Whole-plant growth and leaf formation in ozonated hybrid poplar (Populus × euramericana)

Seasonal growth was studied in potted cuttings of hybrid poplar (one clone of Populus × euramericana) either exposed to ozone in filtered air (0 = control, 0·05, 0·10 μl litre^-1) or in ambient air (mean = 0·03 μl litre^-1). Only at 0·10 μl 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 μl 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 μl 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 at 0·10 μl 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.