Cytokinin Activity in Water-stressed Shoots

Abstract

Water stress applied to the plant shoot or Dowex 50W x 4 50-100 mesh). The column was rinsed with 1 liter of 70%to (v/v) ethanol and with H20. The active substance was eluted with 1 liter of 4 N NH40H. The eluate was concentrated to 300 ml under vacuum at 45 C, and the pH was adjusted to 8.0 and extracted twice on a shaker for 20 hr with 200 ml of n-butanol. The extracts were then combined and dried under vacuum. The residue was dissolved in 5 ml of water at pH 8. This solution was divided into equal portions and bioassayed by means of soybean tissue culture (9).The incorporation of '4C-L-leucine into trichloroacetic acidinsoluble leaf disc extracts was measured according to BenZioni et al. (2). Kinetin treatment was applied prior to incubation in "4C-L-leucine, by placing leaf discs for 45 min between Whatman No. 3 papers wet with kinetin solution.Leaf discs were exposed to two extreme relative humidities by placing them for 30 min in desiccators which contained either wet filter paper or CaCl2. Fresh weight change was recorded.Experiments with labeled kinetin were done as follows. The petiole of a fully expanded leaf was placed in a vial containing 2 ml of 0.5 4c kinetin-8-14C solution (Cal Atomic, Los Angeles, specific radioactivity 24.6 Ac/Mmole). The 2 ml were taken up by the leaf in 60 to 120 min. The leaf was cut in half, and the middle rib was removed. One half was treated as indicated in each experiment, and the other was put in a humid chamber for the duration of the treatment. On termination of treatment the halfleaf was frozen in liquid air and homogenized with 2 ml of cold ethanol. The homogenate was filtered off, and the residue was washed with 2 ml of cold water. To the combined filtrate 1 ml of chloroform was added. The nonpolar fraction contained no label. The polar fraction was separated and applied to Whatman No. 3 chromatography paper. The chromatogram was developed for 30 cm in an ascending-descending fashion in one of the two solvent mixtures: H20-acetic acid-n-butanol (1:1:4) or ethanolchloroform (9:1). The chromatogram was then dried and cut into 10 equal portions which were placed into vials containing dioxane-ethanol-toluene scintillation solution. The vials were then placed into a liquid scintillation counter (Packard TriCarb 3308) and assayed for "4C. RESULTSPlants were exposed for 30 min to an airstream which caused slight wilting of the leaves. The airstream was turned off and the shoots of both the treated and the control plants were sprinkled with water. After 10 min, leaves recovered their turgor, all the plants were topped, and the mature leaves were frozen in liquid air. A rubber hose was mounted on the stump, and exudate was collected for 6 hr. The amount of exudate did not differ between treatments. The data in Table I demonstrate a reduction in the activity of cytokinin in the leaves and in exudate when the stress was applied to the intact plant. This experiment was repeated three times. The range of the reduction was 39 to 60% in exudate and 22 to 59% in leaves.

The Role of Cytokinins in Chloroplast Lamellar Development

“…These observations are in contrast to an earlier report (25) tion was eliminated. In light of these observations and previous studies of Itai and Vaadia (14,15) and Ben-Zioni et al (8) which provided evidence that exogenously applied cytokinin was capable of relieving the effects of small water deficits, it was thought that cytokinin treatment should relieve the effects of small water deficits on the greening process; namely, reduce or eliminate the lag phase in Chl accumulation. However, cytokinin was neither effective in reducing the 3-to 4-hr lag in Chl accumulation nor in eliminating the delay in lowering of the leaf Chl a/b ratio (Figs.…”

Hormonal Activity in Detached Lettuce Leaves as Affected by Leaf Water Content

“…Itai and Vaadia (18), who first reported on decreased cytokinin activity after water stress and its increase following recovery, suggested that these changes resulted from inactivation of the hormone and subsequent reversal of this inactivation. Our data support these findings without having any bearing on the detailed mechanism involved in the modifications in cytokinin activity induced by water stress.…”

“…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).…”

“…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).…”

Reduction of Turgor Induces Rapid Changes in Leaf Translatable RNA

“…Some of these plant responses are the direct result of physical changes accompanying loss of cell turgor (6) while other responses could be due to altered levels of plant growth regulators. Plants exposed to water-limited conditions have been shown to have elevated levels of ABA (25) and ethylene (2) and reduced amounts of cytokinin (16) and gibberellin (1).…”