Journal
JOURNAL OF EXPERIMENTAL BOTANY
Volume 51, Issue 351, Pages 1687-1694Publisher
OXFORD UNIV PRESS
DOI: 10.1093/jexbot/51.351.1687
Keywords
apple; CO2 assimilation; leaf N content; Malus domestica; Rubisco activation state
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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.
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