UPTAKE AND DECARBOXYLATION OF INDOLE-3-ACETIC-ACID DURING AUXIN-INDUCED GROWTH IN LUPIN HYPOCOTYL SEGMENTS - INFLUENCE OF PH AND VANADATE

The elongation growth of etiolated hypocotyl segments of lupin (Lupinu s albus L.) was stimulated by acid pH (4.6 versus 6.5) and by IAA for periods of up to 4 h. After this time, the segments were unable to gro w further. In the presence of an optimal IAA concentration (10 muM), a cid pH increased the growth rate but had no effect on final growth. Wi th suboptimal IAA (0.1 muM), however, acid pH increased growth in a mo re than additive way, suggesting a synergistic action between the two factors. This synergism may be explained by the increased IAA uptake a nd decarboxylation seen at an acid pH. These results reinforce the vie w that the effects of low pH and IAA on growth are not independent. Va nadate inhibited growth and also IAA uptake and decarboxylation. This inhibitor, therefore, probably inhibits growth not only by decreasing ATPase-mediated acidification but also by decreasing H+-dependent IAA uptake from the apoplasm. This dependence of IAA uptake on ATPase may be mediated by apoplasmic acidification. The amount of IAA decarboxyla ted increased when the assay conditions favored the growth of segments , indicating that IAA could be destroyed by decarboxylation during the auxin-induced growth.