The relationship between photosystem II efficiency and quantum yield for CO2 assimilation is not affected by nitrogen content in apple leaves

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO2 assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O-2 (2%) and saturated CO2 (1300 mu mol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO2 assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO2 mol-1 quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO2 assimilation and photorespiration in different N leaves in response to intercellular CO2 concentration (C-i) under normal O-2 conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO2 or O-2 increased with increasing leaf N at any given C-i. The percentage of non-cyclic electron flow to CO2 assimilation, however, remained the same regardless of leaf N content. As C-i increased, the percentage of non-cyclic electron flow to CO2 assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO2 assimilation and the partitioning of electron flow between CO2 assimilation and photorespiration are not affected by N content in apple leaves.