Effects of exogenous sugars on root sugar metabolism and antioxidant system under sub-low root-zone temperature in Malus baccata Borkh

【Objective】In the northern region, the ambient temperature rises faster in the early spring period, and the soil temperature changes are relatively lagging behind, resulting in the roots of Malus baccata Borkh. often being in sub-low temperature.“Sub-low temperature”refers to a low temperature that is above 0 ℃ and lower than the optimal temperature for plant growth and development, but it does not cause plant death. The roots not only anchor the plant and absorb nutrients and water but also sense changes in the soil environment and transmit adverse signals to aboveground part to regulate plant growth and development. Currently, most of the researches on plant response to low temperature stress focus on the aboveground part of the plant, while the root is more sensitive to temperature changes. 1 ℃ change in root zone temperature can cause significant changes in plant growth compared to air temperature. Sugar has an important role in plant metabolism and growth. And, sugar not only serves as structural material and an energy source but also has signaling functions to regulate the expression of relevant genes and activities of relevant enzymes. Sugar accumulation is considered as a defense re-sponse to various abiotic stresses. When plants are under stress, exogenous sugars can regulate ROS levels, secondary metabolism, gene expression, and interactions among hormones. To explore the physiological roles of different exogenous sugars in regulating the response of M. baccata Borkh. roots to sublow temperature, in this study, we used LT treatments with exogenous sucrose, glucose and fructose to analyze the sugar metabolism and antioxidant systems in the roots of M. baccata Borkh., with a view to providing theoretical references for the study on low temperature acclimatization of the apple roots. 【Methods】This experiment was carried out at the research base of Shenyang Agricultural University, and seedlings of M. baccata Borkh. were used as the test material. The seeds were stratified in sand to promote germination, and thereafter they were sown in cavity trays, and transplanted to 16 cm×16 cm plastic pots filled with sand for culture when the seedlings grew to 7-8 true leaves, and the Hoagland nutrient solution was applied every other day. When the true leaves had grown to 15, the robust seedlings with no pests and diseases were selected for conducting the experimental treatments. M. baccata Borkh. seedlings were pre-cultured in an artificial climate chamber with culture conditions set at 20 ℃ (day)/ 10 ℃ (night), 14 h of light (300 μmol·m−2 ·s −1 light intensity), and 70% relative humidity. In this study, five treatments were set up: control (CK), sub-low root zone temperature (L, 5 ℃±0.2 ℃), sub-low root zone temperature + sucrose (LS), sub-low root zone temperature + fructose (LF), and sub-low root zone temperature + glucose (LG), with 15 seedlings per treatment for three biological replicates. The sugar solution was applied in a volume of 250 mL. After 48 h treatment period, root samples were collected. 【Results】After 48 h of L treatment, the activities of sucrose phosphate synthase (SPS), sucrose synthase (SS), acid invertase (AI), neutral invertase (NI), hexokinase (HK) and phosphofructokinase (PFK) were significantly elevated, and the contents of sucrose, glucose, and fructose significantly increased, while the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) significantly increased, and ascorbate peroxidase (APX) and glutathione reductase (GR) activities were not significantly changed. The contents of GSH, DHA and GSSG significantly increased, the ratio of AsA/DHA significantly decreased, and the GSH/GSSG ratio was not significantly changed. Reactive oxygen species (ROS) level, malondialdehyde (MDA) contents and relative conductivity significantly increased. Compared with L treatment, all three exogenous sugar treatments increased the activities of key enzymes of sugar metabolism and antioxidant system in the roots of M. baccata Borkh. to varying degrees, simultaniously promoting the accumulation of antioxidant contents (AsA and GSH) and sugar contents in the roots under sub-low root zone temperature, which in turn reduced the ROS level, MDA contents and relative conductivity of the roots. The exogenous sucrose treatment was better than fructose and glucose treatments in alleviating the oxidative damage in M. baccata Borkh. roots under sublow root zone temperature.【Conclusion】This study provides new insights into the regulation of sugar in the roots of M. baccata Borkh. in response to the L treatment. Sub-low root zone temperature for 48 h increased ROS level, MDA contents and relative conductivity in the roots of M. baccata Borkh., leading to membrane lipid peroxidation in the roots. Exogenous addition of sucrose could alleviate the oxidative damage induced by reactive oxygen species in the roots by regulating the sugar metabolism and antioxidant system, and its effect was significantly better than that with glucose and fructose treatment and it may have the function of signaling molecules. The results of this study provide a scientific foundation for regulating root function in apple cultivation in cold climates.