Relationships among MDA content,plasma membrane permeability and the chlorophyll fluorescence parameters of Puccinellia tenuiflora seedlings under NaCl stress

In order to study the mathematic relationship between changes of the chlorophyll fluorescence parameters of P. tenuiflora seedlings and the fine regulation mechanism of metabolism of cell membrane stability under salt stress, electrolyte leakage rate (EL) using relatively electrical conductivity method and malondialdehyde (MDA) content using the thiobarbituric acid method relating to cell membrane stability were determined as the variable of physiological parameters that could reflect the status of physiological system or injury extent by stress, and the fluorescence parameters such as Fv/Fm、Fv/Fo、qP、Fv′/Fm′、ΦPSⅡ、HDR and qNP were also determined by the method of fluorescence induction kinetics with FMS2- facilitative Fluorescence Monitoring System under normal condition and different degrees of NaCl stress (0.4%,0.8%,1.2%,1.6%,2%,2.4%) after 7d. The result showed that MDA content decreased a little under low NaCl stress (less than 1.2%), then increased significantly with increasing of NaCl concentration (more than 1.2%). Among the chlorophyll fluorescence parameters, Fv/Fo、Fv/Fm、Fv′/Fm′ and qP took a similar trend of increasing firstly then decreasing with the increasing of NaCl concentration, but qNP and HDR increased with the increasing of NaCl concentration accordingly. At the same time, ΦPSⅡ increased slightly under low NaCl stress(less than 0.4%), then decreased significantly, finally increased fastly. The result also showed that with increases in MDA content, Fv/Fo、Fv/Fm、qP and Fv′/Fm′ decreased accordingly, however, EL decreased slightly firstly in MDA content(0.9753～1.1901μmol·g~-1FW), then increased significantly with the increasing of MDA content(1.3080～1.8518μmol·g~-1FW), showing the similar trend of qNP and HDR. On the other hand, ΦPSⅡ displayed ascending trend during MDA content (0.9753～1.0953μmol·g~-1FW), then showed descending trend during MDA content (1.1172～1.1901μmol·g~-1FW), finally increased fastly with the increasing of MDA content(1.3080～1.8518μmol·g~-1FW). The change value could visually characterize the extent of injury of physiological system of plant under salt stress, and the quantitative relationship between the MDA content、EL and the chlorophyll fluorescence parameters probably was the reflection that chlorophyll fluorescence parameters were closely related to membrane lipid peroxidation and plasma membrane permeability of the P. tenuiflora seedlings. It could be concluded that membrane lipid peroxidation of P. tenuiflora, which resulted from the augment of active oxygen, could be mainly eliminated by highly active antioxidant enzymes system under low NaCl concentration(less than 1.2%); while the safeguard mechanism of P. tenuiflora could be different from that of other plants under midst NaCl concentration(more than 1.2%), they dissipated surplus light energy mainly via two approaches: one is to increase light energy absorbance by light-harvesting pigment that used in non-photochemical quenching coefficient (qNP) and heat dissipation rate (HDR), the other is to increase actually photochemical efficiency of PSⅡ in the light (ΦPSⅡ), building up pseudo-cyclic photophosphorylation. By these ways does P. tenuiflora protect photosynthetic apparatus from the damage of surplus of light energy, thus reduce membrane-lipid peroxidation.