The flux of radioactivity from 3,4-114Clmethionine into S-adenosyl_L-methionine (SAM), 1-aminocyclopropane-l-carboxylic acid (ACC), sperm- (11), and by wounded tissue of citrus fruit (12, 29) has been previously reported. The induction of ethylene formation by ABA and the inhibitory effect of AVG on its production in citrus bud culture and leaf explants were also studied (10, 21). In the present investigation, we have used orange peel tissue to study relative incorporation of radioactivity from 3,4-[14CJmethionine into SAM, ACC, spermine, and spermidine while inhibiting biosynthesis of ethylene from ACC by cobalt and phosphate ions. By specifically inhibiting the terminal step of ethylene biosynthesis (Fig. 1)

Free and bound abscisic acid and neutral growth inhibitors were detected in citrus fruit peel using two bioassays and gasliquid chromatography. None of the inhibitor components seems to be directly associated with chloroplasts or chromoplasts in citrus fruit peel.Green, nonsenescent fruits contain mostly neutral inhibitors and relatively low amounts of free and bound abscisic acid. Upon harvest and storage in ethylene, 50 microliters per liter, both free and bound abscisic acid accumulate rapidly, attaining within 24 hours a level of 1 microgram per gram. After 48 hours bound abscisic acid reaches a much higher level than free abscisic acid. Fruits allowed to senesce on the tree follow a similar course of abscisic acid accumulation, attaiuing finally a 10: 1 ratio of bound to free abscisic acid.

No abstract

The role of abscisic acid in the regulation of senescence was investigated in detached tobacco leaves (Nicotiana rustica L.). Leaves senesced in darkness showed a sharp rise in abscisic acid level in the early stage of aging, followed by a rapid decline later. The same trend was found when leaves were aged in light, but the rise in abscisic acid occurred four days later than in darkness. Senescence was slower in light than in darkness, while salt stress accelerated the processes. Leaves treated with kinetin which senesced in light and darkness, did not show an increase in abscisic acid. Application of kinetin led to a transformation from free to bound ABA. These results may indicate that ABA and cytokinin are involved in a trigger mechanism which regulates senescence; the stage at which this trigger is activated determines the rate of senescence.

The objective of the present work was to describe the simultaneous changes in endogenous levels of cytokinins, abscisic acid, indoleacetic acid and ethylene in detached, senescing tobacco (Nicotiana rustica L.) leaves. These measurements were related to changes in chlorophyll contents, '^COj fixation and proline contents -three parameters which have been considered to reflect senescence. Effects of exogenous hormonal treatments on these parameters, as well as on endogenous hormonal levels, provided further evidence for the interrelationships between hormones and for their roles in senescence.Starting with actively growing attached leaves and ending with well-advanced senescence in detached leaves, our data indicate a chronological sequence of three hormonal states: (a) cytokininshigh activity, abscisic acid, auxin and ethylene -low contents (actively growing, attached leaves); (b) cytokinins -low activity, abscisic acid -high, auxin and ethylene -low contents (apparent induction of senescence in detached leaves); and (c) cytokinins and abscisic acid -low, auxin and ethylene -high contents (senescence proper in detached leaves).

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The effects of ambient temperatures between 15° and 43°C were determined on net photosynthesis of ‘French’ prune (Prunus domestica L. cv. Agen) trees maintained under a non-limiting soil water supply. The temperature optimum for photosynthesis was about 30° and net CO2 assimilation decreased rapidly above 35° even when water vapor pressure differences (VPD) were only 5 to 10 mb. Leaf resistance (r1) remained very low (2 to 3 sec cm-1) although leaf temperature reached 47° and the leaf water potential (ψ) decreased to -25 bars. Thus, non-stomatal photosynthetic inhibition was responsible for the 80% decline in net CO2 assimilation. Canopy wetting prevented the decline in net CO2 assimilation by reducing leaf temperature 8° (vs. control) and maintaining the ψ 14 bars higher than the non-misted control.

No abstract