Abstract
The susceptibility of photosystem I (PSI) and photosystem II (PSII) to chilling stress depends on plant species, and cyclic electron flow (CEF) plays an important role in photoprotection for some species under short stress periods. However, little is known about the responses of PSI and PSII to long-term chilling stress. We studied two orchid species—Cymbidium sinense and C. tracyanum— that differ in their capacity to adapt to low temperature, and exposed plants for 19 d to stress conditions that included 4°C and a light intensity of 250 to 350 μmol photons m-2 s-1. Meanwhile, we investigated their dynamic variations in Chl fluorescence and P700 parameters. After exposure to 4°C and 250 μmol photons m-2 s-1 for 6 h, PSI activity was maintained stable in both species, but stronger PSII photoinhibition was observed in C. sinense. During the long-term treatment, the maximum quantum yield of PSII was significantly reduced, with that decrease being greater in C. sinense. After 19 d of chilling treatment, the maximum photo-oxidizable P700 declined only slightly in C. tracyanum but dropped significantly in C. sinense. Linear electron flow was largely depressed during the long-term chilling treatment, especially in C. sinense. Meanwhile, C. tracyanum showed higher CEF activity than C. sinense. These results indicate that PSII is more sensitive to chilling-light stress than PSI in both species. The rate of PSII photodamage at chilling-light stress is higher in C. sinense than C. tracyanum, and CEF contributes to photoprotection for PSI and PSII under long-term chilling stress in C. tracyanum.
Highlights
Members of the genus Cymbidium are well-known horticultural plants
Kuang and Zhang (2015) have reported that, under strong irradiance, the activities of photosystem II (PSII) and photosystem I (PSI) are more drastically decreased in C. sinense than in C. tracyanum, and they have suggested that activation of cyclic electron flow around PSI (CEF) in C. tracyanum under high light is a primary photoprotective mechanism
We addressed the following questions: (1) Do these species show differential responses by PSII and PSI during long-term chilling, and (2) Is CEF relatively stimulated under such stress to alleviate photoinhibition? As part of our examination, we tested the hypothesis that C. tracyanum is less vulnerable to chilling-light stress than C. sinense because plants of the latter species are naturally distributed at lower elevations, are not as well-adapted to long-term low temperature
Summary
Members of the genus Cymbidium are well-known horticultural plants. most of its species are seriously endangered due to human activities (especially over-collection), destruction of natural habitats, and climate change (Luo et al, 2003; Liu et al, 2009). Photoinhibition is a phenomenon in which photosynthetic efficiency declines because the input of photons exceeds photosynthetic requirements (Powles, 1984) In cucumber, both PSII and PSI activities are significantly reduced after treatment for 5 h at 4◦C and 200 μmol photons m−2 s−1 (Sonoike, 1995, 1999). CEF is an important mechanism by which PSI is protected from photoinhibition under high light in Arabidopsis thaliana (Munekage et al, 2002; Kono et al, 2014; Tikkanen et al, 2014). As part of our examination, we tested the hypothesis that C. tracyanum is less vulnerable to chilling-light stress than C. sinense because plants of the latter species are naturally distributed at lower elevations, are not as well-adapted to long-term low temperature We addressed the following questions: (1) Do these species show differential responses by PSII and PSI during long-term chilling, and (2) Is CEF relatively stimulated under such stress to alleviate photoinhibition? As part of our examination, we tested the hypothesis that C. tracyanum is less vulnerable to chilling-light stress than C. sinense because plants of the latter species are naturally distributed at lower elevations, are not as well-adapted to long-term low temperature
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