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Responses of Microstructure, Ultrastructure and Antioxidant Enzyme Activity to PEG-Induced Drought Stress in Cyclocarya paliurus Seedlings

Drought is one of the most important abiotic constraints on agricultural productivity, while global warming leads to the occurrence of more frequent drought events. Cyclocarya paliurus is a multiple-function tree species with medicinal value and timber production, but no information is available on its drought tolerance. In this hydroponic experiment, variations in leaf anatomical morphology, chloroplast ultrastructure, stomatal characteristics, and antioxidant enzyme activities were investigated under six levels of polyethylene glycol 6000 (PEG)-induced drought treatments to assess the drought adaption and physiological response of C. paliurus seedlings. The results showed that PEG-induced drought treatments reduced leaf epidermis, spongy tissue, leaf vein diameter, and spongy ratio, whereas the ratio of palisade tissue to spongy tissue, cell tense ratio, and vein protuberant degree all increased with enhancing the PEG6000 concentrations. Significant differences in stomatal width, stomatal aperture, and stomatal density existed among the treatments (p < 0.01). The stomatal aperture decreased significantly with the increase in PEG6000 concentrations, whereas the greatest stomatal density was observed in the 15% PEG6000 treatment. Compared with the control, higher drought stresses (20% and 25% PEG concentrations) caused damage at the cellular level and chloroplast lysis occurred. PEG6000 treatments also promoted the activities of SOD, POD, and CAT in C. paliurus seedlings, but this increase was insufficient to deal with the membrane lipid peroxidative damage under the high PEG concentrations. Correlation analysis indicated that in most cases there were significant relationships between leaf anatomical characteristics and antioxidant enzyme activities. Our results suggested that C. paliurus seedlings would not survive well when the PEG6000 concentration was over 15% (equal to soil water potential of −0.30 MPa).