Effects of a microbial reagent on the growth and chlorophyll fluorescence characteristics of Pyrus betulifolia Bunge seedlings under saline-alkali stress

【Objective】This study analyzed the effects of a reagent of an alkali-tolerant and halophilic microbial on the growth and chlorophyll fluorescence characteristics of Pyrus betulifolia Bunge seedlings under saline-alkali stress conditions, aiming to explore the potential application of this microbial agent to improve tree performance in saline-alkali soils.【Methods】The experiment used P. betulifolia Bunge as the test material, and composed of three treatment groups: the control group with fresh water treatment (CK), saline-alkali stress by simulating the main saline-alkali components of the soil in the saline-alkali land of the Yellow River Delta region (SA), and microbial reagent + SA (HSA). The liquid reagent of the alkali-tolerant and halophilic microbial was purchased from Shenyang Jiuli Fertilizer Industry Co., Ltd. After four weeks of treatment, samples were collected to determine the growth indicators, chlorophyll content, chlorophyll fluorescence parameters, and antioxidant enzyme activities under different treatments. The alleviating effect of the microbial reagaent treatment on saline-alkali stress in P. betulifolia Bunge seedlings was clarified by comparing and analyzing the changes in tree growth, and in some physiological and biochemical parameters under different treatments.【Results】The results indicated that saline-alkali stress led to the curling, chlorosis and necrosis of the leaves, starting from the tips, and the roots turned brown. After treatment with the microbial reagent, the chlorosis and necrosis symptoms in the seedling leaves disappeared, and the browning degree of the roots was decreased. Saline- alkali stress significantly inhibited the growth of P. betulifolia Bunge seedlings, and the absolute growth of leaves, leaf dry weight and root in terms of dry weight decreased by 82.76%, 12.57% and 26.71%, respectively, while that in the HSA group by 75.86%, 6.29% and 21.12%, respectively. This indicated that the application of the microbial reagent alleviated the inhibitory effect of saline-alkali stress on the growth of seedlings. Saline-alkali stress led to a significant decrease in chlorophyll content. The growth status of seedlings in the HSA group was significantly improved compared with SA, with SPAD value, chlorophyll a and chlorophyll b significantly recovered, increasing by 23.42% , 23.21% , 100.35%, and 44.58%, respectively. This indicated that the application of the microbial reagent alleviated the degradation of chlorophyll in the leaves of P. betulifolia Bunge seedlings caused by saline-alkali stress. In terms of chlorophyll fluorescence parameters, saline-alkali stress reduced the maximum photochemical efficiency of PS Ⅱ (Fv/Fm) and photosynthetic performance index (PIABS) by 14.41% and 76.63%, respectively. After the application of the microbial reagent, Fv/Fm and PIABS increased by 2.34% and 12.43%, respectively. In addition, the analysis of the rapid chlorophyll fluorescence induction kinetics curve (OJIP) showed that compared with CK, SA and HSA treatments significantly increased the fluorescence intensity at K and J phases, while reducing the fluorescence intensity at I phase (30 ms) and P phase (maximum fluorescence) of the leaves. However, the fluorescence intensity at K and J phases in the HSA treatment was lower than that in the SA treatment, while the fluorescence intensities of I and P phases were the opposite. Further standardization o f the OJIP curve showed that the ΔK-band in both SA and HSA treatments increased significantly, with the SA treatment being significantly higher than the HSA treatment. This indicated that the application of the microbial reagent alleviated the damage on the donor-side oxygen-evolving complex (OEC) and the acceptor-side electron transport of PSⅡ caused by saline- alkali stress. Subsequently, we analyzed the changes in antioxidant enzyme activities in the leaves and roots of P. betulifolia Bunge seedlings under different treatments. Compared with the control group, saline- alkali stress significantly increased the activity of superoxide dismutase (SOD) in the leaves of P. betulifolia Bunge seedlings by 29.5%. However, the activities of peroxidase (POD) and catalase (CAT) were reduced, among which the activity of CAT decreased significantly by 59.67%. The application of the microbial reagent increased the activities of SOD and POD in the leaves by 59.53% and 23.19%, and 15.05% and 78.33% compared to CK and SA groups, respectively. However, the CAT activity further decreased. Compared with CK, SOD and CAT activities in the roots in the SA and HSA groups both increased, and the CAT activity reached a significant difference, increasing by 116.67% and 337.50%, respectively; while the POD activity in the roots showed a decreasing trend, but no significant difference was observed.【Conclusion】In summary, the application of the reagent of alkali-tolerant and halophilic microbial effectively alleviated the degradation of photosynthetic pigments and damage to the photosystems, improved photosynthetic performance, enhanced the antioxidant defense system, and reduced the damage of saline-alkali stress to P. betulifolia Bunge seedlings. Therefore, the treatment significantly improved the saline-alkali tolerance of the seedlings. This study provides a technical basis for the ecological restoration of coastal saline-alkali soils and the saline-alkali-tolerant cultivation of P. betulifolia Bunge.