THE EFFECTS OF AUXIN ON THE MECHANICAL-PROPERTIES IN-VIVO OF CELL-WALL IN HYPOCOTYL SEGMENTS FROM GIBBERELLIN-DEFICIENT COWPEA SEEDLINGS

Auxin-induced changes in the mechanical properties of cell wall were e xamined by both positive and negative pressure jump methods using hypo cotyl segments excised from the 3-day-old seedlings of cowpea that has been treated with uniconazole, a potent inhibitor of the biosynthesis of gibberellins. In such segments (U-segments) that were deficient in endogenous gibberellin, auxin increased only the effective turgor (P- i-Y) and did not change the extensibility (Phi) of cell wall. As a res ult, the extent of the auxin-induced promotion of growth was halved. H owever, auxin was able to increase Phi of U-segments that has been pre treated for two hours with GA(3) prior to the application of IAA. Meas urement of intracellular pressure (P-i) with a pressure probe revealed that auxin did not change P-i in either U-segments or GA(3)-pretreate d segments. The results suggest that auxin can decrease the yield thre shold of the cell wall (Y) independently of gibberellin but can increa se Phi only in the presence of gibberellin. The difference between Phi and Y in terms of their requirement for gibberellin to respond to aux in suggests that they are mutually separable mechanical properties tha t originate from different molecular processes that occur in the archi tecture of yielding cell walls.