Research On Growth And Development Of Two Malus Species At Callus And Plantlet Stages And Their Salt-tolerance Mechanisms Under Salt Stress

Apple (Malus domestica L.) is a main economical fruit tree which occupies the largestcultivated area and production in northern China. However, soil salinization has become oneof the most severe environmental factors limiting the productivity of apple in some regionsincluding the Loess Plateau, the Yellow River Delta and the Old Course of Yellow River.Much of the previous studies on salt tolerance mechanisms of Malus have been reported, andmost of them were conducted using whole plant seedlings of rootstock or scion under potexperiments. However, investigations are seriously lacking on salt adaptation mechnism ofMalus at dedifferentiational (callus) and differentiational (plantlets) stages. We studuiedeffects of salt stress on growth and development of two Malus species and thier physiologicalsalt-tolercance mechanisms to explore the germplasm resources of apple with salt-tolerance,and provide scientific and pratical guidance for apple production in soil salinity regions. Inour study, the regeneration system of both rootstock Rehd (Malus robusta Rehd) and scionFuji (Malus domestica Borkh. cv. Fuji) were established in vitro. The callus and plantlets oftwo Malus species were cultured on MS medium containing0mmol/L or150mmol/L NaClto investigate effects of salt stress on their growth and development, anatomical structure ofleaves and physiological adaptation mechanisms at callus and plantlet stages. The main resultsare as following:(1) Growth and development of both two Malus were significantly inhibited under saltstress. The relative growth rate (RGR) in callus of Rehd and Fuji were11.9%and13.6%while46.0%and43.1%in plantlets respectively. Compared with the control, there-differentiation rate of adventitious buds in callus decreased by53.8%and60.0%respectively. The rooting rate of plantlets under salt stress was only50%of that under thecontrol treatment. The roots number and length of plantlets was also lower under salt stress ascompared to controls.(2) The browning rate in callus of Rehd and Fuji reached87.7%and90.8%respeceivelyunder salt stress. Their leaves structure exhibited some damage traits including reducedpalisade tissue and intracellular chloroplast, underdeveloped catheter and serious phloemtissue damage was found in leaves of their plantlets. In the meantime, a couple of adaptivestructure features appeared in leaves including increased thickness of the upper and lower epidermis, elevated proportion of spongy tissue and the formation of lignified vessels.(3) Effective accumulations of the soluble protein, soluble sugar and proline occurred intwo Malus species under salt stress, especially for proline. Proline content of Rehd and Fujiunder salt stress were2.3and4.8times at callus stage and5.4and5.0times at plantlets stagerespectively as compared to the controls.(4) Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and superoxideanion radical (O2) were accumulated rapidly in two Malus species under salt stress conditionleading to cell membrane lipid peroxidation and malondialdehyde (MDA) accumulation. Theincreased rates of H2O2、O2and MDA content in callus were greater than those in plantlets.The H2O2content of Fuji was higher than in Rehd while the reverse was true in O2content,however, the MDA content was similar between them.(5) Activities of superoxide dismutase (SOD), peroxidase (POD) and glutathionereductase (GR) were increased in two Malus species in response to elimination of excess ROSaccumulated under salt stress. More obviously, GR activity recorded3.3-fold in plantlets ofFuji under salt stress as compared to the control. By contrast, catalase (CAT) activity showedslight declines in both of two Malus species. Under salt stress, SOD, CAT and GR activities inplantlets of two Malus species were greater as compared to callus. As for the samedevelopmental stage, the SOD activity in Rehd was greater than that in Fuji whereas thereverse was true for activities of CAT and POD.(6) At dedifferentiational stage, organic osmolytes measured in two Malus species werelower except soluble sugar. The greater concentrations of ROS and the lower activities ofantioxidant enzymes led to higher levels of MDA accumulation and relatively low RGR ascompared with differentiational stage. Thus, two Malus species at the dedifferentiational stageshowed weaker adaptive capacity and more significant sensitivity to salinity stress.(7) Salt-tolerance characteristics of both two Malus species were slightly different inresponse to salt stress, their capacity of salt-resistence and salinity sensitivity were similarbased on their similar responses of the formation of adventitious buds and roots, changes inthe leaf structure and RGR under salt stress.Innovations points were as following in this study:(1) Growth and development as wellas physiological salt-tolerance responses were clarified in two Malus species atdedifferentiational (callus) and differentiational (plantlets) stages under salt stress bycomparative analysis.(2) Both of two Malus species at dedifferentiational stage were moresensitive to salt stress than differentiational stage, and plantlets can be more useful to evaluatesalt-tolerance adaptivity of Malus species.