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In plants changes in cytosolic calcium ion concentrations ([Ca2+]cyt) have been detected after various stress treatments, including salt treatment. The involvement of a Ca2+ signal as an essential component of signalling pathways leading to downstream responses, such as gene expression, is supported only by a few studies. In this study the possible involvement of the salt stress-induced increase in[Ca2+]cyt in the signalling pathway leading to the induction ofATHB7, a homeobox gene encoding a homeodomain leucine zipper (HDZip) transcription factor was analysed. The salt-induced expression ofATHB7was found to be independent of the Ca2+ signal evoked by salt. Instead, it was found thatATHB7expression in shoots was not dependent on a direct contact with salt or osmoticum, whereas in roots,ATHB7seemed to be induced by the direct contact, indicating that signals from roots cause systemic induction ofATHB7. Abscisic acid (ABA) or ABA-dependent components were found to, at least partly, to function as the systemic signal. [ABSTRACT FROM AUTHOR]

ASR1 is a plant-specific, highly charged, low molecular weight polypeptide. Purified ASR1 was shown to posses sequence specific Zn2+-dependent DNA binding activity (Kalifaet al.Biochemical Journal381, 373–378, 2004). Steady-state levels of tomatoAsr1mRNA and protein are transiently increased following exposure of plants to polyethylene glycol, NaCl or abscisic acid. The biological role of ASR1 could not be deduced from sequence analyses or sequence homologies. Tobacco plants over-expressing tomato ASR1 have a decreased rate of water loss and improved salt tolerance. Upon exposure to salt, ASR1-over-expressing plants accumulate less Na+ and proline than wild-type plants, and also results in increased steady-state levels of other gene products under non-stressed plant growth conditions. Therefore, ASR1 is probably involved in the regulation of water- or salt-stress-modulated gene expression. [ABSTRACT FROM AUTHOR]

Two distinct morphological forms characterize the ontogeny of many epiphytic bromeliads. Smaller plants exhibit an atmospheric habit, while larger plants form water-impounding tanks. The study of the functional significance of heteroblasty in epiphytes is severely hampered by considerable size-related variation in morphological, anatomical and physiological parameters. To overcome this problem, plants of varying size of both atmospheric and tank form were included in the present study withVriesea sanguinolenta. The results show that virtually all morphological, anatomical and physiological characteristics vary during ontogeny, but changes were rarely directly related to the step change in gross morphology. Changes were either: (1) gradual from smallest atmospheric to small tank (e.g. leaf divergence angles, reduction in photosystem II efficiency during drought, speed of recovery after drought); (2) there was no change between atmospheric and small tank, but a gradual or step changewithinthe tank form (stomatal density, relationship of leaf N and specific leaf area); or (3) developmental patterns were more complicated with decreases and increases during ontogeny (photosynthetic capacity, carbon isotope ratios, abscisic acid levels during drought). Although the comparisonsbetweenontogenetic phases were always confounded by size differences, a hypothetical small tank plant is expected to suffer higher water loss than a real atmospheric, whereas a hypothetical, large atmospheric plant would show reduced access to resources, such as nutrients, in comparison with the real tank. The present results are consistent with the notion of heteroblasty as an adaptation of early ontogenetic stages to drought, but highlight that size-related variation greatly modifies any difference directly associated with the step change from atmospheric to tank. [ABSTRACT FROM AUTHOR]

This study was to test the hypothesis that the interaction between abscisic acid (ABA) and ethylene may be involved in mediating the effects of water stress on grain filling. Two high lodging-resistant rice ( Oryza sativa L.) cultivars were pot-grown. Three treatments, well-watered, moderate water-stressed (MD), and severe water-stressed (SD), were imposed from 9 d post-anthesis until maturity. Grain filling rate and grain weight were significantly increased under MD but decreased under SD. The two cultivars behaved the same. ABA concentration in the grains was very low during the grain filling stage, reaching a maximum when the grain filling rate was highest. Both the grain filling rate and ABA concentration were substantially enhanced by water stress. In contrast to ABA, concentrations of ethylene and 1-aminocylopropane -1-carboxylic acid (ACC) in the grains were very high at early grain filling stage and sharply decreased during the linear period of grain growth. MD reduced, whereas SD remarkably increased, their accumulation. The ratio of ABA to ACC was increased in MD grains but decreased in SD grains, indicating that there was a greater enhancement of ABA concentration than ethylene production in the MD treatment only. Application of cobalt ion (inhibitor of ethylene synthesis) or ABA at the early grain filling stage significantly increased grain filling rate. Spraying with ethephon (ethylene-releasing agent) or fluridone (inhibitor of ABA synthesis) had the opposite effect. The results suggest that antagonistic interactions between ABA and ethylene mediate the grain filling rate, and a high ratio of ABA to ethylene enhances grain filling rate. [ABSTRACT FROM AUTHOR]

As sessile organisms, plants must adapt to their environment. One approach toward understanding this adaptation is to investigate environmental regulation of gene expression. Our focus is on the environmental regulation of EARLI1, which is activated by cold and long-day photoperiods. Cold activation of EARLI1 in short-day photoperiods is slow, requiring several hours at 4 °C to detect an increase in mRNA abundance. EARLI1 is not efficiently cold-activated in etiolated seedlings, suggesting that photomorphogenesis is necessary for its cold activation. Cold activation of EARLI1 is inhibited in the presence of the calcium channel blocker lanthanum chloride or the calcium chelator EGTA. Addition of the calcium ionophore Bay K8644 results in cold-independent activation of EARLI1. These data suggest that EARLI1 is not an immediate target of the cold response, and that calcium flux affects its expression. EARLI1 is a putative secreted protein and has motifs found in lipid transfer proteins. Over-expression of EARLI1 in transgenic plants results in reduced electrolyte leakage during freezing damage, suggesting that EARLI1 may affect membrane or cell wall stability in response to low temperature stress. [ABSTRACT FROM AUTHOR]

The excision of four out of five primary roots of wheat ( Triticum durum Desf.) seedlings often leads to an enhanced rate of transpiration. Surprisingly this enhancement could be maintained for several hours after root excision and was particularly likely to occur at low irradiances or high atmospheric humidity. This long-term enhancement could not be explained in terms of conventional hydropassive stomatal effects. Elevated rates of transpiration were associated with and possibly caused by increased cytokinin concentrations in shoots of plants with partially excised roots. The single root remaining after excision was able to maintain an adequate water uptake for the continued enhanced transpiration, after only a short transient reduction in leaf water content. The enhanced capacity for water uptake by the remaining root was confirmed by measuring the water flow from detached roots at negative hydrostatic pressure. Even without additional suction, flow from the reduced root system increased about 1.5 h after the start of treatment, suggesting an increase in membrane permeability for water. Although abscisic acid (ABA) concentrations in the roots increased after the root excision treatment, there was no evidence for any enhanced concentration in the xylem sap. The possible role that this accumulation of ABA in roots may have in the apparent increase in hydraulic conductivity after root excision is discussed. [ABSTRACT FROM AUTHOR]

ABSTRACT This study investigated the possibility that abscisic acid (ABA) and cytokinins may mediate the effect of water deficit that enhances plant senescence and remobilization of pre-stored carbon reserves. Two high lodging-resistant wheat (Triticum aestivum L.) cultivars were field grown and treated with either a normal or high amount of nitrogen at heading. Well-watered (WW) and water-stressed (WS) treatments were imposed from 9 d post-anthesis until maturity. Chlorophyll (Chl) and photosynthetic rate (Pr) of the flag leaves declined faster in WS plants than in WW plants, indicating that the water deficit enhanced senescence. Water stress facilitated the reduction of non-structural carbohydrate in the stems and promoted the re-allocation of prefixed [sup 14] C from the stems to grains, shortened the grain filling period and increased the grain filling rate. Water stress substantially increased ABA but reduced zeatin (Z) + zeatin riboside (ZR) concentrations in the stems and leaves. ABA correlated significantly and negatively, whereas Z + ZR correlated positively, with Pr and Chl of the flag leaves. ABA but not Z + ZR, was positively and significantly correlated with remobilization of pre-stored carbon and grain filling rate. Exogenous ABA reduced Chl in the flag leaves, enhanced the remobilization, and increased grain filling rate. Spraying with kinetin had the opposite effect. The results suggest that both ABA and cytokinins are involved in controlling plant senescence, and an enhanced carbon remobilization and accelerated grain filling rate are attributed to an elevated ABA level in wheat plants when subjected to water stress. [ABSTRACT FROM AUTHOR]

ABSTRACT Abscisic acid (ABA) improves the sink strength by promoting the phloem unloading and regulating the assimilate metabolism in the economic sink organs of crops, although its mechanism remains unknown. The present experiment, using the techniques of the in vivo injection of ABA into the intact apple fruit attached to a growing apple tree and the in vivo incubation of the fruit tissue in the ABA-contained medium, showed that ABA strongly activated the fruit ATPase especially P-ATPase, of which the activity was doubled by ABA treatment. This ATPase activation was shown to be in vivo tissue-dependent. The ABA-induced P-ATPase activation was fruit developmental stage-, ABA dose-, medium pH- and incubation time-dependent. Physiological active (+)ABA was shown more effective to stimulate P-ATPase activity than (+/–)ABA, and two ABA analogues (–)ABA and trans -ABA, had no effect on P-ATPase activation, indicating that only physiologically active cis (+)ABA can induce the enzyme activation, and so the ABA-induced effects are stereospecific. The protein synthesis inhibitor cycloheximide was shown to have no effect on P-ATPase activation by ABA, suggesting that synthesis of new proteins was not involved in the enzyme activation. The cytochemical assay revealed that P-ATPase was activated by ABA in both the phloem and its surrounding flesh parenchyma cells, and that the most strongly P-ATPase activation was observed in the plasma membrane of sieve element/companion cell complex. These data suggest that the improvement of phloem unloading by ABA previously reported in this fruit as in other crop sink organs may be attributed, at least partly, to the ABA-induced ATPase activation especially in phloem cells. [ABSTRACT FROM AUTHOR]

ABSTRACT The Arabidopsis gene ATHB7 encodes a transcription factor of the homeodomain-leucine zipper class. The activity of the gene is dependent on the water conditions of the plant; expression being strongly induced in plants subjected to water deficit or to treatment with abscisic acid (ABA). In this report we demonstrate that ATHB7 , when constitutively expressed at levels typical for plants exposed to water-deficit conditions, caused a reduction in elongation growth in the leaf and in the inflorescence stem. The reduction in stem growth mimics the effect on growth caused by water-deficit conditions, but is independent of water conditions in plants that express ATHB7 constitutively. These results indicate that ATHB7 in the wild-type plant may be a mediator of the plant growth response to limiting water conditions in the leaf and in the inflorescence stem. In support of this notion we also show that the ATHB7 promoter in wild-type plants exposed to water-deficit conditions is highly active in the elongating parts of the inflorescence stem as well as leaves. [ABSTRACT FROM AUTHOR]

ABSTRACT Modern-day plants are subjected to various biotic and abiotic stresses thereby limiting plant productivity and quality. It has previously been reported that the use of a strong constitutive 35S cauliflower mosaic virus (CaMV) promoter to drive the expression of Arabidopsis CBF1 in tomato improved tolerance to cold, drought and salt loading, at the expense of growth and yield under normal growth conditions. Hence in the present study, the suitability of expressing the Arabidopsis CBF1 driven by three copies of an ABA-responsive complex (ABRC1) from the barley HAV22 gene in order to improve the agronomic performance of the transgenic tomato plants was investigated. Northern blot analysis indicated that CBF1 gene expression was induced by chilling, water-deficit and salt treatment in the transgenic tomato plants. Under these tested stress conditions, transgenic tomato plants exhibited enhanced tolerance to chilling, water-deficit, and salt stress in comparison with untransformed plants. Under normal growing conditions the ABRC1-CBF1 tomato plants maintained normal growth and yield similar to the untransformed plants. The results demonstrate the promise of using ABRC1-CBF1 tomato plants in highly stressed conditions which will in turn benefit agriculture. [ABSTRACT FROM AUTHOR]

ABSTRACT The signal interactions between calcium (Ca2+ ) and reactive oxygen species (ROS) originated from plasma membrane NADPH oxidase in abscisic acid (ABA)-induced antioxidant defence were investigated in leaves of maize (Zea mays L.) seedlings. Treatment with ABA led to significant increases in the activity of plasma membrane NADPH oxidase, the production of leaf O2 – , and the activities of several antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR). However, such increases were blocked by the pretreatment with Ca2+ chelator EGTA or Ca2+ channel blockers La3+ and verapamil, and NADPH oxidase inhibitors such as diphenylene iodonium (DPI), imidazole and pyridine. Treatment with Ca2+ also significantly induced the increases in NADPH oxidase activity, O2 – production and the activities of antioxidant enzymes, and the increases were arrested by pretreatment with the NADPH oxidase inhibitors. Treatment with oxidative stress induced by paraquat, which generates O2 – , led to the induction of antioxidant defence enzymes, and the up-regulation was suppressed by the pretreatment of Ca2+ chelator and Ca2+ channel blockers. Our data suggest that a cross-talk between Ca2+ and ROS originated from plasma membrane-bound NADPH oxidase is involved in the ABA signal transduction pathway leading to the induction of antioxidant enzyme activity, and Ca2+ functions upstream as well as downstream of ROS production in the signal transduction event in plants. [ABSTRACT FROM AUTHOR]

ABSTRACT Changes in abscisic acid (ABA) contents in Cd-treated rice (Oryza sativa L.) seedlings of two cultivars were investigated. On treatment with CdCl2 , the ABA content rapidly increased in the leaves and roots of Cd-tolerant cultivar (cv. Tainung 67, TNG67) but not in the Cd-sensitive cultivar (cv. Taichung Native 1, TN1). The reduction of transpiration rate of TN1 caused by Cd was less than that of TNG67. Exogenous application of ABA reduced transpiration rate, decreased Cd content, and enhanced Cd tolerance of TN1 seedlings. Exogenous application of the ABA biosynthesis inhibitor, fluridone, reduced ABA accumulation, increased transpiration rate and Cd content, and decreased Cd tolerance of TNG67 seedlings. Fluridone effect on Cd toxicity of TNG67 seedlings was reversed by the application of ABA. The roles of endogenous ABA in Cd tolerance of rice seedlings are discussed and suggested. [ABSTRACT FROM AUTHOR]

ABSTRACT Abscisic acid (ABA) and sucrose are known to induce dehydration tolerance of in vitro plant cells and tissues. The present study reports the presence of different mechanisms by which sucrose and ABA improve dehydration tolerance of Spathoglottis plicata (orchid) protocorms. Orchid protocorms were generated aseptically from seeds on Murashig and Skoog medium, and then treated for 7 d in medium containing 10 mg L-1 ABA and/or 10% (w/v) sucrose. Dehydration tolerance of protocorms was determined at ∼25 °C under various drying conditions at relative humidity from 7 to 93%. The actual rate of water loss (i.e. drying rate) was determined using the rate constant of tissue water loss during drying according to the first-order kinetics. Drying rate affected dehydration tolerance. ABA treatment reduced drying rate and increased dehydration tolerance of protocorms at all relative humidity values tested. However, when compared on the basis of actual drying rates, there was no difference in dehydration tolerance between control and ABA-treated protocorms, suggesting that ABA-induced tolerance was correlated with the drying rate reduction. Sucrose treatment was more effective than ABA treatment for the induction of dehydration tolerance. Interestingly, sucrose only slightly affected drying rate. ABA treatment significantly enhanced the synthesis of dehydrin, whereas sucrose treatment primarily resulted in sucrose accumulation. Sucrose treatment also affected protein turnover during drying, causing a significant decrease in protein content in protocorms. Slow drying promoted the degradation of high molecular weight proteins and enhanced the synthesis of low molecular weight dehydrin. The data suggest that different physiological mechanisms are probably involved in the induction of dehydration tolerance by ABA and sucrose treatment. [ABSTRACT FROM AUTHOR]

ABSTRACT The effect of abscisic acid (ABA) on the tolerance to oxidative stress in a freshwater green alga, Chlamydomonas reinhardtii , was investigated. Exogenously added ABA enhanced the growth of this alga, which was observed under continuous illumination but not in the dark. The cells treated with ABA for 24 h showed tolerance to oxidative stress caused by exposure to paraquat or hydrogen peroxide. In the ABA-treated cells, the activities of two antioxidant enzymes, catalase (CAT) and ascorbate peroxidase (APX), were significantly higher than those in the untreated control. The result suggests that ABA plays a role in the enhancement of tolerance to oxidative stress by increasing the activity of antioxidant enzymes. [ABSTRACT FROM AUTHOR]

Abstract Previous studies of maize suspension-cultured cells showed that abscisic acid (ABA) treatment at warm temperatures improved the tolerance of cells to subsequent chilling. In the present study, it is shown that both ABA-treated and untreated maize cells accumulated proline in response to chilling. However, ABA-treated cells displayed less lipid peroxidation during chilling, and thus, unlike untreated cells, were able to retain the accumulated proline intracellularly. Proline application experiments indicate that an intracellular proline level higher than 2 µ mole (g FW)-1 prior to chilling was needed to meaningfully reduce chilling-enhanced lipid peroxidation and significantly improve chilling tolerance. The results suggest that total proline accumulation in ABA-treated as well as untreated cells during chilling was enough to potentially improve chilling tolerance, but proline leakage rendered the control cells unable to benefit from the endogenous synthesis of proline in relation to the alleviation of chilling injury. Proline participated in chilling tolerance improvement in ABA-treated maize cells, as evidenced by: (1) the inhibition of proline accumulation by l-methionine-d, l-sulphoximine (MSO), an inhibitor of glutamine synthetase, reduced ABA-improved chilling tolerance, and (2) the addition of glutamine into the medium prevented the MSO-induced reduction in chilling tolerance. The revised relationship between proline accumulation and membrane stability at cold is discussed in the light of these current findings. [ABSTRACT FROM AUTHOR]

Abstract Changes in gene expression may lead to cellular adaptation of water-deficit stress, yet all of the induced mRNAs may not play this role. Changes in gene expression must be signalled by transduction mechanisms that first sense a water deficit. This first step triggers changes in gene expression that function to synthesize additional signals such as abscisic acid (ABA). The enzymes involved in ABA biosynthesis have been cloned and their regulation during water-deficit stress is being characterized. Once ABA levels are increased, further signalling mechanisms are initiated to signal new gene expression patterns that are proposed to play a role in cellular adaptation to water-deficit stress. As the genome of Arabidopsis is now completed, much more information can be exploited to characterize these responses. [ABSTRACT FROM AUTHOR]

Abstract Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress. [ABSTRACT FROM AUTHOR]

Abstract Shoot and root growth are differentially sensitive to water stress. Interest in the involvement of hormones in regulating these responses has focused on abscisic acid (ABA) because it accumulates in shoot and root tissues under water-limited conditions, and because it usually inhibits growth when applied to well-watered plants. However, the effects of ABA can differ in stressed and non-stressed plants, and it is therefore advantageous to manipulate endogenous ABA levels under water-stressed conditions. Studies utilizing ABA-deficient mutants and inhibitors of ABA synthesis to decrease endogenous ABA levels, and experimental strategies to circumvent variation in plant water status with ABA deficiency, are changing the view of the role of ABA from the traditional idea that the hormone is generally involved in growth inhibition. In particular, studies of several species indicate that an important role of endogenous ABA is to limit ethylene production, and that as a result of this interaction ABA may often function to maintain rather than inhibit shoot and root growth. Despite early speculation that interaction between these hormones may influence many of the effects of water deficit, this topic has received little attention until recently. [ABSTRACT FROM AUTHOR]

Abstract There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal. [ABSTRACT FROM AUTHOR]

ABSTRACTThe consequences of manipulating abscisic acid (ABA) biosynthesis rates on stomatal response to drought were analysed in wild-type, a full-deficient mutant and four under-producing transgenic lines of N. plumbaginifolia. The roles of ABA, xylem sap pH and leaf water potential were investigated under four experimental conditions: feeding detached leaves with varying ABA concentration; injecting exogenous ABA into well-watered plants; and withholding irrigation on pot-grown plants, either intact or grafted onto tobacco. Changes in ABA synthesis abilities among lines did not affect stomatal sensitivity to ABA concentration in the leaf xylem sap ([ABA]xyl), as evidenced with exogenous ABA supplies and natural increases of [ABA]xyl in grafted plants subjected to drought. The ABA-deficient mutant, which is uncultivable under normal evaporative demand, was grafted onto tobacco stock and then presented the same stomatal response to [ABA]xyl as wild-type and other lines. This reinforces the dominant role of ABA in controlling stomatal response to drought in N. plumbaginifolia whereas roles of leaf water potential and xylem sap pH were excluded under all studied conditions. However, when plants were submitted to soil drying onto their own roots, stomatal response to [ABA]xyl slightly differed among lines. It is suggested, consistently with all the results, that an additional root signal of soil drying modulates stomatal response to [ABA]xyl. [ABSTRACT FROM AUTHOR]

ABSTRACTAbscisic acid (ABA) integrates the water status of a plant and causes stomatal closure. Physiological mechanisms remain poorly understood, however, because guard cells flanking stomata are small and contain only attomol quantities of ABA. Here, pooled extracts of dissected guard cells of Vicia faba L. were immunoassayed for ABA at sub-fmol sensitivity. A pulse of water stress was imposed by submerging the roots in a solution of PEG. The water potentials of root and leaf declined during 20 min of water stress but recovered after stress relief. During stress, the ABA concentration in the root apoplast increased, but that in the leaf apoplast remained low. The ABA concentration in the guard-cell apoplast increased during stress, providing evidence for intra-leaf ABA redistribution and leaf apoplastic heterogeneity. Subsequently, the ABA concentration of the leaf apoplast increased, consistent with ABA import via the xylem. Throughout, the ABA contents of the guard-cell apoplast, but not the guard-cell symplast, were convincingly correlated with stomatal aperture size, identifying an external locus for ABA perception under these conditions. Apparently, ABA accumulates in the guard-cell apoplast by evaporation from the guard-cell wall, so the ABA signal in the xylem is amplified maximally at high transpiration rates. Thus, stomata will display apparently higher sensitivity to leaf apoplastic ABA if stomata are widely open in a relatively dry atmosphere. [ABSTRACT FROM AUTHOR]

ABSTRACT Signalling process of water stress-induced abscisic acid (ABA) accumulation was investigated in maize (Zea mays L.) leaf and root tissues. Potent free-radical scavengers and reducing agents, N-acetyl cysteine (NAC) and cystein (Cys), significantly inhibited or nearly completely blocked dehydration-induced ABA accumulation. Dithiothreitol (DTT), a reducing agent but not a free-radical scavenger, also significantly inhibited such accumulation whereas solely free-radical scavengers, dimethyl sulphoxide (DMSO) and melatonin (Mela), had no effects. Moreover, water stress-induced ABA accumulation was not affected either by free radicals, such as superoxide anion and hydrogen peroxide, or by oxidants such as KIO4. These observations suggest that the blocking of water stress-induced ABA accumulation resulted from the reducing effect, rather than from anything associated with free radicals. The disulphide bond might be crucial to the reactivity of some signal element(s) in the signalling process of water stress-induced ABA accumulation. To test the hypothesis, we used a sulfhydryl modifier, iodoacetamide (IOA), and found that it nearly totally blocked the water stress-induced ABA accumulation. Furthermore, an impermeable sulfhydryl modifier, p-chloromercuriphenylsulphonic acid (PCMBS), could also inhibit the water stress-induced ABA accumulation in the leaf tissues. These results indicate that water stress-perception protein(s) or receptor(s) may be located on the plasmalemma and a sulfhydryl group in the extracellular domain is critical to the reactivity of the speculated water stress receptors. Cys, DTT and IOA did not lead to a decrease of the baseline ABA level, i.e. in non-stressed roots. Result indicates that their blocking of water stress-induced ABA accumulation occurred upstream of the ABA biosynthesis pathway, i.e. in the signalling process that initiates such accumulation. [ABSTRACT FROM AUTHOR]

ABSTRACT This study tested the hypothesis that antagonistic interactions between abscisic acid (ABA) and ethylene mediate the effects of soil compaction on shoot growth. Isogenic wild-type (Ailsa Craig), ABA-deficient (notabilis) and a transgenic (ACO1AS) tomato genotype with a reduced capacity to synthesize ethylene were examined. Exogenous ABA was also applied. Leaf area was comparable when Ailsa Craig and ACO1AS were grown in uncompacted (1·1 g cm-3) or compacted (1·5 g cm-3) soil, but was lower in notabilis. However, a 1·1/1·5 g cm-3 split-pot treatment invoked marked genotypic differences, whereby leaf area was comparable to 1·1 g cm-3 control plants in ACO1AS but was intermediate between the 1·1 and 1·5 g cm-3 treatments in Ailsa Craig and notabilis. ABA may be discounted as the root-sourced signal responsible for reducing leaf area when the roots encountered compacted soil as Ailsa Craig and ACO1AS showed differing responses despite similar increases in xylem sap ABA concentration; leaf area was invariably lower in notabilis. These genotypic differences were correlated with ethylene evolution; thus the greater leaf area in ACO1AS was associated with its reduced ability to synthesize ethylene, whereas the reductions in leaf expansion observed when Ailsa Craig and notabilis encountered compacted soil were accompanied by increased ethylene production. Application of ABA had little effect on ACO1AS, but promoted a recovery of leaf expansion in notabilis, and more surprisingly in Ailsa Craig. These results suggest that antagonistic interactions between ABA and ethylene may regulate leaf expansion when the root system simultaneously encounters uncompacted and compacted soil. [ABSTRACT FROM AUTHOR]

ABSTRACT Stomatal response to leaf water status was experimentally manipulated by pressurizing the soil and roots of potted common bean plants enclosed in a custom-built root pressure chamber. Gas exchange was monitored using a whole-plant cuvette and plant water status using in situ leaf psychrometry. Bean plants re-opened their stomata upon pressurization, but the extent of re-opening was strongly dependent on the time of day when the soil was pressurized, with maximum re-opening in the morning hours and limited re-opening in the afternoon. Neither leaf nor xylem abscisic acid concentrations could explain the reduced response to pressurization in the afternoon. The significance of this phenomenon is discussed in the context of circadian rhythms and of other recent findings on the ‘apparent feed-forward response’ of the stomata of some species to vapour pressure deficit. [ABSTRACT FROM AUTHOR]

To examine the effect of abscisic acid (ABA) on the photo-induced inactivation of the photosystem II (PSII) complex, a suspension culture of Chlamydomonas reinhardtii was treated with ABA for 24 h in darkness and then, after removal of ABA, the cells were exposed to strong light at a photon flux density of 2000 µmol m[sup -2] s[sup -1] at various temperatures. The activity of PSII, as estimated in terms of chlorophyll fluorescence and the evolution of oxygen, decreased significantly during the exposure of cells to the strong light, and the extent of the photo-induced decrease in PSII activity was much greater at lower temperatures. Irrespective of temperature, the decrease in PSII activity in ABA-treated cells was significantly smaller than that in control cells. Moreover, the recovery of PSII activity from the photo-inactivated state in ABA-treated cells was significantly faster than that in control cells. The recovery of PSII activity in both ABA-treated and control cells was almost entirely prevented by the presence of chloramphenicol. These results indicate that ABA protects the PSII complex in C. reinhardtii against photo-induced inactivation by accelerating the recovery of this complex. [ABSTRACT FROM AUTHOR]

ABSTRACT Chilling tolerance was increased in suspension-cultured cells and seedlings of maize (Zea mays L. cv ‘Black Mexican Sweet’) grown in media containing glycinebetaine (GB). A triphenyl tetrazolium chloride (TTC) reduction test indicated that after a 7 d chilling period at 4 °C, cells treated with 1 mm GB at 26 °C for 1 d had a survival rate (30%) that was twice as high as that of untreated controls. The addition of 2·5 mM GB to the culture medium resulted in maximum chilling tolerance (40%). The results of a cell regrowth assay were consistent with viability determined by the TTC method. In suspension-cultured cells supplemented with various concentrations of GB, accumulation of GB in the cells was proportional to the GB concentration in the medium and was saturated at a concentration of 240 μmol (g DW)-1. The degree of increased chilling tolerance was positively correlated with the level of GB accumulated in the cells. The increased chilling tolerance was time-dependent; i.e. it was first observed 3 h after treatment and reached a plateau after 14 h. Feeding seedlings with 2·5 mM GB through the roots also improved their chilling tolerance, as evidenced by the prevention of chlorosis after chilling for 3 d at 4 °C/2 °C. Lipid peroxidation, as expressed by the production of malondialdehyde, was significantly reduced in GB-treated cells compared with the untreated controls during chilling. These results suggest that increased chilling tolerance may be due, in part, to the reduction of lipid peroxidation of the cell membranes in the presence of GB. [ABSTRACT FROM AUTHOR]

ABSTRACTCotton (Gossypium hirsutum L. cv. CS50) plants were grown at two levels of relative humidity (RH) and sprayed daily with abscisic acid (ABA) at four concentrations. Plants grown at lower humidity had higher transpiration rates, lower leaf temperatures and lower stomatal conductance. Plant biomass was also reduced at low humidity. Within each humidity environment, increasing ABA concentration generally reduced stomatal conductance, evaporation rates, superficial leaf density and plant biomass, and increased leaf temperature and specific leaf area. As expected, decreased stomatal conductance resulted in decreased carbon isotope discrimination in leaf material (Δ13Cl). Plants grown at low humidity were more enriched in 18O than those grown at high RH, as theory predicts. Within each humidity environment, increasing ABA concentration increased oxygen isotope enrichment of leaf cellulose (Δ18Oc) and whole-leaf tissue (Δ18Ol). Values of Δ13Cl and Δ18Ol predicted by theoretical models were close to those observed, accounting for 79% of the measured variation in Δ13Cl and 95% of the measured variation in Δ18Ol. Supporting theory, Δ13Cl and Δ18Ol in whole-leaf tissue were negatively related. [ABSTRACT FROM AUTHOR]

In most plant species, a decrease in atmospheric humidity at the leaf surface triggers a decrease in stomatal conductance. While guard cells appear to respond to humidity-induced changes in transpiration rate, as opposed to relative humidity or vapour pressure difference, the underlying cellular mechanisms for this response remain unknown. In the present set of experiments, abscisic acid (ABA)-deficient (aba1) and ABA-insensitive (abi1-1 and abi2-1) mutants of Arabidopsis thaliana were used to test the hypothesis that the humidity signal is transduced by changes in the flux or concentration of ABA delivered to the stomatal complex in the transpiration stream. In gas exchange experiments, stomatal conductance was as sensitive to changes in vapour pressure difference in aba1, abi1-1 and abi2-1 mutant plants as in wild-type plants. These experiments appear to rule out an obligate role for either the concentration or flux of ABA or ABA conjugates as mediators of the guard cell response to atmospheric water potential. The results stand in contrast to the well-established role of ABA in mediating guard cell responses to decreases in soil water potential. [ABSTRACT FROM AUTHOR]

ABSTRACTPolypeptide synthesis and accumulation were examined in the roots of tomato seedlings exposed to a polyethylene glycol-imposed water deficit stress. In these roots, the synthesis of a number of polypeptides was induced, while that of several others was enhanced or repressed. To examine the role played by abscisic acid (ABA) in co-ordinating the accumulation of these proteins, water-deficit-stress-responsive polypeptide synthesis was investigated in the roots of the ABA-deficient mutant flacca. In the roots of this mutant, the ability to accumulate a complete set of water-deficit-stress-responsive polypeptides was impaired, indicating that ABA is required for their synthesis. The role of ABA was further examined by exposing the roots of both genotypes to exogenous ABA, which, with one exception, elicited the accumulation of all water-deficit-stress-responsive proteins. Polyethylene glycol-induced polypeptide accumulation was accompanied by a 1·6-fold increase in the level of endogenous ABA in the roots of wild-type plants and a 5-fold increase in the roots of flc. Thus, although the absolute level was lower than that of the wild-type, flc has the capacity to accumulate ABA in its roots. When fluridone was used to prevent the biosynthesis of ABA, the accumulation of several water-deficit-stress-responsive polypeptides was reduced further. The synthesis of polypeptides was also examined in the roots of salt-treated seedlings. Salt altered the accumulation of several polypeptides, all of which were previously observed in water-deficit-stressed roots, indicating that their synthesis was the result of the osmotic component of the salt stress. However, the accumulation of these polypeptides was not impaired in flc roots, indicating that the role played by ABA in regulating their accumulation in salt-and polyethylene glycol-treated roots differs. As such, salt- and water-deficit-stress-induced changes in gene expression may be effected by different mechanisms, at least at the level of polypeptide accumulation. [ABSTRACT FROM AUTHOR]

This article comments on:Mesophyll conductance decreases in the wild type but not in an ABA‐deficient mutant (aba1) of Nicotiana plumbaginifolia under drought conditions [ABSTRACT FROM AUTHOR]

ABSTRACTNovel techniques were devised to explore the mechanisms mediating the adverse effects of compacted soil on plants. These included growing plants in: (i) profiles containing horizons differing in their degree of compaction and; (ii) split-pots in which the roots were divided between compartments containing moderately (1·4 g cm-3) and severely compacted (1·7 g cm-3) soil. Wild-type and ABA-deficient genotypes of barley were used to examine the role of abscisic acid (ABA) as a root-to-shoot signal. Shoot dry weight and leaf area were reduced and root: shoot ratio was increased relative to 1·4 g cm-3 control plants whenever plants of both genotypes encountered severely compacted horizons. In bartey cultivar Steptoe, stomatal conductance decreased within 4 d of the first roots encountering 1·7 g cm-3 soil and increased over a similar period when roots penetrated from 1·7 g cm-3 into 1·4 g cm-3 soil. Conductance was again reduced by a second 1·7 g cm-3 horizon. These responses were inversely correlated with xylem sap ABA concentration. No equivalent stomatal responses occurred in Az34 (ABA deficient genotype), in which the changes in xylem sap ABA were much smaller. When plants were grown in 1·7: 1·4 g cm-3 split-pots, shoot growth was unaffected relative to 1·4 g cm-3 control plants in Steptoe, but was significantly reduced in Az34. Excision of the roots in compacted soil restored growth to the 1·4 g cm-3 control level in Az34. Stomatal conductance was reduced in the split-pot treatment of Steptoe, but returned to the 1·4 g cm-3 control level when the roots in compacted soil were excised. Xylem sap ABA concentration was initially higher than in 1·4 g... [ABSTRACT FROM AUTHOR]

ABSTRACTPlant osmotic stress responses are associated with changes in gene expression. Osmotic stress-regulated genes (OR genes) have been isolated by various approaches such as differential screening of cDNA libraries or by complementation. We have used differential display of mRNA to isolate partial cDNAs corresponding to transcripts differentially expressed in leaf discs from osmotically stressed rapeseed. The cloning and the sequencing of three OR genes (MC12, MC43 and MC24) isolated by this method are reported here. Differences in corresponding mRNA levels during the first hours of stress were confirmed by Northern blot analysis. The deduced amino acid sequence of the MC12 and MC43 clones, for which mRNA accumulation was detected under osmotic stress, shows significant similarities to saccharopine dehydrogenase and linker histone H1-3 genes, respectively. Furthermore these up- regulated genes are induced by ABA. The nucleotide sequence of MC24 shows strong homology with a chloroplast gene (rps12) encoding a ribosomal protein. mRNA hybridization with MC24 led to a multiple banding pattern resulting from maturation of polycistronic precursor transcripts. Levels of different sized transcript hybridizing with MC24 decreased during osmotic stress, suggesting that rps12 chloroplast gene is down-regulated by osmotic stress. Interestingly, up-regulation of a gene encoding an enzyme involved in lysine catabolism suggests that products from this pathway could be of interest for the adaptation of rapeseed leaf explants subjected to osmotic upshock in vitro. This discovery opens new insights into metabolic adjustment mechanisms under stress. [ABSTRACT FROM AUTHOR]

ABSTRACTElevated levels of abscisic acid (ABA) are required for the expression of several genes during water deficit. The expression patterns of the ABA-requiring genes, le4, le16, le25 and his1-s, differed during a 4 d period of water deficit and a 24 h recovery period in intact tomato plants. During water deficit, a strong correlation between ABA content and mRNA accumulation was observed for le16, with only a moderate or poor correlation observed between the accumulation of ABA and the other mRNAs. The regulation of le4 and le16 can be accounted for by an increase in transcription rate, while components of post-transcriptional regulation are implicated in the regulation of the le25 and his1-s genes. A strong correlation between leaf ABA content and the transcription rate of le4 and le16 during the stress and recovery periods was observed. Protein synthesis was not necessary for the expression of the four genes. However, the mRNA content of le25 increased in the presence of translation inhibitors plus ABA, implicating inhibition of a transcriptional repressor. The regulation of gene expression in response to stress is complex; although ABA was required for the regulation of the genes in this study, there must be additional control mechanisms that interact with components of the ABA regulatory machinery. [ABSTRACT FROM AUTHOR]

The regulation of whole-plant resource allocation during seed development in Arabidopsis thaliana was investigated by examining growth rate and partitioning of 14CO2 in wild-type plants and those carrying the abi3 mutation. Plants carrying the abi3 mutation partitioned more resources into seed development than the wild type. The extra resources were available as a result of delayed senescence of the cauline leaves in the mutant. After supply of 14CO2 at later stages of reproductive development differences in patterns of 14C distribution between mutant and wild type were consistent with long-term changes in growth and allocation. The role of long-distance signals in the regulation of seed yield in Arabidopsis is discussed. [ABSTRACT FROM AUTHOR]

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA (SABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. SABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections). All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. SABA measured in turgid roots changed with root age and among root categories. This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of SABA to changes in root water potential (Ψroot) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of SABA to RWC were due to different RWC values among categories at a given Ψroot, and not to differences in the response of SABA to Ψroot. [ABSTRACT FROM AUTHOR]

Twenty cultivars of spring wheat were examined for variation in abscisic acid (ABA) accumulation following partial dehydration of excised leaves. A-fold range of ABA concentration was obtained. A cross between two cultivars which differed in drought-induced ABA accumulation was used to study the heritability of ABA accumulation and to develop lines differing in their capacity to accumulate ABA. Broad sense heritability was 0-32 between the F2 and F3 generations and 0-70 between the F3 and F4 generations. Apparent homozygosity for ABA accumulation was achieved in several selections at the F4. The possible significance for drought resistance of differences in capacity to accumulate ABA is discussed. [ABSTRACT FROM AUTHOR]

It is widely believed that the root cap participates in geotropism by acting both as a sensor of the direction of gravity and as a source of at least one regulator of root growth, which may be abscisic acid (ABA.). It has been suggested that this regulator accumulates within the prospective concave (or lower) half of the root and there causes a retardation of growth that brings about bending. A re-examination of the evidence upon which this inhibitor hypothesis of geotropic control is based reveals that (1) it is derived almost entirely from microsurgical studies and the deductions from such experiments still require corroborations from analyses of inhibitor content and action; (2) the evidence that ABA is the inhibitor seems poor at present; (3) in maize and lentil, two well-studied species, geocurvature is probably a consequence of accelerated growth within the prospective convex (or upper) half rather than inhibited growth within the concave (or lower) half; (4) the geotropic signal from the cap may be one that redirects a pre-existing basipetal flow of inhibitor away form the upper portion of the root, where growth is de-repressed as a consequence. [ABSTRACT FROM AUTHOR]

Responses of stomata to light and CO2 were smaller when detached epidermis of Commelina communis L. was incubated on a medium containing 50 mol m-3 NaCI than when an equimolar KC1 solution was used. Although opening in the light in the absence of CO2 seemed to be the same whichever salt was present, apertures on KC1 solutions were smaller in the dark or with CO2-containing air. The response to 10-7 mol dm-3 ABA was similarly reduced in the presence of NaCI. If there is an optimal NaC1 concentration for stomatal CO2 and light responses it is at or below 25 mol m-3. These findings point towards control of stomatal movements by light, CO2 and ABA at the level of cation uptake or extrusion. [ABSTRACT FROM AUTHOR]

Dihydrophaseic acid (DPA) has been identified in leaves from Euphorbia lathyrus L and Populus robusta x schnied. The formation of DPA from abscisic acid (ABA) was demonstrated using 14C-ABA. Measurements of ABA, DPA and phaseic acid (PA) concentrations were made in Euphorbia subjected to drought and waterlogging and in Populus subjected to rhythmic water stress. The results are consistent with the proposition that ABA concentrations are controlled by de novo biosynthesis and by metabolism via the PA/DPA pathway. The findings are discussed in relation to effects of the type of stress and its duration and to behaviour during stress relief. [ABSTRACT FROM AUTHOR]

In many woody plants a short photoperiod triggers the onset of cold acclimation, but the nature of this process has remained obscure. We aimed to establish which physiological and genetic factors have a role in short-day-induced acclimation by comparing two types of birch, Betula pubescens Ehrh. and B. pubescens f. hibernifolia Ulv., the latter being unable to increase its abscisic acid (ABA) levels. In the wild type, short-day or natural autumn conditions in the field appeared to elevate the ABA levels before acclimation, which was accompanied by tissue desiccation, osmotic adjustments and accumulation of Group 2 LEA proteins [responsive to ABA (RAB) 16-like; 24, 30 and 33 kDa] and Group 4 LEA proteins [late embryogenesis abundant (LEA) 14-like; 19 kDa]. Under similar conditions the ABA-deficient birch showed reduced water loss, defective osmoregulation, absence of inducible Group 2 LEA proteins, and delayed or reduced tolerance to freezing. In contrast, both birch genotypes showed similar seasonal production patterns of Group 4 LEA proteins. Our results demonstrate that onset of cold acclimation in birch is based on multiple mechanisms, including molecular pathways that are typical of stress responses. ABA may be important for the accurate timing of cold acclimation in trees that are sensitive to photoperiod. [ABSTRACT FROM AUTHOR]

The magnitude of the response of stomatal conductance to a change in the concentration of carbon dioxide external to the leaf from 350 to 700 cm3 m–3 was found to be extremely variable from day to day in the field in Glycine max, Hordeum vulgare and Triticum aestivum. It was found that the leaf-to-air water vapour pressure difference (LAVPD) during the midday measurements of the stomatal response to carbon dioxide affected the magnitude of the response. On days when LAVPD was low, no significant change in conductance occurred with the increase in carbon dioxide concentration. When LAVPD was higher, conductance decreased by 24–52% with the increase in carbon dioxide within a few minutes. The sensitivity of conductance was approximately linearly related to LAVPD in wheat and barley. Experiments with G. max in the field indicated that, on days with low LAVPD, increasing the LAVPD just around the measured portion of a leaflet made stomatal conductance responsive to increased carbon dioxide. This result was also obtained under laboratory conditions with G. max, Helianthus annuus and Amaranthus retroflexus. In G. max, it was determined that leaves in which conductance was not responsive to the increase in carbon dioxide could be made responsive even at low LAVPD by the injection of abscisic acid into their petioles. Because it is known that abscisic acid sensitizes stomata to carbon dioxide, these results are consistent with the idea that abscisic acid may be involved in the response of stomatal conductance to changes in LAVPD. [ABSTRACT FROM AUTHOR]

Bud dormancy in woody plants is released by long-term exposure to non-freezing chilling temperatures, whereas freezing temperatures have been considered to have little or no effect. However, the present results demonstrate that short-term exposure to freezing can release bud dormancy in Betula pubescens (Ehrh.) and B. pendula (Roth). Short-term freezing during the dormancy induction phase improved the release of bud dormancy only if an adequate level of dormancy had been reached. In fully dormant or chilled plants both the percentage and the speed of bud-burst increased, the more so the lower the temperature. Our results rule out the possibility that endogenous abscisic acid could be directly involved in the physiological control of bud dormancy release. The fast, easily applicable method presented here for bud dormancy release could further investigations into the biochemical and biophysical background to the process. The mechanisms of bud dormancy release and its relationship to cold acclimation are discussed in the light of these results, as also are the implications of the findings for modelling of bud dormancy. [ABSTRACT FROM AUTHOR]

There are numerous reports of interactions between light and phytohormones. However, the exact nature of this interaction is not well understood. This article considers the possible roles played in the light signal transduction chain by four phytohormones (auxin, abscisic acid, gibberellins and cytokinins). As the involvement of phytohormones in this signalling mechanism is controversial, the major arguments in the debate are reported, including recently published results based on genetic, biochemical and physiological approaches. [ABSTRACT FROM AUTHOR]

We report the effects of the root hemiparasite Striga hermonthica (Del.) Benth. on the growth and photosynthesis of two cultivars of sorghum: CSH-1, a susceptible variety, and Ochuti, which shows some tolerance to S. hermonthica in the field. Within 4 d of parasite attachment to the host roots, infected plants of both cultivars were significantly shorter than uninfected controls. At 55 d, infected plants of both cultivars had significantly less shoot and root biomass, and significantly smaller leaf areas than uninfected controls. The dry weight of S. hermonthica attached to host roots was insufficient at this stage to explain the decreased growth in terms of a competing sink for carbon and nitrogen. Leaf chlorophyll and nitrogen per unit area were greater in infected plants of both cultivars compared with control plants. However, whereas photosynthesis and transpiration in young leaves of infected CSH-1 plants declined with time when compared with controls, the rates in infected Ochuti plants were similar to those in uninfected controls throughout the time course of observation. In both cultivars, a strong correlation was observed between the rate of photosynthesis and stomatal conductance during photosynthetic induction,but infection resulted in a much slower induction than in control. In CSH-1 plants, both steady-state photosynthesis and stomatal conductance were lower than in controls, whereas in leaves of Ochuti steady-state photosynthesis and stomatal conductance eventually reached the same values as in the control leaves. Results from A/Ci analysis and also from determination of 13C isotope discrimination were consistent with a stomatal limitation to photosynthesis in the leaves of Striga-infected plants. The concentration of the plant growth regulator abscisic acid (ABA) was measured in the xylem sap of infected CSH-1 plants only, and was found to be twice that of uninfected plants. A possible role of ABA in determining host response ... [ABSTRACT FROM AUTHOR]

Three-month-old Cedrella odorata were exposed to a soil-drying treatment. During this period, xylem sap was periodically collected from the plant by applying pneumatic pressure to the roots. This also allowed whole-plant water status to be measured by recording the balancing pressure applied. The concentration of ABA in xylem sap (C) was related to the whole-plant transpiration rate (V) which was measured witha sap flow gauge. The analysis of these paired measurements centred on how the reciprocal of C (R) varied with respect to V. This revealed that (1) the observed increases in C could not be explained by the reductions in V alone, (2) initially, decreases in V were associated with proportional increases in the whole-plant ABA flux (M), and (3) this relationship broke down at low values of V since zero flow was associated with a finite value for C estimated to be 41 pmol ABA mmol-1 H2O. A simple static model is developed from the observations that is able to explain the data well, and the results are discussed in terms of the effects of ABA on stomatal conductance (gsw). [ABSTRACT FROM AUTHOR]

Wild-type and abscisic acid (ABA) -deficient (sitiens) tomato plants were used to analyse the effects of abscisic acid (ABA), butyric acid (BA), jasmonic acid (JA) and linolenic acid (LA) on assimilation and transpiration rates in detached leaves taking up those substances into the transpiration stream. BA did not affect assimilation and transpiration rates. ABA decreased assimilation and transpiration in both wild-type and ABA-deficient mutants. JA reduced the assimilation rate in both lines but induced a significant reduction of transpiration in the wild type only. The response to LA in both lines was slower than that to JA. [ABSTRACT FROM AUTHOR]

The growth patterns of plants subjected to phosphorus starvation resemble those caused by treatment with ABA, suggesting that ABA could mediate the response of the plant to phosphorus starvation. We examined the role of ABA in phosphorus stress by comparing growth and biochemical responses of Arabidopsis thaliana ABA mutants aba-1 and abi2-1 to those of wild-type plants. We first characterized acid phosphatase production of wild-type Arabidopsis in response to phosphorus starvation. We found that several acid phosphatase isozymes are present in roots and shoots, but only a subset of these isozymes are induced by phosphorus stress, and they are induced in both organs. Production of acid phosphatase in response to phosphorus stress was not affected by the aba-1 or abi2-1 mutations. Low phosphorus also resulted in decreased growth of both wild-type and ABA mutant plants, and the root-to-shoot ratio was increased in both wild type and mutants. Anthocyanins accumulated in response to phosphorus stress in both wild-type and mutant plants, but the increase was reduced in the aba-1 mutant. Thus, two different ABA mutants responded normally in most respects to phosphorus stress. Our data do not support a major role for ABA in coordinating the phosphorus-stress response. [ABSTRACT FROM AUTHOR]

Three types of observations were used to test the hypothesis that the response of stomatal conductance to a change in vapour pressure deficit is controlled by whole-leaf transpiration rate or by feedback from leaf water potential. Varying the leaf water potential of a measured leaf by controlling the transpiration rate of other leaves on the plant did not affect the response of stomatal conductance to vapour pressure deficit in Glycine max. In three species, stomatal sensitivity to vapour pressure deficit was eliminated when measurements were made at near-zero carbon dioxide concentrations, despite the much higher transpiration rates of leaves at low carbon dioxide. In Abutilon theophrasti, increasing vapour pressure deficit sometimes resulted in both decreased stomatal conductance and a lower transpiration rate even though the response of assimilation rate to the calculated substomatal carbon dioxide concentration indicated that there was no 'patchy' stomatal closure at high vapour pressure deficit in this case. These results are not consistent with stomatal closure at high vapour pressure deficit caused by increased whole-leaf transpiration rate or by lower leaf water potential. The lack of response of conductance to vapour pressure deficit in carbon dioxide-free air suggests that abscisic acid may mediate the response. [ABSTRACT FROM AUTHOR]

Cytosolic Ca2+ ([Ca2+]i) and elongation growth were measured in the roots of Arabidopsis thaliano. Exposure of plant tissues to high NaCI and abscisic acid (ABA) concentrations results in a reduction in the rate of growth but the mechanism by which growth is inhibited is not understood. Both NaCI and ABA treatments are known to influence [Ca2+, and in this study we measured the effects of salinity and ABA on [Ca2+]i in cells from the meristematic region of Arabidopsis roots. The Ca2+-sensitive dye Fura-2 and ratiometric techniques were used to measure in cells of the root meristem region. Resting [Ca2+]i was found to he between 100 and 200 μmol m-3 in roots of untreated plants. Resting [Ca2+] ichanged in response to changes in the [Ca2+] surrounding growing roots. An increase of external [Ca2+ increased [Ca2+] conversely, a decrease of external [Ca2+] decreased [Ca2+. Exposure of roots to NaCl caused a rapid reduction of [Ca2+]i, a response that was proportional to the external NaCI concentration. Thus, as the NaCl concentration was increased, [Ca2+]i in root meristematic cells decreased. Root elongation was also inhibited in proportion to the external NaCI concentration, with maximal inhibition occurring at 120 mol m-3 NaCI. The [Ca2+]i of root meristem cells also changed in response to ABA. and the magnitude of the effect of ABA was dependent upon ABA concentration. Treatment with 0.2 mmol m-3 ABA caused a momentary increase in [Ca2+]i followed by a decrease after 15 min, but 10 mmol m-3 ABA caused an immediate decline in [Ca2+]i. There was a strong positive correlation between [Ca2+]i and root elongation rates. Experiments with the ABA-deficient Arabidopsis mutant aba-3 indicated that the reduction in [Ca2+]i brought about by NaCI was unlikely to he mediated via changes in endogenous ABA. Experiments with solutes such as sorbitol, KCI and NaNO3 indicated that the effects of NaCl could be mimicked by other solutes and was not specific for NaCl. [ABSTRACT FROM AUTHOR]

Detached barley (Hordeum vulgare L) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub-crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non-significant when LER was modelled against the [ABA] of the elongation zone, [EZ-ABA]. When detached barley shoots were fed sp from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this `unexplained' growth inhibition. The detached shoot experiments indicated that [EZ- ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA. Using a coleoptile growth assay. we applied mild osmotic stresses (ψ=0 to -0.06 MPa) and 10-4 mol m-3 ABA. Individually, these treatments (lid not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants. [ABSTRACT FROM AUTHOR]