EFFECTS OF ABSCISIC-ACID APPLICATION TO ROOTS ON WATER AND ION UPTAKEAND LEAF CONDUCTANCE OF SOYBEAN

Reported effects of applied abscisic acid (ABA) on water and ion uptak e rates of excised leaves and roots are many and contradictory. Seedli ngs (8 d) of soybean (Glycine man (L.) Meer. cv Bragg) were trimmed to three uniform root branches and grown in nutrient solution. Seedlings (18 d) were transferred to a continuously aerated circulating nutrien t solutions in 3 glass tubes mounted together. An automatic system was used to monitor water and ion uptake at 10 min intervals. Concentrati ons of ABA from 10(-5) to 5 x 10(-4) M were applied to one root divisi on. The other two divisions were used as controls. Water uptake was en hanced by 10(-4) to 5 x 10(-4) M ABA in the treated root portion for a short time and then reduced throughout the treatment. When ABA was re placed with nutrient medium, treated roots began to recover slowly. Co mplete recovery required more than 24 h. Water uptake of untreated roo ts was reduced less than treated roots and returned more rapidly to no rmal water uptake. Reductions of water uptake rates were less during a pplications of 3.16 x 10(-5) and 10(-5) M ABA. Ion uptake rates were r educed in treated root portions during application of all ABA concentr ations, while untreated root portions were not affected. Patterns of n utrient uptake reduction were unrelated to ABA concentration. ABA appl ied to roots reduced stomatal aperature as evidenced by consistent red uctions in leaf conductance and in total water uptake.