
corrected sum of squares. Analysis of variance was performed
for OA from pot experiments and leaf area from field experi-
ment for each date separately using the GLM procedure of SAS
(1999). Significance level was set at P < 0.05.
3.1. Pot experiments
The rate of soil drying in the pot experiments was similar for
the genotypes (data not shown). Leaf expansion of droughted
plants in the pot experiments began to decline relative to the
expansion of the well-watered plants at FTSW values around
0.60 with no significant differences between genotypes in both
years (Fig. 1a–c and Table 1). Similar results were obtained for
stem elongation (Fig. 1d–f and Table 1).
The relative water content (RWC) in the droughted plants
declined from 0.75 to 0.60 and 0.78 to 0.56 in 2000 and 2002,
respectively. In the well-watered plants, RWC was in the range
of 0.75–0.84 in both years. Water potential (WP) of droughted
?1.8 MPa in 2002. On the other hand, WP in well-watered
plants varied between ?0.33 and ?0.79 MPain both years. The
ratio of RWC and WP of droughted plants to that of well-
watered plants (RWC and WP ratios) remained close to 1.0
until FTSW fell below about 0.2–0.4, after which it changed
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–1510
The fraction of transpirable soil water threshold for leaf expansion and stem
commercial genotypes (land/com pooled) and the accession (acc) grown in the
glasshouse in 2000 and 2002
YearGenotype Slope (A)FTSWt
The 95% confidence intervals are shown in parentheses.
rapidly in both years (Fig. 2). Droughted plants had lower
osmotic potential at full turgor (OP), significant from 16 days
after the start of the drought treatments in both years (Fig. 3).
The resultant osmotic adjustment was 0.16 MPa in 2000 and
0.19 MPain2002atmaximumstress(FTSW = 0).TheOPratio
also declined when FTSW fell below 0.2–0.4 (data not shown).
The relationship between NTR and FTSW fitted well to the
plateau regression function (Fig. 4). The FTSW thresholds for
the NTR decline were similar for genotypes and also the
thresholds were comparable in both years (Table 2).
3.2. Field experiment
Under field conditions, the gravimetric soil water contents
at 0–20 cm and 20–40 cm were similar, hence only the mean
of the two depths is considered. Fluctuation of the gravimetric
soil water content was high under the well-watered treatment
(Fig. 5a). However, the soil water content was maintained
around 25% (75% FC) for both genotypes in the early stages
of growth. In the later stages, the soil moisture content for the
well-watered commercial plants at both depths remained
significantly lower than that for the well-watered accession
plants (Fig. 5a). The decline in soil water content under the
droughted and moderate stress treatments was similar in the
S. sarrachoides accession and the S. villosum commercial
genotype (Fig. 5a and b). Well-watered plants had sig-
nificantly higher leaf area for both the accession and the
commercial genotype at 22 and 29 days after the start of
drought treatments. Beyond this time, S. sarrachoides
accession plants had significantly higher leaf area compared
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–1511
Fig. 1. The relative leaf expansion rate (RLER) of leaf numbers 11–15 (a–c) and the relative mainstem elongation rate (RSER) (d–f) as functions of FTSW for the
African nightshade genotypes landrace (land), commercial (com) and the accession (acc) grown in the glasshouse in 2000 and 2002. Lines show plateau regression
functions, regression coefficients are shown in Table 1.
The fraction of transpirable soil water threshold (FTSWt) at which normalized
transpiration ratio (NTR) began to decline and the slope of the decline (A) for
the African nightshade genotypes grown in the glasshouse in 2000 and 2002
The 95% confidence intervals are shown in parentheses.
to the S. villosum commercial genotype at each soil water
levels (Fig. 5c and d). Plants in the moderate stress treatment
initially had similar leaf area as the droughted ones. After
rewatering their leaf area increased significantly, although it
remained lower than that in the well-watered treatment for
This study was conducted to evaluate the adaptive responses
of African nightshade to drought. The hypothesis was that plant
processes are affected once the soil moisture falls bellow
certain thresholds and that there exist genotypic differences in
the adaptation of African nightshades to drought stress. These
processes, which are important for yield formation, include leaf
area development, dry matter production and partitioning,
osmotic adjustment and transpiration. The threshold model is a
simplified representation of a relationship, which is continuous
in nature. The shape of this relationship shows no reduction of
the physiological variables over a wide range of FTSW values
followed by a more or less linear decline (Ray and Sinclair,
1998; Liu et al., 2004). This makes it possible to characterize
plant responses to drought in terms of FTSWand is widely used
the validity of the observed thresholds for other soils, but the
use of FTSW has been shown to give fairly consistent response
functions to soil dehydration over a range of conditions (Ray
and Sinclair, 1998).
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–15 12
of transpirable soil water (FTSW) for the African nightshade genotypes grown in the glasshouse in the years 2000 and 2002. The lines show ratio = 1.
Fig. 3. Osmotic potential at full turgor (OP) of African nightshade grown in the glasshouse under droughted and well-watered conditions in 2000 and 2002. Data
points are means across the genotypes and vertical bars show LSD0.05.
Leaf area plays an important role in light interception and
hence influences dry matter production and plant growth
(Jones, 1992). Leaf area development therefore has direct
effects on the yield of leafy vegetables such as African
nightshade. In this study, leaf expansion and stem elongation
declined relatively early in the drying cycle, with an FTSW
threshold range of 0.59–0.64 and 0.53–0.58, respectively. The
lack of significant differences in the thresholds between
genotypes could be attributed to the fact that the genotypes had
similar leaf areas and plant heights under droughted conditions
and hence similar rates of soil drying in the pot experiments.
Similarly high sensitivity of leaf expansion to drought has been
FTSW thresholds for decline of relative water content and
water potential were determined in this study, the general range
of 0.20–0.40 was lower than the thresholds for leaf expansion.
The decline in leaf expansion therefore seems to have preceded
the presence of root signals.
In the early stages during the drying cycle, the root size was
small and it can be assumed that roots were extracting water
mainly from the upper soil layer leading it to dry ahead of the
lower layers. It is therefore probable that roots in the upper
drying soil produced non-hydraulic signals that acted to inhibit
leaf expansion, while roots extracting water from lower soil
layers maintained the plant water status high. Reduction of leaf
growth in the absence of changes in turgor has been reported in
various studies (Munns et al., 2000) and is usually attributed to
signals from roots growing in a drying soil overriding water
relations (Bahrun et al., 2002). Stem elongation in all the
genotypes responded in the same way as leaf expansion. This
resulted in a reduction of plant height under droughted
conditions, which contributed further to the reduction of plant
leaf area through reduced branching.
The genotypes of African nightshade studied showed only
limited osmotic adjustment capacity with an OA of 0.16–
0.19 MPaatthehigheststresslevel(FTSW = 0).ThisOAcould
not maintain turgor positive below a water potential of ?1.8 to
?2.04 MPa and a relative water content of 0.56–0.60. Hence
wilting and leaf shedding could be observed. Similar results
have been reported in genotypes of other crops with similar low
OA capacity (Entz and Fowler, 1990). This contrasts with crop
species able to withstand low WP, for instance up to ?3.7 MPa
reported in winter wheat (Gesch et al., 1992). Crops such as
and spinach (Spinacia oleracea) have also been reported to
have low OA, with values of 0.16 MPa, 0.21 MPa and
0.03 MPa, respectively (Wullschleger and Oosterhuis, 1991).
On the other hand, OA in the range of 1.0–1.3 MPa has been
reported in crops such as soybean (G. max (L.) Merr.),
pigeonpea (Cajanus cajan (L.) Millsp.) and peanut (Arachis
hypogea L.) (Turner et al., 2000).
The decline in cumulative transpiration under drought
production (Jones, 1992). The sensitivity of transpiration to
drought was similar in the African nightshade genotypes, with
FTSW thresholds for NTR of 0.46–0.51. This suggests that in
the event of a terminal drought starting in the early vegetative
stage, transpiration starts to decline when about 50% of the
available soil water has been used. The decline in transpiration
seems to have preceded the decline of plant water status in this
study. This suggests the presence of non-hydraulic root signals
in regulating transpiration. However, the evidence is not strong.
Root signals provide the shoot with a measure of the water
availability, enabling plants growing in a drying soil to reduce
their transpiration even with only small changes in soil water
P.W. Masinde et al./Scientia Horticulturae 110 (2006) 7–1513
Fig. 4. Normalized transpiration ratio (NTR) as a function of the fraction of
transpirable soil water (FTSW) for the African nightshade genotypes landrace
(land) (a), commercial (com) (b) and accession (acc) (c) grown in glasshouse in
2000 and 2002. Points are measured data while lines are plateau regression
functions, regression coefficients are shown in Table 2.
potential and before changes in leaf water status (Davies et al.,
1994, 2002). A decline in transpiration can also be due to
reduced leaf expansion under moderate stress as well as leaf
sheddingunderseverestress (Borrelletal., 2000).Responsesof
the genotypes in terms of the FTSW thresholds for leaf
expansion, stem elongation and normalized transpiration were
comparable between the years. This could be attributed to
similar climatic conditions, especially radiation intensity in
The two African nightshade species exhibited similar soil
water extraction patterns at 0–40 cm depths under droughted
and moderate stress treatments under field conditions. This
could be attributed to similar leaf area between both species
especially in the early stages of growth. Leaf area is known to
be closely correlated with plant water use (Salih et al., 1999). In
the well-watered treatment, soil moisture content was lower in
plots with commercial plants as compared to those with
accession plants towards the end of experimental period. This
could be due to the differences in leaf area of mature plants.
Accessions plants had higher leaf area than commercial plants,
which probably provided a better ground cover thereby
reducing evaporation of soil water.
It can therefore be concluded that the African nightshade
genotypes studied here adapt to drought stress mainly by
avoidance mechanisms, i.e. by conserving water through the
reduction of expansive growth and transpiration. They have
only limited OA. Thus, maintaining the soil moisture at 50–
60% of the transpirable soil water (about 60% water holding
capacity) would be sufficient to prevent a decline in leaf
expansion, stem elongation and transpiration and sustain
relatively high dry matter production. This will translate into
relatively high leaf yields. Under field conditions, the accession
to the commercial genotype, hence higher leaf yield potential.
We are grateful to the German Academic Exchange Service
(DAAD) for providing a PhD scholarship to the senior author,
and to IPK, Gatersleben, Germany and Genebank of Kenya for
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