Effects of High Light and Chilling Stress on Photosystem II Efficiency of Aloe vera L. Plants Probing by Chlorophyll a Fluorescence Measurements

Abstract

In this study, Aloe vera, as a CAM plant, was used to study photosystem II (PSII) activity in response to chilling (2 °C) and high light (HL, 1200 µmol m−2 s−1) stresses. Using chlorophyll (Chl) a fluorescence transient (OJIP) measurements, we indicated that the photosynthetic apparatus of Aloe vera was damaged at chilling, as evaluated from decrease in maximal efficiency of PSII photochemistry (F v/F m) and in performance index (PIABS) coupled with lower values of photosynthetic electron transport chain components including the electron transport flux (φE o) and the inferred oxygen evolving complex activity (F v/F o). Performance index exhibited negative correlation with hydrogen peroxide (H2O2) content, which indicated that the sensitivity of PSII photochemistry exposed to chilling stress was associated with higher H2O2 and malondialdehyde accumulation. In contrary, Aloe vera leaves accumulated Chl b and carotenoids to prevent photoinhibition of PSII under excess light. Interestingly, we found that F v/F m parameter remained unchanged under different light intensities, whereas the HL-treated plants had higher PIABS values than control plants, suggesting that this parameter is more sensitive to different light intensities than commonly used parameter F v/F m.