Effects of salt stress on leaf physiology and anatomical structure of two resistant apple rootstocks

Abstract: 【Objective】Mauls domestica Borkh. is one of the most important cultivated fruit trees in temperate regions. The Northwest Loess Plateau has sufficient sunlight, large diurnal temperature differ- ence, and suitable altitude. It has been the world’s largest apple eugenic area, but more than 70% of this area is saline-alkali land. In addition, due to the lack of organic fertilizers and excessive application of chemical fertilizers, secondary salinization of the soil is becoming serious, which restricts the further de- velopment of apple industry in this region. Therefore, it is of great significance to select and apply root- stocks with strong resistance for high-quality and high yield of fruit trees. Previous studies mostly focused on the comparison of salt tolerance among different varieties, but there were no reports on physio- logical mechanisms behind their difference in tolerance. Related research of perennial plants including fruit trees focus on photosynthesis, antioxidant system and fruit quality, while studies on chlorophyll metabolism and endogenous hormones are rarely reported. M. halliana Koehne is distributed in Hexi Corridor of Gansu, which has the characteristics of drought tolerance and saline alkali tolerance. M. bac- cata Borkh., native to the northeast, is resistant to cold and drought but not tolerant to salt and alkali  stresses. In this study, two apple rootstocks with significant difference in resistances, M. halliana‘9-1- 6’(selected by this research group) and M. baccata, were used to determine the changes in the activi- ties of key enzymes in chlorophyll metabolism under salt stress. The relationship between endogenous hormones and the anatomical structure of the leaves and the saline environment was analyzed. The study would enrich the physiological and ecological understandings of salt tolerance of apple root- stocks.【Methods】The experiment consisted of two treatments, the control (pH 6.8) and salt stress treat- ment (pH 7.0). Each treatment had 5 biological replicates each with 4 pot plants. According to previous experiments, 100 mmol ∙ L- 1 NaCl was used for the salt stress treatment. The materials were one- year- old 9-1-6 and M. baccata seedlings. The Hogland nutrient solution was applied for the potted seed- lings. After short-term (3 d) and long-term (40 d) exposure to salt stress, relative chlorophyll content (SPAD), key enzymes of chlorophyll metabolism (Chlase, MDcase and PaO), contents of endogenous hormones (IAA, ABA, iPA, ZT, JA) and anatomical structure of the leaves were analyzed and com- pared. 【Results】Compared with control, SPAD of the leaf of M. halliana‘9-1-6’and M. baccata was decreased by 44.56% and 74.00%, the activity of chlase increased by 42.83% and 32.44%, MDcase ac- tivity increased by 70.59% and 68.52%, and PaO activity increased by 87.70% and 72.73% by salt stress, respectively. There was no significant difference between the two rootstock leaves in Chlase ac- tivity after 40 days of salt stress. However, MDcase and PaO activities in‘9-1-6’leaves were signifi- cantly lower than those in M. baccata. In addition, under salt stress, IAA content of the leaves de- creased by 7.17% and 20.24%, ZT by 26.98% and 52.65%, iPA by 50.04% and 43.59%, and IAA/ABA by 10.53% and 23.47% in‘9-1-6’and M. baccata, respectively, but ABA increased by 6.28% and 0.96%, JA by 110.88% and 115.03%, respectively. After 40 days of salt stress, the decrease in iPA of‘9-1-6’blades was significantly higher than that of M. baccata, while the declines in IAA, ZT and IAA/ABA were significantly lower in M. baccata. In addition, the content of ABA in leaves of‘9-1-6’was significantly higher than that of M. baccata after salt stress for 40 days, while the content of JA in leaves of‘9-1-6’was significantly lower than that of M. baccata. Under salt stress, leaf thickness, epi- dermal thickness, palisade tissue thickness and CTR of the two rootstocks were significantly reduced, but the decline rates were different. Compared with CK, leaf thickness, epidermal thickness, palisade tissue thickness and CTR of the leaves of‘9-1-6’decreased by 24.52%, 39.71%, 28.04%, 37.08%, and 16.65%, respectively, while that of M. baccata leaves, decreased by 33.87%, 47.85%, 31.02%, 53.62% and 29.89%, respectively. The decline in‘9-1-6’was significantly smaller than in M. baccata.The thickness and SR of the sponge tissue in the two rootstocks increased significantly. The thickness and SR of the sponge tissue of‘9-1-6’leaves were significantly smaller than those of M. baccata.【Conclusion】Under salt stress, the chlorophyll metabolism enzyme activity, endogenous hormones and leaf anatomical structure of the two rootstocks changed significantly. In the high salt-resistant M. halliana‘9-1-6’photosynthetic damage was alleviated by maintaining high SPAD, thick leaves and in- tact double-layered palisade tissue, and by regulating of the activities of chlorophyll metabolizing en- zymes and balancing endogenous hormones content (IAA, ZT and ABA).