The effect of abscisic acid (ABA) on uptake of potassium ((86)Bb(+) or (42)K(+)) by Avena sativa L. coleoptile sections was investigated. ABA lowered the potassium uptake rate within 30 min after its application and inhibition reached a maximum (ca. 75%) after 2 h. The inhibition of K(+) uptake increased with ABA concentration over a range of 0.03 to 10 μg/ml ABA. At a higher K(+) concentration (20 mM) the percentage inhibition decreased. The percentage inhibition of K(+) uptake by ABA remained constant with external K(+) varied from 0.04 to 1.0 mM. After a loading period in 20 mM K(+) ((86)Rb(+)), apparent efflux of potassium was only slightly increased by ABA. Experiments in which growth was greatly reduced by mannitol or by omission of indole-3-acetic acid from the medium indicated there was no simple quantitative correspondence between ABA inhibition of coleoptile elongation and ABA inhibition of K(+) uptake. Chloride uptake was also inhibited by ABA but to a smaller degree than was K(+) uptake. No specificity for counterions was observed for K(+) uptake. Uptake of 3,0-methylglucose and proline were inhibited by ABA to a much smaller extent (14 and 11%) than that of K(+), a result which suggests that ABA acts on specific ion uptake mechanisms.

No abstract

Early events in the evocation of the flower in Pharbitis nil Chois seedlings were investigated by following the incorporation of tritiated uridine into the shoot apex. The uridine was applied to the expanded cotyledons of seedlings at 8 hr into the inductive dark period. The shoot tips were fixed at 20 hr (a 12-hr labeling period). After the 12-hr labeling period there was considerable label throughout the shoot tips of both control plants (dark period interrupted with 5 min of red light at 8 hr) and plants induced to flower. Both RNase and acid hydrolysis removed the nonexchangeable label and the Azure B staining, thus leading to the conclusion that the uridine was incorporated into RNA. Induction in the cotyledons was followed quickly by an increased synthesis of RNA in the rib meristem region of the receptor bud at the time when the floral stimulus is assumed to be arriving. The increase in RNA synthesis is revealed by an increase in the rib meristem/central zone ratio of counts due to the incorporation of tritiated uridine. A comparison of counts in each of the two regions revealed that the change in ratio was due to an increase in the rib meristem and not due to a decrease in the central zone in induced shoot apices. The initial activation of the rib meristem probably occurred by 16 hr from the beginning ofthe dark period. Tendencies in the literature to disregard the role ofthe rib meristem in giving rise to part of the flower are discussed.

Following the application of 2-chloroethyl-tris-(2-methoxyethoxy)-silane (CGA 13586), an ethylenereleasing compound, to olive (Olea europaea L.) shoots, 2 abscission zones were observed. One occurred at the proximal, the other at the distal end of the pedicel. Actual separation occurred mostly at the distal end of the pedicel. Changes observed during the development of the abscission zone included: cell plasmolysis, cell wall and middle lamella dissolution, starch grain accumulation and a general breakdown of cells at the abscission zone.