Reduction of leaf photosynthesis and transpiration rates of potato plants by second‐stage juveniles of Globodera pallida

Net photosynthesis and transpiration rates of potato plants, grown in pots in the greenhouse, were measured at various light irradiances and ambient CO2 concentrations, 3d after inoculation with second stage juveniles of Globodera pallida. Gas exchange rates, both in darkness and in light, and the initial light use efficiency were strongly reduced by nematodes. Stomatal conductance of infected plants was lower than that of control plants and showed little response to decreasing ambient CO2 concentration. The maximum internal CO2 concentration of infected plants was lower than that of control plants. Globodera pallida reduced photosynthesis also by apparent non‐stomatal effects. The effects of G. pallida on gas exchange rates are similar to the effects of abscisic acid in the transpiration stream and of abiotic stresses in the root environment. Apparently, there is a general response of plant roots to adverse conditions. The reduction of photosynthesis may be an important factor in yield reduction by potato cyst nematodes.