CALCIUM CHELATOR AND CHANNEL BLOCKERS SUPPRESS THE IAA-INDUCED MEMBRANE HYPERPOLARIZATION WITHOUT INHIBITING THE FOLLOWING GROWTH PROMOTIONIN HYPOCOTYL SECTIONS OF VIGNA-UNGUICULATA UNDER XYLEM PERFUSION

The effects of the Ca2+ chelator EGTA and the Ca2+ channel blockers La 3+, Gd3+, nifedipine, and verapamil on the IAA-induced early responses of hypocotyl sections of Vigna unguiculata were studied to assess the involvement of Ca2+ in the IAA-induced hyperpolarization followed by growth promotion using the xylem perfusion method. The IAA-induced hyp erpolarization was distinctly inhibited by pretreatment with EGTA, La3 +, and/or Gd3+ (5 mM each). EGTA inhibited by about 80-90% while La3and Gd3+ inhibited by about 60-75%, The sustained hyperpolarization af ter IAA application was also inhibited by these reagents. Nifedipine a nd verapamil (50 mu M each) had little effect on the IAA-induced hyper polarization and growth promotion, The inhibitory effects of EGTA, La3 + and Gd3+ on the membrane potentials were not due to changes in a pas sive component, but mainly to decreases in an electrogenic component. The effect of EGTA was reversible, while those of La3+ and Gd3+ were n ot. Present results suggest that the influx of Ca2+ through the plasma membrane might be required not only for the initiation of hyperpolari zation by IAA but also for the following sustained hyperpolarization a nd may therefore play an essential role in the IAA-induced activation of the electrogenic proton pumps at the plasma membrane in Vigna hypoc otyls. Inhibition of the hyperpolarization by EGTA, La3+, and Gd3+, ho wever, did not involve inhibition of the promotion of growth by IAA, I t seems that activation of the electrogenic proton pumps at the plasma membrane is not always the indispensable primary step of the IAA-indu ced promotion of elongation growth. Some alternative pathway of proton extrusion might take part.