Abstract:

The objective of this study was to identify the regulation effects of exogenous brassinosteroids（BRs）on photosynthetic apparatus of cucumber（Cucumis sativus L.‘Jinyou 4’）under Ca(NO3)2（80 mmol ? L-1）stress. Exogenous 24-epibrassinolide（EBL）markedly increased the reduced ascorbate（AsA）and glutatione（GSH）levels in chloroplast，which were 2.44-fold and 1.40-fold to control. Enzymes activities of APX and GR in AsA-GSH cycle were enhanced by 36.27% and 105.83% respectively，compared to stress treatment. Application of EBL also changed the levels of xanthophyll cycle components，increasing V，Z and total xanthophylls levels and decreasing A level. The de-epoxidation state（DEPS）of xanthophyll cycle by EBL was increased by 9.60% compared to Ca(NO3)2-stressed plant. Under Ca(NO3)2 stress，EBL enhanced the NPQ and alleviated the decrease of ΦPSⅡ. EBL resulted in increases of unsaturated FAs levels，which was favored to the lipid membrane fluidity. These results strongly suggested that exogenous EBL is capable of protecting the chloroplast function through regulating the AsA-GSH cycle and heat dissipation capacity under Ca(NO3)2 stress.

Key words: 24-epibrassinolide, xanthophyll cycle, NPQ, AsA-GSH cycle, fatty acids, chloroplast