Planta

PIanta (BerL) 129, 2 1 7 - 2 1 9 (1976)

9 by Springer-Verlag 1976

Abscisic Acid and the After-Effect of Stress in Tobacco Plants* Samy Boussiba and Amos E. Richmond Department of Biology and The Desert Research Institute, P.O. Box 2053, Ben-Gurion University of the Negev Beer-Sheva, Israel

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.

Introduction When water stressed plants are re-watered and the cell water content and turgor are restored to pre-stress levels, certain symptoms which pertain to the adaptive response of the plant to shortage of water temporarily persist. One of the known after-effects of water stress or wilting is the failure of the stomatal apertures to return to normal opening soon after the water deficit in the plants is restored (Schneider and Childers, 1941; Iljin, 1957; Glover, 1959; Heath and Mansfield, 1962; Stklfelt, 1963; Fischer et al., 1970). Allaway and Mansfield (1969) were the first to suggest the possibility of a causal relationship between the decrease in width of stomatal aperture after wilting and certain hormonal factors such as ABA or cytokinins, a thesis which was also discussed by Livne and Vaadia (1972). Indeed, hormone relations are greatly *

Abbreviations: ABA = abscisic acid; WSD

ration deficit

= water satu-

altered in plants exposed to water deficits (Wright, 1969; Wright and Hiron, 1969; Mizrahi et al., 1970; Hiron and Wright, 1973; Itai and Vaadia, 1965; Itai etal., 1968). While this work was in progress DSrffling et aI. (1974) reported that in plants recovering from water stress, there appeared to be no correlation between the decrease in the content of stressinduced ABA and the rapid rise of turgor associated with the release from this stress. However, since a correlation did seem to exist between the ABA decrease and the increase in stomatal aperture width, they suggested that the after-effect of wilting may be causally related to ABA-inactivation or metabolism. While stomatal closure is a common response of the plant to water deficit, it is not induced by water shortage alone. Considerable evidence from our laboratory (Boussiba et al., 1975; Rikin and Richmond, unpublished data) indicates that transpiration rate decreases significantly in leaves of plants exposed to stresses that do not impair the plant's water balance, e.g. mineral deprivation, BO3 3 toxicity or low temperature. This work supplies evidence of (a) the existence of an after-effect on stomatal opening following a period of mineral deprivation and salination of the root medium; (b) the existence of a close relationship between the content of leaf-ABA and the extent of decrease in stomatal aperture during the two stresses and after their termination.

Materials and Methods Plant Material. Forty five-day old Nicoliana rustica L, plants were used for the experiments. They were grown in a greenhouse under natural conditions with about a 12 h day light period, with daily temperatures fluctuating between 15 and 27 ~ C. Each plant was grown in a 3.3 1 container of half-strength aerated Hoagland solution. The first fully expanded leaves (5th-6th from the cotyledons) were used in all experiments. Samples were taken at 10-11 a.m.

218

S. Boussiba and A.E. Richmond: ABA and After-Effect of Stress

Stress Treatments. (a) Mineral deprivation was imposed by transferring the plants to containers of aerated distilled water for 7 days. Detailed measurements showed that this treatments did not effect a significant difference in leaf-water saturation deficit (leaf-WSD). (b) Salination was imposed by addition of 6 g NaC1/1 (4.5 atm) to the root medium for 4 days, causing a significant increase in leaf-WSD.

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each of the stresses back to aerated half-Hoagland solution.

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Stornatal Aperture. Stomatal aperture was determined microscopically in silicone rubber imprints (Sampson, 1961) of the lower epidermis of two regions in three leaves for each treatment. Twenty stomata were measured randomly in each imprint, bringing the total to 120 stomata.

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The experiments were designed to test the relationship between leaf-ABA content and the extent of opening of the stomata, as measured by the stomatal apertures in the course of stress and during recovery. Two different types of stress were chosen (i) mineral deprivation, which causes a decrease in stomatal aperture and is not associated with a significant change in the plant's water content; (ii) salination of the root medium, known to effect an increase in leaf-WSD and a decrease in the extent of stomatal opening. The rate of recovery from these two stresses, as indicated by many parameters, (unpublished data) differs greatly; plants recovered relatively fast (within two days) from exposure to salination, and slowly (within 4 days) from exposure to mineral deprivation. Thus, the relationship between leaf-ABA content and the degree of opening of the stomata was measured under four different conditions, i.e., in the course of two types of stress and during the periods of recovery from them. Results shown in Figure 1a indicate that, in general, there exists a very close relationship between leaf-ABA content and the extent of stomatal opening in plants exposed to a cycle of mineral deprivation and recovery. This relationship is evident throughout the whole cycle, from the beginning of the stress to its termination, when the after-effect on stomatal opening completely disappears. Thus, the continuous increase in leaf-ABA content during the stress period was accompanied by a continuous decrease in stomatal aperture. This pattern was reversed during the recovery period in which the decrease i n leaf-ABA content was

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Results and Discussion

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Le@ABA Content. Extraction and determination of ABA were done as described by Mizrahi and Richmond (1971) using gasliquid chromatography equipped with flame ionization detector.

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