Effects Of Exogenous Trehalose On Leaf Photosynthesis And Fruit Quality Of "Gala" Apple Under Water Stress

The apple(Malus domestica,L.Borkh)is one of the most important economic fruit in the world and the one whose planting area and output rank among the highest in our country.The Loess Plateau region is the dominant and high quality producing area of apples in our country.But the Loess Plateau is located in arid and semi-arid regions,where water resources are scarce and precipitation is low.Therefore,water is the important limiting factor of apple quality in the Loess Plateau region.The soluble sugar content of apple fruit is not only an important quality index of apple fruit,but also a signal substance to affect the growth and resistance of apple.Trehalose,as a plant growth regulator and signaling substance,is pleiotropic in plants.However,there are few reports on trehalose’s role in apple fruit growth and development,sugar metabolism and drought resistance.In this paper,Gala/M26(Gala(Malus domestica Borkh cv.Gala)/M26(Malus pumila Mill.))apple as the experimental material,to study the effects of exogenous trehalose on leaf photosynthesis and fruit quality of apple under water stress.In order to clarify the theory of regulation of trehalose metabolism on apple quality and to provide theoretical and technical support for improving apple leaf photosynthetic capacity and fruit quality under water stress by using exogenous trehalose.The main research results are as follows:1.The trehalose metabolic pathway was found in ‘Gala’ apple fruit.Five key stages in the growth and development of ‘Gala’ apple were selected,56 days,74 days,92 days,110 days and 128 days after flowering.Through analysis of the transcriptome data of these five periods,it was found that there were trehalose metabolic pathways in apple fruits,and genes related to trehalose metabolic pathways were screened out.The gene expression of important enzymes in trehalose synthesis(trehalose-6-phosphate synthase,trehalose-6-phosphate phosphatase and trehalase),combined with the changes in trehalose-6-phosphate,trehalose and in the activity of trehalose synthesis enzymes,further confirmed the existence of trehalose metabolic pathway during the growth and development of apple fruit.2.Effects of water stress on growth,quality and glucose metabolism of apple fruit.Apple trees were treated with different degrees of water stress at the end of cell division and cell expansion.The results showed that heavy water stress treatment at the end of cell division did not affect the shape of apple fruit,but seriously affected the color and fruit size of apple fruit.Moderate water stress at the late stage of fruit cell expansion did not affect the appearance of‘Gala’ apple fruit,but increased the contents of soluble solids,fructose,glucose and sorbitol in apple fruit.Water stress also increased trehalose content in apple fruit by increasing trehalose 6-phosphate phosphatase activity and decreasing trehalase activity.3.Exogenous trehalose treatment improved photosynthesis and drought tolerance of apple leaves under water stress.Water stress decreased chlorophyll content,net photosynthetic rate,transpiration rate,stomatal conductance and intercellular carbon dioxide concentration in’Gala’ apple leaves,but also decreased the maximum photochemical quantum efficiency,actual photochemical quantum efficiency,photochemical quenching coefficient and relative electron transfer rate,and increased the non-photochemical quenching coefficient.Exogenous trehalose can increase the chlorophyll content in apple leaves and protect the photosynthetic apparatus by reducing the electron acceptor peroxidation of photosystem II through nonphotochemical quenching,thus promoting the electron transfer of photosystem II and maintaining the virtuous cycle of leaf photosynthetic apparatus operation,thus improving the photosynthesis under water stress.At the same time,exogenous trehalose reduced the content of superoxide anion radical and hydrogen peroxide,reduced the accumulation of reactive oxygen species,increased the activity of antioxidant enzymes,improved the antioxidant system,alleviated the oxidative damage caused by water stress,and thus improved the drought tolerance of plants.4.The treatment of exogenous trehalose can promote the accumulation of soluble sugar in apple fruit.Compared with the control,the single fruit weight and soluble solid content of’Gala’ apples were significantly increased after exogenous trehalose treatment.The analysis of key enzymes in sugar metabolism showed that exogenous trehalose could regulate acid invertase,sucrose phosphate synthase,trehalose 6-phosphatase and trehalase,which reduce the content of sucrose in apple fruits,and increase the content of fructose,glucose,sorbitol and trehalose.Meanwhile,exogenous trehalose can alleviate the effect of water stress on the growth and development of apple fruit.Under the same degree of water stress,exogenous trehalose treatment can increase the transverse diameter,longitudinal diameter and single fruit weight of apple fruits.At the same time,the contents of soluble solid,soluble sugar,abscisic acid and trehalose in apple fruit were increased.In conclusion,exogenous trehalose can be used as growth regulator to improve apple fruit quality.Exogenous trehalose can be used as a protective agent of water stress to improve the quality of apple fruit.