EFFECTS OF LEAF NUTRIENT STATUS ON PHOTOSYNTHETIC CAPACITY IN LOBLOLLY-PINE (PINUS-TAEDA L) SEEDLINGS GROWN IN ELEVATED ATMOSPHERIC CO2

We measured needle photosynthesis of loblolly pine seedlings grown in a factorial experiment with two CO2 partial pressures (35 and 65 Pa) and three nutrient treatments (7 mM NH4NO3 + 1 mM PO4; 7 mM NH4NO3 + 0.2 mM PO4; 1 mM NH4NO3 + 1 mM PO4). The data were used to parameterize a physiologically based photosynthetic model that included limitations imposed by ribulose-1,5-bisphosphate carboxylase/oxygenase activity, electron transport capacity and inorganic phosphate availability. With nonlimiting nutrients, seedlings grown at 65 Pa CO2 had significantly higher net photosynthesis and lower stomatal conductance than seedlings grown at 35 Pa CO2. Nutrient limitations by either N or P significantly reduced photosynthetic capacity. When either N or P was limiting, there was no effect of growth CO2 partial pressure on photosynthesis, but stomatal conductance was significantly lower for seedlings grown at 65 Pa CO2. Modeled biochemical parameters suggest that, in all cases, photosynthesis was co-limited by carboxylation, electron transport and phosphate regeneration. Acclimation to growth in elevated CO2 involved a reduction in leaf N content. In the low-N and low-P treatments, modeled parameters indicated that the biochemical processes of photosynthesis were down regulated to the point that there was no effect of increasing CO2 partial pressure. The capacity to regenerate phosphate was reduced in both low nutrient treatments, but was only reduced by elevated C02 when seedlings were grown under low soil P conditions. Increased photosynthetic water use efficiency and nutrient use efficiency in response to CO2 enrichment occurred in all three nutrient treatments and have important implications for whole-plant water and nutrient balance. These data support the contention that soil nutrient status in forest ecosystems will be a critical influence on tree seedling response to increasing atmospheric CO2 partial pressures.