Photosynthetic responses to CO2 at different leaf temperatures in leaves of apple trees (Malus domestica) grown in orchard conditions with differenct levels of soil nitrogen

The temperature-dependency of the photosynthetic response to CO₂ was measured on apple (Malus domestica cv. Cox's Orange) trees growing in orchard conditions with differing levels of nitrogen. Leaf nitrogen content varied from 1.7 to 2.2%. Nitrogen affected leaf attributes; larger areas but lower dry weights in high compared with low nitrogen trees. Across all leaf temperatures, higher rates of photosynthesis occurred in high nitrogen trees but rates differed mostly at 10 °C and progressively diminished to 35 °C. Rates of ribulose 1, 5-bisphosphate (RuBP) carboxylation, oxygenation and electron transport increased in a temperature-dependent pattern but nitrogen had no effect on rates at low (<25 °C) temperatures, whereas triose phosphate utilisation was higher and accounted for the increased photosynthesis. However, higher rates of RuBP carboxylation and regeneration in the high nitrogen trees accounted for the higher assimilation at temperatures above 30 °C. In addition, a stomatal limitation of assimilation at high temperatures was markedly increased by nitrogen but could not account for the increased photosynthesis, similarly, mesophyll conductance could not explain the effect. Basal (fruit subtending) leaves had lowest rates of assimilation and rates increased along the shoot. In addition, leaf nitrogen content also increased along the shoot and assimilation, carboxylation and regeneration all increased in concert, consistent with a causal relationship.