PITH AUTOLYSIS IN PLANTS .4. THE ACTIVITY OF POLYGALACTURONASE AND CELLULASE DURING DROUGHT STRESS-INDUCED PITH AUTOLYSIS

The water potential, amount of pith autolysis and activities of apopla stic cellulase and polygalacturonase of tomato stems were measured dur ing 24 h of drought stress (DS) and for 24 h following reirrigation. D uring DS the water potential dropped from -5.5 to -10.4 bars and rose to -8.3 bars soon after reirrigation. Drought stress induced considera ble pith autolysis, more of which occurred after reirrigation. Pretrea tment with mechanical perturbation (MP) of the stems or application of exogenous ethephon on the buds hardened the tomato plants against DS- induced pith autolysis. Drought stress caused an increase in apoplasti c polygalacturonase and an even greater increase in apoplastic cellula se. Reirrigation caused a large transient increase in the former and a decrease in the latter. The apoplastic reducing sugar content (as gal acturonic acid) of the stem rose in parallel with the activity of the enzymes. Both DS and MP caused an increase in ethylene evolution, alth ough the former was significantly greater than the latter. However, wh en MP preceded DS, the amount of ethylene produced was significantly l ess than DS alone induced. Pretreatment with either MP or exogenous et hephon inhibited the increase in apoplastic cellulytic enzymes. It is concluded that DS induces ethylene evolution from the tomato stem, cau sing an increase in the stem apoplastic cellulytic enzymes, which in t urn start the autolysis of the pith cell walls. Pretreatment with MP o r ethephon, each of which induces ethylene evolution, hardens the stem so that it does not produce more ethylene during DS, and thus becomes resistant to DS-induced pith autolysis.