The effect of rapid changes in the water potential of the root medium or the atmosphere was investigated with particular respect to water and ion uptake in young wheat plants. Either PEG 400 or PEG 6000 were added to the nutrient solution, so that the water potential was decreased from about 0 to -5 bars and -13 bars, respectively. Both water and ion uptake immediately decreased when the water potential of the medium was reduced. By raising the humidity around the shoots from 1400 N m-^ (60% RH) to about 2300 N m"^ (100% RH) water uptake decreased, whereas ion uptake transiently increased. If sodium chloride was added to reduce the water potential of the nutrient solution, ion uptake was found to decrease to essentially the same level as when PEG was used. It is suggested that a change in the water potential of the plants has effects on the active-ion-transport mechanism.

Short‐term effects of ABA, ABA + kinetin and kinetin on ion (86Rb‐potassium and phosphate) and water uptake in sunflower plants (Helianthus annuus var. californicus) were examined with a continuous‐recording technique. Ion uptake in the roots and transport to the shoots were also investigated by conventional tracer uptake experiments and by sap bleeding experiments with excised roots. After addition of 5 × 10−6‐4 × 10−5M ABA to the root medium there was an immediate decrease (30–70%) in the rate of ion uptake which lasted 30–70 min. The rate of water uptake was not significantly affected as measured with this method. Ion transport to the shoots and to the bleeding sap of excised roots was decreased by ABA. ABA‐induced inhibition of ion uptake was abolished by the presence of kinetin, and uptake was slightly stimulated by 2 × 10−5M kinetin alone. We suggest that concentration gradients of ABA or rapid changes in the ABA‐kinetin balance in the roots affect ion uptake and transport.

Effects of rapid or stepwise decrease of the water potential by polyethylene glycol in the root medium on the efflux and uptake of **Rb-potassium, and the effect of abscisic acid on the efflux of ^^Rbpotassium were investigated in roots of intact wheat plants (Triticum aestivum L. cv. Welbulls Original Starke II). The efflux of ions increased at both a rapid and a stepwise reduction in water potential, but the effect was less at a stepwise reduction in water potential. Active uptake of ions decreased as much at a stepwise as at a rapid reduction in water potential. Addition of 4 x 10"-M abscisic acid to the root medium did not affect the efflux of ions. It is suggested that low water potentials affect both membrane permeability and active uptake of ions. The increase of efflux is not induced by increase of ABA only. 0O31-9317/79/05OOOl-O6$O3.OO/O © 1979 Physiologia Plantarum GUNVOR ERLANDSSON Physiol. Plant. 47. 1979