THE RELATIONSHIP BETWEEN LEVELS OF ENDOGENOUS GIBBERELLINS AND THE RESPONSE OF VIGNA HYPOCOTYLS TO EXOGENOUS INDOLE-3-ACETIC-ACID

Segments excised from the upper and the lower parts of cowpea (Vigna u nguiculata L.) hypocotyls were compared in terms of their responses to exogenous indole-3-acetic acid (IAA) in relation to their endogenous levels of gibberellin. Growth of the segments was measured continuousl y during xylem perfusion with a linear differential transformer. IAA i nduced a burst of elongation in the upper segments but only slight pro motion of growth in the lower segments. Treatment with uniconazole, a potent inhibitor of the biosynthesis of gibberellins, reduced the resp onsiveness of the upper segments to exogenous IAA to about one half of the control value. Pre-perfusion with GA(3) of such segments for two hours prior to application of IAA, partially restored the responsivene ss to IAA. Analysis by GC/MS identified GA(1), GA(4), GA(9), GA(20) an d GA(51) as native gibberellins in the hypocotyls of cowpea seedlings. Analysis by GC/SIM also showed that the physiologically active gibber ellins (GA(1) and GA(4)) were located mainly in the upper part of the hypocotyl and the treatment with uniconazole marketly reduced the endo genous level of gibberellins there to less than 11% of the control lev el. These results suggest that levels of endogenous gibberellins possi bly control the response to IAA in these segments.