Evidence for the regulation of leaf movement by photosystem II activity

Abstract

Some light-demanding plants fold leaves under high light and open leaves under low light. Since paraheliotropism leaf movement under high light could be induced by environmental stresses that aggravate photoinhibition of photosystem II (PSII), we hypothesize that the leaf movement is regulated by PSII activity. To test this hypothesis, we examined photosynthetic gas exchange and chlorophyll fluorescence, diurnal changes in leaf angle, and maximum quantum yield of PSII (Fv/Fm) in a leaf-foldable canopy liana, Bauhinia tenuiflora, grown in an open field. The rate of CO2 assimilation, electron transport rate, and non-photochemical quenching was saturated under light of 1000μmolphotonsm−2s−1. After exposure to high light of 1000μmolphotonsm−2s−1 and 2000μmolphotonsm−2s−1 for 8h, Fv/Fm decreased to 54% and 19%, respectively. The repair of photodamaged PSII was strongly suppressed when illuminated at high light. After high light treatment, the recovery of PSII activity under low light functions efficiently. During the daytime, the minimum values of Fv/Fm and leaf angle were 0.75° and 9°, respectively. In the early morning, high Fv/Fm was accompanied by large leaf angle. At noon, when Fv/Fm was lower than 0.8, B. tenuiflora strongly folded its leaves. In the afternoon, when Fv/Fm was higher than 0.8, leaf angle gradually increased. After treatment with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, an inhibitor of PSII), Fv/Fm and leaf angle decreased significantly. Based on these results, the leaf movement in B. tenuiflora can be regulated by PSII photoinhibition, suggesting that PSII photoinhibition plays a potential role in the regulation of leaf movement under environmental stresses.