Daily irrigation attenuates xylem abscisic acid concentration and increases leaf water potential of Pelargonium × hortorum compared with infrequent irrigation

The physiological response of plants to different irrigation frequencies may affect plant growth and water use efficiency (WUE; defined as shoot biomass/cumulative irrigation). Glasshouse‐grown, containerized Pelargonium × hortorum BullsEye plants were irrigated either daily at 100% of plant evapotranspiration (ET) (well‐watered; WW), or at 50% ET applied either daily [frequent deficit irrigation (FDI)] or cumulatively every 4 days [infrequent deficit irrigation (IDI)], for 24 days. Both FDI and IDI applied the same irrigation volume. Xylem sap was collected from the leaves, and stomatal conductance (g_s) and leaf water potential (Ψ_leaf) measured every 2 days. As soil moisture decreased, g_s decreased similarly under both FDI and IDI throughout the experiment. Ψ_leaf was maintained under IDI and increased under FDI. Leaf xylem abscisic acid (ABA) concentrations ([X‐ABA]_leaf) increased as soil moisture decreased under both IDI and FDI, and was strongly correlated with decreased g_s, but [X‐ABA]_leaf was attenuated under FDI throughout the experiment (at the same level of soil moisture as IDI plants). These physiological changes corresponded with differences in plant production. Both FDI and IDI decreased growth compared with WW plants, and by the end of the experiment, FDI plants also had a greater shoot fresh weight (18%) than IDI plants. Although both IDI and FDI had higher WUE than WW plants during the first 10 days of the experiment (when biomass did not differ between treatments), the deficit irrigation treatments had lower WUE than WW plants in the latter stages when growth was limited. Thus, ABA‐induced stomatal closure may not always translate to increased WUE (at the whole plant level) if vegetative growth shows a similar sensitivity to soil drying, and growers must adapt their irrigation scheduling according to crop requirements.     

Stomatal closure with soil water depletion not associated with changes in Bulk leaf water status

Summary It has previously been reported that canopy water loss by cowpea (Vigna unguiculata) decreases with small depletions in soil water. In these studies, under field conditions, it was demonstrated that with small changes in soil water status leaf conductance of cowpea decreases in a manner which is consistent with the sensitive regulation of canopy water loss.However, treatments which differed in leaf conductance, and presumably stomatal aperture, had similar leaf water potentials. It is hypothesized that the stomatal closure which results from soil water depletion is mediated by changes in root water status through effects on the flow of information from root to shoot. An efficient mechanism of this type could be partially responsible for the extreme drought avoidance exhibited by this plant.Dedicated to Dr. K. Springer     

Stomatal response to leaf water potential in almond trees under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L. cv. Garrigues) were grown in the field under drip irrigated and non irrigated conditions. Leaf water potential () and leaf conductance (g₁) were determined at three different times of the growing season (spring, summer and autumn). The relationships between and g₁ in both treatments showed a continuous decrease of g₁ as decreased in spring and summer. Data from the autumn presented a threshold value of (approx. –2.7 MPa in dry treatment, and approx. –1.4 MPa in wet treatment) below which leaf conductance remained constant.     

Nodule gas exchange and water potential response to rapid imposition of water deficit

The permeability (P) of the gaseous diffusion barrier in the nodules of soybean [Glycine max (L.) Merr.] decreases when water deficits are extended over a 7 to 10 d period. The mechanism controlling P changes is unclear, but may result from the release of water to intercellular pathways, and an associated change in the nodule water potential. The purpose of these experiments was to impose water deficit treatments rapidly in order to determine the early sequence of the responses of nodule water potential and nodule gas exchange without the complications that arise from long-term water deficit treatments. A vertical, split-root system was used to separate nodule drying effects from plant water deficits by replacing humidified air that was passed over upper root nodules in well-watered plants with dry air, or by replacing the nutrient solution that surrounded lower roots with -1.0 MPa polyethylene glycol (PEG) solution, or by a combination of the dry air and PEG treatments. The PEG treatment caused large decreases in both the components of nodule water potential and nodule relative water content, but there was no indication that these factors had immediate, direct effects on either nitrogenase activity or P. After 7 h of the PEG treatment a significant decrease in nitrogenase activity was found but no decrease in P was detected. These results indicate that changes in nitrogenase activity in response to water deficits precede decreases in P. Exposure of nodules to dry air in well-watered plants had no significant effect on either nitrogenase activity or P during the 7 h treatment.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

The branch-thorn occurrence of Lycium ruthenicum is associated with leaf DNA hypermethylation in response to soil water content

Molecular Biology Reports - Lycium ruthenicum is an eco-economic shrub which can exist in two forms, thorny and thornless under varying soil moisture conditions. The aim of this study was to...     

Stomatal control in tomato with ABA-deficient roots: response of grafted plants to soil drying

The hypothesis that ABA produced by roots in drying soil is responsible for stomatal closure was tested with grafted plants constructed from the ABA-deficient tomato mutants, sitiens and flacca and their near-isogenic wild-type parent. Three types of experiments were conducted. In the first type, reciprocal grafts were made between the wild type and sitiens or flacca. Stomatal conductance accorded with the genotype of the shoot, not the root. Stomates closed in all of the grafted plants in response to soil drying, regardless of the root genotype, i.e. regardless of the ability of the roots to produce ABA. In the second type of experiment, wild-type shoots were grafted onto a split-root system consisting of one wild-type root grafted to one mutant (flacca or sitiens) root. Water was withheld from one root system, while the other was watered well so that the shoots did not experience any decline in water potential or loss of turgor. Stomates closed to a similar extent when water was withheld from the mutant roots or the wild-type roots. In the third type of experiment, grafted plants with wild-type shoots and either wild-type or sitiens roots were established in pots that could be placed inside a pressure chamber, and the pressure increased as the soil dried so that the shoots remained fully turgid throughout. Stomates closed as the soil dried, regardless of whether the roots were wild type or sitiens. These experiments demonstrate that stomatal closure in response to soil drying can occur in the absence of leaf water deficit, and does not require ABA production by roots. A chemical signal from roots leading to a change in apoplastic ABA levels in leaves may be responsible for the stomatal closure.     

Heterogenous stomatal closure in response to leaf water deficits is not a universal phenomenon

The extent and occurrence of water stress-induced “patchy” CO₂ uptake across the surface of leaves was evaluated in a number of plant species. Leaves, while still attached to a plant, were illuminated and exposed to air containing [¹⁴C]CO₂ before autoradiographs were developed. Plant water deficits that caused leaf water potential depression to −1.1 megapascals during a 4-day period did result in heterogenous CO₂ assimilation patterns in bean (Phaseolus vulgaris). However, when the same level of stress was imposed more gradually (during 17 days), no patchy stomatal closure was evident. The patchy CO₂ assimilation pattern that occurs when bean plants are subjected to a rapidly imposed stress could induce artifacts in gas exchange studies such that an effect of stress on chloroplast metabolism is incorrectly deduced. This problem was characterized by examining the relationship between photosynthesis and internal [CO₂] in stressed bean leaves. When extent of heterogenous CO₂ uptake was estimated and accounted for, there appeared to be little difference in this relationship between control and stressed leaves. Subjecting spinach (Spinacea oleracea) plants to stress (leaf water potential depression to −1.5 megapascals) did not appear to cause patchy stomatal closure. Wheat (Triticum aestivum) plants also showed homogenous CO₂ assimilation patterns when stressed to a leaf water potential of −2.6 megapascals. It was concluded that water stress-induced patchy stomatal closure can occur to an extent that could influence the analysis of gas exchange studies. However, this phenomenon was not found to be a general response. Not all stress regimens will induce patchiness; nor will all plant species demonstrate this response to water deficits.     

Correction to: Linear relation between leaf xylem water potential and transpiration in pearl millet during soil drying

Plant and Soil - In the original version of this article, equations 4 and 9 unfortunately contained errors     

Biomass partition,leaf gas exchange and water relations of alfalfa and milkvetch seedlings in response to soil drying

This study compared physiological and growth responses to water stress of two legume species during the seedling stage. Potted alfalfa (Medicago sativa L. cv. Algonquin) and milkvetch (Astragalus adsurgens Pall. cv. Pengyang earlymaturing vetch) seedlings were grown under well-watered [soil water content (SWC) maintained at 14.92% daily] or water-stressed conditions (drying) for 15 days. Net photosynthetic rate (P _N), transpiration rate (E) and stomatal conductance (g _s) of both species decreased parabolically. When SWC decreased to 7.2% and 10.3%, g _s values for alfalfa and milkvetch were significantly different from those of the respective well-watered plants (p<0.05). When SWC decreased to 6.6% for alfalfa and 6.8% for milkvetch, leaf water potentials (ψ_L) were significantly different from those of the well-watered plants (p<0.05). Thus the difference between the SWC thresholds for a nonhydraulic root signal (nHRS) and a hydraulic root signal (HRS) were 0.6% and 3.5% for alfalfa and milkvetch, respectively. Milkvetch had a lower g _s than alfalfa for a given SWC (p<0.05). Although alfalfa seedlings had a higher dry mass (DM) and root:shoot ratio (R/S) than milkvetch in both treatments (p<0.05), we concluded that milkvetch seedlings had greater drought tolerance than alfalfa.     

Rice leaf growth and water potential are resilient to evaporative demand and soil water deficit once the effects of root system are neutralized

Rice is known to be sensitive to soil water deficit and evaporative demand, with a greatest sensitivity of lowland‐adapted genotypes. We have analysed the responses of plant water relations and of leaf elongation rate (LER) to soil water status and evaporative demand in seven rice genotypes belonging to different species, subspecies, either upland‐ or lowland‐adapted. In the considered range of soil water potential (0 to ?0.6 MPa), stomatal conductance was controlled in such a way that the daytime leaf water potential was similar in well‐watered, droughted or flooded conditions (isohydric behaviour). A low sensitivity of LER to evaporative demand was observed in the same three conditions, with small differences between genotypes and lower sensitivity than in maize. The sensitivity of LER to soil water deficit was similar to that of maize. A tendency towards lower sensitivities was observed in upland than lowland genotypes but with smaller differences than expected. We conclude that leaf water status and leaf elongation of rice are not particularly sensitive to water deficit. The main origin of drought sensitivity in rice may be its poor root system, whose effect was alleviated in the study presented here by growing plants in pots whose soil was entirely colonized by roots of all genotypes.     

The homeobox genes ATHB12 and ATHB7encode potential regulators of growth in response to water deficit in Arabidopsis

The Arabidopsis thaliana homeodomain leucine-zipper gene ATHB7, which is active specifically under water deficit conditions, is proposed to act as a negative regulator of growth (Söderman et al., 1996, Plant J. 10: 375 381; Hjellström et al., 2003, Plant Cell Environ 26: 1127 1136). In this report we demonstrate that the paralogous gene, ATHB12, has a similar expression pattern and function. ATHB12,like ATHB7,was up-regulated during water deficit conditions, the up-regulation being dependent on abscisic acid (ABA) and on the activity of the Ser/Thr phosphatases ABI1 and ABI2. Plants that are mutant for ATHB12, as a result of T-DNA insertions in the ATHB12 gene, showed a reduced sensitivity to ABA in root elongation assays, whereas transgenic Arabidopsis plants expressing ATHB12 and/orATHB7 as driven by the CaMV 35S promoter were hypersensitive in this response compared to wild-type. High-level expression of either gene also resulted in a delay in inflorescence stem elongation growth and caused plants to develop rosette leaves with a more rounded shape, shorter petioles, and increased branching of the inflorescence stem. Transgenic Arabidopsisplants expressing the reporter geneuidA under the control of the ATHB12promoter showed marker gene activity in axillary shoot primordia, lateral root primordia, inflorescence stems and in flower organs. Treatment of plants with ABA or water deficit conditions caused the activity of ATHB12to increase in the inflorescence stem, the flower organs and the leaves, and to expand into the vasculature of roots and the differentiation/elongation zone of root tips. Taken together, these results indicate that ATHB12 and ATHB7 act to mediate a growth response to water deficit by similar mechanisms.     

Aberrant protein expression is associated with decreased developmental potential in porcine cumulus–oocyte complexes

Oocyte developmental competence is progressively obtained during pubertal development in females. Poor developmental potential in oocytes derived from prepubertal females suggests that essential processes required for oocyte development have not been fulfilled. The objective of this experiment was to analyze the protein profiles of porcine cumulus–oocyte complexes (COC) derived from cyclic and prepubertal females to identify alterations in protein abundance that correlate with developmental potential. COC complexes, aspirated from prepubertal and cyclic ovaries, were pooled into three replicates of 400 COCs each per treatment in ～100 µl SOF‐HEPES medium. Protein samples were extracted and analyzed by two‐dimensional differential in gel electrophoresis (2D‐DIGE). Over 1,600 proteins were resolved on each of the three replicate gels. Sixteen protein spots were identified by mass spectrometry, representing 14 unique, differentially expressed proteins (volume ratio greater than 1.3). Glutathione‐S‐transferase and pyruvate kinase 3 were more abundant in COCs derived from cyclic females, whereas soluble epoxide hydrolase and transferrin were more abundant in prepubertal derived COCs. Abundance of several glycolytic enzymes (enolase 1, pyruvate kinase 3, and phosphoglycerate kinase) was increased in COCs derived from cyclic females, suggesting glucose metabolism is decreased in prepubertal derived COCs. We conclude that the abundance of proteins involved in metabolism and oxidative stress regulation is significantly altered in prepubertal derived COCs and may play a role in the mechanisms resulting in developmental competence. Mol. Reprod. Dev. 77: 51–58, 2010. © 2009 Wiley‐Liss, Inc.     

Changes in plant–soil hydraulic pressure gradients of soybean in response to soil drying

Plant gas‐exchange response to drying soil in many instances tracks a common pattern when expressed as a function of fraction of transpirable soil water (FTSW). There is little decrease in gas exchange until FTSW decreases to a value in the range of about 0.3–0.45, then with further drying gas exchange declines approximately linearly. This unique pattern is hypothesised to reflect mainly changes in the water potential gradient between bulk soil and plant. The primary objective was to directly document the basis of this response by measuring the hydrostatic pressure gradient required in the soil to maintain leaf xylem at zero potential with decreasing FTSW. Pots in which soybean (Glycine max) plants were grown were placed in a pressure chamber and the pressure adjusted to maintain zero water potential in a leaf petiolule. These results showed a small, relatively constant hydrostatic pressure had to be applied to the soil to maintain zero leaf xylem water potential until FTSW decreased to approximately 0.3–0.45. Thereafter, the required hydrostatic pressure gradient increased as FTSW continued to decrease. Hydraulic conductance was calculated to be relatively stable early in the drying cycle, and then decrease as the soil dried to comparatively high FTSW of 0.5–0.7.     

Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. II. Stomatal sensitivity to abscisic acid imported from the xylem sap

Sunflower plants [Helianthus annuus L.) were subjected to soil drought. Leaf conductance declined with soil water content even when the shoot was kept turgid throughout the drying period. The concentration of abscisic acid in the xylem sap increased with decreasing soil water content. No general relation could be established between abscisic acid concentration in the xylem sap and leaf conductance due to marked differences in the sensitivity of leaf conductance of individual plants to abscisic acid from the xylem sap. The combination of these results with data from Gollan, Schurr & Schulze (1992, see pp. 551–559, this issue) reveals close connection of the effectiveness of abscisic acid as a root to shoot signal to the nutritional status of the plant.     

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings

In this study, tree hydraulic conductance (K _tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g _s), net photosynthesis (A) and bulk leaf water potential (Ψ_leaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K _tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO₂ concentration. As expected, we found a strong and positive relationship between g _s and K _tree (r = 0.92, P = 0.0001) and between A and K _tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψ_leaf to K _tree depended on the direction of change in K _tree: increases in K _tree caused Ψ_leaf to decrease from around −1.0 to −0.6 MPa, but reductions in K _tree were accompanied by homeostasis in Ψ_leaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g _s and Ψ_leaf, with Ψ_leaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψ_leaf approached −1 MPa.     

A neural crest deficit in Down syndrome mice is associated with deficient mitotic response to Sonic hedgehog

Trisomy 21 results in phenotypes collectively referred to as Down syndrome (DS) including characteristic facial dysmorphology. Ts65Dn mice are trisomic for orthologs of about half of the genes found on human chromosome 21 and exhibit DS-like craniofacial abnormalities, including a small dysmorphic mandible. Quantitative analysis of neural crest (NC) progenitors of the mandible revealed a paucity of NC and a smaller first pharyngeal arch (PA1) in Ts65Dn as compared to euploid embryos. Similar effects in PA2 suggest that trisomy causes a neurocristopathy in Ts65Dn mice (and by extension, DS). Further analyses demonstrated deficits in delamination, migration, and mitosis of trisomic NC. Addition of Sonic hedgehog (Shh) growth factor to trisomic cells from PA1 increased cell number to the same level as untreated control cells. Combined with previous demonstrations of a deficit in mitogenic response to Shh by trisomic cerebellar granule cell precursors, these results implicate common cellular and molecular bases of multiple DS phenotypes.     

Responses of leaf gas exchange, water relations, and water consumption in seedlings of four semiarid tree species to soil drying

To understand the response patterns to soil drying and the water use properties of commonly reforested trees in the semiarid Loess Plateau region of China, a glasshouse experiment was carried out with the seedlings of four species, i.e., Robinia pseudoacacia, Armeniaca sibirica, Syringa oblata, and Quercus liaotungensis. Severe water stress induced by withholding water resulted in permanent wilting of most of the seedlings pot-cultured with sandy soil in 8–12 days. Predawn and midday leaf water potentials and gas exchange characteristics (e.g., stomatal conductance) in the seedlings did not show marked changes until the volumetric soil water content decreased to about 0.05. As the soil water content decreased further, these physiological parameters rapidly declined, approaching their minimal levels at the stage of permanent wilting. The response of each parameter to soil water content changes was fitted with a non-linear saturation curve. Though the results suggested that the general pattern of responses to soil drying was identical among the species, quantitative differences in drought tolerance and water use properties were detected. Leaf stomatal conductance in R. pseudoacacia and A. sibirica showed earlier responses to reduced predawn leaf water potentials. However, water use characteristics and specific leaf area indicated that these two species consumed more water and may not be as drought tolerant as S. oblata and Q. liaotungensis. These results may provide important information to compare the reforestation species with respect to soil drying.     

Ecophysiological relevance of cuticular transpiration of deciduous and evergreen plants in relation to stomatal closure and leaf water potential

The water permeability of the leaves of three deciduous plants (Acer campestre, Fagus sylvatica, Quercus petraea) and two evergreen plants (Hedera helix, Ilex aquifolium) was analysed in order to assess its role as a mechanism of drought resistance. Cuticular permeances were determined by measurement of the water loss through adaxial, astomatous leaf surfaces. Minimum conductances after complete stomatal closure were obtained by leaf drying curves. The comparison of the water permeabilities determined with these two experimental systems revealed good agreement in the case of Acer, Fagus, Quercus, and Ilex. For Hedera the minimum conductance was 3-fold higher than the cuticular permeance indicating a significant contribution of residual stomatal transpiration. The leaf water potential was measured as a function of water content and analysed by pressure-volume curves. The influence of water potential as a component of the driving force for transpirational water loss was assessed in order to identify modifications of the cuticular barrier by the leaf water content. The ecophysiological meaning of the water relations parameters describing transpiration under drought conditions (cuticular transpiration, minimum transpiration, residual stomatal transpiration, effect of leaf water content on transpiration) and the water relations parameters derived from pressure-volume curves (osmotic potential at full saturation, turgor loss point, bulk modulus of elasticity) are discussed with regard to adaptations for drought resistance.     

Mechanisms contributing to seasonal homeostasis of minimum leaf water potential and predawn disequilibrium between soil and plant water potential in Neotropical savanna trees

Seasonal regulation of leaf water potential (_L) was studied in eight dominant woody savanna species growing in Brazilian savanna (Cerrado) sites that experience a 5-month dry season. Despite marked seasonal variation in precipitation and air saturation deficit (D), seasonal differences in midday minimum _L were small in all of the study species. Water use and water status were regulated by a combination of plant physiological and architectural traits. Despite a nearly 3-fold increase in mean D between the wet and dry season, a sharp decline in stomatal conductance with increasing D constrained seasonal variation in minimum _L by limiting transpiration per unit leaf area (E). The leaf surface area per unit of sapwood area (LA/SA), a plant architectural index of potential constraints on water supply in relation to transpirational demand, was about 1.5–8 times greater in the wet season compared to the dry season for most of the species. The changes in LA/SA from the wet to the dry season resulted from a reduction in total leaf surface area per plant, which maintained or increased total leaf-specific hydraulic conductance (G_t) during the dry season. The isohydric behavior of Cerrado tree species with respect to minimum _L throughout the year thus was the result of strong stomatal control of evaporative losses, a decrease in total leaf surface area per tree during the dry season, an increase in total leaf-specific hydraulic conductance, and a tight coordination between gas and liquid phase conductance. In contrast with the seasonal isohydric behavior of minimum _L, predawn _L in all species was substantially lower during the dry season compared to the wet season. During the dry season, predawn _L was more negative than bulk soil estimated by extrapolating plots of E versus _L to E=0. Predawn disequilibrium between plant and soil was attributable largely to nocturnal transpiration, which ranged from 15 to 22% of the daily total. High nocturnal water loss may also have prevented internal water storage compartments from being completely refilled at night before the onset of transpiration early in the day.