Severe Water Deficit Treatments Research Articles

Investigating the impacts of different degrees of deficit irrigation and nitrogen interactions on assimilate translocation, yield, and resource use efficiencies in winter wheat

Water and nitrogen (N) are recognized as the primary determinants influencing the development and output of winter wheat. They affect the growth of winter wheat not only through their own changes, but also through their interaction. The translocation and accumulation of pre- and post-anthesis assimilates (including dry matter and N) are important physiological processes in winter wheat, which are closely related to the resource use efficiency and yield. However, a dearth of research exists that examines the complex interplay between varying water levels, especially severe water deficit, and N deficiency levels on the growth dynamics of winter wheat. The purpose of this study was to quantify and compare the effects of different degrees of water deficit and N interaction on assimilate translocation, water and N use efficiency and yield of winter wheat. A four-year long-term field experiment conducted under the rain-out shelter including four irrigation and two N levels was launched during 2019–2023. This method avoided the impact of precipitation on water treatment and helped to achieve serious water deficit treatment. The findings indicated that the higher N application improved the wheat's ability by 49.6 % - 362.3 % to utilize the available soil water. The pre- and post-anthesis translocation and accumulation of assimilates (including dry matter and N) in winter wheat increase alongside elevated levels of water and N application, except under severe water deficit treatment. Plants under mild to moderate water deficit were better able to translocate the pre-anthesis assimilates. However, the severe water deficit hindered the re-translocation of assimilates before anthesis. Providing additional N during periods of severe water scarcity leads to a 13.4 % - 44.7 % reduction in water use efficiency (WUE). Furthermore, both the irrigation water use efficiency and WUE diminished by 10.3 % - 60.5 % and 4.5 % - 41.4 % with higher levels of irrigation, while improved by 24.4 % - 48.2 % and 12.6 % - 39.4 % with higher N application rates. Conversely, N partial factor productivity and N use efficiency followed the opposite trend. Similar to WUE, increasing N application under severe water deficit would result in a yield reduction of 38.7 %. The reduction in crop yield resulting from severe water stress was primarily ascribed to the decline in the kernels number per spike (up to 74.9 %), with the reduction in spike number per unit area following closely behind (up to 29.1 %). This integrated approach, combining resource use efficiency with a detailed assessment of assimilate dynamics, provides a holistic view of how water and N management strategies impact winter wheat performance. The findings will offer precise insights for developing targeted irrigation and N scheduling strategies for sustaining both winter wheat production and environmental sustainability.

Transcriptomic analyses provide new insights into jujube fruit quality affected by water deficit stress

Water availability is a crucial factor affecting fruit quality and productivity. A period of mild drought stress can increase fruit sweetness and improve fruit quality. However, the mechanism underlying the transcriptional regulation in Chinese jujube under water deficit conditions remains relatively uncharacterized. In this study, we analyzed the protein and sugar contents, the average weight, and transcript expression (RNA sequencing) of jujube fruit at different ripening stages (i.e., white mature and half-red fruit) under mild and severe water deficit conditions. The sucrose and protein contents increased significantly under water deficit conditions. The increase in water deficit stress from mild to severe did not significantly alter the sugar accumulation, but it decreased the average fruit weight. The sugar- and amino acid-related pathways, flavonoid biosynthesis, xyloglucan metabolism, and phenylpropanoid biosynthesis were affected by water deficit stress. Gene co-expression analyses identified many ABA- and ethylene-related genes with expression levels modulated by water deficit conditions, especially in the half-red stage. Additionally, we identified the key genes affecting fruit sugars, pigments, cell wall polysaccharides, and phenylpropanoids, which influence fruit quality. Notably, genes related to sugar transporters rather than sugar metabolism contributed to the sucrose accumulation. Flavonoid synthesis-related gene expression contributed to the earlier coloration of the stressed fruit than of the control fruit. Severe water deficit stress up-regulated the expression of pectinesterase and endotransglucosylase/hydrolase genes, which accelerated fruit ripening and softening. Regarding the phenylpropanoid biosynthesis pathway, the exposure to mild water deficit stress promoted the synthesis of secondary metabolites useful to humans (e.g., coniferin and syringin), whereas the severe water deficit treatment increased lignin accumulation. Considered together, these findings revealed the transcriptional regulatory mechanism that improved jujube fruit quality under mild drought conditions. Furthermore, the results of this study may be useful for optimizing the irrigation during jujube cultivation.

Exogenous abscisic acid coordinating leaf senescence and transport of assimilates into wheat grains under drought stress by regulating hormones homeostasis

Drought at the grain filling stage of wheat will cause premature leaf senescence, thus leading to considerable loss of wheat yield. Therefore, this paper aims to establish a cultivation technology for strong drought resistance, delayed senescence, and yield improvement based on the analysis of hormones homeostasis obtained by applying chemical control substances. Experiments were conducted with two genotypes of wheat. Four water irrigation treatments were applied to impose the water deficit, including well-watered control treatment (WW), mild water deficit (MiWD), moderate water deficit (MoWD), and severe water deficit (SWD). Exogenous abscisic acid (ABA) was sprayed on the plants at the anthesis stage of the wheat. As a result, exogenous ABA reduced initial senescence rate (r0), total duration of chlorophyll (Chltotal), rapid senescence phase (Chlloss), and the accumulated temperature at an inflection point (M) but improved the persistence phase (Chlper) of flag leaves under all of the four treatments. However, exogenous ABA produced inconsistent effects on photoassimilate relocation and grain weight under different treatments. It produced positive regulatory effects on grain weight under WW, MiWD, and MoWD treatments. On the one hand, spraying ABA during the persistence phase of flag leaves reduced the ratios of zeatin to gibberellin (Z/GA3), spermine to spermidine (Spm/Spd), and salicylic acid to ABA (SA/ABA), which prolonged active photosynthesis by stimulating high level of proline (Pro) and increased the activities of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). Therefore, drought tolerance was enhanced, and more photosynthetic assimilates were accumulated. On the other hand, the rapid senescence phase and the transport rate of assimilates into grains were accelerated, resulting in higher grain weight, yield, and water use efficiency (WUE). However, under SWD treatment, exogenous ABA improved the ratio of SA/ABA, leading to low Pro content and low antioxidant enzyme activity of flag leaves in the rapid loss phase. Meanwhile, drought resistance declined and the transport duration of assimilates into grains was shortened, thus making photosynthetic assimilates redundant. Therefore, exogenous ABA can lead to the reduction in grain weight, yield, and WUE of wheat under SWD treatment.

Variability among Young Table Grape Cultivars in Response to Water Deficit and Water Use Efficiency

Climate change will lead to higher frequencies and durations of water limitations during the growing season, which may affect table grape yield. The aim of this experiment was to determine the variability among 3-year old table grape cultivars under the influence of prolonged water deficit during fruit development on gas exchange, growth, and water use efficiency. Six own rooted, potted table grape cultivars (cv. ‘Muscat Bleu’, ‘Fanny’, ‘Nero’, ‘Palatina’, ‘Crimson Seedless’ and ‘Thompson Seedless’) were subjected to three water deficit treatments (Control treatment with daily irrigation to 75% of available water capacity (AWC), moderate (50% AWC), and severe water deficit treatment (25% AWC)) for three consecutive years during vegetative growth/fruit development. Water deficit reduced assimilation, stomatal conductance, and transpiration, and increased water use efficiencies (WUE) with severity of water limitation. While leaf area and number of leaves were not affected by treatments in any of the tested cultivars, the response of specific leaf area to water deficit depended on the cultivar. Plant dry mass decreased with increasing water limitation. Overall, high variability of cultivars to gas exchange and water use efficiencies in response to water limitation was observed. ’Palatina’ was the cultivar having a high productivity (high net assimilation) and low water use (low stomatal conductance) and the cultivar ‘Fanny’ was characterized by the highest amount of total annual dry mass as well as the highest total dry mass production per water supplied during the experiment (WUEDM). Hence, ‘Fanny’ and ‘Palatina’ have shown to be cultivars able to cope with water limiting conditions and should be extensively tested in further studies.

Performance of potato (Solanum tuberosum) advanced hybrid MS/6-1947 under high temperature stress and water deficit conditions

In present investigation, potato (Solanum tuberosum L.) advanced stage hybrid MS/6-1947 was evaluated along with controls under high temperature and water stress regimes for three successive winter crop seasons. Under high temperature at Modipuram, MS/6-1947 (13.3 t/ha) showed yield enhancement of 20, 9 and 11% over the controls Kufri Bahar, Kufri Pukhraj and Kufri Surya. It also exhibited at par tolerance to hopper with Kufri Surya and damages due to mite burn were lower than Kufri Bahar and Kufri Pukhraj. The hybrid possessed 18% tuber dry matter. Results ondrought toleranceat Modipuram revealed superior performance of MS/6-1947 under normal irrigation, mild water and severe water stress conditions as compared to best control Kufri Pukhraj as yield reduction was comparatively lower under mild and severe water stress treatments. Drought tolerance index (DTI) values were maximum in MS/6-1947 (1.02 and 0.79) followed by Kufri Pukhraj (0.86 and 0.51) and Kufri Bahar (0.72 and 0.58) under mild and severe water deficit treatments, respectively. Drought tolerance studies at Jodhpur revealed better performance of MS/6-1947 under normal and mild water stress conditions as compared to the controls Kufri Pukhraj and Kufri Surya as yield reduction was comparatively lower under mild water stress conditions. DTI values were maximum in MS/6-1947 (1.08) followed by Kufri Puhkraj (1.01) and Kufri Surya (0.41) under mild water deficit treatment. Tuber dry matter content increased under water stress over normal irrigation at both the locations. MS/6-1947 is suitable for table potatoes. MS/6-1947 may be a better option for integrating in cereal based crop sequences, for exploiting its yield potential in early season and for attaining sustainable productivity in areas where high temperature and lower water availability are constraint for raising the successful potato crop.

Exogenous application of gamma-aminobutyric acid (GABA) alleviates the effect of water deficit stress in black cumin (Nigella sativa L.)

Water deficit is an abiotic stress factor that negatively affects black cumin (Nigella sativa L.) production. Gamma aminobutyric acid (GABA), an endogenous signaling molecule and metabolite, has high physiological and molecular activity in plant cells, which can promote tolerance to water deficit stress, but little information is available on the effect of exogenous application on growth of black cumin. A field experiment over two years was carried out at a farm located in Naqadeh-Urmia, West Azerbaijan, Iran to evaluate the effects of GABA on some agronomic and biochemical attributes of black cumin under water deficit stress conditions. Three irrigation treatments (irrigation after 50, 100, and 150 mm evaporation based on evaporation from class A pan) and four levels of GABA application (0, 0.5, 1.0, and 2.0 mg L−1) were tested. Irrespective of GABA application, the severe water deficit treatment (i.e., irrigation after 150 mm evaporation) provided the lowest seed number per follicle, 1000-seed weight, and seed yield. Increasing water deficit, significantly reduced chlorophyll a by 8.2 to 15.8% and chlorophyll b by 18.4 to 41.5%, whereas GABA application significantly improved these traits. The application of 2.0 mg L−1 GABA increased chlorophyll a content by 6.2% and chlorophyll b content by 19.2% compared with control. In addition, GABA application showed a positive and significant effect on soluble sugars content, proline accumulation, and catalase (CAT) activity. The maximum values of these variables were obtained with the application of GABA at 2.0 mg L−1. CAT, peroxidase (POX), and superoxide dismutase (SOD) activity increased with decreasing chlorophyll a and chlorophyll b contents, whereas soluble sugars and proline content increased with increasing activity of those antioxidant enzymes. Overall, in addition to cellular mechanisms, such as osmoregulation and antioxidant defense, GABA application can improve growth and productivity of black cumin under water deficit stress conditions.

Regulated deficit irrigation effects on physiological parameters, yield, fruit quality and antioxidants of Vaccinium corymbosum plants cv. Brigitta

Highbush blueberries require sufficient water throughout the growing season, especially during fruit development. Regulated deficit irrigation (RDI) has been used in many fruit crops to save water and potentially increase fruit quality without reducing yield. Our aim was to determine the effects of RDI on yield, plant–water relations, and fruit quality at harvest. Three irrigation treatments based on actual evapotranspiration (ETa) were applied: 50, 75 and 100% (control) to six-year-old ‘Brigitta’ highbush blueberries in Colbun, Maule Region, Chile, for seasons 2013–2014 and 2014–2015 and to 26-year-old ‘Brigitta’ plants in South Haven, Michigan, USA, during the 2014 season. Severe water deficit treatment (50% ETa) decreased photosynthetic rate, vegetative growth (second season in Colbun and South Haven’s trial), and fruit quality (berry size, titratable acidity, soluble solids, weight), and increased oxidative stress during both seasons in Colbun. The 50% ETa treatment also had the highest yield reduction during Colbun’s second season. In contrast, mild water stress (75% ETa) resulted in similar fruit yields and quality (firmness, fruit size, titratable acidity, soluble solids and berry weight) but with higher water productivity than the 100% ETa treatment, while it achieved intermediate values for antioxidant capacity compared to the 50% ETa treatment.

Different physiological responses under drought stress result in different recovery abilities of two tropical woody evergreen species

ABSTRACT The effects of water deficit on physiological and biochemical variables of young plants of two tropical woody species, Pachira aquatica and Sterculia foetida, and their recovery abilities were measured. Leaf water potential, gas exchange and selected carbon metabolism components were measured in a greenhouse experiment with control, moderate and severe water deficit treatments. Under severe drought stress, gas exchange and chlorophyll fluorescence decreased in both species compared to their respective controls. After rehydration, P. aquatica and S. foetida showed a partial and full recovery of the measured variables, respectively. In addition, a decrease and an increase in photosynthetic pigments were observed for P. aquatica and S. foetida, respectively, compared to their controls. In conclusion, the two species showed differing responses regarding photosynthetic pigment content dynamics for tolerating water deficit. Individuals of P. aquatica in the severe water deficit treatment showed a decrease in pigments, which may have impaired the recovery of metabolism (gas exchange) after rehydration, while S. foetida experienced an increase, favoring a full recovery of gas exchange and biochemical metabolism after rehydration.

Grain yield and water use efficiency of super rice under soil water deficit and alternate wetting and drying irrigation

This study investigated if super rice could better cope with soil water deficit and if it could have better yield performance and water use efficiency (WUE) under alternate wetting and drying (AWD) irrigation than check rice. Two super rice cultivars and two elite check rice cultivars were grown in pots with three soil moisture levels, well watered (WW), moderate water deficit (MWD) and severe water deficit (SWD). Two cultivars, each for super rice and check rice, were grown in field with three irrigation regimes, alternate wetting and moderate drying (AWMD), alternate wetting and severe drying (AWSD) and conventional irrigation (CI). Compared with that under WW, grain yield was significantly decreased under MWD and SWD treatments, with less reduction for super rice than for check rice. Super rice had higher percentage of productive tillers, deeper root distribution, higher root oxidation activity, and greater aboveground biomass production at mid and late growth stages than check rice, especially under WMD and WSD. Compared with CI, AWMD increased, whereas AWSD decreased grain yield, with more increase or less decrease for super rice than for check rice. Both MWD and SWD treatments and either AWMD or AWSD regime significantly increased WUE compared with WW treatment or CI regime, with more increase for super rice than for check rice. The results suggest that super rice has a stronger ability to cope with soil water deficit and holds greater promising to increase both grain yield and WUE by adoption of moderate AWD irrigation.

Reproductive and Fiber Quality Responses of Upland Cotton to Moisture Deficiency

Quantitative information between water deficit and cotton (Gossypium hirsutum L.) reproductive potential and fiber quality is needed to improve cotton model predictability. An experiment was conducted by seeding Upland cotton cultivar, Texas Marker (TM)‐1, in sunlit growth chambers. Four water stress treatments, 100, 80, 60, and 40% of daily evapotranspiration of the control, were imposed during flowering for plants grown at optimum temperature and nutrient supply. Soil moisture content and midday leaf water potential (LWP) were measured twice weekly during treatment period. Photosynthetic measurements taken during the stress treatments were correlated with midday LWP. New flowers and bolls were tagged daily to estimate boll maturation period (BMP). Plant‐ and boll‐component dry weights were recorded at end of the experiment. Lint sample collected, grouped based on average LWP during BMP, were analyzed for fiber quality parameters. Declining photosynthetic rates as plants experience water deficits were mostly to stomatal limitation, but non‐stomatal factors played a role as stress progressed. Seedcotton and seed weight, boll numbers, and total biomass declined significantly at severe water deficit treatments reflecting declining trends of photosynthesis. Fiber length, strength, and uniformity declined linearly with decrease in LWP, whereas fiber micronaire increased with decrease in LWP. Fiber strength was most responsive to changes in LWP followed by micronaire, length, and uniformity. Immature fiber content increased and fiber maturity ratio declined with diminishing LWP. The functional relationships between LWP and fiber properties will be useful to develop fiber sub‐models under optimal temperature and nutrient, but varying moisture conditions.

Effect of varied summer deficit irrigation on components of olive fruit growth and development

The timing, duration, and intensity of summer water restrictions differentially affect overall olive fruit growth and production, based on the underlying fruit developmental processes. For that, the fruit component and tissue morphogenetic response to different irrigation strategies during summer was examined in a hedgerow olive orchard cv. Arbequina. Control trees (CON) were irrigated to maintain the root zone close to field capacity throughout fruit growth. From budburst to 4 weeks after full bloom (WAFB) (Period 1) and from 14 WAFB to harvest (at 23 WAFB) (Period 4) trees of all treatments were irrigated as CON. Two severe water deficit treatments were applied during summer by irrigating 30% CON from 4 to 9 WAFB (Period 2) in DI-P2 or from 9 to 14 WAFB (Period 3) in DI-P3. Moderate water deficit was applied in Periods 2 and 3 by irrigating 50% CON in DI-P2&3. Growth and development of the fruit and its component tissues (exocarp, mesocarp and endocarp), fruit composition, mesocarp cell area and cell number, and epidermal characteristics at the end of each period were evaluated. Water deficits significantly reduced fruit volume at the time when they were applied. Mesocarp size was more sensitive to water deficit than endocarp size and showed a high recovery capacity after rewatering. Although the majority of cells were developed in Period 1, a substantial number of mesocarp cells were also formed later. While mesocarp cell number was unaffected by water reduction in any of the deficit periods, cell size was highly affected but with high recoverability. Endocarp size was reduced when water restriction was applied in DI-P2 and its effect continued until harvest. Fruit oil content at harvest was not significantly affected by the applied water restrictions, whereas water was the fruit component which most responded to both the increases and decreases in irrigation. Cuticle thickness, epidermal cell size and number at harvest appeared to respond to both irrigation regime and fruit expansion pressures.

Water deficit affects plant and soil water status, plant growth, and ginsenoside contents in American ginseng

American ginseng (Panax quinquefolius L.) produces pharmacologically active secondary compounds known as ginsenosides which have been shown to be influenced by both genetic and environmental factors. In a greenhouse experiment, effects of water deficit on ginseng plant growth, predawn leaf water potential (ΨLeaf), soil water potential (ΨSoil), leaf abscisic acid (ABA) concentration, and root ginsenoside contents as well as photosynthesis-related physiological responses were studied. Three-year-old seedlings, grown in 200 mL volume of plastic pots, were well watered for 45 days prior to the initiation of water deficit treatments. Plants in the water deficit treatments were irrigated every 10 or 20 days for the mild and severe water deficit treatments, respectively, while the control plants were watered every 4 days. The experiment was terminated after 15, 6, and 3 dry down cycles (60 days) for the control, mild, and severe water deficit treatments, respectively. As water deficit progressed, both ΨSoil and ΨLeaf decreased, but foliar ABA concentration increased. Other physiological responses to water deficit, including transpiration rate, stomatal conductance, and CO2 assimilation rate, were decreased. Water deficit decreased root growth, but unaffected shoot growth. Foliar chlorophyll content was also decreased in the water deficit treatments. The contents of individual ginsenosides Re, Rb1, Rc and Rd, and total ginsenosides were increased in the storage roots of water deficit-treated plants as compared with well-watered controls. Rootlet fresh weight before transplanting (RFWBT) as a covariate had a significant effect on the contents of ginsenoside Rb1, Rc, and Rb2. Overall, the results indicate that water deficit could contribute not only to reducing plant performance but also increasing the levels of ABA and certain ginsenoisdes.

Effects of water stress on dry matter accumulation and translocation in winter wheat cultivars planted at different ages

为了明确河南中部地区冬小麦品种改良过程中物质积累与转运特性及对水分亏缺响应的差异, 选取新中国成立以来6个不同年代主栽冬小麦品种, 采用测坑试验研究了冬小麦品种在不同水分胁迫(CK: 充分供水田间持水量的75%~85%; MD: 轻度干旱, 田间持水量的60%~70%; SD: 重度干旱, 田间持水量的45%~55%)下干物质生产与积累转运特性的演进特征及其与产量的关系。结果表明, 品种改良协调了干物质在各生育阶段的分配, 使花前和花后干物质积累与转运对籽粒的贡献更加平衡。在品种更替过程中, 株高和穗下节降低, 千粒重、籽粒产量和收获指数增加, 干物质积累总量无显著差异, 千粒重和收获指数的提高对产量增加起重要作用。在CK、MD和SD处理下, 20世纪90年代和2002年品种比20世纪50年代品种平均株高降低35.2%、36.2%和38.2%, 平均千粒重比增加31.7%、17.4%和56.3%, 平均籽粒产量增加40.4%、43.0%和52.4%; 20世纪90年代和2002年品种的收获指数比20世纪80年代及以前品种增加31.4%、22.3%和24.6%。CK处理早期品种干物质积累在抽穗至开花阶段超过现代品种。MD和SD处理的20世纪90年代及以后的品种花前干物质转运能力高, 茎秆干物质输出率增加, 花后贮藏物质积累降幅小, 干物质贮藏转运分配比例适宜, 对籽粒贡献率高, 是水分胁迫条件下现代品种高产的基础。

Analysis of drought responsive proteins in wheat ( Triticum durum) by 2D-PAGE and MALDI-TOF mass spectrometry

Drought is an abiotic stress that strongly influences plant growth, development and productivity. By proteomics study it is possible to identify the complex mechanism of water-stress response. The aim of this research was the analysis of wheat ( Triticum durum) proteome changes under stress conditions by applying a severe water deficit treatment for 7 days. Stress-induced proteome changes were analyzed by two-dimensional gel electrophoresis in conjunction with matrix-assisted laser desorption ionization-time of flight mass spectrometry. Thirty-six protein spots showed a reproducible significant change between control and stressed samples. The reasonable implications in drought response of the identified proteins were discussed. Results provide new insights that can lead to a better understanding of the molecular basis of drought-sensitivity in plants. Therefore, the obtained data could suggest the development of drought resistant wheat varieties, in order to improve agricultural production in dry regions.

Interactive Effects of Deficit Irrigation and Crop Load on Cabernet Sauvignon in an Arid Climate

Regulated deficit irrigation (RDI) and crop-load adjustment are regarded as important viticultural practices for premium-quality wine production, although little is known about their interactive effects. Crop loads were altered on field-grown, own-rooted Cabernet Sauvignon grapevines exposed to RDI varying in severity and timing in the arid Columbia Valley (Washington) from 1999 to 2003. Following a dry-down period through fruit set to stop shoot growth, vines were irrigated at 60 to 70% of full-vine evapotranspiration until harvest. Other vines either received the same amount of water up to veraison, after which the irrigation rate was cut in half, or had their irrigation halved before veraison but not thereafter. Clusters were thinned within irrigation treatments during the lag phase of berry growth to achieve a target yield of 6.7 t/ha, compared with an unthinned control. The severity and timing of RDI had only minor effects on vegetative growth, yield formation, fruit composition (soluble solids, titratable acidity, pH, K⁺, color), and cold hardiness. The more severe water-deficit treatments slowed berry growth while the treatments were being imposed, but final berry weights were similar in three of five years. Although cluster thinning reduced yields by 35% and crop loads by 32%, crop load had little or no influence on vegetative growth and cluster yield components and advanced fruit maturity at most by three to four days. Very few interactive effects of RDI and crop load were observed, indicating that the crop load did not influence the response of vines to RDI.

Inhibition of nitrogenase activity and nodule oxygen permeability by water deficit

Short-term effects of water deficit on nitrogenase activity were investigated with hydroponically grown soybean plants (Glycine max L. Merr. cv. Biloxi) by adding polyethylene glycol (PEG) to the hydroponic solution and measuring nitrogenase activity, nodule respiration, and permeability to oxygen diffusion (P o ). These experiments showed a rapid decrease in acetylene reduction activity (ARA) and nodule respiration. A consequence of the decreased respiration rate was that P o calculated by Fick's Law also decreased. However, these results following PEG treatment were in direct conflict with a previous report of stability in P o determined by using an alternative technique. To resolve this conflict, an hypothesis describing a sequence of responses to the initial PEG treatment is presented. An important finding of this study was that the response to water deficit induced by PEG occurred in two stages. The first stage of decreased nodule activity was O 2 -limited and could be reversed by exposing the nodules to elevated pO 2 . The second stage which developed after 24h of exposure to PEG resulted in substantial loss in nodule activity and this activity could not be recovered with increased pO 2 . Severe water deficit treatments disrupt nodule activity to such a degree that O 2 is no longer the major limitation.

Maize Kernel Set at Low Water Potential: I. Sensitivity to Reduced Assimilates during Early Kernel Growth

Plant water deficits during flowering and early kernel growth reduce the yield potential of maize (Zea mays L.) by decreasing kernel number per ear. Our objective was to determine if zygotic abortion, which increases in water‐deficient plants, is caused by a decrease in assimilate supply to the ear. Plants were grown in soil/sand media in a controlled environment. Beginning at silk emergence and continuing for 3 wk after pollination, leaf photosynthesis was inhibited ≈50% (moderate) or 100% (severe) by withholding water (water‐deficit treatment) or decreasing light intensity (low‐light treatment). Silk water potential (ψw) at pollination was about −0.7 and −1.1 MPa in the moderate and severe water‐deficit treatments, compared to about −0.4 MPa in well‐watered plants. Mean kernel number per ear was 581 in control plants and was decreased 48 and 99% by the moderate and severe water‐deficit treatments, respectively. In the corresponding low‐light treatments, kernel number was decreased 40 and 100%. Thus, losses in kernel number pe ear reflected the relative inhibition of photosynthesis during early kernel growth, regardless of plant water status. Compared to controls, the water‐deficit and low‐light treatments decreased partitioning of dry matter, carbohydrates, and reduced N to the kernels. However, carbohydrates continued to accumulate immediately after pollination in the stalk and leaf tissue. This suggests that some stalk and leaf reserves were unavailable to support early kernel growth at low ψw. Collectively, these data indicate that early kernel development is highly dependent on a continued supply of carbohydrates from concurrent photosynthesis. Water deficits increase the probability of developmental failure by limiting this supply.

Water deficits change dry matter partitioning and seed yield in narrow-leafed lupins (Lupinus angustifolius L.)

lrrigation treatments were imposed in the field on an indeterminate cultivar of narrow-leafed lupins (Lupinus angustifolius L., cv. Danja) and on a breeding line with reduced branching (75A/329) so that they experienced no water-deficits (frequently irrigated), a transient mild water-deficit or a transient severe water-deficit during early reproductive growth, or continuous severe water-deficit during reproductive growth (unirrigated). Both leaf water potential and leaf conductance declined in all treatments in which a water-deficit was imposed. Differences in leaf conductance were apparent before differences in leaf water potential: conductance declined to 40% and 30% of the frequently irrigated controls in the transient mild and severe water-deficit treatments, respectively. Leaf water potential declined to -1 - 1 MPa and -1.6 MPa, respectively, in the transient mild and severe water-deficit treatments, compared to between -0 - 65 and -0 - 95 MPa for the frequently irrigated controls. Seed yield and total dry weight were reduced in the transient severe water-deficit and unirrigated treatments, but were no different from the frequently irrigated treatment when the water-deficit was transient and mild. However both transient water-deficit treatments produced more main-stem seed yield than the frequently irrigated treatment, especially in the reduced-branching line 75A/329. The transient mild water-deficit treatment also produced more first-order apical axis yield than the frequently irrigated treatment. These yield increases were mainly due to a greater yield of seed per pod, although on the first-order apical axes there was also a tendency to set more pods. The greater seed yield per pod in the transient water-deficit treatments was due to an apparent redirection of assimilate from vegetative to reproductive growth. This was not due to a smaller reduction in reproductive growth rates than in vegetative growth rates, but to an acceleration of reproductive growth that was maintained after stress relief. The same early acceleration of reproductive growth was also observed in unirrigated treatments, but the severe stress which persisted throughout later reproductive growth reduced pod growth rates and negated the early advantage.