The Optimal Allocation of Nitrogen Within the C₃ Photosynthetic System at Elevated CO₂

Abstract

The distribution of nitrogen among compounds involved in photosynthesis varies in response to changes in environmental conditions such as photon flux density. However, the extent to which the nitrogen distribution within leaves adjusts in response to increased atmospheric CO₂ is unclear.

A model was used to determine the nitrogen distribution which maximises photosynthesis under realistic light regimes at both current and elevated levels of CO₂, and a comparison was made with observed leaf nitrogen distributions reported in the literature.

The model accurately predicted the distribution of nitrogen within the photosynthetic system for leaves grown at current levels of CO₂, except at very high leaf nitrogen contents. The model predicted that, under a doubling of CO₂ concentration from its current level, the ratio of electron transport capacity to Rubisco activity (J_max : V_cmax) should increase by 40%. In contrast, measurements of J_max : V_cmax taken from the literature show a slight but non-significant increase in response to an increase in CO₂. The discrepancy between predicted and observed J_max : V_cmax suggests that leaf nitrogen distribution does not acclimate optimally to elevated CO₂. Alternatively, the discrepancy may be due to effects of CO₂ which the model fails to take into account, such as a possible decrease in the conductance to CO₂ transfer between the intercellular spaces and the sites of carboxylation at elevated CO₂.