The Mitigative Effects Of Exogenous Strigolactones On Apple Seedlings Under Alkaline Stress

Apples(Malus domestica Borkh.)are one of the most widely grown fruit trees in the world,and China ranks first in the world in terms of apple cultivation area and total production.Apples are suitable for cultivation in slightly acidic to neutral soils,but at present the salinisation of soils in China’s main apple-producing regions is becoming more and more severe,with the p H of some apple orchards even exceeding 8.5,which is seriously limiting the sustainable development of the apple industry.Therefore,how to improve the alkali tolerance of apples has become an urgent problem.Strigolactones(SLs)are new plant hormones that play an important role in plant growth and development and in response to abiotic stresses.However,it is not yet clear whether the exogenous strigolactones can improve the alkaline tolerance of apple seedlings.In this study,we used apple rootstock‘Malus hupehensis Rehd.’seedlings as experimental material and analysed the effects of exogenous monogamma lactone analogue(rac-GR24)on growth and development,photosynthesis,antioxidant levels,osmotic and ionic balance,and organic acid synthesis and secretion of apple seedlings by leaves spraying GR24.The main results were as follows.The main results are as follows:(1)Exogenous GR24 could alleviate the damage caused by alkali stress to apple seedlings,and there was a significant dose effect,with 1μmol/L GR24 having the best alleviation effect.(2)Exogenous GR24 alleviated the inhibition of growth and development of apple seedlings by alkali stress.The growth of apple seedlings was significantly inhibited by the alkali stress treatment,while the spraying of GR24 under alkali stress conditions alleviated the yellowing of the leaves margins of apple seedlings,the leaves became larger and the number of new roots increased significantly.In addition,plant height,above-ground fresh weight,above-ground dry weight,root length,below-ground fresh weight,below-ground dry weight,and root vigour were up-regulated by 33.33%,21.05%,20.11%,96.17%,24.28%,61.36%and 36.36%,respectively.(3)Exogenous GR24 alleviated the inhibition of photosynthesis in apple seedlings by alkali stress.The photosynthetic rate of apple seedlings was significantly reduced after the alkali stress treatment.However,the chlorophyll a,chlorophyll b,total chlorophyll and carotenoid contents of apple seedlings showed a significant increase after spraying GR24under alkali stress conditions;photosynthetic parameters Pn,Tr,Gs and WUE increased by41.8%,37.39%,39.76%and 41.95%,respectively.(4)Exogenous GR24 increased the activity of antioxidant enzymes and the content of non-enzymatic antioxidant substances under alkali stress.The activities of four antioxidant enzymes(SOD,POD,CAT and APX)were increased by 63.63%,16.07%,2.46%and 27.16%,respectively,and the total antioxidant capacity,phenylalanine,flavonoids,and total plant phenols were increased by 22.14%,9.94%,16.34%,and 12.10%,respectively.(5)Exogenous GR24 maintained the ionic and osmotic equilibria of apple seedlings under alkali stress.The ionic and osmotic equilibria were disrupted after alkali stress,while the Pro and SS contents in apple seedlings were increased by 55.76%and 24.74%,respectively,and the Na~+uptake in leaves and roots was decreased by 21.21%and 19.08%,respectively,while the K~+uptake was increased by 4.2%and 57.87%,respectively,after spraying GR24 under alkali stress conditions,thus stabilizing Na~+/K~+.(6)Exogenous GR24 promoted the accumulation and release of acid in the root system of apple seedlings and alleviated the damage caused by high soil p H stress to seedlings.Soil p H increased after alkaline stress treatment,while soil p H was decreased by 2.6%after spraying GR24 under alkaline stress conditions,while H~+content in the roots and organic acid content inside and outside the roots were increased significantly,and root H~+-ATPase activity also was increased significantly.In conclusion,exogenous GR24 can affect the growth and photosynthetic capacity of apple seedlings by regulating the antioxidant capacity,ion homeostasis and organic acid synthesis under alkali stress conditions,thereby alleviating the damage caused by alkali stress to grape seedlings.This study elucidated the mechanism of the effect of strigolactone on improving the alkali stress tolerance of apple seedlings,which also provided the theoretical basis and technical support for apple cultivation in saline alkali soil.