NITROGEN SUPPLY AS A LIMITING FACTOR DETERMINING THE SENSITIVITY OF POPULUS-TREMULOIDES MICHX TO OZONE STRESS

Seedlings of trembling aspen (Populus tremuloides Michx.) were grown a t six rates of supplemental nitrogen (N). Five of the N rates were sub -optimal to optimal for growth, whilst the sixth provided an excess of N with no additional growth benefit. The biomass of plants stressed b y ozone (O-3) in open-top chambers was significantly reduced at optima l rates of N. Plants which received sub-optimal or excess N showed no significant effects of O-3 on biomass accumulation. Ozone induced two compensatory responses; increased shoot/root ratio and accelerated sen escence, at all N rates. Growth rate was strongly influenced by the am ount of N supplied, and O-3-induced reductions in total biomass were h ighly correlated with growth rate. We concluded that when growth rate was small as a result of N limitation, compensatory responses to O-3 s tress were sufficient to prevent detectable losses in biomass.