Effects of Ca²⁺ on membrane lipid peroxide, antioxidation enzymes and thermal dissipation of squash seedlings under high temperature and light intensity

Vegetable growth and development have suffered restraints and damages by high temperatures and strong lights under rising global temperatures in recent years. The yield, quality and economic benefits of summer vegetables substantially reduced due to dual stresses of high temperatures and strong lights. Therefore an experiment was carried out in 2009 to explore the methods of alleviating of high temperature/light adversities in vegetable production. In the experiment, a squash (Cucurbitapepo L.) variety of “Alan” was used to study Ca²⁺ effects on the characteristics of chlorophyll fluorescence, membrane lipid peroxide and antioxidation enzymes of squash seedlings by exogenous application of CaCl₂₊ solution under the stresses of high temperature and light intensity. The results showed that squash seedlings under the application of 5~20 mmol·L^-1 of Ca²⁺ had higher superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT) activities and GSH content than the control experiment (CK). However, the contents of malondialdehyde (MDA), H₂2O₂, and relative electrical conductivity were lower than those of over-40 mmol·L^-1 Ca²⁺ and CK treatments. The maximum PSII efficiency (Fv /Fm), actual PSII efficiency (ΦPSⅡ) and photochemical queching coefficient (qP) improved under the 5~20 mmol·L^-1 Ca²⁺ treatment. However, non-phochemical fluorescence quenching coefficient (NPQ) decreased under the 5~20 mmol·L^-1 Ca²⁺ treatment. This indicated that the 5~20 mmol·L^-1 Ca²⁺ treatment remarkably buffered damages caused by high temperatures and light intensity stresses and decreased excitation energy of dissipation. The best buffer effect for high temperatures and strong lights occurred under 10 mmol·L^-1 Ca²⁺ application. No buffer effect was noted when Ca²⁺ concentration was more than 40 mmol·L^-1.