Photosynthesis and physiological characteristics of ‘Yanfu 6' apple under saline-alkali stress on different rootstocks

Abstract: 【Objective】Apple is the most important temperate deciduous fruit tree in the world. In the Loess Plateau area of Northwest China, apple industry has become the pillar industry and main income source. However, salination and alkalization of soil which interferes the normal growth and metabolism of plants, is increasingly serious and seriously restricting the apple industry in the northwest loess plateau. Salt-resistant apple rootstocks are commonly used to alleviate saline-alkali stress. It is important to reveal the mechanism of rootstock responding to the stress. Rootstocks affect the stress resistance, adaptability and productivity of scion varieties. In this study, photosynthesis and physiological characteristics of two year old Malus pumila‘Yanfu6' (Y6) grafted on M. halliana (Malus halliana Koehen) (CS) and M. hupehensis (Malus hupehensis) (PY) , grown under saline-alkali stress were analyzed, These scion-rootstock combinations are considered suitable in central Gansu province.【Methods】Tenleaf grafted plants grown in pots were used in the experiment and assigned to the control (Hoagland nutrient solution) and saline-alkali stress (Hoagland nutrient solution + 100 mmol · L^-1 NaCl + NaHCO3) groups. The effects of saline-alkali stress on photosynthetic pigments (chlorophyll a (Chl a) and chlorophyll b (Chl b) ) , photosynthetic characteristic indices (net photosynthetic rate (Pn) , stomatal conductance (Gs) , transpiration rate (Tr) and intercellular CO2 concentration (Ci) ) , chlorophyll fluorescence parameters (such as quantum yield Y (NPQ) , non-regulated energy dissipation quantum yield Y (NO) , non-photochemical quenching coefficient (qN) and photochemical quenching coefficient (qP) ) , and antioxidant enzymes (superoxide dismutase (SOD) and peroxidase activities (POD) ) , and relative conductivity (REC) were examined every twenty days from the beginning of treatment. Finally, correlations among the indices were analyzed.【Results】The results showed that with the prolongation of salt-alkali stress, the REC, Y (NO) and qN of the two combinations showed an increased trend. For chlorophyll content, Pn, Gs, Tr and qP showed a decreasing trend. Whereas the change trends in Ci, Y (NPQ) , SOD and POD were different between Y6 + CS and Y6+PY. After 40 d of stress, the REC of Y6+CS was significant lower than that of Y6 + PY, and was increased by 20.77% and 42.39%, respectively. The decrease in chlorophyll content of Y6+CS was significantly lower than that of Y6 + PY. Compared to the control, chlorophylls decreased by 23.90% and 52.20% in Y6+CS and Y6+PY, respectively. The Y (NO) of Y6 + CS was significant lower than that of Y6 + PY, and decreased by 32.00% and 49.18%, respectively. The Ci of Y6 + CS leaves showed a constantly decreasing trend, while that of Y6 + PY leaves decreased first and then increased. Y (NPQ) , SOD and POD of Y6 + CS leaves showed an upward trend, while Y (NPQ) of Y6 +PY leaves showed a decreasing trend. With the extension of stress, SOD and POD activities first increased and then decreased.【Conclusion】The correlation analysis of the 12 indices showed that Pnof the two combinations had a positive relation with Gs, Ci, Tr, Chl and qP, but a negative correlation with REC, POD, SOD, Y (NPQ) , Y (NO) and qN. Under saline-alkali stress, the structure and function of cell membrane were destroyed, leading to increased membrane leakage, function loss of enzymes and disturbed metabolism. Damage of cell membrane in trees with strong resistance was relatively small, and the POD activity of Y6 + PY increased first and then decreased with the prolongation of saline-alkali stress.The enzyme increase helps to remove hydrogen peroxide. The decrease might be due to the increased severity of stress. Then senescence of the organs occurred with the outbreak of reactive oxygen species, which damaged the cell membrane integrity and functions. Therefore, Y6+CS had a better adaptability to saline alkali soils by increasing SOD and POD activities that protect cells from oxidative stress. The study provides reference for selecting suitable apple rootstocks in Northwest China saline-alkali soils.