PARENT SESSION
Oral Session #31: Plant Ecology: Gas Exchange.
Presiding: R. Montgomery
Tuesday, August 6. 8:00 AM to 11:30 AM. Cochise Meeting Room, TCC.

Interactions between the effects of atmospheric [CO₂] and P nutrition on photosynthesis in white lupin (Lupinus albus L.).

Campbell, Catherine^*,1, Sage, Rowan¹, ¹ University of Toronto, Department of Botany, Toronto, Ontario, Canada

ABSTRACT- An adequate supply of reduced carbon is essential for plants to capture mineral nutrients. Low atmospheric [CO₂], such as was present during the Pleistocene, might have limited carbon resources available to roots with which to take up soil P. Limits to P uptake can in turn limit photosynthesis because P is required for carbon assimilation. P is the nutrient most limiting to plant growth for nitrogen-fixing legumes such as white lupin. It is known that photosynthetic acclimation occurs at high CO₂ in response to nitrogen deficiency, but the response to P deficiency is less well characterised. This study examined the response of photosynthesis to low P treatment under three [CO₂]: 200, 400 and 750 ppm. We differentiated between factors limiting photosynthesis by determining Rubisco activity, fluorescence yield, and the oxygen sensitivity of photosynthesis. We also examined assimilation and respiration to determine the daily carbon balance of plants under each treatment, in order to estimate the whole plant carbon budget. Daily photosynthetic rates were higher at higher growth [CO₂] only in the high-P treated plants; photosynthetic rates in low-P grown plants were the same at all [CO₂]. The oxygen sensitivity of photosynthesis was reduced at low P only in plants grown at 400 and 750 ppm CO₂, indicating that photosynthesis in these plants was limited by the speed at which P could be liberated from triose phosphates. Initial results suggest, however, that Rubisco activity also dropped in response to P deficiency in plants grown at 400 and 750 ppm CO₂. This means that photosynthetic acclimation to elevated CO₂ in response to P deficiency takes place not just at the predicted [CO₂] of the future; it also occurs at current atmospheric [CO₂], when compared with the low [CO₂] of Pleistocene.

KEY WORDS: CO₂, phosphorus, photosynthesis, Lupinus albus