Rubisco activation state decreases with increasing nitrogen content in apple leaves

Based on the curvilinear relationship between leaf nitrogen content and the initial slope of the response of CO2 assimilation (A) to intercellular CO2 concentrations (C-i) in apple, it is hypothesized that Rubisco activation state decreases with increasing leaf N content and this decreased activation state accounts for the curvilinear relationship between leaf N and CO2 assimilation. A range of leaf N content (1.0-5.0 g m(-2)) was achieved by fertilizing bench-grafted Fuji/M.26 apple (Malus domestica Borkh,) trees for 45 d with different N concentrations, using a modified Hoagland's solution. Analysis of A/Ci curves under saturating light indicated that CO2 assimilation at ambient CO2 fell within the Rubisco limitation region of the A/C-i curves, regardless of leaf N status. Initial Rubisco activity showed a curvilinear response to leaf N, In contrast, total Rubisco activity increased linearly with increasing leaf N throughout the leaf N range. As a result, Rubisco activation state decreased with increasing leaf N. Both light-saturated CO2 assimilation at ambient CO2 and the initial slope of the A/C-i curves were linearly related to initial Rubisco activity, but curvilinearly related to total Rubisco activity. The curvatures in the relationships of both light-saturated CO2 assimilation at ambient CO2 and the initial slope of the A/C-i curves with total Rubisco activity were more pronounced than in their relationships with leaf N, This was because the ratio of total Rubisco activity to leaf N increased with increasing leaf N. As leaf N increased, photosynthetic N use efficiency declined with decreasing Rubisco activation state.