SEASONAL TRENDS OF LIGHT-SATURATED NET PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE OF LOBLOLLY-PINE TREES GROWN IN CONTRASTING ENVIRONMENTS OF NUTRITION, WATER AND CARBON-DIOXIDE

Repeated measures analysis was used to evaluate the effect of long-ter m CO2 enhancement on seasonal trends of light-saturated rates of net p hotosynthesis (A(sat)) and stomatal conductance to water vapour (g(sat )) of 9-year-old loblolly pine (Pious taeda L.) trees grown in a 2 x 2 factorial experimental design of nutrition and water. A significant i nteraction effect of CO2 and nutrition on mean A(sat) was observed for juvenile foliage. Also, juvenile foliage exposed to + 350 mu mol mol( -1) CO2 had a higher rate of increase of A(sat) between late summer an d early autumn. This would lead to a greater potential for recharging carbohydrate reserves for winter. Mature foliage was affected by CO2, water and nutrient treatments in two ways. First, A(sat) was significa ntly increased as a result of elevated CO2 in January, a period when s tomatal conductance was only 47% of the maximum observed rate. Secondl y, the rate of increase of A(sat) from winter to early spring was acce lerated asa result of both nutrient + water and + 350 mu mol mol(-1) C O2 treatments. This accelerated response resulted in a greater potenti al for photosynthate production during the period when growth initiati on occurred. Nutrient, water or carbon dioxide treatments did not sign ificantly alter trends in g(sat) for mature or juvenile foliage. A sig nificant nutrition x CO2 interaction was observed for the mature folia ge, suggesting that g(sat) increased with increasing CO2 and nutrition . These results may have important consequences for the determination of the water use efficiency of loblolly pine. In spite of low g(sat) i n the winter to early spring period, there was a substantial gain in A (sat) attributable to elevated CO2 concentrations.