The decrease in Fv/Fo in ungrafted plants, 5/cultivar, and8/cultivar plants may be as a result of an increase in protective non-radiative energy dissipation associated with a regulated decreasein photochemistry – described as down-regulation and/or chronicphotodamage of the PSII centers (Genty et al., 1989; Osmond, 1994).The Fv/Foratio seems a robust parameter, and several authorshave concluded that PSII photochemistry cannot be impaired byrelatively severe water stress; although ANand gscan decrease sig-nificantly (Lawlor and Tezara, 2009). In our experiment, all plant combinations, regardless of the Fv/Fovalues, showed a significantde crease in the net carbon gain, due in part to stomatal closure that restricts water losses.