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 (Amax), 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 Amax 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 Amax 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 (AmaxA and N(A)) and per unit leaf weight (AmaxW and N(W)), but the relationship was stronger when data were expressed on a leaf area basis. Both AmaxA, and N(A) were higher for outer canopy leaves than for inner canopy leaves and AmaxW and N(W) were higher for leaves in the high-N treatment than for leaves in the low-N treatment. The relationship between Amax and N resulted in a similar photosynthetic nitrogen-use efficiency at light saturation (AmaxNUE) 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 (Ci), 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.