ADAPTIVE GROWTH-RESPONSES TO OSMOTIC-STRESS OF HYPOCOTYL SECTIONS OF VIGNA-UNGUICULATA - ROLES OF THE XYLEM PROTON PUMP AND IAA

The growth responses to osmotic stress of hypocotyl sections of Vigna unguiculata were studied by the xylem perfusion method. Hypocotyl sect ions shrank upon exposure to osmotic stress. Sections showed no adapti ve responses to osmotic stress when they were in an IAA-depleted condi tion as a result of perfusion with solutions that lacked IAA for 3-4 h . The correlation between the growth rate and the membrane potential o f the xylem/symplast boundary (V-px) was very limited in the absence o f IAA. By contrast, hypocotyl sections showed distinct adaptive respon ses to osmotic stress after perfusion with solutions that contained 10 mu M IAA. In the presence of IAA, V-Px increased in the negative dire ction and growth resumed in spite of the osmotic stress. The growth ra te was closely correlated with the xylem membrane potential. Hyperpola rization of the membranes of the xylem/symplast boundary always preced ed the recovery of growth under osmotic stress. It appears that IAA is essential for the adaptive recovery of growth under osmotic stress an d, moreover, that the xylem proton pump plays an indispensable role in modulating the growth of hypocotyl sections. This result confirms pre diction of an earlier simulation study using the apoplast canal model [Katou and Furumoto (1986) Protoplasma 133: 174, Katou and Enomoto (19 91) Plant Cell Physiol. 32: 343].