Comparative physiological responses and adaptive strategies of apple Malus halliana to salt, alkali and saline-alkali stress

Malus halliana Koehne (M. halliana) is a native rootstocks to the saline-alkali soil of Northwest China which shows a higher saline-alkali tolerance than other apple rootstocks. However, there are few studies of saline-alkali resistance on M. halliana. This study was aimed to elucidated the physiological adaptive mechanisms of M. halliana involved in the responce to three different stress factors. The salt stress (SS), alkali stress (AS) and saline–alkali stress (MAS) were applied to study the different responses of ion, photosynthesis, Chl fluorescence, antioxidant enzymes and osmolytes for 40 days. The results showed that M. halliana accumulated more Na+ in roots and less K+ in leaves under AS compared to the SS and MAS treatments. M. halliana plants adapted to three stresses by decreasing leaf water content (WC), stomatal conductance (Gs) and intercellular CO2 concentration (Ci), increasing water use efficiency (WUE) and accumulating osmolytes. However, the respond mechanisms of M. halliana to SS, AS and MAS were different. Under SS, M. halliana plants adapted to stress by improving the activity of superoxide dismutase (SOD) and peroxidase (POD) and starting thermal dissipation protection mechanism. Under AS, plants mianly accumulated the organic acids (OA) to adjust osmotic balance and triggered xanthophyll cycle to dissipate excess energy. Under MAS, plants kept the dynamic balance of photosynthetic system between injury and repair by starting heat dissipation mechanism and xanthophyll cycle. And it also mianly accumulated osmolytes to maintain osmotic regulation and improved the activity of antioxidant enzyme to avoid oxidative damage. In addition, the analysis of principal component found that Gs, Ci, WC, Chl a + b (Chlorophyll), Fm (maximum fluorescence), Fv/Fm (maximal photochemical efficiency), ETR(photosynthetic electron transport rate), ΦPSII (actual photochemical efficiency) and qP (photochemical quenching coefficient) were major influencing factors to the decreasing of Pn (net photosynthetic rate). In summary, the inhibitory effects of growth and photosynthesis on M. halliana under three stresses were AS > MAS > SS. Under MAS, the salt and alkail had a certain synergistic effect.