Influence of canopy light environment and nitrogen availability on leaf photosynthetic characteristics and photosynthetic nitrogen-use efficiency of field-grown nectarine trees

Relationships between CO2 assimilation at light saturation (A(max)), nitrogen (N) content and weight per unit area (W-A) were studied in leaves grown with contrasting irradiances (outer canopy versus inner canopy) and N supply rates in field-grown nectarine trees Prunus persica L. Batsch. cv. Fantasia. Both A(max) and N content per unit leaf area (N-A) were linearly correlated to W-A, but leaves in the high-N treatment had higher N-A and A(max) for the same value of W-A than leaves in the low-N treatment. The curvilinear relationship between photosynthesis and total leaf N was independent of treatments, both when expressed per unit leaf area (A(maxA) and N-A) and per unit leaf weight (A(maxW) and N-W), but the relationship was stronger when data were expressed on a leaf area basis. Both A(maxA) and N-A were higher for outer canopy leaves than for inner canopy leaves and A(maxW) and N-W were higher for leaves in the high-N treatment than for leaves in the low-N treatment. The relationship between A(max) and N resulted in a similar photosynthetic nitrogen-use efficiency at light saturation (A(max) NUE) for both N and light treatments. Photosynthetic nitrogen-use efficiency was similar among treatments throughout the whole light response curve of photosynthesis. Leaves developed in shade conditions did not show higher N-use efficiency at low irradiance. At any intercellular CO2 partial pressure (C-i), photosynthetic CO2 response curves were higher for outer canopy leaves and, within each light treatment, were higher for the high-N treatments than for the low-N treatments. Consequently, most of the differences among treatments disappeared when photosynthesis was expressed per unit N. However, slightly higher assimilation rates per unit N were found for outer canopy leaves compared with inner canopy leaves, in both N treatments. Because higher daily irradiance within the canopies of the low-N trees more than compensated for the lower photosynthetic performances of these leaves compared to the leaves of high-N trees, daily carbon gain (and N-use efficiency on a daily assimilation basis) per leaf was higher for the low-N treatment than for the high-N treatment in both outer and inner canopy leaves.