Effect of Pumpkin Rootstock on Antioxidant Enzyme Activities and Photosynthetic Fluorescence Characteristics of Cucumber under Ca(NO3)2 Stress

The aim of this study was to determine whether the documented induction of cucumber (Cucumis sativus L.) seedling tolerance to Ca(NO3)(2) stress by grafting onto pumpkin (Cucurbita maxima x C. moschata) is associated with the protection of antioxidant enzymes and photosynthetic fluorescence characteristics. Greenhouse experiments were carried out to determine antioxidant enzyme activities, gas exchange, and photosystem II (PSII) efficiency of self-grafted and pumpkin-grafted (rootstock-grafted) 'Jinyou No. 3' cucumber plants. Plants were grown hydroponically and supplied with two nutrient solutions: a non-salinized control and a salinized solution containing 80 mM Ca(NO3)(2). Under Ca(NO3)(2) treatment, antioxidant enzyme activities first increased and then decreased, with maximal levels observed upon 12 h of treatment. The induction of antioxidant enzyme activity by Ca(NO3)(2) stress was more significant when cucumber plants were grafted onto pumpkin rootstock. Relative to the control, reductions in net photosynthetic rate, intercellular CO2 concentration, and stomatal conductance under 12 h of Ca(NO3)(2) treatment were significantly lower in rootstock-grafted plants (8.2, 19.9, and 35.6%, respectively) than in self-grafted ones (18.5, 39.4, and 90.9%, respectively). In rootstock-grafted plants subjected to Ca(NO3)(2) stress, relative reductions in the ratio of maximal to variable fluorescence, relative quantum efficiency of PSII photochemistry and the photochemical quenching coefficient were also significantly lower than in self-grafted plants; enhancement of non-photochemical quenching was significantly higher, however, suggesting that rootstock-grafted plants had higher potential to dissipate excess excitation energy and to reduce the probability of photodamage to PSII. Taken together, the use of salt-tolerant pumpkin rootstock can improve cucumber antioxidant defense and photosynthetic capacity under Ca(NO3)(2) stress and consequently lead to enhanced plant performance.