WHOLE-PLANT GROWTH AND LEAF FORMATION IN OZONATED HYBRID POPLAR (POPULUS X EURAMERICANA)

Seasonal growth was studied in potted cuttings of hybrid poplar (one clone of Populus x euramericana) either exposed to ozone in filtered air (0 = control, 0.05, 0.10 mul litre-1) or in ambient air (mean = 0.03 mul litre-1). Only at 0.10 mul litre-1 was biomass production reduced and related to leaf loss rather than leaf formation, since the latter was similar in all treatments. Stem length at 0.10 mul litre-1 approached that of the control, whereas starch concentration in the green stem bark tended to be reduced, as were the ratios of stem weight/length and root/shoot biomass. The changes in carbon allocation and biomass production gradually became established during the second half of the growing season. At the altered carbon allocation at 0.10 mul litre-1, the ratio of whole-plant production/attached foliage area resembled that of the other O3 regimes. However, the latter ratio was strongly reduced al 0.10 mul litre-1 when calculated on the basis of the potential foliage area, as compensated for the O3-induced leaf loss. Thus the carbon return/cost balance of the totally formed foliage was low, although the relative-growth rate of ozonated plants temporarily reached that of the control. The relation between leaf differentiation under ozonation (lowered stomatal density) and whole-plant production remains uncertain. The plant behavior found is discussed in terms of passive response or acclimatization to O3 stress.