NITROGEN NUTRITION OF C-3 PLANTS AT ELEVATED ATMOSPHERIC CO2 CONCENTRATIONS

The atmospheric CO2 concentration has risen from the preindustrial lev el of approximately 290 mu 1 1(-1) to more than 350 mu 1 1(-1) in 1993 . The current rate of rise is such that concentrations of 420 mu 1 1(- 1) are expected in the next 20 years. For C-3 plants, higher CO2 level s favour the photosynthetic carbon reduction cycle over the photorespi ratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthe tic cycles appears to alter nitrogen and carbon metabolism in the leaf , possibly causing decreases in nitrogen concentrations in the leaf. T his may result from increases in the concentration of storage carbohyd rates of high molecular weight (soluble or insoluble) and/or changes i n distribution of protein or other nitrogen containing compounds. Upta ke of nitrogen may also be reduced at high CO2 due to lower transpirat ion rates. Decreases in foliar nitrogen levels have important implicat ions for production of crops such as wheat, because fertilizer managem ent is often based on leaf chemical analysis, using standards estimate d when the CO2 levels were considerably lower. These standards will ne ed to be re-evaluated as the CO2 concentration continues to rise. Lowe r levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.