| With the figleaf gourd (Cucurbita ficifolia) as rootstock and Jinchun 4(Cucumis sativus L. cv. Jinchun4) as scion, we studied the chlorophyll fluorescence parameter, root activity, RUBP enzyme, active changes of protective enzymes, the malondialdehyde (MDA) content, absorbance of mineral elements and so on at chilling temperature (4°C) under low irradiance (100μmol m-2 s-1 PFD) in the own-rooted cucumber plants, grafted cucumber plants and figleaf gourd plant, to probe into the possible mechanisms of grafted cucumber plants enduring chilling temperature and low irriadiance. The results as follow:1. With the figleaf gourd (Cucurbita ficifolia) as rootstock and Jinchun 4(Cucumis sativus L. cv. Jinchun4) as scion, the roots of figleaf gourd (Cucurbita ficifolia, as rootstock) could improve the resistance of cucumber plants (Cucumis sativus L. cv. Jinyan 4, as scion) to low temperature. We studied the root activity and photosynthetic activity of photosystems at chilling temperature (4°C) under low irradiance (100μmol m-2 s-1 PFD) in the own-rooted cucumber plants and grafted cucumber plants. Compared with dark adaptation seedlings, the chlorophyll a fluorescence transient curve and the oxidizable P700 (P700+) of both own-rooted seedlings and grafted seedlings decreased, PS2 and PS1 of own-rooted seedling leaves were more inhibited relative to grafted ones at the end of chilling stress. The reduced triphenyltetrazolium chloride (TTC) content, which was used to reflect the root activity, kept stable in rootstock roots at end of chilling stress, while it decreased noticeably in cucumber roots. These results implied that the root system activity of figleaf gourd were higher than cucumber's. Superoxide dismutase (SOD) activity was higher in rootstock roots than in cucumber roots at both room temperature and chilling temperature. Upon exposure to chilling stress, the malondialdehyde (MDA) content, which reflects the degree of lipid peroxidation, increased markedly in cucumber roots and kept stable in rootstock roots.2. At chlling temperature and under low irradiance in a short time (8h), Fv/Fm,Fo,φPSâ…¡,ETR,Fv′/Fm′,qP etc of descreased during stress, but the NPQ increased gradually. This indicated that at chlling temperature and under low irradiance, the reactive center of PSâ…¡was induced photoinhibition. As to 8 hour in the treament every index of grafted cucumber plants was higher than the own-rooted cucumber plants. And the difference was evident (P<0.05). This indicated that the reactive center of PSâ…¡of grafted cucumber plants had higher resistance to chilling temperature and low irradiance than the own-rooted cucumber plants.With the time lasted in the treatment, the oxidizable P700 (P700+) of both own-rooted seedlings and grafted seedlings decreased. It indicated that chilling stress under low irradiance could inhibit the PSâ… . And grafted cucumber plants were apparently lower than that of own-rooted cucumber plants during stress(P<0.05). It indicated that PSâ… of own-rooted cucumber plants was sensitive to chilling temperature and low irridiance than the grafted cucumber plants.During chilling stress under low irradiance, activity of RuBPCase decreased. It indicated that chilling temperature and low irradiance could inhibit the active of RuBPCase. But at the end of treatmen the activity of RuBPCase in grafted cucumber plants were higher than in own-rooted cumube plants and the difference (P<0.01). RuBPCase is the speed-restrained enzyme in plant photosynthesis. It indicated that photosynthesis of grafted cucumber plants was lower inhibited than own-root cucumber plants by chilling temperature and low irradiance. And it could be one of the factors to endure chiliing temperature and low irradiance.With the time lasted at chilling temperature under low irradiance, the activity SOD of both grafted cucumber plants and own-root cucumber plants increased. It indicated that the ativity of protective enzyme enhanced. At the end of treament, the activity SOD of grafted cucumber plants was higher than that of own-root cucumber plants (P<0.05). So the enhanced ptotective enzyme could be one of the factors of grafted cucumber plants to endure chiliing temperature and low irradiance than own-root cucumber plants.In conclusion, at chilling temperature under low irradiance, the activity of reactive centers of PSâ…¡decreased (Fv/Fm). And the electron transfer was limited (φPSâ…¡). But the reactive center was not destroyed (Fo). At the same time, PSâ… of cucumber was induced photoinhibition, and the activity of RuBPCase was decreased. It indicated that dark reaction also was influenced. In the three parts, the dark reaction was inhibited further.3. Under low irradiance stress and at low temperature(4℃) in root zone, Fv/Fm, the oxidizable P700 (P700+), RuBPCase had changes unconspicuous at beginning of stress (2h). ButφPSâ…¡, ETR, qP were increased evidently With the stressing time lasted. It indicated that cucumber was not inhibited at short time(2h) of stress. Fv/Fm,φPSâ…¡, ETR, qP, the oxidizable P700 (P700+) and RuBPCase were in a similar. It indicated that PSâ…¡,PSâ… and RuBPCase of cucumber were inhibited with the stress. At the end of stress, Fv/Fm,φPSâ…¡, ETR, qP, the oxidizable P700 (P700+) and RuBPCase of grafted cucumber plants were higher than own-root cucumber plants evidently and the difference was prominent (P<0.05). It indicated that grafted cucumber plants had higher fastness to low temperature at root zone than that of own-root cucumber plants under low irradiance.4. At low temperature and under low irradiance stress for a quite long time, Fv/Fm, X dissipation,φPSâ…¡, ETR, Fv′/Fm′, qP etc of grafted cucumuer plants and own-root cucumber plants decreased gradually with the stress time delaying. It indicated that reaction center of PSâ…¡of cucumber was induced photoinhibition at low temperature under low irradiance. Fv/Fm,φPSâ…¡, ETR, Fv′/Fm′of grafted cucumber plants recovered rapidly at suitable conditions. It indicated that low temperature and irradiance could be slightly induced photoinhibition. At low temperature and low irradiance, NPQ of grafted plants ascended evidently. It indicated that NPQ could effectively dissipate a portion of energy. The strengthened NPQ could reduce ETR and decreased the energy transferred to PSâ… . The oxidation station of P700 could be reflected by light absorbtion in 820 nm in leaves.5. The antioxidation enzymes(SOD, POD, CAT and APX) of grafted and own-root cucumber leaves had been affected and reactive oxygen were accumulated under low temperature and low irradiance. During treatment, the decreased SOD activity restrained the reaction of O2 , inhibited the water-water circle, so it made the O2 accumulated, membrane peroxided and the content of MDA increased with the stressing time lasted, the activity of POD and APX increased and it inflected the degree of chilling harm.6. The contents of Ca, Mg, Cu, Mn and Zn in roots and leaves of grafted cucumber were evidently higher than that of own-root cucumber before the treatment of low temperature and low irradiance stress and recovery treatment. Ca2+-Mg2+ could make biology membrane keep stable under low temperature and low irradiance and promoted some biochemical process. For example: the reciprocal reaction of membrane and membrane protein, active the reaction centre of photosynthesis the circle electron transfer of energy dissipation etc. And it should be propition for membrane integrality to make energy dissipate and the biochemical process. Cu, Mn and Zn were the cofactor of SOD and Zn also could maintain the CAT activity. The high contents of Cu, Mn and Zn played an important role in keeping the high activities of SOD and CAT, eliminating the O2 and increasing the cold tolerance. |
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