Physiological Differences Of Apple Rootstocks With Different Salt Tolerance Response To Salt Stress And Mitigation Effects Of Melatonin

Salt stress seriously affects plant growth and development, and is one of the major threatfor agricultural crop production. Apple is one of the world's major fruit trees of cultivatedspecies but is salt-sensitive. To study and master the mechanism of apple trees to salttolerance is of great significance to the cultivation of salt-tolerant varieties and finding waysto improve salt tolerance. In this study, we comparably studied the growth characteristics,photosynthesis, anatomical structure, Na~+and K~+content and the regulation of different genesto salt stress between the salt-tolerant Malus hupehensis Rehd. and salt-sensitive Malusprunifolia 'yingyehaitang'. The mitigation effects of exogenous melatonin on salinity-inducedstress in Malus hupehensis Rehd. were also studied. The main results are as follows:1. Salt stress inhibited growth of seedlings of both Malus species, but with significantdifferences in intensity between genotypes originating from the two habitats. After15d oftreatment, both of Malus species biomass was inhibited, but the growth of M. prunifolia wasmore inhibited by salinity than was that of M. hupehensis. Salt stress inhibited root growth ofseedlings of both Malus species. The total root length, root volume, the number of root tip,root surface area, average root diameter and forks numbers of two Malus species under saltstress was decreased. However, the decreasing was significantly different in intensity betweenthe two Malus species.2. Under salinity stress, the lamina anatomical structure in leaves of two Malus specieschanges adaptively, but the difference between the two species is significant. Scanningelectron microscopy showed that salt stress increased the thickness of the stratum corneum ofboth Malus species. However, much higher degree of thickening of the cuticle of thesalt-tolerant rootstocks of M. hupehensis was showed. Stomatal of two Malus species showeddifferent changes in response to salt stress. Stomatal density was increased in stressed newleaves as well as decreased in stressed mature leaves of M. prunifolia compared with controls.However, stomatal density was increased in both stressed new and mature leaves of M.hupehensis compared with controls. Moreover, stomatal aperture was significantly decreasedby33.3%in stressed mature leaves of M. prunifolia, while the stomatal aperture in stressed new leaves did not change significantly. Stomatal aperture was significantly decreased by21.71and11.81%in stressed new and mature leaves of M. hupehensis, respectively.3. Although the rates of net photosynthesis decreased rapidly for both genotypes undersalinity stress, M. prunifolia had lower values compared with M. hupehensis throughout thetreatment period. At day15of salinity treatment, the Pn value of M. prunifolia were decreasedby94.7%while M. hupehensis were decreased by83.2%compared with control.4. Exposure to salinity altered the levels of Na~+and K~+contents in roots, stems andleaves in both Malus seedlings, but with significant differences in intensity betweengenotypes originating from the two habitats. The level of sodium rose significantly after salttreatment for the leaves, stems, and roots of both genotypes. However, the average increaseover the control was much greater for M. hupehensis than for M. prunifolia. By contrast, inthe leaves, the average increase over the control was much greater for M. prunifolia than forM. hupehensis. Potassium contents in the leaves, stems, and roots decreased significantly after15d of salinity treatment for both genotypes, but with no significant difference. Salinitysignificantly increased the Na~+/K~+ratios in all tissues from both genotypes. Although valuespeaked in stems from M. hupehensis, the leaves and roots from that species had lower ratioscompared with M. prunifolia.5. Differential expression of ion transporters and aquaporins may contribute to different salttolerance in Malus species. Both MdHKT1and MdSOS1of M. hupehensis transcript morethan that of M. prunifolia, implying that M. hupehensis had better capacity of excludingsodium out of the leaves. M. hupehensis had higher amount of MdNHX1transcripts, whichcould help exclude excess Na~+into the vacuoles and sustain better cellular environment.Relative higher transcript of tested aquaporin was found in M. hupehensis, implying it hadbetter capacity of maintaining better leaf water status and diluting excess ions effectivelyunder high salinity condition.6. Melatonin has the mitigation effects on salinity-induced stress in apple rootstocks.Exogenous melatonin reduced salt injury index of Malus hupehensis Rehd. seedlings. Leavesof melatonin-treated seedlings maintained a relatively higher Fv/Fm compared with thetreated one without melatonin, with those values declining very slowly during the salinitytreatment. Melatonin pretreatment significantly maintained cell membrane stability andalleviated growth inhibition. The mitigation effects of exogenous melatonin onsalinity-induced stress in Malus hupehensis Rehd. depends on the concentrations, and0.1μmol/L melatonin showed the most obvious mitigation effect.