DOES A LOW NITROGEN SUPPLY NECESSARILY LEAD TO ACCLIMATION OF PHOTOSYNTHESIS TO ELEVATED CO2

Long-term exposure of plants to elevated partial pressures of CO, (pCO (2)) often depresses photosynthetic capacity. The mechanistic basis fo r this photosynthetic acclimation may involve accumulation of carbohyd rate and may be promoted by nutrient limitation. However, our current knowledge is inadequate for making reliable predictions concerning the onset and extent of acclimation. Many studies have sought to investig ate the effects of N supply but the methodologies used generally do no t allow separation of the direct effects of limited N availability fro m those caused by a N dilution effect due to accelerated growth at ele vated pCO(2). To dissociate these interactions, wheat (Triticum aestiv um L.) was grown hydroponically and N was added in direct proportion t o plant growth. Photosynthesis did not acclimate to elevated pCO(2) ev en when growth was restricted by a low-N relative addition rate. Ribul ose-l, 5-bisphosphate carboxylase/oxygenase activity and quantity were maintained, there was no evidence for triose phosphate limitation of photosynthesis, and tissue N content remained within the range recorde d for healthy wheat plants. In contrast, wheat grown in sand culture w ith N supplied at a fixed concentration suffered photosynthetic acclim ation at elevated pCO(2) in a low-N treatment. This was accompanied by a significant reduction in the quantity of active ribulose-l, 5-bisph osphate carboxylase/oxygenase and leaf N content.