Physiological Mechanism of AMF Regulating the Growth of Trifoliate Orange Under Low Temperature Stress

In recent years, low temperature and freezing weather has seriously threatened the development of citrus industry. Arbuscular mycorrhizal (AMF) can enhance the absorption and utilization of nutrients and water and the tolerance to abiotic stresses. In this study, potted pot experiments were conducted to study the effects of AMF on low temperature stress of citrus (trifoliate orange, Poncirus aurantius) with AMF (Diversispora epigaea, D.e). The results showed that AMF inoculation significantly increased plant height, stem diameter, leaf number, above-ground and underground fresh weight, maximum light quantum efficiency (QY_max), steady-state light quantum efficiency (QY_Lss), net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci). The contents of soluble sugar, soluble protein, proline (Pro), catalase (CAT) activity and superoxide Dismutase (SOD) activity were significantly increased. Compared with 25℃, -5℃ stress significantly increased the relative electrical conductivity of plants, increased the contents of malondialdehyde (MDA), hydrogen peroxide (H2O2), soluble sugar, soluble protein and Pro, and enhanced the activities of CAT and SOD. Significantly reduced NPQ_Lss, QY_max, QY_Lss, Pn, Gs, Ci, and Tr. Under -5℃ stress conditions, QY_max and QY_Lss of AMF treatment were significantly higher than those of untreated group. It can be concluded that AMF can increase the activity of PSⅡ reaction center in leaves, improve the light energy conversion efficiency and electron transfer ability of leaves, promote the photosynthetic function of PSⅡ, and improve the tolerance to low temperature. Under -5℃ stress, Pn and Tr values of AMF treatment were significantly higher than those of the untreated group, but Gs and Ci had no significant changes. So, AMF could alleviate the restriction effect of low temperature stress on photosynthetic capacity of plants, that is, AMF-infected plants could enhance photocooperation to improve the cold resistance of trifoliate orange. Under -5℃ stress, the contents of soluble sugar and Pro as well as the activities of CAT and SOD in AMF group were significantly higher than those in AMF unvaccinated group. It can be concluded that AMF can enhance the antioxidant capacity and cold resistance of plants by increasing osmotic regulatory substances and antioxidant oxidase activities. In conclusion, AMF inoculation can promote the growth of aboveground and underground parts of trifoliate orange seedlings, and enhance their stress resistance under low temperature stress by enhancing photosynthesis, increasing the content of osmoregulatory substances and enhancing their antioxidant.