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 °C under low irradiance (100 µmol 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 PS1 was more sensitive to chilling treatment under low irradiance than PS2. Sweet pepper leaves had lower F v /F m , higher nonphotochemical 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 (H 2 O 2 , 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.
The effects of chilling treatment (4 °C) under low irradiance, LI (100 µmol m -2 s -1 ) and in the dark on subsequent recovery of photosynthesis in chilling-sensitive sweet pepper leaves were investigated by comparing the ratio of quantum yields of photosystem (PS) 2 and CO 2 assimilation, Φ PS2 /Φ CO2 , measured in normal air (21 % O 2 , NA) and low O 2 -air (2% O 2 , LOA), and by analyzing chlorophyll (Chl) a fluorescence parameters. Chilling treatment in the dark had little effect on F v /F m and Φ PS2 /Φ CO2 , but it caused the decrease of net photosynthetic rate (P N ) under saturating irradiance after 6-h chilling treatment, indicating that short-term chilling alone did not induce PS2 photoinhibition. Furthermore, photorespiration and Mehler reaction also did not obviously change during subsequent recovery after chilling stress in the dark. During chilling treatment under LI, there were obvious changes in F v /F m and Φ PS2 /Φ CO2 , determined in NA or LOA. F v /F m could recover fully in 4 h at 25 °C, and Φ PS2 /Φ CO2 increased at the end of the treatment, as determined in both NA and LOA. During subsequent recovery, Φ PS2 /Φ CO2 in LOA decreased faster than in NA. Thus the Mehler reaction might play an important role during chilling treatment under LI, and photorespiration was an important process during the subsequent recovery. The recovery of P N under saturating irradiance determined in NA and LOA took about 50 h, implying that there were some factors besides CO 2 assimilation limiting the recovery of photosynthesis. From the progress of reduced P700 and the increase of the Mehler reaction during chilling under LI we propose that active oxygen species were the factors inducing PS1 photoinhibition, which prevented the recovery of photosynthesis in optimal conditions because of the slow recovery of the oxidizable P700.
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