DARK-INDUCED ASCORBATE DEFICIENCY IN LEAF CELL-WALLS INCREASES PLASMALEMMA INJURY UNDER OZONE

To assess protection of the mesophyll cell plasmalemma against O-3 by apoplasmic reduced ascorbate (AA), its concentration in the leaf cell wall of common bean (Phaseolus vulgaris L.) was lowered from 0.6 mM to 0.1 mM by pre-exposing plants to continuous darkness for up to 48 h. Subsequent ozonization of ascorbate-deficient leaves with 350-450 nmol O-3 mol(-1) resulted in a rapid rise of apoplasmic AA within the seco nd hour of the treatment, the concomitant appearance of cytoplasmic ma rker enzymes in cell. wall solute extracts and the development of wate r-logged spots on leaves. Prior to these events, stomatal conductances had just reached values close to those observed in AA-nondeficient le aves, whereas AA concentration in the cell wall was still 2-4 times lo wer than in leaves pre-exposed to the normal 10-h dark period. In AA-n ondeficient leaves the inital apoplasmic AA level of 0.6 mM was mainta ined under O-3 for 2.5 h; thereafter, it increased moderately. There a ppeared to be no signs of injury even 2 d after the whole 4.5-h treatm ent. During the period of equal stomatal conductances, the O-3 decay r ate in direct reaction with AA in AA-deficient cell walls was estimate d to be 50-70% of that occurring in AA-nondeficient leaves. It is sugg ested that under AA deficiency some threshold for the stability of the plasmalemma was surpassed owing to the more ''O-3-permeable'' cell wa ll. The mesophyll conductance was found to be stable throughout O-3 ex posure, indicating that the cytoplasmic O-3 defense barrier was not ex ceeded. Possible changes in oxyradical reactions and in cell wall phen olics are discussed. It is suggested that after prolonged darkness the flow rate of reactive oxygen intermediates to the plasmalemma may als o be higher because they are less trapped in direct and peroxidase-cat alyzed reactions.