Artificially-induced leaf nitrate accumulation affects photosynthesis in micropropagated apple plants with different water supply

One-year-old micropropagated 'Gala' apple (Malus X domestica Borkh.) plants with split root systems were supplied with tungsten (Na2WO4), an inhibitor of nitrate reductase activity, and NaNO3, separately, or in combination. The chemicals were supplied either through the xylem (40 ml 0.3 mM Na2WO4 or 3 mM NaNO3), or as foliar sprays (30 mM NaNO3 and 20 mM Na2WO4). When applied separately, neither increased the NO3- content of leaves, although Na2WO4 inhibited foliar nitrate reductase activity completely, and NaNO3 supplied additional nitrate to the leaves. However, when both were applied together, either through the xylem or as sprays, NO3- accumulated in the leaves of well-irrigated plants, as well as in those subjected to half- or whole-root drought stress. Such artificially induced NO3- accumulation in leaves significantly decreased the net photosynthetic rate, stomatal conductance, and rate of transpiration at approx. 12.00 h following the application, compared to controls supplied with deionised water. Net photosynthesis, stomatal conductance, and transpiration rate were negatively correlated with the foliar concentration of NO3-. None of the treatments influenced leaf temperatures or sub-stomatal CO2 concentrations, indicating that the decrease in net photosynthesis was not due to stomatal limitation. It is concluded that low foliar NO3- concentrations are a characteristic of the species, and important for efficient photosynthesis in apple.