Relative Leaf Expansion Rate Research Articles

Relative Leaf Expansion Rate as an Indicator of Compensatory Growth of Defoliated Vines (Vitis vinifera L. cv. Prokupac)

Relative Leaf Expansion Rate as an Indicator of Compensatory Growth of Defoliated Vines (Vitis vinifera L. cv. Prokupac)

Relative leaf expansion rate as an indicator of compensatory growth of defoliated vines of Prokupac (Vitis vinifera L.)

Reaction of the grapevine to the early defoliation is to mitigate its effects through compensatory growth by producing more lateral shoots with a greater number of leaves. In this study, we evaluated the usage of non-destructive and continuous measurements of mean and lateral leaf area on the same shoots for the purpose of monitoring the leaf area development and calculating relative leaf expansion rate - RLER during vegetation. This study has shown that the grapevine's ability to recover its leaf area after defoliation depends mainly on the time when the defoliation occurs. Early defoliated vines had time to compensate removed leaves by producing more lateral shoots with a greater number of leaves, which resulted in a larger leaf area. With the decrease in the intensity of shoot growth during vegetation, the recovery ability decreases, therefore, compensatory growth is not enough to restore the reduced leaf area. Based on the value of RLER, its defoliation is performed in the period of intensive growth of shoots, it affects the stagnation of the emergence of new shoots and leaves over several days, followed by a period of re-growth. Very slow or no growth of shoots and leaves occurred after the veraison stage.

Soil moisture levels affect growth, flower production and yield of cucumber

Abstract Field trial to determine the optimum soil moisture level for enhanced performance of cucumber was conducted in polyethylene covered rain shelter for two seasons in 2017 and repeated in 2018. The research was conducted at Rongo University Research and Teaching field. Three seeds of cucumber ‘Ashley’ were sown directly in 3.5 litre plastic pots containing 8 kg of sterilised air-dried growth medium made up of sand and top soil in the ratio of 1:2. The treatments were four levels of water applied at 100 % (control), 80 %, 60 % and 40 % pot capacity. The experimental design was completely randomised block design replicated three times. Data on the plant growth, flower production and yield were subjected to Analysis of Variance (ANOVA) and mean separation tests at p ≤ 0.05 level of significance. There were significant differences on the vine growth, internodes length and diameter as a result of Water Stress (WS) levels. The treatments also significantly affected the Relative Leaf Water Content (RLWC) and Relative Leaf Expansion Rate (RLER) but did not affect the number of leaves per vine. Fruit firmness was also affected by water stress. To optimise the productivity of cucumber, the soil moisture level should be maintained to at least 80 % of the field capacity throughout its productive phase.

Tolerance of common bean (Phaseolus vulgaris L.) to different durations of simulated shallow water table condition

Shallow water table problem has lessened farmer’s interest in cultivating vegetables at riparian wetland in Indonesia. At present, riparian wetlands are mostly underutilized. Objective of this study was to evaluate tolerance of common bean (Phaseolus vulgaris) to shallow water table condition. Common bean variety of PV072 was used in this study. Research was conducted in Jakabaring, South Sumatra, Indonesia; from November 2016 to February 2017. Leaf and root growth or damage can be used as an indicator of stress tolerance in plants exposed to unfavorable soil conditions. Results of our study indicated that roots of common bean were unable to survive in water saturated condition below water table, but the plants were able to compensate for this constraint by increasing root biomass within aerobic substrate above water table. Moreover, after termination of shallow water table treatment, common bean was able to regrow their roots into the ex-saturated zone. Common bean plants produced marketable yield albeit limited rhizosphere space. Interestingly, the plants exposed to 24 days of shallow water table produced the highest yield. In conclusion, common bean can be considered as tolerant vegetable to shallow water table. It should be noted that bean leaf reached 90 percent and full size in less than 6 days and 9 days, respectively, after the young leaf was unfolded. Therefore, long term study on bean leaf growth and development requires measurement on sequential and multiple leaves. Also, nocturnal relative leaf expansion rate (RLER) was significantly faster than diurnal RLER.

Relative leaf expansion rate and other leaf-related indicators for detection of drought stress in chili pepper (Capsicum annuum L.).

Relative leaf expansion rate and other leaf-related indicators for detection of drought stress in chili pepper (Capsicum annuum L.).

Kinematic evidence that atmospheric nitrogen dioxide increases the rates of cell proliferation and enlargement to stimulate leaf expansion in Arabidopsis

To elucidate the stimulation of leaf growth by atmospheric nitrogen dioxide (NO2), we performed a kinematic analysis of the eighth leaves of Arabidopsis thaliana (accession C24) plants grown for 17–35 days after sowing in the presence or absence of 50 ppb NO2 (designated +NO2 plants and –NO2 plants, respectively). We found that the peak and mean values of the relative rates of leaf expansion, cell division and cell expansion were always greater in +NO2 plants than in –NO2 plants. No evidence for prolonged duration was obtained. Thus, NO2 treatment increased the rates of both cell proliferation and enlargement to increase leaf size. Furthermore, a fold-change analysis showed that cell proliferation and enlargement differentially regulated NO2-induced leaf expansion.

Growth, photosynthesis and H+-ATPase activity in two Jerusalem artichoke varieties under NaCl-induced stress

In order to evaluate differential growth, photosynthesis and H+-ATPase activity responses to salt-induced stress, two Jerusalem artichoke (Helianthus tuberosus L.) genotypes (Nanyu No. 1 and Qingyu No. 2) were used in sand-culture experiment with different concentrations of NaCl (0, 30, 60, 90, 120 and 150mM). After 20 days of growth, the NaCl stress resulted in a decrease of biomass accumulation, relative leaf expansion rate and photosynthetic rate, but an increase of proline content in both genotypes. Compared with Qingyu No. 2, Nanyu No. 1 had lower biomass, photosynthetic rate, gas exchange and transpiration rate, but higher proline content, activities of plasma membrane H+-ATPase (PM H+-ATPase) and vacuolar membrane H+-ATPase (VM H+-ATPase). Hence, the NaCl adaptation strategy in Nanyu No. 1 was by lowering photosynthetic rate, stomatal conductance and transpiration rate while maintaining high H+-ATPase activities, whereas the adjustment of Qingyu No. 2 was by keeping much higher rate of proline accumulation and concentration of chlorophyll. The differences in salt tolerance showed that different adaptation mechanisms existed between cultivars of Jerusalem artichoke. The findings offered the possibility of selecting salt-tolerant genotypes of Jerusalem artichoke.

Seedling vigor variation among 80 recombinant chromosome substitution lines (RCSL) of barley ( Hordeum vulgare)

L. Inostroza, C. Quezada, W. Inostroza, I. Matus, M. Tapia, and A. del Pozo. 2011. Seedling vigor of 80 recombinant chromosome substitution lines (RCSL) of barley (Hordeum vulgare). Cien. Inv. Agr. 38 (1): 137-147. The seedling vigor of 80 barley recombinant chromosome substitution lines (RCSLs) was measured for selecting high seedling vigor genotypes. The RCSLs were derived from a cross between H. vulgare subsp. spontaneum and H. vulgare subsp. vulgare ‘Harrington’. The work was carried out under greenhouse conditions during the summer of 2008. The experimental design was an alpha lattice with 3 replicates. In each replicate, 40 plants were established, distributed in eight polyethylene pots containing sand as substrate, which were fertilized an irrigated. The emergence of seedlings, number of leaves and shoots, dry matter of leaves, shoots and roots and leaf area of fully expanded leaves were measured. In addition, growth indices were calculated: emergence rate, leaf appearance rate, relative leaf expansion rate, relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR) and the allometric coefficient (K) between root and shoot dry matter. All calculated indices varied significantly among genotypes (P≤0.05) and some RCSLs showed better early vigor associated traits than ‘Harrington’. The dry matter accumulation 30 DAS (when the experiment ended) was significant and positively correlated with the RGR (r=0.61; P≤ 0.05) and NAR (r=0.41; P≤0.05). The K coefficient was negatively correlated with NAR (r=-0.40, P≤0.05). The genotypes with the greatest seminal vigor were the RCSLs 45, 92, 112 and ‘Harrington’, whereas the RCSLs 5, 19, 47 and 121 presented the lowest seminal vigor.

Seedling vigor of 80 recombinant chromosome substitution lines (RCSL) of barley (Hordeum vulgare)

The seedling vigor of 80 barley recombinant chromosome substitution lines (RCSLs) was measured for selecting high seedling vigor genotypes. The RCSLs were derived from a cross between H. vulgare subsp. spontaneum and H. vulgare subsp. vulgare ‘Harrington’. The work was carried out under greenhouse conditions during the summer of 2008. The experimental design was an alpha lattice with 3 replicates. In each replicate, 40 plants were established, distributed in eight polyethylene pots containing sand as substrate, which were fertilized an irrigated. The emergence of seedlings, number of leaves and shoots, dry matter of leaves, shoots and roots and leaf area of fully expanded leaves were measured. In addition, growth indices were calculated: emergence rate, leaf appearance rate, relative leaf expansion rate, relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR) and the allometric coefficient (K) between root and shoot dry matter. All calculated indices varied significantly among genotypes (P≤0.05) and some RCSLs showed better early vigor associated traits than ‘Harrington’. The dry matter accumulation 30 DAS (when the experiment ended) was significant and positively correlated with the RGR (r=0.61; P≤ 0.05) and NAR (r=0.41; P≤0.05). The K coefficient was negatively correlated with NAR (r=-0.40, P≤0.05). The genotypes with the greatest seminal vigor were the RCSLs 45, 92, 112 and ‘Harrington’, whereas the RCSLs 5, 19, 47 and 121 presented the lowest seminal vigor.

Effect of phosphorus nutrition on growth of potato genotypes with contrasting phosphorus effeciency

Potato (Solanum tuberosum L.) has high phosphorus requirement for optimum growth and yield. Thus, under P deficiency, growth and yield are considerably reduced. An experiment was conducted in a controlled growth chamber on the effect of P supply on morphological and physiological plant parameters of three potato genotypeshaving contrasting P efficiency. The genotypes were CGN 17903, CIP 384321.3 and CGN 18233. They were grown under two P levels [100 mg P kg-1 soil (low P) and 700 mg P kg-1 of soil (high P)]. Treatments were arranged in a completely randomised block designs with six replications. Low P supply reduced shoot dry matter yield, relative growth rate, leaf number, whole plant relative leaf expansion rate, total leaf area per plant, plant height and net assimilation rate of P-inefficient genotype, more than that of the P-efficient genotypes. However,low P supply did not affect net photosynthetic rate per unit leaf area, leaf dark respiration rate, chlorophyll fluorescence rate and electron transport rate of both P-efficient and inefficient genotypes. P-efficient genotype CGN 17903 allocated more dry matter yield to the leaf which might have enabled higher light harvesting, hence contributing to high biomass accumulation of this genotype.

THE INFLUENCE OF DIFFERENT SUBSTRATE COMPOSITION ON GROWTH PARAMETERS AND DRY MASS PARTITIONING OF CUCUMBER (CUCUMIS SATIVUM L.) SEEDLINGS

The influences of substrate composition on growth parameters and dry mass portioning of cucumber (Cucumis sativum L.) seedlings were estimated in an experiment conducted in a plastic greenhouse. Graded seeds of cv. Ekron F1, were individually seeded in foam trays filled with different substrates, namely peat compost (Klassmann product), vermin compost (locally produced) and a mixture (50 % to 50%) of peat compost and vermin compost. Plants were equally irrigated, but no additional nutrient elements were supplied during the experiment time. Roots, stems and leaf dry matter were weighted and stem length and plant leaf area was successfully measured after the first and the second true leafs were appeared. Based on that data, the relative growth rates (RGR) and its components; net assimilation rate (NAR) and leaf area ratio (LAR) were computed. The relative stem elongation rate (RSER), the relative leaf expansion rate (RLER), root to whole plant dry weight ratio (RWR), root to shoot dry weight ratio (RSR), and the respective fractions of roots, stems and leaf dry mass against total plant weight (RMF, SMF, LMF) were also calculated for each experimental plot. A significantly higher relative growth rate (RGR) was found for young seedlings grown in vermin compost compared to commercial peat compost. Though there was a difference on net assimilation rate (the physiological component of RGR), the increased growth rate was mostly due to differences found for leaf assimilation ratio (the morphological component of RGR). Higher dry matter per plant, higher relative stem elongation rate and higher relative leaf expansion rate were found for vermin compost, but no effect was found regarding dry mass partitioning of young cucumber seedlings.

Effects of Cadmium on Carbon and Nitrogen Assimilation in Shoots of Mungbean [Vigna radiata (L.) Wilczek] Seedlings

AbstractIncreased cadmium (Cd) uptake from contaminated soils damages plant metabolism. The purpose of this study was to determine Cd‐induced time‐related changes in some shoot growth and physiological attributes, and their interrelationships in Cd‐tolerant (NM‐98) and sensitive (NM‐28) mungbean varieties. Shoot Cd and leaf chlorosis increased with a concomitant reduction in shoot dry weight, leaf area, relative growth rate (RGR), net assimilation rate (NAR) and relative leaf expansion rate. Reduction in transpiration rate (E) and stomatal conductance (gs) and increase in substomatal CO2 level (Ci), indicated that Cd reduced net photosynthesis (Pn) by reducing CO2 fixation by Rubisco, albeit these changes were less pronounced in NM‐98. A positive correlation of chlorosis with shoot Cd, and negative relationships of chlorosis and shoot Cd with Pn revealed that Cd damages the photosynthetic apparatus in mungbean. Time course decrease in in vivo nitrate reductase activity (NRA) and an increase in soluble nitrate in NM‐28 revealed that Cd markedly hampers nitrogen assimilation. Positive correlations of RGR and NAR with Pn and NRA and negative ones with chlorosis, shoot dry weight, shoot Cd and Ci in NM‐98 suggested that mungbean sensitivity to Cd is due to perturbed C and N assimilation.

Day length affects the dynamics of leaf expansion and cellular development in Arabidopsis thaliana partially through floral transition timing.

Plant aerial development is well known to be affected by day length in terms of the timing and developmental stage of floral transition. Arabidopsis thaliana is a 'long day' plant in which the time to flower is delayed by short days and leaf number is increased. The aim of the work presented here was to determine the effects of different day lengths on individual leaf area expansion. The effect of flower emergence per se on the regulation of leaf expansion was also tested in this study. Care was taken to ensure that day length was the only source of micro-meteorological variation. The dynamics of individual leaf expansion were analysed in Ler and Col-0 plants grown under five day lengths in five independent experiments. Responses at cellular level were analysed in Ler plants grown under various day lengths and treatments to alter the onset of flowering. When the same leaf position was compared, the final leaf area and both the relative and absolute rates of leaf expansion were decreased by short days, whereas the duration of leaf expansion was increased. Epidermal cell number and cell area were also altered by day-length treatments and some of these responses could be mimicked by manipulating the date of flowering. Both the dynamics and cellular bases of leaf development are altered by differences in day length even when visible phenotypes are absent. To some extent, cell area and its response to day length are controlled by whole plant control mechanisms associated with the onset of flowering.

Interrelationships between p-Coumaric Acid, Evapotranspiration, Soil Water Content, and Leaf Expansion

Increasing concentrations of p-coumaric acid applied to (cucumber seedling)-[Cecil A( p ) soil-sand mixture (or soil)] systems inhibited evapotranspiration (primarily transpiration) and leaf area expansion of cucumber seedlings and increased soil moisture. Higher soil moisture resulting from the inhibition of evapotranspiration lowered soil solution concentrations of p-coumaric acid by 14-40% but did not significantly influence the inhibitory effects of p-coumaric acid on seedlings. Inhibition of evapotranspiration and total leaf area and increases in lowest daily soil water were observed 1-3 d after the first p-coumaric acid treatment, whereas inhibition of absolute and relative rates of leaf expansion was observed within a 24-hr period. Development of the maximum effects of p-coumaric acid required several additional days. Recovery from effects, i.e., return to control levels, after p-coumaric acid depletion from soil solution was a gradual process requiring days for evapotranspiration, lowest daily soil water, and total leaf area, but was slightly faster for leaf area expansion. It appears, at least for short-term studies, that the initial input or treatment concentrations of p-coumaric acid represented a reasonable estimate of dose despite the dynamic nature of soil solution concentrations, and that the lowering of available p-coumaric acid concentrations, associated with the elevation of soil moisture, did not result in a concurrent detectable seedling response. However, increased soil moisture associated with p-coumaric acid treatments of sensitive species suggests a means by which the magnitude of some allelopathic interactions may be modified and resource competition and allelopathy could interact.

Estimation of the whole‐plant CO2compensation point of tobacco (Nicotiana tabacumL.)

AbstractThe whole‐plant CO2compensation point (Γplant) is the minimum atmospheric CO2level required for sustained growth. The minimum CO2requirement for growth is critical to understanding biosphere feedbacks on the carbon cycle during low CO2episodes; however, actual values of Γplantremain difficult to calculate. Here, we have estimated Γplantin tobacco by measuring the relative leaf expansion rate at several low levels of atmospheric CO2, and then extrapolating the leaf growth vs. CO2response to estimate CO2levels where no growth occurs. Plants were grown under three temperature treatments, 19/15, 25/20 and 30/25°C day/night, and at CO2levels of 100, 150, 190 and 270 μmol CO2 mol−1 air. Biomass declined with growth CO2such that Γplantwas estimated to be approximately 65 μmol mol−1for plants grown at 19/15 and 30/25°C. In the first 19 days after germination, plants grown at 100 μmol mol−1had low growth rates, such that most remained as tiny seedlings (canopy size <1 cm2). Most seedlings grown at 150 μmol mol−1and 30/25°C also failed to grow beyond the small seedling size by day 19. Plants in all other treatments grew beyond the small seedling size within 3 weeks of planting. Given sufficient time (16 weeks after planting) plants at 100 μmol mol−1eventually reached a robust size and produced an abundance of viable seed. Photosynthetic acclimation did not increase Rubisco content at low CO2. Instead, Rubisco levels were unchanged except at the 100 and 150 μmol mol−1where they declined. Chlorophyll content and leaf weight per area declined in the same proportion as Rubisco, indicating that leaves became less expensive to produce. From these results, we conclude that the effects of very low CO2are most severe during seedling establishment, in large part because CO2deficiency slows the emergence and expansion of new leaves. Once sufficient leaf area is produced, plants enter the exponential growth phase and acquire sufficient carbon to complete their life cycle, even under warm conditions (30/25°C) and CO2levels as low as 100 μmol mol−1.

Salinity effect on plant growth and leaf demography of the mangrove, Avicennia germinans L.

We assessed the effect of salinity on plant growth and leaf expansion rates, as well as the leaf life span and the dynamics of leaf production and mortality in seedlings of Avicennia germinans L. grown at 0, 170, 430, 680, and 940 mol m−3 NaCl. The relative growth rates (RGR) after 27 weeks reached a maximum (10.4 mg g−1 d−1) in 170 mol m−3 NaCl and decreased by 47 and 44% in plants grown at 680 and 940 mol m−3 NaCl. The relative leaf expansion rate (RLER) was maximal at 170 mol m−3 NaCl (120 cm m−2 d−1) and decreased by 57 and 52% in plants grown at 680 and 940 mol m−3 NaCl, respectively. In the same manner as RGR and RLER, the leaf production (P) and leaf death (D) decreased in 81 and 67% when salinity increased from 170 to 940 mol m−3 NaCl, respectively. Since the decrease in P with salinity was more pronounced than the decrease in D, the net accumulation of leaves per plant decreased with salinity. Additionally, an evident increase in annual mortality rates (λ) and death probability was observed with salinity. Leaf half-life (t0.5) was 425 days in plants grown at 0 mol m−3 NaCl, and decreased to 75 days at 940 mol m−3 NaCl. Thus, increasing salinity caused an increase in mortality rate whereas production of new leaves and leaf longevity decreased and, finally, the leaf area was reduced.

Relationships between Phenolic Acid Concentrations, Transpiration, Water Utilization, Leaf Area Expansion, and Uptake of Phenolic Acids: Nutrient Culture Studies

Phenolic acid treatments of cucumber seedlings (Cucumis sativus cv "Early Green Cluster") inhibited transpiration, water utilization, leaf area, and absolute and relative rates of leaf expansion. The cinnamic acids, ferulic and p-coumaric acids, were two to five times more inhibitory than the benzoic acids, p-hydroxybenzoic acid and vanillic acid. When phenolic acid concentrations were maintained at inhibitory concentrations through multiple successive treatments, percent inhibition of water utilization remained relatively constant for a given concentration and phenolic acid, percent inhibition of leaf area initially increased and then leveled off to a constant percent, and percent inhibition of transpiration and rates of leaf area expansion declined over time. Subsequently, p-coumaric acid was chosen as the model compound for further study. When p-coumaric acid was inhibitory, percent inhibition of transpiration, water utilization, and rates of leaf area expansion of actively growing leaves rapidly declined (i.e., was lost) as p-coumaric acid concentrations surrounding roots decreased. Absolute and relative rates of leaf expansion, for example, declined approximately 12 and 14%, respectively, for every 0.1 mM decline in p-coumaric acid concentration. Uptake of p-coumaric acid by cucumber seedling roots was continuous over the 24- or 36-hr periods monitored, but was not consistently related to the initial p-coumaric acid treatment concentrations. However, declining p-coumaric acid concentrations monitored at 6- or 12-hr intervals over the 24- or 36-hr periods continued to be highly correlated to the initial p-coumaric acid treatment concentrations. A 25% depletion by 1 3-d-old cucumber seedlings took 8.5, 12, 19.5, 25, and 29.5 hr for 0.125-, 0.25-, 0.5-, 0.75-, and 1-mM treatments, respectively. Uptake during periods when phenolic acid concentrations and root uptake (depletion from solution) were related appeared to represent periods dominated by apoplastic movement into the intercellular spaces of roots. Uptake during periods without this relationship likely represented periods dominated by symplastic movement. The ability of cucumber seedlings to modify active phenolic acid concentrations surrounding their roots suggests that cucumber seedling can directly influence the magnitude of primary and secondary effects of phenolic acids through feedback regulation.

Integrated responses of rosette organogenesis, morphogenesis and architecture to reduced incident light in Arabidopsis thaliana results in higher efficiency of light interception.

Plants have a high phenotypic plasticity in response to light. We investigated changes in plant architecture in response to decreased incident light levels in Arabidopsis thaliana (L.) Heynh, focusing on organogenesis and morphogenesis, and on consequences for the efficiency of light interception of the rosette. A. thaliana ecotype Columbia plants were grown under various levels of incident photosynthetically active radiation (PAR), with blue light (BL) intensity proportional to incident PAR intensity and with a high and stable red to far-red light ratio. We estimated the PAR absorbed by the plant, using data from precise characterisation of the light environment and 3-dimensional simulations of virtual plants generated with AMAPsim software. Decreases in incident PAR modified rosette architecture; leaf area decreased, leaf blades tended to be more circular and petioles were longer and thinner. However, the efficiency of light interception by the rosette was slightly higher in plants subjected to lower PAR intensities, despite the reduction in leaf area. Decreased incident PAR delayed leaf initiation and slowed down relative leaf expansion rate, but increased the duration of leaf expansion. The leaf initiation rate and the relative expansion rate during the first third of leaf development were related to the amount of PAR absorbed. The duration of leaf expansion was related to PAR intensity. The relationships identified could be used to analyse the phenotypic plasticity of various genotypes of Arabidopsis. Overall, decreases in incident PAR result in an increase in the efficiency of light interception.

Evaluation of physiological traits as indirect selection criteria for drought tolerance in sugar beet

Drought is the most important limitation to sugar beet ( Beta vulgaris L.) production in the UK and other areas dependent on usually insufficient summer rainfall. As increased irrigation is not a viable answer to the problem, an economically and environmentally desirable solution is new varieties with decreased sensitivity to water deficits. However, there is little genotypic information on drought tolerance in sugar beet, and breeders are not equipped to make these selections. The objectives of this study were to assess the degree of genotypic diversity for drought-related morpho-physiological traits and to measure the strength of association between these traits and indicators of crop performance. 46 sugar beet genotypes with diverse genetic backgrounds were tested in three field experiments from 1999 to 2001. Drought was imposed by covering plots with large polythene tunnels, allowing the crop to grow solely on stored soil moisture. Agronomic characters and drought tolerance indices are described in a companion paper [Ober et al., Assessing the genetic resources to improve drought tolerance in sugar beet. Agronomic traits of diverse genotypes under droughted and irrigated conditions. Field Crops Res., in press]. In this report, we show that there were significant genotypic differences for stomatal conductance, succulence index, specific leaf weight, and osmotic adjustment, but not for photosynthetic rate, relative water content or total water use. Patterns of water use within the soil profile differed between genotypes; some extracted more water from deep soil layers. The maintenance of green foliage cover during drought was positively correlated with drought tolerance index, which was negatively correlated with succulence index and scores for wilting and leaf senescence. Droughted sugar yield was positively correlated with soil water extraction and negatively correlated with relative leaf expansion rate measured in late summer. Under irrigated conditions, there was high positive correlation between transpiration rate and sugar yield. Genotype × trait biplots showed superior genotypes with relatively greater expression of combinations of favourable traits. The results suggest that succulence index and wilting score could be used to cull inferior plants in early stages of breeding programs, and green foliage cover and patterns of water use could help identify superior genotypes in elite germplasm. These data should enable tools to be developed for indirect selection of genotypes suited to drought-prone environments.

Hydraulic and chemical signals in the control of leaf expansion and stomatal conductance in soybean exposed to drought stress.

Both hydraulic and chemical signals are probably important in regulating leaf growth and stomatal conductance of soybean (Glycine max L. Merr.) under drought stress. However, until now they have not been investigated concomitantly in this species. To explore this, a pot experiment in a temperature-regulated greenhouse was conducted, in which plants were subjected to progressive drought during early reproductive stages. Biophysical parameters, viz. relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were followed in control and stressed plants. Drought stress decreased relative leaf expansion rate, stomatal conductance and leaf turgor, whereas it increased leaf [ABA], xylem pH and xylem [ABA]. As soil dried, significant differences between water treatments for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA], xylem pH and xylem [ABA] were observed at 14, 9, 14, 14, 14 and 9 d after imposition of stress, respectively. The relationships of relative values for relative leaf expansion rate, stomatal conductance, leaf turgor, leaf [ABA] and xylem pH to the fraction of transpirable soil water (FTSW) were well described by linear-plateau functions that allowed calculation of the soil-water thresholds at which processes in stressed plants began to diverge from well-watered controls. The soil-water threshold for stomatal conductance (0.64) was significantly higher than that for relative leaf expansion rate (0.29), xylem pH (0.28), leaf [ABA] (0.27) and leaf turgor (0.25). Relative xylem [ABA] increased, first linearly (when FTSW > 0.5) and then exponentially (when FTSW < 0.5) with decreasing FTSW. Relative stomatal conductance decreased exponentially with increasing relative xylem [ABA] (r2=0.98). Decreased stomatal conductance coincided with an increase in xylem [ABA] and occurred before any significant change of leaf turgor could be detected, indicating that chemical signals (seemingly root-originated ABA) control stomatal behaviour at moderate soil water deficits. Relative relative leaf expansion rate was linearly correlated with relative leaf turgor (r2=0.93), relative xylem pH (r2=0.97) and relative leaf [ABA] (r2=0.98), implying that both hydraulic and chemical signals were probably involved in regulation of leaf expansion at severe soil water deficits.