Effect of Na₂SiO₃ on leaf photosystem Ⅱ (PS Ⅱ) under alkaline salt stress in Shine Muscat grape

Abstract: 【Objective】Soil salinization poses as a significant environmental factor that constrains grape productivity and quality. Silicon, recognized as a beneficial element, demonstrates a commendable mitigating effect against a range of biotic and abiotic stresses. Its influence on photosynthetic rate plays a pivotal role in determining crop yield. Nevertheless, the underlying mechanism by which silicon alleviates the damage inflicted on photosystem Ⅱ (PSⅡ ) by stress remains elusive. In this experiment, the chlorophyll fluorescence parameters were measured and the effect of silicon on the PSII of Shine Muscat grape leaves was explored under salt stress, so as to provide not only a theoretical support for the alleviation of fluorescence characteristics of grape leaves under salt stress by silicon salt, but also a technical guidance for grape cultivation in saline alkali soil.【Methods】Forty annual grapevine cuttings of Shine Muscat, exhibiting uniform and robust growth without any signs of pests or diseases, were se-lected as the experimental subjects. Four treatments were established: control (distilled water), sodium silicate (Na2SiO3, 2 mmol·L-1 ), alkaline salt (NaHCO3, 100 mmol·L-1 ), and a combined treatment with both Na2SiO3 and NaHCO3 (2 mmol·L-1 + 100 mmol·L-1 ). Each treatment was replicated for ten times to ensure reproducibility and statistical reliability. The grape cuttings were irrigated with 1 L of the respective treatment solutions, administered in four equal installments of 0.25 L each, to gradually achieve the desired concentration gradient. All other management practices remained uniform across all treatments. Chlorophyll fluorescence parameters were measured using a Handy PEA rapid fluorescence analyzer (Hansatech, UK). The rapid chlorophyll fluorescence induction kinetic curves (OJIP) and associated parameters were recorded for grape leaves from each treatment. Ten leaves per treatment were assayed, with each measurement repeated for five times to ensure accuracy. Prior to measurement, the leaves were dark- adapted for 20 minutes using leaf clips, followed by a 1- second saturation pulse of 3000 μmol·m-2 ·s -1 light to induce fluorescence. The OJIP fluorescence induction curves were subjected to JIP-test analysis to normalize the relative fluorescence values. The differences in fluorescence values between the treatments and the control (CK) were represented as ΔVt. Additionally, JIP- test analysis was performed on the OJIP curves to extract additional fluorescence parameters. Based on these findings, the schematic diagrams were constructed to illustrate the structure and function of leaf and chloroplast membranes, providing a visual representation of the physiological responses of grape leaves to the treatments.【Results】The results showed that, in comparison to the control (CK), the Fv/Fm and PIabs values of Shine Muscat grape leaves exhibited an increase, following the treatment with Na2SiO3 alone. Conversely, the application of NaHCO3 led to a significant decrease in these parameters. The combined treatment with Na2SiO3 and NaHCO3 resulted in a significant enhancement compared to the NaHCO3 treatment alone, indicating that silicon effectively mitigated the inhibitory effect of the alkaline salt on leaf PSII activity. The analysis of the OJIP curve revealed that, compared to CK, the relative fluorescence intensity and relative variable fluorescence values at points L, K, and J were significantly elevated with NaHCO3 treatment, and meanwhile φPo, φEo, ΨEo and Sm decreased by 15.04% , 38.17% , 27.31% and 4.44%, respectively. However, after the combined treatment with Na2SiO3 and NaHCO3, the trend was opposite to that of NaHCO3 treatment, and φPo, φEo and ΨEo values significantly increased by 15.52%, 54.71% and 34.18%, respectively. Additionally, the photosynthetic performance indices, including ABS/CSm, TRo/CSm, ETo/CSm, RC/CSm and ETo/RC, were found to be the lowest under NaHCO3 treatment condition, with decreases of 11.06%, 24.42%, 45.01%, 31.12% and 20.27%, respectively, compared to CK. The combined application of Na2SiO3 and NaHCO3 significantly increased these values by 15.50%, 33.37%, 78.44%, 28.58% and 39.37%, respectively, compared to the NaHCO3 treatment. The DIo/CSm and ABS/RC values showed opposite trends. Compared to CK, the difference was not significant after Na2SiO3 treatment, while it increased significantly under NaHCO3 treatment condition, increasing by 104.86% and 28.72%, respectively. Compared to NaHCO3 treatment, the combined treatment with Na2SiO3 and NaHCO3 significantly decreased by 10.93% and 64.09%.【Conclusion】In summary, the detrimental effects of NaHCO3 on grape leaves were primarily evident in diminishing the density of reaction centers per unit leaf area and suppressing the electron transfer capacity of individual reaction centers. Conversely, Na2SiO3 exhibited beneficial properties, augmenting the number of reaction centers, elevating the absorption and entrapment capabilities of light energy, and mitigating the electron transfer impediment in each reaction center. Consequently, it alleviated the adverse impact of salt stress on plants.