[Show abstract][Hide abstract] ABSTRACT: Independently controlled irrigation plots were designed to test two container nursery irrigation regimes on oakleaf hydrangea (Hydrangea quercifolia ‘Alice’) in both nursery and controlled greenhouse environments. The experiments were conducted in both 3.8 and 11.4 L containers. Plants were automatically irrigated by one of two soil moisture sensor-based regimes: (1) a daily water use (DWU) system that delivered the exact amount of water that had been lost in the previous 24 h and (2) an on-demand (OD) irrigation system based on a specific substrate moisture content derived from the relationship between substrate moisture and photosynthetic rate. In this system, irrigation was applied when the substrate moisture level fell below 33% container capacity, which corresponded to 90% maximum predicted photosynthetic rate. Both treatments delivered the volume of water required to return the containers to container capacity by overhead irrigation, but the DWU system was static, irrigating once per day, whereas OD was dynamic and irrigated whenever the substrate moisture reached the 33% threshold level. Gas exchange was measured at the driest point prior to the next irrigation event. Periodical growth index, water use, and final dry weight were recorded. OD used less water than DWU outdoors, reduced leaching fraction among greenhouse experiments, and had either no or a positive impact on biomass in all but one trial. For 3.8 L plants, photosynthesis and stomatal conductance were consistently greater when irrigated by the OD program. Both treatments used significantly less water than the industry standard of 2.5 cm per day. This research demonstrated that both DWU and OD are a dramatic improvement over conventional irrigation scheduling and could be adopted as conservative irrigation systems for nursery production.
Scientia Horticulturae 01/2014; 179:132–139. · 1.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background and AimsPhysical dormancy (PY) occurs in seeds or fruits of 18 angiosperm families and is caused by a water-impermeable palisade cell layer(s) in seed or fruit coats. Prior to germination, the seed or fruit coat of species with PY must become permeable in order to imbibe water. Breaking of PY involves formation of a small opening(s) (water gap) in a morpho-anatomically specialized area in seeds or fruits known as the water-gap complex. Twelve different water-gap regions in seven families have previously been characterized. However, the water-gap regions had not been characterized in Cucurbitaceae; clade Cladrastis of Fabaceae; subfamilies Bombacoideae, Brownlowioideae and Bythnerioideae of Malvaceae; Nelumbonaceae; subfamily Sapindoideae of Sapindaceae; Rhamnaceae; or Surianaceae. The primary aims of this study were to identify and describe the water gaps of these taxa and to classify all the known water-gap regions based on their morpho-anatomical features.Methods
Physical dormancy in 15 species was broken by exposing seeds or fruits to wet or dry heat under laboratory conditions. Water-gap regions of fruits and seeds were identified and characterized by use of microtome sectioning, light microscopy, scanning electron microscopy, dye tracking and blocking experiments.Key ResultsTen new water-gap regions were identified in seven different families, and two previously hypothesized regions were confirmed. Water-gap complexes consist of (1) an opening that forms after PY is broken; (2) a specialized structure that occludes the gap; and (3) associated specialized tissues. In some species, more than one opening is involved in the initial imbibition of water.Conclusions
Based on morpho-anatomical features, three basic water-gap complexes (Types-I, -II and -III) were identified in species with PY in 16 families. Depending on the number of openings involved in initial imbibition, the water-gap complexes were sub-divided into simple and compound. The proposed classification system enables understanding of the relationships between the water-gap complexes of taxonomically unrelated species with PY.
[Show abstract][Hide abstract] ABSTRACT: Background and AimsPhysical dormancy (PY)-break in some annual plant species is a two-step process controlled by two different temperature and/or moisture regimes. The thermal time model has been used to quantify PY-break in several species of Fabaceae, but not to describe stepwise PY-break. The primary aims of this study were to quantify the thermal requirement for sensitivity induction by developing a thermal time model and to propose a mechanism for stepwise PY-breaking in the winter annual Geranium carolinianum.Methods
Seeds of G. carolinianum were stored under dry conditions at different constant and alternating temperatures to induce sensitivity (step I). Sensitivity induction was analysed based on the thermal time approach using the Gompertz function. The effect of temperature on step II was studied by incubating sensitive seeds at low temperatures. Scanning electron microscopy, penetrometer techniques, and different humidity levels and temperatures were used to explain the mechanism of stepwise PY-break.Key ResultsThe base temperature (Tb) for sensitivity induction was 17·2 °C and constant for all seed fractions of the population. Thermal time for sensitivity induction during step I in the PY-breaking process agreed with the three-parameter Gompertz model. Step II (PY-break) did not agree with the thermal time concept. Q10 values for the rate of sensitivity induction and PY-break were between 2·0 and 3·5 and between 0·02 and 0·1, respectively. The force required to separate the water gap palisade layer from the sub-palisade layer was significantly reduced after sensitivity induction.Conclusions
Step I and step II in PY-breaking of G. carolinianum are controlled by chemical and physical processes, respectively. This study indicates the feasibility of applying the developed thermal time model to predict or manipulate sensitivity induction in seeds with two-step PY-breaking processes. The model is the first and most detailed one yet developed for sensitivity induction in PY-break.
[Show abstract][Hide abstract] ABSTRACT: The involvement of two steps in the physical dormancy (PY)-breaking process previously has been demonstrated in seeds of Fabaceae and Convolvulaceae. Even though there is a claim for a moisture-controlled stepwise PY-breaking in some species of Geraniaceae, no study has evaluated the role of temperature in the PY-breaking process in this family. The aim of this study was to determine whether a temperature-controlled stepwise PY-breaking process occurs in seeds of the winter annuals Geranium carolinianum and G. dissectum.
Seeds of G. carolinianum and G. dissectum were stored under different temperature regimes to test the effect of storage temperature on PY-break. The role of temperature and moisture regimes in regulating PY-break was investigated by treatments simulating natural conditions. Greenhouse (non-heated) experiments on seed germination and burial experiments (outdoors) were carried out to determine the PY-breaking behaviour in the natural habitat.
Irrespective of moisture conditions, sensitivity to the PY-breaking step in seeds of G. carolinianum was induced at temperatures ≥20 °C, and exposure to temperatures ≤20 °C made the sensitive seeds permeable. Sensitivity of seeds increased with time. In G. dissectum, PY-break occurred at temperatures ≥20 °C in a single step under constant wet or dry conditions and in two steps under alternate wet-dry conditions if seeds were initially kept wet.
Timing of seed germination with the onset of autumn can be explained by PY-breaking processes involving (a) two temperature-dependent steps in G. carolinianum and (b) one or two moisture-dependent step(s) along with the inability to germinate under high temperatures in G. dissectum. Geraniaceae is the third of 18 families with PY in which a two-step PY-breaking process has been demonstrated.
Annals of Botany 06/2012; 110(3):637-51. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: a b s t r a c t A demand-based irrigation system was developed for Hibiscus rosa-sinensis 'Cashmere Wind' based on the relationship between photosynthesis and substrate moisture level (moisture response curve). An experiment was conducted to evaluate the premise that biomass would decrease only when substrate moisture levels caused a significant reduction in photosynthetic rate. Irrigation set points were based on the moisture response curve and corresponded to 49, 41, 30, and 22 m 3 m −3 volumetric water content (89–61% container capacity). Gas exchange and leaf water potential were greater for plants in the three wettest irrigation treatments. Plants under these treatments used 1.4, 1.2, and 1.05 times more water during the experiment than plants in the driest treatment. Biomass metrics were generally unaffected by treatments or were greater for one or both intermediate treatments. This research demonstrates that a demand-based irrigation system with a physiological basis (predicated on the relationship between pho-tosynthesis and substrate moisture potential) could be an effective biorational approach for scheduling irrigation and reducing water consumption in container-grown nursery crops.
[Show abstract][Hide abstract] ABSTRACT: Chia, Salvia hispanica L., was well developed into a cultivated crop and an important component of Mesoamerican cultures and nutrition. Early Mesoamerican
breeders produced lines with well developed agronomic characteristics including good, uniform seed yield and retention. Seed
retention in particular is disadvantageous for survival in the wild. Maize, beans and squash were developed into important
crops concomitant with chia in Mesoamerica but unlike these other crops lack of photoperiodic variability in floral induction
limited the spread of chia cultivation into North America. There has been renewed interest in chia as an excellent source
of ω3 fatty acids and dietary fiber for healthy diets. Such highly unsaturated oils also are useful starting materials for
many renewable chemicals. Further we find chia grows very well in Midwestern and Eastern USA but flowers too late in the season
for seeds to mature before killing frosts. We set out to develop the genetic diversity in floral induction to provide germplasm
for production in the US and other temperate areas of the world. We demonstrate that new early flowering lines are able to
flower under a photoperiod of 15 h under greenhouse conditions. In field conditions, some selected new lines flowered at a
photoperiod of 14 h and 41 min during the 2009 growing season in Kentucky and can produce seeds in a range of environments
in temperate areas.
Genetic Resources and Crop Evolution 01/2012; · 1.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The 'hinged valve gap' has been previously identified as the initial site of water entry (i.e. water gap) in physically dormant (PY) seeds of Geranium carolinianum (Geraniaceae). However, neither the ontogeny of the hinged valve gap nor acquisition of PY by seeds of Geraniaceae has been studied previously. The aims of the present study were to investigate the physiological events related to acquisition of PY and the ontogeny of the hinged valve gap and seed coat of G. carolinianum.
Seeds of G. carolinianum were studied from the ovule stage until dispersal. The developmental stages of acquisition of germinability, physiological maturity and PY were determined by seed measurement, germination and imbibition experiments using intact seeds and isolated embryos of both fresh and slow-dried seeds. Ontogeny of the seed coat and water gap was studied using light microscopy.
Developing seeds achieved germinability, physiological maturity and PY on days 9, 14 and 20 after pollination (DAP), respectively. The critical moisture content of seeds on acquisition of PY was 11 %. Slow-drying caused the stage of acquisition of PY to shift from 20 to 13 DAP. Greater extent of cell division and differentiation at the micropyle, water gap and chalaza than at the rest of the seed coat resulted in particular anatomical features. Palisade and subpalisade cells of varying forms developed in these sites. A clear demarcation between the water gap and micropyle is not evident due to their close proximity.
Acquisition of PY in seeds of G. carolinianum occurs after physiological maturity and is triggered by maturation drying. The micropyle and water gap cannot be considered as two separate entities, and thus it is more appropriate to consider them together as a 'micropyle--water-gap complex'.
Annals of Botany 07/2011; 108(1):51-64. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A controlled environment chamber was developed to quantify transpiration in dogwood seedlings (Cornus florida and Cornus kousa) and poinsettia (Euphorbia pulcherrima) cuttings. The chamber included both incandescent and fluorescent lights mounted on an adjustable shelf to accommodate a range of plant heights and replicated transpiration stations consisting of one infrared temperature sensor and scale per station. This system was used to detect transpiration on seedlings gravimetrically and by an increase in leaf temperature. For cuttings, the chamber consisted of solely an incandescent light source and three scales. Both systems used an air conditioning system which also permitted control of the vapor pressure deficit air . In both the seedlings and cuttings, the chamber system was able to maintain a constant VPD air and detect and record transpiration. INTRODUCTION Water is critical to plant survival as a carrier for nutrients, a substrate in reactions, and the hydraulic force behind growth. Transpiration is the loss of water through the stomata or small openings in leaves. As CO 2 enters and oxygen exits leaves, water vapor also exits the leaves. Transpirational water loss in a crop impacts a range of plant production issues, from photosynthesis and growth to irrigation scheduling. By measuring and understanding water loss, models can be developed and irrigation practices can be refined. The development of transpiration chambers allows investigation of water loss in a controlled environment. Transpiration can be measured for an individual leaf, excised stem, or for the whole plant (Reigosa Roger, 2001). Single leaf measurements use a steady-state porometer and extrapolation to whole plant transpiration must be mathematically derived. Excised stem and whole-plant transpiration methods measure water transfer from a solution or growing substrate through the plant/leaf into the atmosphere. This is either measured gravimetrically, using container lysimeters or estimates of sap flow using stem heat balance. Regardless of the technique used to measure transpiration, care must be taken to control the environment during the measurements. Whole-plant measurements have several advantages to single-leaf methods including the ability to evaluate root and shoot communication as plants become increasingly drier, or in graft combinations found in ornamental plants. These measurements can be made for plants growing in containers in ambient or greenhouse conditions, but using a controlled environment chamber allows for greater manipulation and uniformity for leaf temperature and VPD air (VPD air calculation substitutes air temperature for leaf temperature). Also, Ramirez et al. (2006)
[Show abstract][Hide abstract] ABSTRACT: A photosynthesis-based irrigation system is a novel approach to nursery crops irrigation management and could conceivably reduce water use during production. Experiments were conducted to determine the plausibility of a photosynthesis-based irrigation system. Cuttings, seedlings, and grafted (self and reciprocal) plants from two woody genera (Hibiscus rosa-sinensis, Cornus florida and Cornus kousa) were tested. To determine if the plants responded similarly to reduced substrate moisture, gas exchange was measured over a range of substrate moisture contents. Photo-synthetic rates remained near maximum over a wide range of substrate moisture contents. Moisture response curves were similar among the species and among cuttings, seedlings, and grafted plants. A sigmoidal equation best represented the relationship between photosynthetic rate and substrate moisture content. INTRODUCTION Nursery crop production is a substantial portion of the U.S. agricultural economy, valued at 6.6 billion USD annually (USDA, 2009). Nursery production, especially container production, is dependent on irrigation for plant survival and optimal growth. Over-irrigation is not uncommon in part due to inefficient irrigation delivery techniques i.e. overhead irrigation, as well as poor scheduling of irrigation (Beeson and Knox, 1990; Yeager et al., 2007). Numerous technologies have been developed for estimating plant water use and refining irrigation scheduling. Unfortunately, these techniques have not been adopted by nursery growers (Beeson et al., 2004). A photosynthesis-based irrigation system has promise for nursery crops. Such a system would irrigate based on the relationship between photosynthesis and substrate moisture content, using substrate moisture probes to trigger irrigation events. A photo-synthesis-based irrigation system is a plant-based irrigation system. Plant-based systems are generally considered to be highly relevant and allow for environmental influence (Jones, 2004). One distinct advantage of plant-based systems is that they can respond to the physiological changes that occur directly due to changes in plant water status. However, in most cases, some defined level of deficit is reached before triggering the irrigation. For a photosynthetic-based system to be viable, moisture response curves must demonstrate that multiple species respond similarly and predictably to decreasing substrate moisture. Additional criteria for a feasible system include establishing that 1) propagation technique, 2) substrate, and 3) environmental conditions do not affect the moisture response curve. The objective of this research was to evaluate the photosynthetic response of two woody genera propagated by multiple techniques to decreasing substrate moisture content in an effort to assess the feasibility of developing a photosynthesis-based irrigation system.
[Show abstract][Hide abstract] ABSTRACT: Physical dormancy in seeds of species of Geraniaceae is caused by a water-impermeable palisade layer in the outer integument of the seed coat and a closed chalaza. The chalazal cleft has been reported to be the water gap (i.e. location of initial water entry) in innately permeable seeds of Geraniaceae. The primary aim of this study was to re-evaluate the location of the water gap and to characterize its morphology and anatomy in physically dormant seeds of Geraniaceae, with particular reference to G. carolinianum.
Length, width, mass, anatomy and germination of two seed types (light brown and dark brown) of G. carolinianum were compared. Location, anatomy and morphology of the water gap were characterized using free-hand and microtome tissue sectioning, light microscopy, scanning electron microscopy, dye tracking, blocking and seed-burial experiments.
Treatment with dry heat caused a colour change in the palisade cells adjacent to the micropyle. When placed in water, the 'hinged valve' (blister) erupted at the site of the colour change, exposing the water gap. The morphology and anatomy in the water-gap region differs from those of the rest of the seed coat. the morphology of the seed coat of the water-gap region is similar in G. carolinianum, G. columbinum, G. molle and G. pusillum and differs from that of the closely related species Erodium cicutarium.
Dislodgment of swollen 'hinged valve' palisade cells adjacent to the micropyle caused the water gap to open in physically dormant seeds of G. carolinianum, and it was clear that initial water uptake takes place through this gap and not via the chalazal opening as previously reported. This water gap ('hinged valve gap') differs from water gaps previously described for other families in morphology, anatomy and location in the seed coat.
Annals of Botany 06/2010; 105(6):977-90. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Burning bush (Euonymus alatus) is a popular ornamental shrub and an easily grown, profitable nursery crop. Many states, however, have designated burning bush as an invasive plant. Rudy Haag burning bush (E. alatus ‘Rudy Haag’) is a nearly seedless cultivar that may have a significant marketing advantage for nursery producers if data to support a non-invasive character can be substantiated. Fruit display is often promoted as a desirable characteristic in landscape ornamentals, but prolific seed production is often cited as an indicator of invasive potential. Preliminary observation indicated that cultivars of burning bush differ in fecundity under natural pollination conditions. The objective of this project was to document the potential risk of ‘Rudy Haag’ burning bush as an invasive plant species based on seed production. E. alatus ‘Compactus’ and ‘Rudy Haag’ plants of a flowering size were established in field plots at two locations, Lexington and Princeton, KY. In Kentucky, both cultivars of burning bush flowers as plants are leafing out in late April through May. Seeds mature by late fall and usually develop as a single ovule encased within a brightly colored aril inside a dry capsule. Under full sun conditions, ‘Compactus’ and ‘Rudy Haag’ produced an average of 1238 and 12 seed per plant, respectively, across three years and two production sites. In Lexington, a 75% shade treatment was also included, but had no effect on seed production. ‘Compactus’ averaged 1514, 1061 and 240 seed per plant in 2005, 2006 and 2007; significantly fewer seed were produced in the last harvest year. ‘Rudy Haag’ plants produced no seeds the first year and an average of less than six in the subsequent two harvest years. Field and greenhouse pollination combinations (self-, intra- and cross-pollinated) between ‘Compactus’ and ‘Rudy Haag’ were unsuccessful with no subsequent seed development. In a simulated understory seed bank study using E. alatus ‘Compactus’, no germination occurred in the field or under germination conditions following exhumation as compared to bush honeysuckle (Lonicera), which germinated readily under both conditions. Burning bush seed had limited persistence in the soil with an estimated viability of 38, 29, 9 and 2% after 3, 6, 9 and 12 months in soil, respectively. Minimal germination and limited persistence as well as extremely low seed production under natural and supplemented pollination conditions support a low invasive risk of using E. alatus ‘Rudy Haag’ in landscape plantings.
[Show abstract][Hide abstract] ABSTRACT: Many techniques exist for estimating plant water use and refining irrigation requirements. However, grower adoption of existing irrigation technology is low. Development of an easily transferrable irrigation model could aid in grower adoption of irrigation technology. Previously, a photosynthesis-based irrigation model was developed for Hibiscus ‘Cashmere Wind’ under controlled-environment conditions. This model was based on the relationship between substrate moisture content and photosynthetic rate. A sigmoidal equation best fit the curve (r2=0.62). Photosynthesis was maintained at or near maximum rates from 100% container capacity to 62% container capacity. A distinct decrease in photosynthesis was apparent at 61% container capacity and this was used as an irrigation setpoint. Subsequent experiments showed that biomass and quality could be maintained equally among irrigation treatments so long as substrate moisture content was maintained above this setpoint. Additionally, a model was developed for two Cornus species under controlled environment conditions and exhibited similar characteristics. In order to determine the transferability of a photosynthesis-based irrigation model from controlled environment to outdoor conditions and to determine if different setpoints would be necessary among closely related Cornus taxa, the relationship between photosynthetic rate and container water content was measured for three related Cornus taxa (C. florida ‘Cherokee Princess’, C. kousa ‘National’, and C. x Constellation) under outdoor, pot-in-pot conditions. Photosynthesis was measured as substrate moisture content decreased from well-watered conditions (90% or greater container capacity) to water deficit conditions. Again, a sigmoidal equation best fit the data (r2=0.70-0.74), however, photosynthesis could not be maintained at near maximum levels over as wide range of substrate moisture contents as observed for greenhouse-grown plants. All three field-grown dogwood taxa showed a similar photosynthetic rate (near 80% of maximum) at 88% container capacity. Therefore, a revised model using this setpoint will be evaluated for its impact on water use and biomass accumulation in pot-in-pot dogwood production.
[Show abstract][Hide abstract] ABSTRACT: The most common Cornus taxa in the nursery industry are those from the big-bracted tribe. Within this group a substantial amount of variation in environmental tolerances has been reported. Our objectives were 1) to determine the variation in a suite of foliar micromorphology parameters and gas exchange for seedlings of C. kousa and C. florida grown under controlled environment conditions and for three related grafted selections: C. florida ‘Cherokee Princess’, C. kousa ‘National’ and C. florida x C. kousa hybrid Constellation® grown in outdoor conditions, and 2) to determine variation in stomatal number for a range of eight established big-bracted selections, and a C. mas selection for comparison, in order to better understand their response to stresses and performance in landscapes. Of the controlled-environment produced-plants, the number of stomates per unit area was 60% greater for C. florida than for C. kousa, 83.5 and 51.5, respectively. However, area per individual stomate and total stomatal area per leaf, an integrated value of stomate number and stomatal complex area, were greater for C. kousa. Epidermal cell thickness was greater for C. kousa than C. florida. The pattern of wax deposition on the abaxial surface differed between C. kousa and C. florida. The reduced total stomatal area per leaf and abaxial wax may be an adaptation to protect C. florida, which is considered drought-adverse. The number of stomates per unit leaf area varied significantly among the established big-bracted selections from a low of 51.8 for C. mas ‘Spring Sun’ to a high of 209.6 for C. x Constellation®. In general, the C. kousa cultivars and the C. kousa x C. florida hybrids had a greater number of stomates per unit leaf area than did C. mas ‘Spring Sun’ and C. kousa seedlings. The hybrids had a range of stomates, from 209.6 to 72.8 for Constellation® and Ruth Ellen®, respectively. Among Constellation®, ‘Cherokee Princess’ and ‘National’, Constellation® had the greatest number of stomates. The average stomatal complex size and average total stomatal area were greater for Constellation® and National than for ‘Cherokee Princess’. Constellation® had greater photosynthetic rate, stomatal conductance, and transpiration rates than both C. kousa ‘National’ and C. florida ‘Cherokee Princess'. Among the two species growing the controlled environment, C. florida had a greater photosynthetic rate, stomatal conductance, and transpiration rate than C. kousa.
[Show abstract][Hide abstract] ABSTRACT: Sensitivity cycling to physical dormancy (PY) break in seeds is known to occur in some Fabaceae and Convolvulaceae species. PY in seeds of species of Convolvulaceae and of some other angiosperm plant families can be broken by storing them dry. However, the mechanism of opening the water gap in the seed coat (dormancy break) during dry storage has not been investigated. In research reported here, we determined whether sensitivity cycling occurs in seeds of Ipomoea hederacea (Convolvulaceae) and investigated the effect of dry storage on opening of the water gap. Seeds can cycle between insensitive and sensitive states to dormancy break, and dormancy can be broken in sensitive seeds by storing them dry at high to moderate temperatures. Seeds with the lower part of the hilum blocked lost a minimal amount of water during storage. Percentage water loss was significantly correlated with percentage dormancy break. Desorption of water through the hilar fissure during dry storage reduced the vapor pressure below the bulges (water gap), which caused slits to form around the bulges (opening of water gap). The amount of water desorption was low in insensitive seeds because the hilar fissure remained closed and thus the water gap did not open.
International Journal of Plant Sciences 05/2009; 170(4):429-443. · 1.54 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The water gap is an important morphoanatomical structure in seeds with physical dormancy (PY). It is an environmental signal detector for dormancy break and the route of water into the non-dormant seed. The Convolvulaceae, which consists of subfamilies Convolvuloideae (11 tribes) and Humbertoideae (one tribe, monotypic Humberteae), is the only family in the asterid clade known to produce seeds with PY. The primary aim of this study was to compare the morphoanatomical characteristics of the water gap in seeds of species in the 11 tribes of the Convolvuloideae and to use this information, and that on seed dormancy and storage behaviour, to construct a phylogenetic tree of seed dormancy for the subfamily.
Scanning electron microscopy (SEM) was used to define morphological changes in the hilum area during dormancy break; hand and vibratome sections were taken to describe the anatomy of the water gap, hilum and seed coat; and dye tracking was used to identify the initial route of water entry into the non-dormant seed. Results were compared with a recent cladogram of the family.
Species in nine tribes have (a) layer(s) of palisade cells in the seed coat, a water gap and orthodox storage behaviour. Erycibe (Erycibeae) and Maripa (Maripeae) do not have a palisade layer in the seed coat or a water gap, and are recalcitrant. The hilar fissure is the water gap in relatively basal Cuscuteae, and bulges adjacent to the micropyle serve as the water gap in the Convolvuloideae, Dicranostyloideae (except Maripeae) and the Cardiochlamyeae clades. Seeds from the Convolvuloideae have morphologically prominent bulges demarcated by cell shape in the sclereid layer, whereas the Dicranostyloideae and Cardiochlamyeae have non-prominent bulges demarcated by the number of sub-cell layers. The anatomy and morphology of the hilar pad follow the same pattern.
PY in the subfamily Convolvuloideae probably evolved in the aseasonal tropics from an ancestor with recalcitrant non-dormant seeds, and it may have arisen as Convolvulaceae radiated to occupy the seasonal tropics. Combinational dormancy may have developed in seeds of some Cuscuta spp. as this genus moved into temperate habitats.
Annals of Botany 02/2009; 103(1):45-63. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The water-impermeable seeds of Ipomoea lacunosa undergo sensitivity cycling to dormancy breaking treatment, and slits are formed around bulges adjacent to the micropyle during dormancy break, i.e. the water gap opens. The primary aim of this research was to identify the mechanism of slit formation in seeds of this species.
Sensitive seeds were incubated at various combinations of relative humidity (RH) and temperature after blocking the hilar area in different places. Increase in seed mass was measured before and after incubation. Scanning electron microscopy (SEM) and staining of insensitive and sensitive seeds were carried out to characterize these states morphologically and anatomically. Water absorption was monitored at 35 and 25 degrees C at 100 % RH.
There was a significant relationship between incubation temperature and RH with percentage seed dormancy break. Sensitive seeds absorbed water vapour, but insensitive seeds did not. Different amounts of water were absorbed by seeds with different blocking treatments. There was a significant relationship between dormancy break and the amount of water absorbed during incubation.
Water vapour seals openings that allow it to escape from seeds and causes pressure to develop below the bulge, thereby causing slits to form. A model for the mechanism of formation of slits (physical dormancy break) is proposed.
Annals of Botany 01/2009; 103(3):433-45. · 3.45 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dormancy in seeds of Cuscuta (Convolvulaceae, tribe Cuscuteae) is due to a water-impermeable seed coat (physical dormancy). In nondormant seeds of several species of this family, bulges adjacent to the micropyle have been identified as the initial route of water entry into seeds (water gap). However, there are claims that water enters seeds of Cuscuta spp. via the entire seed coat. Although several studies have been done on seed coat anatomy of Cuscuta, none has identified and/or characterized the morphology/anatomy of a water gap. Thus, the primary aim of this research was to identify and describe the morphology and anatomy of the water gap in seeds of Cuscuta australis. It was also determined if sensitivity cycling to dormancy-breaking treatments occurs in seeds of this species.
Light microscopy, scanning electron microscopy, tissue-sectioning and dye-tracking and blocking experiments were used to investigate the morphology and anatomy of the water gap. Treatments simulating natural conditions were used to break seed dormancy. Storage of seeds at different temperatures was tested for their effect on sensitivity to dormancy-breaking treatment.
Dormancy-breaking treatments caused the tightly closed hilar fissure to open. Staining was observed in cells below the hilum area but not in those below the seed coat away from the hilum. Sensitivity to dormancy-breaking treatment was induced by storing seeds dry and reduced by storing them wet.
Whereas bulges adjacent to the micropyle act as the water gap in other species of Convolvulaceae with physical dormancy, the hilar fissure serves this function in Cuscuta. Cuscuta australis can cycle between insensitivity <--> sensitivity to dormancy-breaking treatments.
Annals of Botany 07/2008; 102(1):39-48. · 3.45 Impact Factor