Effects of Kinetin on Transpiration Rate and Abscisic Acid Content of Water Stressed Young Wheat Plants

Effects of kinetin on transpiration rate and abscisic acid content were determined. Leaves from 9-day-old wheat plants (Triticum aestivum L. cv. Weibull's Starke II) were used. —Transpiration rate decreased in excised leaves put in water, but it was maintained at a higher rate when kinetin was supplied. When excised leaves were water stressed by air-drying for 1 h, addition of kinetin resulted in a considerable stimulation of transpiration rate. The effect reached its maximum after 15 h and this level remained relatively unchanged for at least 10 h. Intact seedlings which were stressed before leaf excision, showed only a slight stimulation of kinetin on transpiration rate. — Abscisic acid content slowly increased up to three-fold in 2 days in excised leaves put in water. In excised and water-stressed leaves the abscisic acid content was reduced during the first 24 h and then increased. As the leaves were fully turgid, the increase could not have been caused by water stress. However, both in stressed and unstressed leaves kinetin addition reduced the increase in abscisic acid content. — It is suggested that the stimulation by kinetin on transpiration rate in excised and water stressed leaves was mainly due to the combined effect of (1) a reduction in the activity of endogenous cytokinins, (2) kinetin acting as a ‘substitute’ for the inactivated cytokinins but exerting a stronger effect on transpiration than the endogenous cytokinins, and (3) the ‘extra’ reduction in abscisic acid content caused by the kinetin treatment. Furthermore, the results indicate that changes in cytokinins might be partly responsible for the aftereffect on transpiration.     

An abscisic acid-related reduced transpiration promotes gradual embolism repair when grapevines are rehydrated after drought

* Proposed mechanisms of embolism recovery are controversial for plants that are transpiring while undergoing cycles of dehydration and rehydration. * Here, water stress was imposed on grapevines (Vitis vinifera), and the course of embolism recovery, leaf water potential (Psi(leaf)), transpiration (E) and abscisic acid (ABA) concentration followed during the rehydration process. * As expected, Psi(leaf) and E decreased upon water stress, whereas xylem embolism and leaf ABA concentration increased. Upon rehydration, Psi(leaf) recovered in 5 h, whereas E fully recovered only after an additional 48 h. The ABA content of recovering leaves was higher than in droughted controls, both on the day of rewatering and the day after, suggesting that ABA accumulated in roots during drought was delivered to the rehydrated leaves. In recovering plants, xylem embolism in petioles, shoots, and roots decreased during the 24 h following rehydration. * A model is proposed to describe plant recovery after rehydration based on three main points: embolism repair occurs progressively in shoots and further in roots and in petioles, following an almost full recovery of Psi(leaf); hydraulic conductance recovers during diurnal transpiring hours, when formation and repair of embolisms occurs in all plant organs; an ABA residual signal in rehydrated leaves hinders stomatal opening even when water relations have recovered, suggesting that an ABA-induced transpiration control promotes gradual embolism repair in rehydrated grapevines.     

The After-Effect of Water Stress on Chlorophyll Formation during Greening and the Levels of Abscisic Acid and Proline in Dark Grown Wheat Seedlings

Cut seedlings of wheat plants (Triticum aestivum L. cv. Starke II Weibull) between 6 and 7 days old were water stressed in darkness by exposing them to air of 35% relative humidity 2.5 to 20 h. This treatment resulted in a water potential of -11 bars in the leaves after 20 h. The leaves were then rewatered and irradiated. The chlorophyll formation that took place in fully turgid leaves during the greening was markedly decreased in the case of the water-stress pretreatmet. and especially the lag phase was prolonged. The longer the stress pretreatment the more evident was the subsequent effect on chlorophyll formation. However, no linear relationship was found between the amount of stress and the chlorophyll content. Protochlorophyllide regeneration from endogenously formed δ-aminolevulinic acid was markedly decreased even after the shortest water-stress period. However, protochlorophyllide accumulation from exogenously supplied δ-aminolevulinic acid was only slightly decreased following the water-stress pretreatment. Further more, the ratio of protochlorophyllide₆₅₀ to protochlorophyllide₆₂₈ was slightly reduced by the same conditions. During the stress period both abscisic acid and proline were accumulated in the leaves. The content of abscisic acid increased up to six times the normal level during water stress lasting for 20 h. The increase of proline was about three-fold for similar treatment. After rewatering the leaves the levels of both abscisic acid and proline rapidly declined and reached. 10 h later, the levels found in unstressed seedlings. The increase in abscisic acid during water stress associated with impaired chlorophyll metabolism suggested that the after-effect of water stress might be linked to chlorophyll metabolism through abscisic acid or some of its metabolites. The changes in proline content open the possibility that this substance could function as a reserve substance for the formation of chlorophyll after the discon tinuation of the stress.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Stomatal behaviour and leaf water potential of chilled and water-stressed Solanum melongena, as influenced by growth history

Abstract Leaf diffusion resistance and leaf water potential of intact Solanum melongena plants were measured during a period of chilling at 6 °C. Two pretreatments, consisting of a period of water stress or a foliar spraying of abscisic acid (ABA), were imposed upon the plants prior to chilling. The control plants did not receive a pretreatment. In addition to intact plant studies, stomatal responses to water loss and exogenous abscisic acid were investigated using excised leaves, and the influence of the pretreatment observed. Chilled, control plants wilted slowly and maintained open stomata despite a decline in leaf water potential to –2.2 MPa after 2 d of chilling. In contrast plants that had been water stressed or had been sprayed with abscisic acid, prior to chilling, did not wilt and maintained a higher leaf water potential and a greater leaf diffusion resistance. In plants that had not received a pretreatment, abscisic acid caused stomatal closure at 35 °C, but at 6°C it did not influence stomatal aperture. The two pretreatments greatly increased stomatal sensitivity to both exogenous ABA and water stress, at both temperatures. Stomatal response to water loss from excised leaves was greatly reduced at 6°C. These results are discussed in relation to low temperature effects on stomata and the influence of preconditioning upon plant water relations.     

Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m⁻²·s⁻¹ photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.     

Hormonal activity in detached lettuce leaves as affected by leaf water content

The interrelationship between water deficiency and hormonal makeup in plants was investigated in detached leaves of romaine lettuce (Lactuca sativa L. cv. `Hazera Yellow'). Water stress was imposed by desiccating the leaves for several hours in light or darkness at different air temperatures and relative humidity. In the course of desiccation, a rise in abscisic acid content and a decline in gibberellin and cytokinin activity were observed by gas-liquid chromatography, by both the barley endosperm bioassay and radioimmunoassay and by the soybean callus bioassay. Gibberellin activity began to decline in the stressed leaves before the rise in abscisic acid, the rate of this decline being positively correlated with the rate of increase in leaf water saturation deficit. Recovery from water stress was effected by immersing the leaf petioles in water while exposing the blades to high relative humidity. This resulted in a decrease in leaf water saturation deficit, a reduction in abscisic acid content, and an increase in gibberellin and cytokinin activity.     

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3C) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.Abbreviations ABA = abscisic acid - ABAGE = abscisic acid-glucose ester - DW = dry weight - FW = fresh weight - RIA = radioimmunoassay - RWC = relative water content - w = water potential - o = osmotic potential - p = turgor potential     

Effects of Water Stress and Abscisic Acid on Transpiration Regulation in Wheat

The transpiration response to recurrent light periods was studied'n water-stressed wheat seedlings. Seedlings were stressed by three methods: addition of mannitol to the root medium, root cooling and drving of the roots in air. All three methods induced almost equal effects on transpiration regulation during alternating dark and light intervals. Exogenous abscisic acid supplied to the shoots of excised plants had qualitatively the same effect as water stress. Water stress and ABA increased the time lapse between light-on and the onset of transpiration increase and lowered the amplitude of transpiration increase in light. Weak light introduced before strong light shortened the delay times.     

Photosynthetic responses and proline content of mature and young leaves of sunflower plants under water deficit

In mature and young leaves of sunflower (Helianthus annuus L. cv. Catissol-01) plants grown in the greenhouse, photosynthetic rate, stomatal conductance, and transpiration rate declined during water stress independently of leaf age and recovered after 24-h rehydration. The intercellular CO₂ concentration, chlorophyll (Chl) content, and photochemical activity were not affected by water stress. However, non-photochemical quenching increased in mature stressed leaves. Rehydration recovered the levels of non-photochemical quenching and increased the F_v/F_m in young leaves. Drought did not alter the total Chl content. However, the accumulation of proline under drought was dependent on leaf age: higher content of proline was found in young leaves. After 24 h of rehydration the content of proline returned to the same contents as in control plants.     

Interdependence of growth, water relations and abscisic acid level in Phaseolus vulgaris during waterlogging

The interdependence between changes in growth and water relations after waterlogging was investigated by recording simultaneously growth, transpiration, water potential, turgor, leaf diffusion resistance and abscisic acid content in Phaseolus vulgaris L. cv. bruine Noord-Hollandse. Growth was inhibited immediately after flooding, whereas transpiration decreased gradually to a low level in about three days. The first two days after flooding a small increase in abscisic acid content in the leaves was observed which was accompanied by an increase in diffusion resistance. The increase in abscisic acid content could result from an inhibited export from the leaves. After the first two days a decrease in water potential and turgor was accompanied by a drastic increase in both abscisic acid content and diffusion resistance. This large increase in abscisic acid content occurred before the turgor had reached its minimum value. The change in diffusion resistance kept showing a lag of about one day with the change in abscisic acid content. The possibility is discussed that besides abscisic acid also its metabolite phaseic acid is involved in stomatal closure. After the formation of adventitious roots on the hypocotyl, abscisic acid level, diffusion resistance, water potential and turgor returned to the control values. Transpiration showed a slow recovery from the sixth day after flooding, whereas growth was inhibited for at least nine days. A remarkable similarity exists between our observations on the responses of bean plants to flooding and the well known responses to drought.     

Metabolism of Abscisic Acid and Its Regulation in Xanthium Leaves during and after Water Stress

Metabolism of abscisic acid was compared in stressed and in rehydrated leaf blades of Xanthium strumarium L. Chicago strain that were either detached or left intact on the plant. Under all conditions, phaseic acid was the major metabolite. The high level of phaseic acid that was observed in intact plants 1 day after recovery from stress declined slowly and had not yet reached the prestress level 1 week later. The glucosyl ester of abscisic acid, β-d-glucopyranosyl abscisate, accumulated at a low rate during periods of prolonged stress. Repeated stress-recovery cycles resulted in a gradual increase in the level of the glucosyl ester, which did not decline following relief of stress for at least 34 days. The level of the glucosyl ester of abscisic acid may therefore serve as a cumulative indicator of the water stresses to which a particular leaf has been exposed.     

Effects of light, temperature and nitrogen treatments upon the fatty acid composition of galactolipids of young and older leaves from winter rape plants

40 aromatic and chlorocyclohexenic structural analogues of abscisic acid were synthesized stereospecifically, and inhibition of transpiration was investigated following two experimental procedures (cut barley leaves and water stressed cotton plants). Structure-activity relationships are discussed. – Two chlorosubstituted cyclohexenic compounds are the most active; their inhibition of transpiration can be compared to that of abscisic acid.     

Effects of water stress on cellular ultrastructure and on concentrations of endogenous abscisic acid and indole-3-acetic acid in Fatsia japonica leaves

Ultrastructural alterations in mesophyll cells as well as variations in bulk leaf endogenous ABA and IAA concentrations were studied in water-stressed field-grown plants of Fatsia japonica. Under water deficit cellular membranes were modified and an increase in vesicles was observed. The main damage to the chloroplasts included thylakoid swelling and disruption of the chloroplast envelope. Concomitant variations in abscisic acid and indole-3-acetic acid were observed. Despite the expected increased in endogenous ABA concentration in relation to water stress, after the highest concentration of ABA, observed at predawn in severely stressed plants (29-1), there was a sharp decline from 2768 pmol g fw^–1 to 145 pmol g fw^–1; thus in severely stressed plants ABA levels were not related to changes in bulk leaf ABA contents. Water stress did not influence the concentrations of indole-3-acetic acid, although the increase in the endogenous abscisic acid concentration could be related with the ultrastructural changes.Abbreviations ABA abscisic acid - IAA indole-3-acetic acid - leaf water potential     

Water Stress in Plants

The abscisic acid (ABA) content was determined quantitatively in the leaves from wilted and unwilted tomato plants (Lycopersicon esculentum Mill. CV. Revermun) by the use of the wheat coleoptile test and gas-liquid chromatography (GLC). Plants which have received an insufficient daily water supply for 18 days showed adaptation to wilting conditions. The plants adjust to the added amount of water by regulating their water loss through transpiration. The concentration of ABA was not higher in the leaves of plants adapted to water stress than in plants that were watered abundantly. Wilted detached leaves and leaves from rapidly wilted intact plants showed the well-known reaction by increasing the ABA level. A possible role of ABA in the early stages of the adaptation process is discussed.     

Effects of Water Stress on Cuticular Transpiration Rate and Amount and Composition of Epicuticular Wax in Seedlings of Six Oat Varieties

Six varieties of oat (Avena sativa L. cv. Stormogul II, Risto, Sol II, Selma, Sang and Pendek, arranged according to decreasing drought resistance) were cultivated under controlled conditions and exposed to water stress on 4 consecutive days. Seven-day-old seedlings were stressed by cooling the roots for 3 h to 1.0°C. During this treatment the leaf water potential decreased from -7 to -12 bars. Cuticular transpiration rate, total amount of epicuticular wax and amounts of some wax components (primary alcohols, alkanes, fatty acids) were determined. Unstressed seedlings of the most drought resistant variety (Stormogul II) showed the highest cuticular transpiration rate. After stress treatment the cuticular transpiration rate was most strongly reduced in this variety and at the same time it showed the largest increase in amount of epicuticular wax of the tested varieties. In Pendek and Sang, showing the least increase in epicuticular wax, the cuticular transpiration rate was only 5% lower after stress treatment. In all varieties the primary alcohol content of the epicuticular wax was slightly higher in stressed seedlings than in controls. Further, in Stormogul and Risto the content of the predominant alkanes was much lower in stressed seedlings than in controls. On the contrary, in Pendek the stressed seedlings showed a higher alkane content. In Stormogul II, Risto and Sol II the total amount of fatty acids was higher in stressed seedlings than in controls while the opposite was true in Sang. The relation between the epicuticular wax (amount and composition) and the cuticular transpiration rate is discussed as well as the possibility of using the tested parameters in a screening test for drought resistance.     

Abscisic acid and the after-effect of stress in tobacco plants

Summary Tobacco plants (Nicotiana rustica L.) were exposed to a period of stress of either mineral deprivation or salination of the root medium. Thereafter the plants were transferred back to the pre-stress growth medium, for study of the pattern of recovery. Abscisic acid (ABA) content and the extent of stomatal opening in leaves of tobacco plants were found to be inversely related. The results support the possibility that the phenomenon know as after-effect of stress may not be exclusive to recovery from water stress, but may be typical of the pattern of plant recovery from the effects of several growth restricting environments. It is suggested that the after-affect results from the delay in resumption of the pre-stress hormonal balance in the plant, particularly with regard to ABA, after termination of the stress.Abbreviations ABA abscisic acid - WSD water saturation deficit     

Gas exchange is related to the hormone balance in mycorrhizal or nitrogen-fixing alfalfa subjected to drought

The beneficial effect of mycorrhization on photosynthetic gas exchange of host plants under drought conditions could be related to factors other than changes in phosphorus nutrition and water uptake. Our objective was to study the influence of drought on phytohormones and gas exchange parameters in Medicago sativa L. cv. Aragón associated with or in the absence of arbuscular mycorrhizal (AM) fungi and/or nitrogen-fixing bacteria. Four treatments were used: (1) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR); (2) plants inoculated with only Rhizobium (R); (3) plants inoculated with only mycorrhizae (M); and (4) non-inoculated plants (N). When endophytes were well established, treatments received different levels of phosphorus and nitrogen in the nutrient solution in order to obtain plants similar in size. Sixty days after planting, plants were subjected to two cycles of drought and recovery. Midday leaf water potential (Ψ), CO₂ exchange rate (CER), leaf conductance (g_w) and transpiration (T), as well as leaf and root abscisic acid (ABA) and cytokinin concentrations were measured after the second drought period. Gas exchange parameters were determined by infrared gas analysis. Cytokinins and ABA levels in tissues were analysed by ELISA and HPLC, respectively. Nodulated R and MR plants had the lowest ABA concentrations in roots under well-watered conditions. Water stress increased ABA concentrations in leaves of N, R and MR plants, while ABA concentration in M plants did not change. The highest production of ABA under water deficit was in the roots of non-mycorrhizal plants. The ratio of ABA to cytokinin concentration strongly increased in leaves and roots of non-mycorrhizal plants under drought. By contrast, this ratio was lowered in roots of M plants and remained unchanged in leaves and roots of MR plants when stress was imposed. The highest leaf conductances and transpirational fluxes under well-watered conditions were those of nitrogen-fixing R and MR plants, but these results were not impaired with increased CO₂ exchange rates. Photosynthesis, leaf conductance and transpiration rates decreased in all treatments when stress was imposed, with the strongest decrease occurring in non-mycorrhizal plants. The relationships found between these gas exchange parameters and the hormone concentrations in stressed alfalfa tissues suggest that microsymbionts have an important role in the control of gas exchange of the host plant through hormone production in roots and the ABA/cytokinin balance in leaves. The most relevant effect of mycorrhizal fungi was observed under drought conditions.     

Leaf Age as a Determinant in Stomatal Control of Water Loss from Cotton during Water Stress

The stomatal resistance of individual leaves of young cotton plants (Gossypium hirsutum L. var. Stoneville 213) was measured during a period of soil moisture stress under conditions of constant evaporative demand. When plants were subjected to increasing soil water stress, increases in stomatal resistance occurred first on the lower leaves and the stomata on the upper surfaces were the most sensitive to decreasing leaf-water potential. Stomatal closure proceeded from the oldest leaves to the youngest as the stress became more severe. This apparent effect of leaf age was not due to radiation differences during the stress period. Radiation adjustments on individual leaves during their development altered the stomatal closure potential for all leaves, but did not change the within-plant pattern. Our data indicate that no single value of leaf water potential will adequately represent a threshold for stomatal closure in cotton. Rather, the stomatal resistance of each leaf is uniquely related to its own water potential as modified by age and radiation regime during development. The effect of age on stress-induced stomatal closure was not associated with a loss of potassium from older leaves. Increases in both the free and bound forms of abscisic acid were observed in water-stressed plants, but the largest accumulations occurred in the youngest leaves. Thus, the pattern of abscisic acid accumulation in response to water stress did not parallel the pattern of stomatal closure induced by water stress.