| Photosynthesis is the basis of growth and development of plants, which provides the basic organic substances and energy for all life activities. Salinity inhibits seriously photosynthesis and decreases crop productivity. Therefore, it is important to study the mechanisms that how salt stress affects photosynthesis. Up to now, the reasons for the decline of photosynthetic rate of fruit crops under salt stress and the effect of rootstocks on it have not yet been well understood. In the paper, we studied effects of salt stress on Pyrus. betulaefolia (B) and P. calleryana (C), Hohsui (H)-B and H-C. We used 0.3%, 0.6% soil salt content to treat them respectively, and measured their photosynthetic properties, photosynthetic daily dynamics, and their response curves to CO2 and PFD by CIRAS-1 portable photosynthesis system; and moreover, studied the generation speed of O2-, change of MDA and chlorophyll contents in the laboratory. The main results were as follows:1 Effects of salt stress on photosynthesis of pear (1) There were obvious discrepancy of photosynthesis between two pear rootstocks, i.e Pyrus. betulaefolia and P. calleryana, and between "Hohsui" trees grafted on them, when they were treated by salinity. (2) Pn, Gs, Tr, and CE were higher in the leaves of B, and H-B than those in the leave of C, and H-C respectively, the former with higher Ci than the latter under high salinity. (3) That photosynthesis of pear blade was inhibited by salinity most possibly was caused by stomatal limitation under low salinity at early period, but it was caused by nonstomatal limitation under high salinity and low salinity at the late phase. (4) In the late phase of salt treatment, the discrepant effects by two rootstocks on photosynthetic properties were diminished under severe salinity. (5) P. calleryana was more sensitive to salinity than P. betulaefolia, and H-C more sensitive to salinity than H-B.2 Effect of salt stress on photosynthetic daily dynamics of pear The diurnal curve of pear photosynthesis was changed by salt stress, and the CO2 fixation ability and light quantum utilization were declined under severely salt stress, and all these changes were affected by rootstocks. Tr was linearly related with Gs in the leaves of Hohsui grafted on P. Betulaefolia, but it was linearly related with VPD in the leaves of Hohsui grafted on P. Calleryana under salinity, In addition, Pn of pear leaves was mainly affected by its CO2 use efficiency and light use efficiency.3 Effects of sal inity on the capacity of carbon metabol ism of pear Carboxylation efficiency (CE), photon respiration and dark respiration ascended firstly and then declined;except that CE dropped rapidly, all the other indicators were recovered or declined slowly in late period of treatment under slight salt stress. The CE increment of H-B0.3 and H-C0.6 at the early stage was more than H-C0.3 and H-B0.6, respectively, but the CE reduction of H-B0.3 and H-B0.6 were less than H-C0.3 and H-C0.6, respectively. Apparent quantum yield (AQY) of H-B0.3, H-B0.6 and H-C0.3 temporarily rose at a small scale, while H-C0.6 was tend to decrease. The AQY of pear grafted on P. betulaefolia declined less than that on P. calleryana. Photon respiration (Rp) of the most trees declined, while the Rp of H-B0.3 and H-C0.3 ascended firstly and then declined. The Rp of pear grafted on P. betulaefolia declined less than that on P. calleryana. Rp of trees treated with high salinity declined more than low salinity. The variance tendency of dark respiration (Rd) was similar with Rp.4 Effect of salt stress on photosynthetic pigment and membrane system In pear leave, generation speed of superoxide radicle (O2- )was raised under salt stress, and the increment is ascended with salinity and duration. The generation speed of O2-of P. calleryana was quicker than P. Betulaefolia , and H-C than H-B. The contents of MDA changed in the similar manner with the generation speed of O2- under salinity. All photosynthetic pigments declined under salt stress, where Chlb declined most signifi...
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