| The physiological metabolism of leaf senescence in M. hupehensis and M. sieversii seedlings under water stress was studied with polyethylene glycol (PEG 6000) treatment. The results showed as follow:Under water stress, the chlorophyll in both seedlings degraded greatly with the increase of stress time, however, the contents of O2 , H2O2, MDA and PMP increased significantly. The declining extents of these results in the drought tolerant variety (M. sieversii) were lower than the non-drought variety (M. hupehensis). The results indicated that there existed high significant correlation between O2, H2O2, MDA, PMP and chlorophyll in both Mains seedlings. So it is concluded that Chi degradation in Malus seedlings induced by water stress had significant relationship with the damage of active oxygen.The results of nucleic acid metabolisms under water stress indicated that the activities of DNase and RNase, the contents of O2 and H2O2 increased gradually in both seedlings with the increasing time of water stress. However, the contents of DNA and RNA declined significantly. The further study showed that the less decrease in DNA content was accompanied with high increase of DNase activity. So it is concluded that RAN was more sensitive to water stress compared with DNA. On the other hand, There existed high significant correlation between DNA, RNA and DNase, RNase, RWC, (V, H2O2 contents.Therefore it is the accumulation of free radical and active oxygen that promote the leaf senescence process and further influence the nucleic acid metabolisms.The changes of endopeptidases activities in M. hupehensis leaves under water stress could be divided into two phases: relatively low steady phase and rapid increase phase, which consisted with the varieties of chlorophyll contents. It is inferred that there were more than three subclasses of endopeptidases during leaf aging process, namelythiol-proteinases, serine proteinases and metallo-proteinases. Among of them, thiol-proteinases was the most active one in the process of protein degradation. However, the acid endopeptidases played a key role in leaf senescence during aging process. The optimum temperature for endopeptidases were all 45℃. The results also showed that BA and actidione inhibited endopeptidases activity. However, ABA and H2O2 could enhance its activity.The physiological results related to the primary conversion of light system II (including Fv/Fo, Fv/Fitu qp, qN, yield and Ca/Fo) and the peroxidation of membrane in both seedling leaves under water stress showed that the changes of chlorophyll fluorescence parameters were closely related to the difference of Malus seedlings to drought resistance. Moreover, there existed significant correlation between the changes of these chlorophyll fluorescence parameters and MDA, plasma membrane permeability in both M. hupehensis and M. sieversii seedlings. So it can be concluded that the inhibition degree of primary conversion of light energy was closely related to membrane lipid peroxidation in both seedling under water stress.Water stress caused changes in the contents of chlorophyll (Chi), carotenoid (Car), antioxidants ASA, Chl-Pro, photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci) and the activities of SOD, POD, CAT in both Malus seedlings. Moreover, the changing extents of these parameters were consistent with the ability of drought resistance. On the other hand, there existed significant correlation between photosynthetic rate and Chi, Chl-Pro O2-, H2O2, 1O2, Gs in these seedlings. So it is indicated that the decline of photosynthetic rate in Malus seedlings under water stress had significant relationship with non-stomatal factors induced by metabolic obstacle of active oxygen beside the limitation of stamatal factors. Moreover, the ultrastructure of mesophyll cell in M. hupehensis and M. sieversii seedling leaves were examined with transmission electron microscope. The results showed that the ultrastructure of mesophyll cell revealed some changes und...
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