CO2 assimilation in relation to nitrogen in apple leaves

Summary Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated for 45 d with different nitrogen concentrations in modified Hoagland's solutions. A wide range of leaf N content was noted in recently fully expanded leaves (from 0.9–4.4 g m–2). Apparent quantum efficiency for CO2 assimilation was relatively constant except for a slight decrease at the lower end of the leaf N range. The light saturation point for CO2 assimilation increased with increasing leaf N. Curvilinear relationships were found between leaf N and 1) light-saturated CO2 assimilation at ambient CO2 2) the initial slope of the response of CO2 assimilation to intercellular CO2 concentration, and 3) CO2-saturated photosynthesis. All three initially increased linearly with increasing leaf N, then reached a plateau at a leaf N content of approximately 3 g m–2. The relationship between leaf N and stomatal conductance was similar to that of CO2 assimilation with leaf N. Calculated intercellular CO2 concentration, however, tended to decrease with increases in leaf N, indicating that stomatal conductance did not limit photosynthesis in leaves with low N content. Light-saturated CO2 assimilation, expressed on a leaf N basis, decreased with increasing leaf N. A curvilinear relationship existed between leaf N content and photosynthetic capacity in apple leaves. Photosynthetic N use efficiency decreased with increasing leaf N content.