The decay of O-3 through direct reaction with cell wall ascorbate is not sufficient to explain the different degrees of O-3-sensitivity in two poplarclones

Ozone (O-3) flux to the mesophyll cell walls and its decay in the direct re action with cell wall-ascorbate were quantified by measuring stomatal condu ctance, cell wall ozone-exposed area, cell wall thickness and cell wall-asc orbate concentration in two differently-sensitive poplar clones exposed to O-3 (0.150 mu l L-1 for 5 h). Stomatal closure under O-3 was more pronounce d in the sensitive Eridano than in the resistant: I-214 poplar clone. The r elative internal area in the ozonated sensitive clone was significantly (-2 1%) lower than that in the control one. The concentration of reduced ascorb ate in the cell wall ([ASA(cw)]) of the sensitive clone was initially 81% h igher than in the resistant one. Following O-3 treatment, [ASA(cw)] increas ed more than 3-fold in both clones, while there was a much more rapid incre ase in dehydroascorbic acid concentration [DHA(cw)] in the sensitive clone. Calculated ozone flux to mesophyll cell wall and to plasmalemma was compar ed between the two clones. The decay of O-3 flux in cell wall was more rapi d in the sensitive clone, due to the more rapid rise of [ASA(cw)] under O-3 , bur the total (stomatal+cell wall) attenuation of O-3 flow during the exp osure was similar in both of the clones. It is concluded that the decay of O-3 through direct reaction with cell wall ascorbate is not sufficient to e xplain the different O-3-sensitivity in two poplar clones.