Contributions of photosynthetic parameters to overall gas exchange performance varied between vinifera and hybrid grape cultivars

The dense trichomes on the abaxial side of the leaves of hybrid grapes might limit the diffusion rates of gas and water vapor, consequently reducing assimilation rate (A). To evaluate the difference in gas exchange efficiency, CO₂ response curve (A-C_i curve) and light response curve of mature leaves of ‘Riesling’ (Vitis vinifera), ‘Black Queen’ (V. hybrid) and ‘Golden Muscat’ (V. hybrid) vines were measured. Data were fitted to modified FvCB models and then integrated using a numerical integration approach. At 25°C, A of ‘Black Queen’ and ‘Golden Muscat’ were 40 and 38%, respectively, less efficient than that of ‘Riesling’. In addition to the diffusional factors, i.e., stomatal and mesophyll conductance, biochemical efficiency, mainly J_max, had an equal or greater contribution to the lower A in hybrid grapes compared to the vinifera at optimal temperatures. Biochemical factors, mainly J_max, contributed 3/4 of the inferior A performance in ‘Black Queen’, while biochemical and diffusional factors had equal shares to A in ‘Golden Muscat’.