Rubisco activation state decreases with increasing nitrogen content in apple leaves

Abstract

Based on the curvilinear relationship between leaf nitrogen content and the initial slope of the response of CO 2 assimilation (A) to intercellular CO 2 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 CO 2 assimilation. A range of leaf N content (1.0±5.0 g m À2) 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/C i curves under saturating light indicated that CO 2 assimilation at ambient CO 2 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 CO 2 assimilation at ambient CO 2 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 CO 2 assimilation at ambient CO 2 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.