Effect of saline-alkali stress on leaf water deficit, gas exchange and microdistribution of eight mineral elements in Shatangju mandarin (Citrus reticulata)

Abstract:【Objectives】The experiment was undertaken to explore the physiological and ecological characteristics of Shatangju mandarin (Citrus reticulata‘Shatangju’) under saline-alkali stress, and to provide a scientific support for the introduction, cultivation and soil management in calcareous and saline- alkali soils.【Methods】The 3- year- old Shatangju mandarin (Citrus reticulata‘Shatangju’) trees grafted on the trifoliate orang [Poncirus trifoliate (L.) Raf.] were used as the test materials, and CaCO3, NaHCO3 and NaCl were used to simulate the main chemical components in calcareous soil, alkaline soil and saline soil, respectively. Two levels of addition of 0.3% and 0.7% compounds to the cultivation substrate were designed, and the trees without the addition of saline-alkali served as the control. During 90 d of cultivation, the leaf water status, gas exchange characteristics and effects on the microdomain distribution and balance of eight mineral elements were tested and analyzed.【Results】(1)Under salinealkali stress, the relative water content (RWC) in leaves decreased and the water saturation deficit (WSD) increased; the difference rose with the increase of saline- alkali stress intensity and treatment time, but was mainly dominated by the types of saline-alkali components in the substrate; the difference of WSD was greater than that of RWC; after 90 d of stress, compared with the control, under the stress of 0.3% CaCO3, 0.3% NaHCO3, 0.3% NaCl, 0.7% CaCO3, 0.7% NaHCO3 and 0.7%NaCl, the RWC decreased by 3.2% , 7.6% , 9.6% , 13.8% , 15.5% and 22.5% , respectively, and the WSD increased by 46.2%, 109.2%, 138.5%, 198.5%, 223.1% and 323.1%, respectively. (2)Under saline-alkali stress, the chlorophyll content (Chl C), net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) in leaves significantly decreased, the reduction range increased with the extension of the stress time, and the difference was large under different kinds of saline-alkali stress, which was generally consistent with the response of RWC and opposite to that of WSD; upto 90 d of stress, under the stress of 0.3% CaCO3, 0.7% CaCO3, 0.3% NaHCO3, 0.7% NaHCO3, 0.3% NaCl and 0.7% NaCl, Chl C content decreased by 4.5%, 16.5%, 15.4%, 39.7%, 31.4% and 49.4%, Pn decreased by 7.2%, 23.1%, 18.3%, 46.5%, 42.3% and 61.4%, Tr decreased by 3.8%, 8.3%, 11.8%, 32.9%, 38.4% and 63.0%, Gs decreased by 2.5% , 10.4%, 19.0%, 36.2%, 46.0% and 65.6%, respectively. (3) After 90 d of cultivation under saline-alkali stress, the relative contents of major mineral elements in the cells of different tissues in the leaves of Shatangju mandarin changed significantly. Among them, under CaCO3 stress, the relative contents of S, Cl and Ca in leaf palisade tissue cells increased by 18.8%, 12.2% and 13.6%, respectively; the relative contents of Na, Mg, P, K and Fe decreased by 24.6% , 24.0% , 22.5% , 9.8% and 43.5%. Under NaHCO3 stress, the relative contents of Na, S and Cl in palisade tissue cells increased by 163.6% , 31.8% and 73.8% , respectively; the relative contents of Mg, P, K, Ca and Fe decreased by 39.0%, 24.5%, 6.5%, 12.1% and 45.6%, respectively. Under NaCl stress, the relative contents of Na, S and Cl in palisade tissue cells increased by 183.1%, 183.1% and 78.1%, respectively; the relative contents of Mg, P, K, Ca and Fe decreased by 50.0%, 37.3%, 17.1%, 15.6% and 60.9%, respectively. (4) Under saline-alkali stress, the balance relationship of the relative contents of eight mineral elements in the leaf cells changed significantly, with the smallest change occurring under CaCO3 stress and the largest change under NaCl stress, and the change in phloem cells was significantly greater than that in xylem cells, and the change in palisade tissue cells was significantly smaller than that in sponge tissue cells. Among ten ratios of mineral elements in palisade cells, the ratios of Na/Cl, Ca/K, S/P and Mg/Fe increased significantly, but Ca/K ratio increased to a minimum, S/P ratio increased significantly, the other 6 ratios all decreased significantly, and K/Na ratio and K/Cl ratio decreased to a maximum, while S/ Cl and Mg/P decreased less. Under the stress of CaCO3, NaHCO3 and NaCl, the S/P ratios with the largest increase in palisade tissue cells increased by 61.1%, 80.6% and 141.7% on average, and the K/Na ratios with the greatest decrease reduced to 2.1%, 39.4% and 58.6% on average, respectively.【Conclusion】Shatangju mandarin was sensitive to NaCl stress, followed by NaHCO3 stress, and 0.7% CaCO3 content of could cause significant stress. Saline-alkali stress may lead to significant leaf water deficit and aggravation of photosynthetic physiology of Shatangju mandarin; the microdomain distribution, balance and ion homeostasis of the main mineral elements in the leaves also changed significantly under saline-alkali stress. Therefore, soil chemical properties and improvement should be paid a great attention to in the introduction, cultivation and orchard management of Shatangju mandarin.