Cytokinin and inhibitor activities in the avocado fruit mesocarp

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.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.

Endogenous cytokinins in maturing manzanillo olive fruits

Cytokinin activity was found in mature black olive fruits while in green fruits the activity was markedly lower. The substances with cytokinin activity showed chromatographic properties similar to those of nucleotides. Acid hydrolysis caused a shift of the active zone in the organic phase of 1-butanol-acetic acid-water (4:1:5) from R(F) 0.1 to R(F) 0.8. The possible relation of these endogenously appearing cytokinins to anthocyanin accumulation and aging of the fruit is discussed.

The Effect of Indole-3-acetic Acid and Other Growth Regulators on the Ripening of Avocado Fruits

Observations were made of the effects of several plant regulators, indole-3-acetic acid, kinetin, abscisic acid, and gibberellic acid, as well as of extracts prepared from leaves and fruit stalks on the respiration pattern, ethylene production, and the number of days to ripen of avocado fruits (Persea americana Mill.). These substances were vacuum infiltrated to insure good penetration and distribution. Kinetin, abscisic acid, gibberellic acid, and the extracts had no effect on either ripening time or on the respiration pattern and ethylene production of the fruits. Indoleacetic acid, however, had a marked effect on ripening. At high concentrations (100 and 1000 mum), indoleacetic acid stimulated respiration and induced preclimacteric ethylene production, resulting in accelerated ripening of the fruits. At the low concentrations (1 and 10 mum), it delayed ripening of fruits and suppressed the climacteric respiration and ethylene production. The results reinforce several previous observations with other fruits that auxins may largely constitute ;resistance to ripening' and may be responsible for the lack of ripening shown by unpicked fruits.

Abscisic Acid and cytokinin contents of leaves in relation to salinity and relative humidity

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. The amount of abscisic acid in the leaves of salinized plants rose markedly in low, but not in high, relative humidity. No change in the amount of extractable cytokinins was detected in any treatment. It is tentatively suggested that variations in the water content of leaves constitute a primary signal for modification of plant hormonal balance.

Cytokinin Activity in Avocado Seeds during Fruit Development

The soybean callus bioassay was used to determine levels of cytokinin activity in avocado (Persea americana) seeds.In the embryo, levels are high during the early stages of development, but diminish as the fruit grows. The level of cytokinin activity in the endosperm is very high throughout the period that this tissue exists. The seed coats have very high activity levels while the fruit is young, reaching values comparable with those found in the endosperm. The activity level falls as the rate of fruit growth slows down and disappears completely by the time the seed coats shrivel at approximately the same time the fruit reaches "horticultural maturity".