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 sectionsof Vigna unguiculata were studied by the xylem perfusion method.Hypocotyl sections shrank upon exposure to osmotic stress. Sectionsshowed no adaptive responses to osmotic stress when they werein an IAA-depleted condition as a result of perfusion with solutionsthat lacked IAA for 3–4 h. The correlation between thegrowth rate and the membrane potential of the xylem/symplastboundary (V^px) was very limited in the absence of IAA. By contrast,hypocotyl sections showed distinct adaptive responses to osmoticstress after perfusion with solutions that contained 10 µMIAA. In the presence of IAA, V^px increased in the negative directionand growth resumed in spite of the osmotic stress. The growthrate was closely correlated with the xylem membrane potential.Hyper-polarization of the membranes of the xylem/symplast boundaryalways preceded the recovery of growth under osmotic stress.It appears that IAA is essential for the adaptive recovery ofgrowth under osmotic stress and, moreover, that the xylem protonpump plays an indispensable role in modulating the growth ofhypocotyl sections. This result confirms prediction of an earliersimulation study using the apoplast canal model [Katou and Furumoto(1986) Protoplasma 133: 174, Katou and Enomoto (1991) PlantCell Physiol. 32: 343]. (Received June 27, 1996; Accepted October 28, 1996)