The adaptive response of plants to a decrease in osmotic potential of the root medium involves adjustments that facilitate the maintenance of their water balance. Thus Bernstein (2,3) showed that turgor pressure may be maintained under salinization through a decrease in the osmotic potential of the whole plant. However, actual turgor may also be maintained by a decrease in transpirational water loss. Indeed, the turgor of salinized cotton plants was even higher than the controls according to Gale et al. (4) Steveninck (10) reported that ABA3 affected closure of stomata and reduced transpiration in excised first leaves of wheat and barley. Earlier, Tal (18) had described a wilting tomato mutant in which transpiration was excessive owing to the inability of the stomata to close. Imber and Tal ("Phenotypic reversion of flacca, a wilting mutant of tomato, by abscisic acid." Science. In press.) recently discovered that when ABA was sprayed on these mutants, the stomata would close as in normal plants; transpiration was reduced and the wilting mutant resumed a normal appearance. There is thus good evidence for the involvement of both cytokinins and ABA in regulating stomatal aperture and transpiration.This study is part of a broader project to test the hypothesis that ABA plays a controlling role in the adaptation of plants to decreased osmotic potential in the root medium. This particular investigation queries whether ABA could be involved in the reduction of transpiration that generally follows a decrease in the osmotic potential of the root medium.Sixty-day-old tobacco plants (Nicotiana tabacum) grown in a (Fig. 1). Treatment with kinetin and ABA increased and decreased, respectively, the rate of transpiration of leaves from both stressed and nonstressed plants. In leaves of stressed plants, treatment with kinetin increased the rate of transpiration to that of the nonstressed, water-sprayed plants. In leaves of the nonstressed plants, ABA reduced the level of transpiration to that of the stressed, watersprayed plants. The same patterns were obtained whether the root stress was imposed with NaCl or with mannitol. These patterns proved reproducible in several repetitions of the experiment. The transpiration rate exhibited by leaves sprayed with both ABA and kinetin lies between those rates obtained by application of each of the two hormones alone (Table I). These experiments were run three times, with two replications each, and the same patterns of transpiration were observed each time. Thus in our system, one hormone seemed to negate the effect of the other hormone, as observed for other physiological phenomena (7,13,14,17). This is reminiscent of other cases where the ratio of hormones plays a regulatory role in the control of a physiological process (12,15). Possible differences in water content in the various test plants were evaluated by determining the relative water content of the leaf tissue (16), with the use of leaf discs measuring 23 mm in diameter. Although initially, 4 hr after the application of the ...
The question is raised whether the hormonal modifications in a plant exposed to osmotic root stress result directly from the decrease in water potential of the root environment or from disturbances of the plant's water balance.Tobacco plants were held for 24 hours under either high or low relative humidities, with or without salt.
Preclimacteric avocado (Persea americana Mill.) fruits produced very little ethylene and had only a trace amount of l-aminocyclopropane-lcarboxylic acid (ACC) and a very low activity of ACC synthase. In contrast, a significant amount of l-(malonylamino)cyclopropane-l-carboxylic acid (MACC) was detected during the preclimacteric stage. In harvested fruits, both ACC synthase activity and the level of ACC increased markedly during the climacteric rise reaching a peak shortly before the climacteric peak. The level of MACC also increased at the climacteric stage. Cycloheximide and cordycepin inhibited the synthesis of ACC synthase in discs excised from preclimacteric fruits. A low but measurable ethylene forming enzyme (EFE) activity was detected during the preclimacteric stage. During ripening, EFE activity increased only at the beginning of the climacteric rise. ACC synthase and EFE activities and the ACC level declined rapidly after the climacteric peak. Application of ACC to attached or detached fruits resulted in increased ethylene production and ripening of the fruits. Exogenous ethylene stimulated EFE activity in intact fruits prior to the increase in ethylene production. The data suggest that conversion of S-adenosylmethionine to ACC is the major factor limiting ethylene production during the preclimacteric stage. ACC synthase is first synthesized during ripening and this leads to the production of ethylene which in turn induces an additional increase in ACC synthase activity. Only when ethylene reaches a certain level does it induce increased EFE activity.
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 caflus 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 defcit.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.Application of abscisic acid to the leaves caused partial stomatal closure in turgid lettuce leaves, whereas treatment with gibberellic acid and kinetin of such leaves had no effect on the stomatal aperture. In desiccating leaves, however, gibberellic acid and kinetin treatment considerably retarded stomatal closure, thus enhancing the increase in leaf water saturation deficit. These results suggest that the effect of desiccation in changing leaf hormonal make-up, i.e. a rapid increase in abscisic acid and a decrease in both cytokinin and gibberellin activity, is related to a mechanism designed to curtail water loss under conditions inducing water deficiency.In leaves, water stress causes a rapid increase in ABA content (15, 31) and a sharp decrease in cytokinin activity (18). ABA is known to induce stomatal closure with resulting decreased transpiration in many types of leaves (9,16,19,23,27,28). Furthermore, a very close relationship between leaf ABA content and the extent of stomatal opening was found in leaves of plants exposed to a cycle of mineral deprivation or salination and subsequent recovery (7). Kinetin was reported to enhance transpiration in leaves (22,24,25,27,29), and partially to overcome the ABA effect of reducing transpiration in both attached (28) and detached (27) leaves. It was thus suggested that these hormonal changes were probably conducive to the maintenance of a balanced water economy in the intact plant, effecting increased water intake by the root system (12) and reducing water loss from transpiration in the leaves (15,23,28).There is as yet no firm evidence for the involvement of other hormones in the course of plant adaptation to water stress (17). Treatment with GA3 increased transpiration in excised barley leaves (22) but the hormone was ineffective when applied to excised oat leaves (24). Reid et al. (30) reported a marked reduction in the movement of GAs from the root to the shoot in flooded tomato roots where water stress occur...
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