Low Ψw Research Articles

AT Hook-Like 10 phosphorylation determines ribosomal RNA processing 6-like 1 (RRP6L1) chromatin association and growth suppression during water stress.

Phosphorylation of AT Hook-Like 10 (AHL10), one of the AT-hook family of plant-specific DNA binding proteins, is critical for growth suppression during moderate severity drought (low water potential, ψw ) stress. To understand how AHL10 phosphorylation determines drought response, we identified putative AHL10 interacting proteins and further characterized interaction with RRP6L1, a protein involved in epigenetic regulation. RRP6L1 and AHL10 mutants, as well as ahl10-1rrp6l1-2, had similar phenotypes of increased growth maintenance during low ψw . Chromatin precipitation demonstrated that RRP6L1 chromatin association increased during low ψw stress and was dependent upon AHL10 phosphorylation. Transcriptome analyses showed that AHL10 and RRP6L1 have concordant effects on expression of stress- and development-related genes. Together these results indicate that stress signalling can act via AHL10 phosphorylation to control the chromatin association of the key regulatory protein RRP6L1. AHL10 and RRP6L1 interaction in meristem cells is part of a mechanism to downregulate growth during low ψw stress. Interestingly, the loss of AHL13, which is homologous to AHL10 and phosphorylated at a similar C-terminal site, blocked the enhanced growth maintenance of ahl10-1. Thus, AHL10 and AHL13, despite their close homology, are not redundant but rather have distinct roles, likely related to the formation of AHL hetero-complexes.

Size and activity of the root meristem: A key for drought resistance and a key model of drought-related signaling.

Plants make many adjustments to their growth and development in response to even small changes in water availability. Under such conditions, root elongation can be actively restricted by stress-related signaling mechanisms. Here we look at how the Arabidopsis thaliana root meristem can be affected by moderate water limitation (low water potential, ψw ). Recent characterization of the clade E Growth-Regulating (EGR) protein phosphatases and Microtubule Associated Stress Protein 1 (MASP1) provides an example of how active restriction of root meristem size allows the plant to downregulate root elongation during low ψw stress. EGR2 protein accumulation in cortex cells of the transition zone at the distal end of the root meristem illustrates how the balance of cell division versus cell expansion signals at this critical location can determine meristem size and root elongation during low ψw . These characteristics of EGRs also raise the question of whether they may also be involved in hydrotropism, and, more broadly, whether hydrotropism is a distinct response or a specific manifestation of more general mechanisms used to adjust root growth under moderate severity low ψw whether or not a gradient of water availability is present. These questions, as well as a better understanding of how specific cell layers (cortex and endodermis) seem to have an outsized role in growth regulation and better understanding the roles of plasma membrane-based signaling and polar-localized proteins in the regulation of root meristem size and cell division activity are key to elucidating the cellular mechanisms that determine root growth behavior during soil drying.

Spatial differences in stoichiometry of EGR phosphatase and Microtubule-associated Stress Protein 1 control root meristem activity during drought stress

During moderate severity drought and low water potential (ψw) stress, poorly understood signaling mechanisms restrict both meristem cell division and subsequent cell expansion. We found that the Arabidopsis thaliana Clade E Growth-Regulating 2 (EGR2) protein phosphatase and Microtubule-Associated Stress Protein 1 (MASP1) differed in their stoichiometry of protein accumulation across the root meristem and had opposing effects on root meristem activity at low ψw. Ectopic MASP1 or EGR expression increased or decreased, respectively, root meristem size and root elongation during low ψw stress. This, along with the ability of phosphomimic MASP1 to overcome the EGR-mediated suppression of root meristem size and the observation that ectopic EGR expression had no effect on unstressed plants, indicated that during low ψw EGR activation and attenuation of MASP1 phosphorylation in their overlapping zone of expression determines root meristem size and activity. Ectopic EGR expression also decreased root cell size at low ψw. Conversely, both the egr1-1 egr2-1 and egr1-1 egr2-1 masp1-1 mutants had similarly increased root cell size but only egr1-1egr2-1 had increased cell division. These observations demonstrated that EGRs affect meristem activity via MASP1 but affect cell expansion via other mechanisms. Interestingly, EGR2 was highly expressed in the root cortex, a cell type important for growth regulation and environmental response.

NCED expression is related to increased ABA biosynthesis and stomatal closure under aluminum stress

Aluminum (Al)-induced decrease in leaf hydration has been associated with low gas exchange, especially stomatal conductance (gs). However, the mechanisms explaining these responses are unclear. Citrus limonia was exposed to 0 and 1480 μM Al in nutrient solution for 90 days to test whether the low gs and leaf hydration in plants exposed to Al is associated with increased 9-cis-epoxycarotenoid dioxygenase (NCED) gene expression and abscisic acid (ABA) biosynthesis. Relative leaf water content (RWC), water potential (Ψw) and gas exchange in the leaves, as well as leaf and root ClNCED3, ClNCED1 and ClNCED5 expression and accumulation of ABA and its metabolites (phaseic acid, dihydrophaseic acid, (+)-7′-hydroxy-ABA and ABA-β-d-glucosyl ester) were measured. Aluminum up-regulated ClNCED3 and induced ABA accumulation in the roots before impairments in leaf water status (low Ψw, RWC and gs) could be observed. Leaf ABA concentration increased from 7 to 90 days and this could be partially explained by the up-regulation of ClNCED3, ClNCED1 and ClNCED5 in this organ. Stomatal closure occurred concomitantly with the increase of ABA concentration, and this result provides further evidence of the role of ABA modulation of plant hydration under Al stress.

Water availability effects on germination, membrane stability and initial root growth of Agave lechuguilla and A. salmiana

Water availability during seed germination and seedling development is critical to plant development and establishment. The aim of this study was to determine the effects of four water potentials (ΨW) (−0.03, −0.48, −1.19 and −2.04 MPa) on seed germination, seedling root growth and root membrane stability of Agave lechuguilla Torr. and A. salmiana Otto ex Salm-Dyck. to explain the paradoxical situation of vegetatively propagated species producing large quantities of seeds in a dry climate. The results showed that seed water uptake and germination percentage were significantly lower at −1.19 and −2.04 MPa than at −0.03 MPa. The time to reach maximum seed germination increased at low ΨW. The final root length and root growth rate were higher in A. salmiana than in A. lechuguilla and decreased at ΨW of −1.19 and −2.04 MPa. Both Agave species increased root electrolyte leakage at ΨW of −1.19 MPa, which suggests changes in membrane integrity and stability in response to the water deficit. These germination and initial root growth responses could ensure the establishment of both Agave species during the rare periods with sufficient moisture availability and allow them to tolerate initial periods of low water availability, although with low establishment probability.

Highly ABA-Induced 1 (HAI1)-Interacting protein HIN1 and drought acclimation-enhanced splicing efficiency at intron retention sites

The Highly ABA-Induced 1 (HAI1) protein phosphatase is a central component of drought-related signaling. A screen for HAI1-interacting proteins identified HAI1-Interactor 1 (HIN1), a nuclear protein of unknown function which could be dephosphorylated by HAI1 in vitro. HIN1 colocalization and interaction with serine-arginine rich (SR) splicing factors and appearance of nuclear speckle-localized HIN1 during low water potential (ψw) stress suggested a pre-mRNA splicing-related function. RNA sequencing of Arabidopsis Col-0 wild type identified more than 500 introns where moderate severity low ψw altered intron retention (IR) frequency. Surprisingly, nearly 90% of these had increased splicing efficiency (decreased IR) during stress. For one-third of these introns, ectopic HIN1 expression (35S:HIN1) in unstressed plants mimicked the increased splicing efficiency seen in stress-treated wild type. HIN1 bound to a GAA-repeat, Exonic Splicing Enhancer-like RNA motif enriched in flanking sequence around HIN1-regulated introns. Genes with stress and HIN1-affected splicing efficiency were enriched for abiotic stress and signaling-related functions. The 35S:HIN1 plants had enhanced growth maintenance during low ψw, while hin1 mutants had reduced growth, further indicating the role of HIN1 in drought response. HIN1 is annotated as an MYB/SANT domain protein but has limited homology to other MYB/SANT proteins and is not related to known yeast or metazoan RNA-binding proteins or splicing regulators. Together these data identify HIN1 as a plant-specific RNA-binding protein, show a specific effect of drought acclimation to promote splicing efficiency of IR-prone introns, and also discover HAI1-HIN1 interaction and dephosphorylation that connects stress signaling to splicing regulation.

Assessing seasonal drought stress response in Norway spruce (Picea abies (L.) Karst.) by monitoring stem circumference and sap flow

AbstractSummer drought frequency is expected to increase with climate change in forested regions of Europe. To examine the physiological impacts of low soil moisture on Norway spruce [Picea abies (L.) Karst.], we conducted an irrigation experiment in a Norway spruce‐dominated forest. We monitored sap flow (Qs), stem circumference and soil water potential (Ψw), measured needle water potential (Ψl), and estimated potential evapotranspiration (PET) in control and irrigated plots. Soil water availability influenced the response of Qs to PET and the impact of Qs on maximum daily stem shrinkage (MDS). The positive relationship between Qs and PET was constrained below a threshold Ψw near −0.3 MPa. MDS was higher beyond this threshold, for a given value of Qs. Higher MDS and lower tree water status (ΔW) were observed at low Ψw in control plants, suggesting the lower water potential of stems' conducting tissues. Stem circumference increase (SCI) was 62% lower in control trees following the irrigation treatment. Slight SCI recovery was observed in these trees in response to early autumn rainfall, which caused ΔW to return to its predrought state. The results demonstrate that low water availability not only reduced Qs, ΔW, SCI, Ψl and increased MDS but also altered their mutual relations. Copyright © 2014 John Wiley & Sons, Ltd.

Role of the Putative Osmosensor Arabidopsis Histidine Kinase1 in Dehydration Avoidance and Low-Water-Potential Response

The molecular basis of plant osmosensing remains unknown. Arabidopsis (Arabidopsis thaliana) Histidine Kinase1 (AHK1) can complement the osmosensitivity of yeast (Saccharomyces cerevisiae) osmosensor mutants lacking Synthetic Lethal of N-end rule1 and SH3-containing Osmosensor and has been proposed to act as a plant osmosensor. We found that ahk1 mutants in either the Arabidopsis Nossen-0 or Columbia-0 background had increased stomatal density and stomatal index consistent with greater transpirational water loss. However, the growth of ahk1 mutants was not more sensitive to controlled moderate low water potential (ψ(w)) or to salt stress. Also, ahk1 mutants had increased, rather than reduced, solute accumulation across a range of low ψ(w) severities. ahk1 mutants had reduced low ψ(w) induction of Δ(1)-Pyrroline-5-Carboxylate Synthetase1 (P5CS1) and 9-cis-Epoxycarotenoid Dioxygenase3, which encode rate-limiting enzymes in proline and abscisic acid (ABA) synthesis, respectively. However, neither Pro nor ABA accumulation was reduced in ahk1 mutants at low ψ(w). P5CS1 protein level was not reduced in ahk1 mutants. This indicated that proline accumulation was regulated in part by posttranscriptional control of P5CS1 that was not affected by AHK1. Expression of AHK1 itself was reduced by low ψ(w), in contrast to previous reports. These results define a role of AHK1 in controlling stomatal density and the transcription of stress-responsive genes. These phenotypes may be mediated in part by reduced ABA sensitivity. More rapid transpiration and water depletion can also explain the previously reported sensitivity of ahk1 to uncontrolled soil drying. The unimpaired growth, ABA, proline, and solute accumulation of ahk1 mutants at low ψ(w) suggest that AHK1 may not be the main plant osmosensor required for low ψ(w) tolerance.

Spatial distribution of transcript changes in the maize primary root elongation zone at low water potential

BackgroundPrevious work showed that the maize primary root adapts to low Ψw (-1.6 MPa) by maintaining longitudinal expansion in the apical 3 mm (region 1), whereas in the adjacent 4 mm (region 2) longitudinal expansion reaches a maximum in well-watered roots but is progressively inhibited at low Ψw. To identify mechanisms that determine these responses to low Ψw, transcript expression was profiled in these regions of water-stressed and well-watered roots. In addition, comparison between region 2 of water-stressed roots and the zone of growth deceleration in well-watered roots (region 3) distinguished stress-responsive genes in region 2 from those involved in cell maturation.ResultsResponses of gene expression to water stress in regions 1 and 2 were largely distinct. The largest functional categories of differentially expressed transcripts were reactive oxygen species and carbon metabolism in region 1, and membrane transport in region 2. Transcripts controlling sucrose hydrolysis distinguished well-watered and water-stressed states (invertase vs. sucrose synthase), and changes in expression of transcripts for starch synthesis indicated further alteration in carbon metabolism under water deficit. A role for inositols in the stress response was suggested, as was control of proline metabolism. Increased expression of transcripts for wall-loosening proteins in region 1, and for elements of ABA and ethylene signaling were also indicated in the response to water deficit.ConclusionThe analysis indicates that fundamentally different signaling and metabolic response mechanisms are involved in the response to water stress in different regions of the maize primary root elongation zone.

Responses of germination and radicle growth of two Populus species to water potential and salinity

The effects of water potential, NaCl and Na2SO4 on germination and radicle growth of two riparian tree species, Populus euphratica Oliv. and P. pruinosa Schrenk (Salicaceae), were tested. Growth chamber studies revealed an optimum temperature range for seed germination of both species between 15–35°C. The final germination percentage of both species decreases with decreasing water potential in all types of solution applied in the experiments. P. pruinosa was less tolerant to low ψW stress than P. euphratica, especially in salt solutions. Germination percentages fell below 20% for P. pruinosa at −0.6 MPa (NaCl) or −0.4 MPa (Na2SO4) and for P. euphratica at −1.2 MPa (NaCl) or −0.6 MPa (Na2SO4). Radicle growth of both species was inhibited by high concentrations of PEG, NaCl and Na2SO4. However, growth was enhanced at −0.13 and −0.29 MPa in PEG or at −0.13 MPa in NaCl solutions compared to distilled water. Radicle growth of P. euphratica was higher than that of P. pruinosa. Germination and radicle growth of both species exhibited ion toxicity. Na2SO4 was more toxic than iso-osmotic solutions of NaCl. Radicle growth proved to be more sensitive than seed germination. Thus, flooding does not only yield the necessary soil moisture for germination but also favors seedling establishment of both species through leaching of salts from the soil surface. The different sensitivity of the species during their early growth stages might, moreover, contribute to the observed differences in their distribution in the Talim Basin (northwest China).

Induction of Heat Stress Tolerance in Barley Seedlings by Pre-Sowing Seed Treatment with Glycinebetaine

Heat stress adversely affects plant growth and development, while glycinebetaine (GB) plays a protective role under stressful conditions. The objective of this study was to assess the optimum level of GB for use as a presowing seed treatment and the subsequent effect on the heat tolerance of barley (Hordeum vulgare L. cv. Haider-93) seedlings. Among a range of GB levels, the 20 mM concentration emerged as the most effective in enhancing seed germination, shoot fresh and dry weight and shoot water content under heat stress, and this level was selected for further studies. Time course changes revealed that the seedlings developing from 20 mM GB treated seeds had greater shoot dry weight, net photosynthetic rate (PN), leaf water potential (ψw) and reduced relative membrane permeability (RMP), compared to no-GB treated plants under heat stress. Correlations between dry weight and high PN (r = 0.881), low ψw (r = −0.938) and RMP (r = −0.860) of shoots suggested the involvement of GB in heat stress tolerance. Leakage of Ca2+ and NO3− was the greatest followed by K+ and PO43− under no-GB seed treatment, and GB application under heat stress appreciably reduced the leakage of all these ions, particularly Ca2+, K+ and NO3−. In conclusion GB absorbed by seeds, after translocation to the seedlings, enhanced their capacity to maintain greater water content, and higher seedling vigor by virtue of increased PN, reduced RMP and leakage of important ions under heat stress. These results have implications for final field stand under the conditions where the ambient temperature is supra-optimal for barley growth.

Growth responses of seminal roots of wheat seedlings to a reduction in the water potential of vermiculite

We examined the elongation rate, water status and solute accumulation in the seminal roots of wheat seedlings (Triticum aestivum L.) that were growing in vermiculite with a water potential (Ψw) ranging from −0 03 to −1 10 MPa. The elongation rate of the primary seminal root was similar to that of the first pair of seminal roots but that of the second pair of seminal roots was lower at all values of Ψw tested. The elongation rate was highest in vermiculite with a Ψw of −0.03 MPa but did not decrease significantly until the Ψw was reduced to −0.15 MPa. Further reductions in Ψw reduced the elongation rate markedly. The Ψw of mature tissues was always similar to that of vermiculite. The osmotic potential (Ψo) decreased to the same extent as the decrease in Ψw. Thus, the turgor pressure (Ψp) remained unchanged even in vermiculite with a low Ψw. In elongating tissues, Ψw and Ψo were far lower than they were in mature tissues and, thus, reductions in turgor were not significant. Even when the Ψw of vermiculite changed, there were no consistent changes in terms of a difference in Ψw between elongating plus mature tissues and vermiculite. There were also no consistent changes in levels of osmotica, calculated using the van’t Hoff’s law, in the elongating tissues but the levels in mature tissues increased in vermiculite with a low Ψw. Our results suggest that (1) reductions in root elongation in vermiculite with a low Ψw were caused by reductions in the extensibility and/or increases in the yield threshold of cell walls and by reductions in the hydraulic conductivity of the tissues; and (2) a seminal root regulates its growth to keep turgor pressure unchanged.

Sugar-responsive gene expression, invertase activity, and senescence in aborting maize ovaries at low water potentials.

Ovary abortion can occur in maize (Zea mays) if water deficits lower the water potential (psiw) sufficiently to inhibit photosynthesis around the time of pollination. The abortion decreases kernel number. The present work explored the activity of ovary acid invertases and their genes, together with other genes for sucrose-processing enzymes, when this kind of abortion occurred. Cytological evidence suggested that senescence may have been initiated after 2 or 3 d of low psiw, and the expression of some likely senescence genes was also determined. Ovary abortion was assessed at kernel maturity. Acid invertase activities were localized in vivo and in situ. Time courses for mRNA abundance were measured with real time PCR. Sucrose was fed to the stems to vary the sugar flux. Many kernels developed in controls but most aborted when psiw became low. Ovary invertase was active in controls but severely inhibited at low psiw for cell wall-bound forms in vivo and soluble forms in situ. All ovary genes for sucrose processing enzymes were rapidly down-regulated at low psiw except for a gene for invertase inhibitor peptide that appeared to be constitutively expressed. Some ovary genes for senescence were subsequently up-regulated (RIP2 and PLD1). In some genes, these regulatory changes were reversed by feeding sucrose to the stems. Abortion was partially prevented by feeding sucrose. A general response to low psiw in maize ovaries was an early down-regulation of genes for sucrose processing enzymes followed by up-regulation of some genes involved in senescence. Because some of these genes were sucrose responsive, the partial prevention of abortion with sucrose feeding may have been caused in part by the differential sugar-responsiveness of these genes. The late up-regulation of senescence genes may have caused the irreversibility of abortion.

Proline accumulation in maize (Zea mays L.) primary roots at low water potentials. II. Metabolic source of increased proline deposition in the elongation zone

The proline (Pro) concentration increases greatly in the growing region of maize (Zea mays L.) primary roots at low water potentials (psiw), largely as a result of an increased net rate of Pro deposition. Labeled glutamate (Glu), ornithine (Orn), or Pro was supplied specifically to the root tip of intact seedlings in solution culture at high and low psiw to assess the relative importance of Pro synthesis, catabolism, utilization, and transport in root-tip Pro deposition. Labeling with [3H]Glu indicated that Pro synthesis from Glu did not increase substantially at low psiw and accounted for only a small fraction of the Pro deposition. Labeling with [14C]Orn showed that Pro synthesis from Orn also could not be a substantial contributor to Pro deposition. Labeling with [3H]Pro indicated that neither Pro catabolism nor utilization in the root tip was decreased at low psiw. Pro catabolism occurred at least as rapidly as Pro synthesis from Glu. There was, however, an increase in Pro uptake at low psiw, which suggests increased Pro transport. Taken together, the data indicate that increased transport of Pro to the root tip serves as the source of low-psiw-induced Pro accumulation. The possible significance of Pro catabolism in sustaining root growth at low psiw is also discussed.

Root Growth and Oxygen Relations at Low Water Potentials. Impact of Oxygen Availability in Polyethylene Glycol Solutions1

AbstractPolyethylene glycol (PEG), which is often used to impose low water potentials (ψw) in solution culture, decreases O2 movement by increasing solution viscosity. We investigated whether this property causes O2 deficiency that affects the elongation or metabolism of maize (Zea mays L.) primary roots. Seedlings grown in vigorously aerated PEG solutions at ambient solution O2partial pressure (pO2) had decreased steady-state root elongation rates, increased root-tip alanine concentrations, and decreased root-tip proline concentrations relative to seedlings grown in PEG solutions of above-ambientpO2 (alanine and proline accumulation are responses to hypoxia and low ψw, respectively). Measurements of root pO2 were made using an O2 microsensor to ensure that increased solutionpO2 did not increase rootpO2 above physiological levels. In oxygenated PEG solutions that gave maximal root elongation rates, rootpO2 was similar to or less than (depending on depth in the tissue) pO2 of roots growing in vermiculite at the same ψw. Even without PEG, high solution pO2 was necessary to raise rootpO2 to the levels found in vermiculite-grown roots. Vermiculite was used for comparison because it has large air spaces that allow free movement of O2 to the root surface. The results show that supplemental oxygenation is required to avoid hypoxia in PEG solutions. Also, the data suggest that the O2 demand of the root elongation zone may be greater at low relative to high ψw, compounding the effect of PEG on O2 supply. Under O2-sufficient conditions root elongation was substantially less sensitive to the low ψwimposed by PEG than that imposed by dry vermiculite.

Reversing Drought‐Induced Losses in Grain Yield: Sucrose Maintains Embryo Growth in Maize

Large losses in yield can occur in maize (Zea mays L.) growing in water‐limited conditions (low Ψw), particularly during flowering and early embryo growth where grain number is decreased. However, embryo growth is maintained where otherwise none would occur if the stems are infused with photosynthate (sucrose) and other nutrients (amino acids, salts, vitamins, and hormones) during exposure to low Ψw. In the present investigation, we deleted or supplied various components of the complete infusion mediumto identify the active solute. Infusions involved making a cavity in a stem internode, filling the cavity with water, sealing the cavity, and connecting the interior of the cavity to an external reservoir of the medium. A field experiment at high Ψw showed that wounds and enhanced nutrient supplies from the infusions did not alter embryo growth or yield. In controlled environment chambers, low Ψw was imposed by withholding water for 6 d beginning 5 d prior to pollinations. A new infusion was made each day for 5 d as the low Ψw developed around pollination. The low Ψw virtually eliminated grain development. Sucrose infusion reversed this loss and no other component of the medium showed activity under our conditions. In plants receiving no sucrose at low Ψw, ovary sucrose content was moderately lower and starch much lower than in controls. In plants receiving sucrose at low Ψw, ovary sucrose content was higher than in controls and starch recovered moderately. The difference in sucrose and starch responses indicates that carbohydrate metabolism was altered in the ovaries at low Ψw. Embryo survival correlated more closely with the content of starch than sucrose. We conclude that there were two effects of low Ψw that contributed to arrested embryo growth: a decreased sucrose flux and an altered carbohydrate metabolism in the ovaries.

Assimilate Flux Determines Kernel Set at Low Water Potential in Maize

Plant water deficits during flowering cause maize (Zea mays L.) kernels to abort. Lack of current and reserve photosynthate account for much of the kernel loss, but partitioning to ovaries at low ovary water potential (ψw) may also be limited by lack of assimilate demand. To test this possibility, we measured the water status, carbohydrate content, and growth of ovaries on plants grown in the field in pots containing 22 kg of soil under one of three light environments [Control, 75‐cm rows, 43 055 plants ha−l; Shade, same as Control except under shade cloth (55% fight interception) from the sixth leaf stage until physiological maturity; or Isolated, 122‐cm rows, 6727 plants ha−1] and exposed to a water deficit during pollination. Water was withheld at silk emergence and plants were hand‐pollinated 4 d later when silk ψ was ≈ − 1.1 MPa, leaf ψw was ≈ − 1.8 MPa, and photosynthesis was completely inhibited. The water deficit decreased kernel set, which was correlated with the inhibition of ovary dry matter accumulation. The concentration of sucrose and glucose increased in ovaries of water‐deficient plants, and ovary turgor remained at or above control levels. Thus, inhibition of ovary growth at low ψw was not related to a loss of turgor, nor was it caused by a depletion of ovary sugars. Sugar accumulation at low ψw, suggested that metabolism may have been impaired. Coupled with a low level of reserves, failure to utilize available sugars at low ψw would severely inhibit assimilate flux to the ear and render kernel set highly vulnerable to water deficits during pollination.

Increasing Assimilate Reserves Does Not Prevent Kernel Abortion at Low Water Potential in Maize

Water deficits during anthesis cause maize (Zea mays L.) kernels to abort soon after fertilization. Inhibition of photosynthesis accounts for much of the kernel loss, but lack of assimilate reserves might also limit kernel development. We tested whether varying the level of C and N reserves prior to anthesis affected kernel set of plants pollinated at low water potential (Ψv). Plants were grown in sand/soil media in pots in the field under one of three light environments: Control, 75‐cm rows, 4.3 plants m−2; Shade, same as Control except under 55% shade cloth from V6 until maturity; or Isolated, 122‐cm rows, 0.7 plants m−2. These treatments produced a 3.3‐fold range in total extractable carbohydrate (TEC) and a 1.7‐fold range in total Kjeldahl nitrogen (TKN) per plant at silk emergence. Water was withheld at silk emergence and plants were pollinated by hand with excess pollen 4 d later when silk Ψw was = −1.1 MPa, leaf Ψw was = −1.8 MPa, and photosynthesis was completely inhibited. Plants were rewatered 2 d after pollination and remained well watered until physiological maturity. The brief water deficit decreased kernels per ear by 45% in the Controls, 72% in the Shade treatment, and 49% in the Isolated treatment. Lower kernel set in the Shade treatment was associated with low levels of TEC and TKN in the vegetative stalk, leaves, and reproductive stalk, compared with Control plants. However, accumulation of TEC and TKN in excess of Control values in the Isolated plants did not prevent kernel loss. Kernel set in all three treatments was more closely related to the rate of photosynthesis prior to imposing the water deficit than to the level of reserves accumulated prior to flowering. These results suggest that selection for high levels of reserves at pollination will not improve kernel set at low Ψw in maize.

Water Deficit Affects Receptivity of Maize Silks

Drought during anthesis decreases seed set in maize (Zea mays L.) even if pollination occurs. To test whether low silk water potential (ψw) decreases silk receptivity, we examined silk development, pollen‐silk interaction, and kernel set in two hybrids grown in a greenhouse and exposed to a brief water deficit after silks emerged from the husks. Low silk ψw directly affected silk development. Silk elongation was inhibited as silk ψw decreased, and no growth occurred at silk ψw below −0.8 MPa. Silk senescence (collapse of basal silk tissue) was delayed, compared with controls, when silk elongation was arrested by low ψw within 3 days after the first silks appeared (DAFS). similar water deficit 5 DAFS, however, hastened silk senescence. The change in silk development had a direct effect on kernel set. Low silk ψw decreased set 20 to 40% 3 DAFS, and decreased set almost completely 5 DAFS. Pollen tubes were observed in >90% of silks pollinated at ψw between −1.0 and −1.3 MPa, although there were fewer tubes per silk. Tube growth was slow at low silk ψw, but tubes failed to reach the ovary only after silks had senesced. These results indicate that low ψw imposed <4 DAFS limited kernel set primarily by causing developmental failure within the ovary, not loss of silk function. Low ψw imposed >5 DAFS limited kernel set by hastening silk senescence. Thus, water deficits at anthesis can cause loss of silk receptivity in maize, but only when low silk ψw occurs >5 d after silks emerge from the husks.

Flower and Pod Development in Water-Deficient Soybeans (Glycine maxL. Merr.)

Water deficits during flowering decrease the number of seed-bearing pods in soybean (Glycine max L. Merr.). Failure to set pods may indicate an inherent sensitivity to low tissue water potential (ψw) at critical stages of flower development. To test this, we measured the water status and pod set of flowers exposed to water deficits at various stages of development from bud to early ovary (pod) expansion. In well-watered (control) plants, the ψw of buds, flowers, and pods were about 0·2 to 0·3 MPa lower than leaf ψw at midday. The ψw of the reproductive structures decreased in concert with leaf ψw when water was withheld. Turgor, estimated from measurements of ψw and ψ$$$, was maintained at low Ww by osmotic adjustment at all flower stages. Upon rewatering, ψw of buds, anthesis-flowers, and expanding pods returned to control values within 1 d. The ψw of pods that had just formed but were not ‘set’ recovered more slowly. Water deficits imposed when flowers reached anthesis, pod set, or early pod expansion decreased the percentage of flowers that initiated pod expansion and maintained pod development. Inhibition of development was most pronounced in the anthesis-stress treatment in which only one-third as many flowers at mid-raceme positions set pods, compared to controls. None of these pods developed beyond 20 mm in length. The results indicate that sensitivity of soybean flowers to low ψw varies with raceme position and stage of flower development. Flowers in mid-raceme positions are particularly vulnerable to low ψw. Early ovary expansion is the critical stage in soybean reproductive development under drought conditions.