The susceptibility of cucumber and sweet pepper to chilling under low irradiance is related to energy dissipation and water-water cycle

The photoprotection of energy dissipation and water-water cycle were investigated by comparing chilling sensitivity of photosystems 2 (PS2) and 1 (PS1) in two chilling-sensitive plants, cucumber and sweet pepper, upon exposure to 4 degreesC under low irradiance (100 mumol m(-2) s(-1)) for 6 h. During chilling stress, the maximum photochemical efficiency of PS2 (F-v/F-m) decreased only slightly in both plants, but the oxidisable P700 decreased markedly, which indicated that PSI was more sensitive to chilling treatment under low irradiance than PS2. Sweet pepper leaves had lower F-v/F-m, higher non-photochemical quenching (NPQ), and higher oxidisable P700 during chilling stress. Activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in cucumber leaves was higher, but APX activity decreased apparently compared to that at room temperature. The productions of active oxygen species (H2O2, O-2(-.)) increased in both plants, faster in cucumber leaves than in sweet pepper leaves. In sweet pepper leaves, a stronger de-epoxidation of the xanthophyll cycle pigments, a higher NPQ could act as a major protective mechanism to reduce the formation of active oxygen species during stress. Thus sensitivity of both plants to chilling under low irradiance was dominated by the protective mechanisms between PS1 and PS2, especially the energy dissipation and the water-water cycle.