Effects of drought-rehydration on photosynthetic capacity, chlorophyll fluorescence, and microstructure of Cyclobalanopsis glauca seedling leaves in Karst Area of Northwest Guangxi

Abstract

[Objective] To explore the mechanism of Cyclobalanopsis glauca seedlings adapting to the drought-rehydration environment in Karst area, China, using the seedlings of C. glauca as materials, which providds theoretical basis for vegetation restoration and artificial afforestation in the region. [Methods] Water control experiments in pots were carried out. The effects of four different drought treatments, including control (-0.1 MPa), mild drought (-0.5 MPa), moderate drought (-0.9 MPa), severe drought (-1.5 MPa), and rehydration treatments, on photosynthesis, chlorophyll fluorescence, and anatomical structure of leaves were studied. [Results] (1) With the increase in drought stress intensity, the leaf relative water content, water potential, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci) were significantly decreased, while stomatal limitation (Ls) was increased significantly. Photosynthetic parameters under mild stress and instantaneous water use efficiency (WUE) under mild and moderate drought stress were not significantly affected. After rehydration, leaf water parameters, Pn, Tr, Gs, Ci, and WUE under drought stress were higher than those before rehydration, while Ls was lower than those before rehydration. The leaf water conditions and photosynthetic parameters under mild drought stress were better than those of the control after rehydration. Only Ls under moderate drought stress did not recover to the level of the control after rehydration. Neither leaf water content nor photosynthetic parameters under severe drought stress were recovered to the level of the control after rehydration. (2) With the increase in drought stress intensity, the initial fluorescence (Fo) of leaves were increased significantly, while the maximum fluorescence (Fm), maximum photochemical quantum yield (Fv/Fm) and potential photochemical efficiency (Fv/Fo) of leaves were decreased significantly. There were significant differences in these parameters between mild drought stress and control. After rehydration, Fm, Fv/Fm, and Fv/Fo under drought stress were higher than those before rehydration, whereas Fo was slightly lower than that before rehydration. All the chlorophyll fluorescence parameters after rehydration under mild stress recovered to or better than those of the control. Fo under moderate and severe drought stress did not return to the control level after rehydration, and Fv/Fm under severe stress was only 0.75 after rehydration. (3) With the increase in drought stress, leaf thickness, upper and lower epidermis thickness, stomatal density, and main vein conduit diameter were increased significantly, the length and width of stomatal apparatus, stomatal opening area, and sponge tissue thickness were decreased significantly, while palisade tissue thickness, ratio of palisade tissue thickness to spongy tissue thickness, and main vein thickness were changed in the order of moderate > mild > control > severe drought stress. After rehydration, only the stomatal opening area and the thickness of main leaf veins under drought stress were increased significantly compared with that before rehydration. After rehydration, the leaf structural parameters under mild drought stress were restored to or better than those of the control. The stomatal opening area after rehydration in moderate drought stress was still significantly lower than that of the control. After rehydration, the stomatal opening area under severe drought stress failed to recover, and the thickness of main veins under severe drought stress was lower than that of the control. [Conclusion] C. glauca seedlings have strong drought tolerant ability and postdrought recovery ability, which are suitable for ecological restoration in Karst area. The seedlings need to be protected from moderate drought stress (-0.9 MPa) in the nurturing stage, to facilitate post-drought recovery