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Abstract
Calcium (Ca2+) is an essential nutrient element for plants as it stabilizes the membrane system structure and controls enzyme activity. To investigate the effects of Ca2+ on plant growth and leaf photosynthetic electron transport in oat (Avena sativa) under NaCl stress, oat seeds and plants were cultivated in nutrient solutions with single NaCl treatment and NaCl treatment with CaCl2 amendment. By measuring the seed germination rate, plant growth, Na+ and Cl- accumulation in leaves, ion leakage in seedlings and leaves, prompt chlorophyll a fluorescence (PF) transient (OJIP), delayed chlorophyll a fluorescence (DF), and modulated 820 nm reflection (MR) values of the leaves at different growth phases, we observed that Ca2+ alleviated the inhibition of germination and plant growth and decreased Na+ and Cl- accumulation and ion leakage in the leaves under NaCl stress. NaCl stress changed the curves of the OJIP transient, induced PF intensity at P-step (FP) decrease and PF intensity at J-step (FJ) increase, resulted in obvious K and L bands, and altered the performance index of absorption (PIABS), the absorption of antenna chlorophyll (ABS/RC), electron movement efficiency (ETo/TRo), and potential maximum photosynthetic capacity (FV/FM) values. With the time extension of NaCl stress, I1 and I2 in the DF curve showed a decreasing trend, the lowest values of MR/MRO curve increased, and the highest points of the MR/MRO curve decreased. Compared with NaCl treatment, the extent of change induced by NaCl in the values of OJIP, DF and MR was reduced in the NaCl treatment with CaCl2 amendment. These results revealed that Ca2+ might improve the photosynthetic efficiency and the growth of salt-stressed plants by maintaining the integrity of oxygen-evolving complexes and electron transporters on the side of the PSI receptor and enhancing the relationship between the functional units of the photosynthetic electron transport chain. The findings from this study could be used for improving crop productivity in saline alkali lands.
Highlights
Soil salinity can seriously affect crop growth at any stage of plant growth, and can reduce economic yield [1]
Under the NaCl treatment, the lowest point of decline in the modulated reflection (MR)/MRO transient emerged earlier than that of the NaCl treatment with CaCl2 amendment. These results showed that Ca2+ alleviated the damage on photosystem II (PSII) and on some sites of the photosynthetic electron transport chain under NaCl stress
In the present study, according to the increase in the ion leakage in the oat seedlings and leaves, we concluded that the 150 mM NaCl treatment damaged the cells in the oat seedlings, and the 300 mM NaCl treatment damaged the cells in the leaves of oat plants
Summary
Soil salinity can seriously affect crop growth at any stage of plant growth, and can reduce economic yield [1]. Sodium chloride (NaCl), one of the major salts in saline soils, usually causes Na+ and Cl−toxicity and a low external osmotic potential, inducing the formation of reactive oxygen species (ROS), which could lead to oxidative damage in plants under salinity stress [2,3]. ROS, Na+ and Cl-, can damage various cell components through oxidative stress and ion toxicity, and can interfere with the normal physiological processes of plant cells [2,3]. Some photosynthetic electron transfer signals, such as instantaneous fluorescence (PF), delayed fluorescence (DF), and 820 nm modulated reflection (MR), measured synchronously by M-PEA, can reflect the changes in the light energy absorption, transmission, and dissipation, revealing the changes in the photosynthetic processes in stressed plants [9,10,11,12,13]
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