Depletion Of Plant Available Water Research Articles

Responses of Vitis vinifera L. cv. Sultanina to Water Deficits During Various Pre- and Post-Harvest Phases Under Semi-Arid Conditions

Performance of Sultanina grapevines irrigated at 60% plant-available water depletion (Tl) were compared to situations where water deficits were induced at budbreak (T2), before flowering (T3), after flowering (T4), at pea size (TS) and during ripening (T6). The field trial was performed on a fine sandy soil at Upington in the Lower Orange River region in South Africa. Treatments Tl to T6 were irrigated at 60% plant-available water (PAW) depletion during the post-harvest period and winter. A further four treatments were irrigated at 60% PAW depletion from budbreak until irrigation was terminated three weeks (T7), seven weeks (TS), eleven weeks (T9) and fifteen weeks (TlO) after harvest, respectively. The periods of water stress induced during the various pre-harvest phenological phases were too short to have a significant effect on vegetative growth. Water deficits during the early season (T2, T3 and T4) tended to affect yield more negatively than deficits applied between pea size and harvest (TS and T6). Yield reduction was associated with tendencies toward reduced bunch differentiation, berry shed and smaller berries. Berry size was significantly reduced by water deficits induced after flowering (T4). Water deficits had no significant effect on juice sugar content or on total titratable acids. No choice-grade sun-dried raisins could be produced when grapevines were subjected to water deficits during ripening (T6). In comparison to continued irrigation at 60% PAW depletion and water deficits induced up to pea size, significant browning of dipped raisins produced from TS and T6 grapevines occurred during some seasons. In combination with low relative humidity, water deficits between pruning and budbreak (T7 to TlO) reduced yields significantly compared to continued irrigation at 60% PAW depletion throughout the post-harvest period. Low relative humidity and dry soil also induced delayed budbreak and stimulated excessive shoot growth after harvest to such an extent that cane mass was significantly higher compared to irrigation at 60% PAW depletion. Neither water deficits nor significant shoot regrowth during the post-harvest period had any effect on cane starch content at pruning.

Responses of Vitis vinifera L. cv. Sultanina to Level of Soil Water Depletion Under Semi-Arid Conditions

Sultanina grapevines were irrigated at 90%, 60% and 30% plant-available water (PAW) depletion during the growing season in a field trial on a fine sandy soil in the Lower Orange River region of South Africa. Grapevines of all treatments were irrigated at 60% PAW depletion during winter. Measurement of diurnal leaf water potential (tJ'1) cycles at different stages during the season showed that plant water status was not only a function of soil water availability, but also of meteorological conditions. Increasing vapour pressure deficit (VPD) decreased pre-dawn leaf water potential, suggesting nocturnal water loss via transpiration. On the other hand, tpl measured at 14h00 increased with an increase in VPD, indicating partial stomatal closure during the warmest part of the day. Vegetative growth, berry mass and production increased with increased plant water status as imposed by the different PAW depletion levels. PAW depletion level had almost no effect on sugar accumulation or acid breakdown. Soil water depletion level had no effect on the quality of dipped raisins. Hence, the ideal balance between vegetative growth, yield and water use efficiency for the production of dipped Sultanina raisins was obtained with irrigation at 60% PAW depletion. Since quality of sun-dried raisins was considerably reduced by irrigation at 60% as well as 90% PAW depletion, relative to 30% depletion, the latter seems to be the optimum depletion level for sun-dried raisin production. This optimum depletion level might not be applicable for irrigation of Sultanina on heavier, more fertile alluvial soils.

Response of Vitis vinifera L. cv. Barlinka/Ramsey to Soil Water Depletion Levels with Particular Reference to Trunk Growth Parameters

Vitis vinif era L. cv Barlinka/Ramsey was irrigated with micro sprinklers at 10 % , 40 % and 60 % depletion of plant available water (PAW) in a field trial on a sandy soil in the Hex River Valley. A fourth treatment was trickle-irrigated at 40% PAW depletion. Irrigations were scheduled with tensiometers. A system which measures trunk diameter was used for automatic trickle irrigation of a fifth treatment. Measuring trunk growth continuously using linear variable differential transformers revealed no increase in diameter between budbreak and bloom. From bloom until veraison trunk diameter increased, whereafter it decreased slightly. Average seasonal trunk diameter increases were 0,8 mm, 1,2 mm and 2,1 mm, for the 60%, 40% and 10% PAW depletion levels, respectively. When well supplied with water, diurnal trunk contraction ranged between 0 mm and 0,01 mm per day. As water stress increased, trunk contraction increased curvilinearly to a maximum value of ca 0,1 mm per day. The most acceptable combination of growth, yield, berry size and eating quality was obtained by irrigation using micro sprinklers at 40% PAW depletion. Irrigation at 10% as well as 60% PAW depletion reduced berry taste and colour significantly. Root studies by means of the profile wall method showed that irrigation at 10 % PAW depletion limited fine root development in comparison to 40 % PAW depletion. Trickle irrigation stimulated development of fine roots within 500 mm of the grapevine as opposed to micro sprinklers. Trickle irrigation at 40% PAW depletion tended to increase water stress in comparison to micro sprinklers. This tended to improve grape quality, but reduced production and berry size. Although automatic irrigation held no significant advantages regarding yield and quality in comparison to manual scheduling with the aid of tensiometers, it did simplify irrigation management.

Water Relations of Vitis vinifera L. cv. Sunred Seedless in Response to Soil Water Depletion Before Harvest

Effects of soil water depletion level and irrigation cut off during ripening on water stress in Sunred Seedless table grapes were studied in a field trial in the Hex River Valley of South Africa. Water status of grapevines subjected to four irrigation treatments was determined prior to harvest during the 2000/01 season. Grapevines of all treatments were irrigated at 40% plant available water (PAW) depletion before veraison. From veraison, i.e. the onset of ripening in December, until harvest at the end of January, two treatments were irrigated at 20% and 40% PAW depletion, respectively. Two more treatments were applied by cutting off irrigation at 20% PAW depletion when total soluble solids reached 12°B and 15°B, respectively. Pre-dawn water stress in grapevines irrigated at 20% PAW depletion until harvest was less than in those where irrigation was cut off at 12°B. Sap flow measurements indicated that partial stomatal closure seemed to have occurred over the warmest part of the day. Apparently, stomatal control limited Ψl differences between treatments although extensive soil water depletion occurred where irrigation was cut off during ripening. If accumulated leaf water potential (ΨT) is considered, irrigation cut off at 12°B and 15°B induced more water stress in grapevines from 04:00 to 24:00 compared to those vines where irrigation was continued until harvest. Where irrigation was continued throughout the ripening period, the level of PAW depletion did not affect ΨT. However, during the sunshine period (08:00 until 18:00), ΨT in grapevines subjected to 40% PAW depletion, as well as those where irrigation was cut off, was higher than in the ones that were irrigated at 20% depletion until harvest. During the first half of the night, i.e. from 18:00 until 24:00, water potential in grapevines where irrigation was cut off at 12°B was unable to recover to the same level as in those exposed to the other treatments.

Root dynamics of Themeda triandra Forsk. in relation to water stress and defoliation at different phenological stages

The impact of water stress on root growth for the indigenous apomectic C4-grass species Themeda triandra was determined over a growing season covering three growth stages (vegetative, pipe and reproductive), as well as for regrowth occurring to one, three and six days after defoliation. Four water treatments (T1 = 0–25%, T2 = 25–50%, T3 = 50–75% and T4 = 75–100% depletion of plant-available water) were applied to plants grown in pots under greenhouse conditions. Root length and root mass increased significantly, with a decrease in plant-available water over all the growth stages. Root lengths and root masses were significantly higher during the reproductive growth stage for all the water treatments. In addition, root lengths for the different defoliation treatments, within the same water treatment, did not differ significantly. The influence of water stress has more effect over the short term on root growth than defoliation or growth stage. These measures will be useful to resource managers and should be considered when predicting ecosystem responses to global climate change.

Response to terminal water deficit stress of cowpea, pigeonpea, and soybean in pure stand and in competition

The response to terminal water deficit stress of three grain legumes, soybean, cowpea and pigeonpea, was evaluated in plants grown in large tubes, in competition with either the same species or one of the other two species. The aim was to explore how species differences in drought response affected water use, growth and survival of plants in pure stand and in competition. Two plants, comprising the test species and its competitor, were grown in each tube. Water was withheld 26 days after sowing by which time each plant had at least three fully expanded trifoliolate leaves. Leaf water status and plant growth were measured through destructive samples when 80% and 90% of the estimated plant available water (PAW) was depleted and at plant death, while PAW depletion, node growth and leaf survival were monitored at 2–3 day intervals until the last plants died (61 days after water was withheld). In pure stand, the rate of PAW depletion was initially slowest in cowpea despite its much larger leaf area, and fastest in soybean. Node growth was most sensitive in cowpea, ceasing at 65% PAW depletion compared with 85% PAW depletion in pigeonpea and soybean, so that the latter two species produced relatively more nodes after water was withheld. However, senescence of the lower leaves was most rapid in soybean and slowest in cowpea. Cowpea and pigeonpea extracted almost all PAW and died an average 18 days and 14 days, respectively, after maximum PAW depletion. In contrast, soybean died before 90% of PAW was depleted and so in pure stand used less water. There were otherwise only minor differences between the species combinations in the timing and maximum level of PAW depletion. The ability of cowpea and pigeonpea to maintain leaf water status above lethal levels for longer was achieved through different means. Cowpea relied primarily on dehydration avoidance and maintained tissue water status higher for longer, whereas pigeonpea demonstrated greater dehydration tolerance. While significant levels of osmotic adjustment (OA) were identified in soybean and pigeonpea, OA appeared to be of limited benefit to leaf survival in soybean. Pigeonpea invested significantly more total dry matter (TDM) in roots than either cowpea or soybean. Cowpea survived longest in pure stand whereas pigeonpea and soybean survived shortest in pure stand, suggesting that the dehydration avoidance response of cowpea was more effective in competition with like plants whereas the dehydration tolerance strategies of pigeonpea and soybean were least effective when competing against like plants. On average, TDM per plant ranked in the order cowpea > soybean > pigeonpea, largely reflecting initial differences in plant size when water was withheld. However, there was an inverse relation between TDM of a species and that of its competitor, so that in effect, water not used by a given plant to produce TDM was used by its competitor and there were no differences in TDM production per tube.

Genotypic differences in leaf area maintenance contribute to differences in recovery from water stress in soybean

Genotypic effects on leaf survival during water deficit stress and subsequent recovery were evaluated using soybean plants grown in tall cylinders in the glasshouse. An initial experiment sought to verify reported genotypic differences in leaf area maintenance under severe water deficit stress. A second experiment sought to test the hypothesis that these putative differences might affect recovery after stress was relieved. Two shoot genotypes, G2120 and cv. Valder, reported to have high and low leaf area retention, respectively, were used in both experiments. In order to preclude the possibility that the reported differences between G2120 and Valder were related to root rather than shoot traits, each shoot was grafted at the cotyledonary stage onto 2 non-self root genotypes, cv. Leichhardt and PI416937. Leichhardt has an apparently normal root, while PI416937 has been reported to be ‘extensively fibrous-rooted’. In the first experiment, water was withheld at the first trifoliolate leaf stage and the plants subjected to terminal water deficit stress. Consistent with the previous report, leaf area was maintained for longer into the stress by the G2120 shoots, with rapid loss of lower leaves not starting until c. 90% of plant-available water (PAW) had been depleted, compared with c. 80% for Valder. The Valder leaves also showed more ‘firing’ damage, with large patches of dead leaf tissue on the retained leaves. Also consistent with the previous report, leaf epidermal conductance to water vapour was lower in G2120 than in Valder. There were no apparent root effects. In the second experiment, water was again withheld at the first trifoliolate leaf stage, and treatments were re-watered when 80%, 85%, 90%, and 95% of the estimated PAW was extracted. Again, G2120 shoots showed better leaf area maintenance during the drying cycle, and less firing damage. When the plants were re-watered, the re-growth of G2120 generally exceeded that of Valder at all levels of PAW depletion. The differences in recovery between G2120 and Valder shoots were sufficient to have agronomic relevance, and confirmed the hypothesis that leaf area retention can affect recovery after severe water deficit stress. Root effects were relatively small. During the drying cycle, leaflet growth was marginally enhanced by Leichhardt relative to PI416937 roots. After re-watering, there was stronger recovery of plants with PI416937 roots, especially those with G2120 shoots. The basis of the differences between the root genotypes is not known but the stronger recovery of PI416937 may reflect its putative ‘extensively fibrous’ nature.

Evapotranspiration of Vitis vinifera L. cvs Sunred Seedless and Muscat Supreme in response to soil water depletion and irrigation cutoff during berry ripening

The effects of soil water depletion and irrigation cutoff during ripening on yield, quality and evapotranspiration (ET) of table grapes were studied to refine irrigation strategies needed during hot, dry summers. Six irrigation treatments were applied to two cultivars, viz. Sunred Seedless and Muscat Supreme, from the 1998/99 until 2000/01 seasons in a field trial in the Hex River valley of South Africa. Grapevines of all treatments were irrigated with micro-sprinklers at 40% plant available water (PAW) depletion before véraison and after harvest. Two treatments were irrigated at 20% and 40% PAW depletion from véraison to harvest. Four more treatments were applied by cutting off irrigation at these two depletion levels when total soluble solids reached 12°B and 15°B, respectively. According to earlier results, best overall performance for Sunred Seedless was obtained where irrigation at 40% PAW depletion was cut off at 15°B compared to the worst, i.e. where irrigation at 20% depletion was cut off at 15°B. For Muscat Supreme, irrigation at 20% PAW depletion, either continued until harvest, or cut off at 15°B, resulted in optimum overall performance. Although yield and quality responded differently, this study showed that the ET of the two cultivars responded in a similar way to the irrigation treatments under the given conditions. The ET of grapevines irrigated at 40% PAW increased from 1.6 mm day−1 in September to a maximum of 5.4 mm day−1 in January. Irrigation at 20% PAW depletion increased the maximum ET to 7.3 mm day−1. For both soil water depletion levels, irrigation cutoff at 12°B and 15°B resulted in, respectively, ca. 75% and 65% PAW depletion at harvest. Mean daily ET during January decreased to 4.1 mm day−1 and 5.0 mm day−1 after the irrigation had been cut off at 12°B and 15°B, respectively. The crop coefficients, which decreased accordingly, can be used to reduce irrigation to obtain optimum table grape yield and quality, particularly for Sunred Seedless.

Protein concentration in response to water stress in Themeda triandra Forsk

The influence of water stress on the concentration of water-soluble protein growth reserves was determined during three growth stages (vegetative, pipe and reproductive) as well as in three plant parts (roots, stubble and leaves) for the indigenous C4 semi-arid grass species, Themeda triandra. Four water stress treatments (T1 = 0–25%, T2 = 25–50%, T3 = 50–75% and T4 = 75–100% depletion of plant available water) were applied to plants grown in pots under greenhouse conditions. The water-soluble protein concentration (WSPC), determined in fractionated material obtained from stressed plants after drying in an oven, differed significantly (P<0.05) between plant parts as well as between growth stages. The WSPC in the leaves and stubble increased significantly with increased water stress for the vegetative, pipe and reproductive growth stages, while a similar increase was observed in the roots during the reproductive growth stage. The WSPC in the roots was variable among treatments during the vegetative and pipe growth stages. The results illustrate that under severe water stress, the decrease in aboveground vegetative growth causes an increase of water-soluble protein in aerial plant parts. Therefore, under severe drought conditions, the same number of animal units can still be carried owing to the higher nutritional value of the available fodder. This amplifies the importance of careful grazing and stock management in T. triandra dominant grassland, thus ensuring sustainable production and survival in an ecologically sensitive semi-arid climate.

Impact of water stress on growth reserves and re-growth of Themeda triandra (Forssk) following defoliation

The influence of water stress and defoliation on the concentration of total non-structural carbohydrates (TNCC) and water-soluble protein (WSPC) as growth reserves after defoliation were determined in the indigenous C4 semi-arid grass species Themeda triandra during three growth stages (vegetative, piping and reproductive) and in three plant parts (roots, stubble and leaves). Four water stress treatments (T1 = 0-25%, T2 = 25-50%, T3 = 50-75% and T4 = 75-100% depletion of plant available water) were applied to the plants in pots in a glasshouse. The TNCC declined drastically after severe defoliation over all the water treatments (P < 0.05), in all the plant parts (P < 0.05) and for all the growth stages (P < 0.05), while the TNCC did not recover (P < 0.05) (to the TNCC values as before defoliation) within 6 days after defoliation. The WSPC in the leaves increased from after defoliation, up to day 6 over all the water treatments (P < 0.05) and for all the growth stages (P < 0.05). Non-structural carbohydrates serve as the most important growth reserve and are utilised for re-growth after severe defoliation. Water-soluble protein can also be utilised as growth reserve, but only as an emergency measure for up to two days after defoliation. These results could make a meaningful contribution to ensure sustainable utilisation of the grassland ecosystem in semi-arid areas.

REDIRECTION OF PRECIPITATION BY A CORN CANOPY AND RELATED SOILWATER STATUS1*

Redirection of precipitation by a crop canopy significantly affects the distribution of water entering soil. This knowledge can be used to improve crop management practices for protecting ground water. Over a 4-y period, rain and/or irrigation water was collected at 8 equidistant positions between corn (Zea mays L.) rows. Soil water status was measured to a 0.3-m depth in the same relative positions. Wind direction and speed, and crop canopy development influenced the location where throughfall entered the soil surface and resultant soil water content. Stemflow increased to > 60% of incoming precipitation with canopy closure in most years. These large amounts of water entering the soil as stemflow increased percolation in the row and soil water content below the row, the zone of greatest depletion of plant available water. Associated with canopy closure, reduced throughfall reached the soil surface within 0.2 m of the row; hence, a reduced potential for leaching of agrichemicals applied in this area. * Minnesota Agric. Exp. Stn. Scientific J. Series No. 991250097.

Productivity and water relations of field-grown cashew: a comparison of sprinkler and drip irrigation

Cashew is an emerging crop in the seasonally ‘wet–dry’ tropical regions of northern Australia. In North Queensland flowering and fruiting of cashew coincides with the dry season (May–November). During this period growers sprinkler irrigate at 500 L/tree.week. A 3-year (1996–98) experiment compared this strategy with alternatives, including no irrigation or drip irrigation in which 115 or 230 L/tree.week was applied by drippers placed near the tree trunk and near the canopy drip line throughout the dry season. Measurements of soil water to 1.3 m, leaf gas exchange, chlorophyll fluorescence, tree sap flow and yield were made. Data collected in the first 2 years showed that the water requirement of the trees increased progressively as the crop load and evaporative demand increased during the dry season. During the final year of the study, additional sprinkler and drip treatments, in which water applications were progressively increased during the dry season, were introduced. The productivity of cashew in this experiment was strongly influenced by irrigation treatments, ranging (over all years) from 42 to 160 g nut/m 2 canopy surface area. Depletion of plant-available water in the root zone was associated with a reduction in photosynthesis mediated by partial stomatal closure. These effects of soil drying were evident in all irrigated treatments during the mid and late stages of the dry season but were more severe in treatments receiving the least water. When irrigation was withheld until the mid-stage of the dry season the trees had similar yields to those that were irrigated throughout, emphasising the importance of providing adequate irrigation between nut set and harvest. When rainfall from January to September in each year of the study was taken into account, there was a strong linear relationship between nut yield and water applied (rainfall + irrigation), with each extra kilolitre of water applied resulting in about 6 extra g nut/m 2 canopy surface area. This linear relationship was based on water application in the range 25–50 kL per season. It is possible that if the seasonal water application had exceeded 50 kL the marginal response to extra water may have diminished. Using drippers was slightly more efficient than sprinklers, with drip-irrigated trees requiring about 5% less water applied to achieve a given nut yield. In years when rainfall is average, and subject to other economic factors, growers in North Queensland should aim to irrigate about 500 L/tree.week. In years of low rainfall between January and September it is likely that yield will be improved by applying more irrigation water; high rainfall during these months of the year may reduce the irrigation requirement. In all cases growers should be careful to accurately monitor water applications, particularly when the total (from rainfall &amp;plus; irrigation) exceeds 40 kL/tree for the season.

Impact of irrigation scheduling practices on pesticide leaching at a regional level

Information about how current and proposed management practices impact environmental quality is required to develop best management practices. A modeling approach was used to evaluate the scheduling practices of local farmers and two other irrigation scheduling practices for their potential impact on groundwater pollution in Doña Ana County, New Mexico. Data about farmers’ practices came from historical information about the timing and quantity of water delivered to the farms. The irrigation scheduling practices were: tensiometer-based with the tensiometers placed at 50% or 75% of the root-zone depth and irrigations started when tensiometer’s readings reached 6 kPa for sandy soils, 23 kPa for sandy loam soils, 44 kPa for loamy soils, and 74 kPa for clay loam soils; and at 50% plant available water depletion (PAWD) level regardless of soil type. The objective was to use irrigation scheduling model ( IRRSCHM), a volume balance, mixing-cell, type irrigation scheduling and pesticide transport model, to assess and compare the impact of different irrigation scheduling practices on cyanazine (Bladex) and metolachlor (Dual) concentrations at 180 cm below the soil surface during a 30-year cropping sequence. The region was divided into different soil textural classes to facilitate rapid estimation of soil parameters needed for the model. Very low Bladex and Dual concentrations were predicted at 180 cm below the soil surface. However, the predicted pesticide concentrations increased as soil sand fractions increased, regardless of the irrigation scheduling practice. The tensiometer based irrigation scheduling resulted in the highest Bladex and Dual concentrations. The lowest concentrations were predicted under the farmer’s practices due to deficit irrigation. Dual concentrations at 180 cm depth of the sandy soil class were about 20 times less than the 5.25 × 10 −1 mg l −1 Health Advisory Level under the tensiometer-based irrigation scheduling practices, while, the farmer’s practices resulted in Dual concentration about 625 times less than the Health Advisory Level. Similarly, the predicted Bladex concentration in sandy soil class was 3125 times less than the 1.30 × 10 −2 mg l −1 Bladex Health Advisory Level under the tensiometer-based irrigation scheduling and about 416 000 times less than the Health Advisory Level under farmer’s practices. Simulation results suggested that current farmer’s practices do not pose a threat to the area’s groundwater quality and result in a 15–40% leaching fraction depending on soil type. Tensiometer-based irrigation scheduling was similar to scheduling irrigations at 50% plant available water depletion and resulted in a 35–50% leaching fraction depending on soil type as long as the tensiometer was placed in the proper root zone depth. The model’s calculated leaching fractions using farmer’s practices were similar to measured leaching fractions in Doña Ana County, giving credibility to the use of simulation models for assessing and comparing the potential impact of different irrigation scheduling practices on environmental quality at a regional level.

Radiation Balance of Dryland Grain Sorghum as Affected by Planting Geometry1

AbstractThe effects of planting geometry on the radiation balance of dryland grain sorghum (Sorghum bicolor L. cv. DK 46) were studied in a field experiment at the USDA‐ARS Conservation and Production Research Laboratory at Bushland, TX, in 1984 on a Pullman clay loam (a fine, mixed, thermic Torrertic Paleustoll). The objective was to reduce the radiation load on a crop through manipulation of planting geometry and to determine whether that would affect crop productivity. Net radiation was 5% higher over wide compared to narrow rows (0.76 and 0.38 m, respectively) when averaged over three population levels from 33 to 110 days after sowing. East‐west rows had 14% higher net radiation than north‐south rows, averaged over two row spacings. The differences in net radiation were due to daytime responses, presumably shortwave albedo differences. Leaf photosynthesis and transpiration rates and stomatal resistance were measured twice during the grain‐filling period. These measurements, taken 5 days after a period of moderate rains, showed no differences in the plant response due to row spacing or direction treatments. Slightly higher leaf temperatures in the high‐population plots may have been related to greater depletion of plant available water than in medium and low population plots (72, 66, and 67% depletion, respectively). After 2 weeks of drying, narrow‐row and high‐population treatments showed greater stress as evidenced by lower transpiration and photosynthesis rates and higher stomatal resistance and leaf temperature. Soil water depletion was 20% less in narrow than wide rows during a 2‐week period spanning the leaf transpiration measurement dates. Leaf photosynthesis, transpiration, temperature, and stomatal resistance were not affected by differences in net radiation due to row spacing or row direction.

Soybean Leaflet Movements as an Indicator of Crop Water Stress1

Vertical orientation of soybean [Glycine max (L.) Merr] leaflets during the onset of water stress, and the usefulness of this phenomenon as an indicator of crop water stress, were investigated in relation to leaf water potential (Ψ), stomatal diffusive resistance (ra) and soil water status in a field study. Observations were made during the reproductive stage on plants in a field with a Rhodic Paleustalf soil. Terminal leaflet movements were influenced by plant water stress and were not influenced by their azimuth direction. Increases in the angle of the terminal trifoliolate leaflet to the soil coincided with the earliest notable changes in Ψ and ra and thus provided a clear indication of the onset of crop water stress. Leaflet inversion began when Ψ was approximately ‐1.4 MPand was associated with a greater than 60% depletion of plant available water. The possible mechanism of leaflet inversion is related to changes in osmotic and pressure potentials and potassium concentration across the pulvinus.

Responses of Vigna unguiculata (L.) Walp. To Atmospheric and Soil Drought

Growth responses, water relations, leaf conductance, and gas exchange of V.unguiculata were studied under conditions of drought in both the aerial and the soil environment. Dry air caused a significant reduction in whole plant biomass and all its components despite a plant water status which was not different from plants in humid air. Stomata closed in dry air and water loss per plant was lower in dry than in humid air because of both lower leaf conductances and a smaller total leaf area. Dry soil also caused decreases in biomass growth, but leaf growth continued at the soil water status at which stomata began to close (45% depletion of plant available water). Relative leaf expansion rate started to decline below the rate of control plants when 60% (humid air) or 65% (dry air) of plant available water was depleted. Therefore, in V. unguiculata, stomata appear to be more sensitive to soil drought than growth of leaves. Leaf relative water content and osmotic potential remained at the level of control plants when stomata started to close and decreased only slightly below the level of control plants even under severe water stress. A trend of decreasing osmotic potentials at 55-60% depletion of plant available water was observed, but leaf growth ceased only when 90% of plant available water was depleted. Leaf conductance decreased more rapidly than CO2 assimilation during development of soil drought.