| The physiological disorder caused by high temperatures and/or intense light frequently results in a great loss to pear fruit quantity and quality annually, although pears have been grown as a principal fruit worldwide. Therefore, it is meaningful, theoretically and practicably, to study the effect of High-temperature and Intense-light Stresses on Leaf Photosynthetic Rate and Antioxidant Characteristics. With both laboratory assay and field trials, the experiment was undertaken to reveal relationship between leaf photosynthetic rates and antioxidant characteristics. The main results are as follows:1. Under 100%, 75% and 35% natural light conditions, the changes of leaf Pn rates and antioxidation characteristics were different with adult trees. With full natural light, the net photosynthetic rates, superoxide anion productive rates and the activities of SOD, APX and GR showed a typical“double peaks”shaped curve, while a“parabolic”curve was shown with both 75% and 35% natural lights. The results implied that there was an considerable“midday rest”under intense light stress, and the emergence of‘midday rest’was closely related to oxidative stress caused by intense light stress. Based on the whole-day Pn accumulation, 100% light intensity was basically the same as 75% treatment which could avoid the“midday rest”of photosynthesis, indicating that lower light intensity contributed to avoid oxidative stress and thus enhance photosynthetic efficiency.2. The antioxidant enzyme activity, the superoxide anion content, LOX activity and net photosynthetic rates in leaf tissues were responsive to heat stress extents. With the rise of temperature, the superoxide anion productive rates and LOX activity showed an upward change, and conversely Pn rates showed a downward trend, indicating that the high tenperatures aggrevated oxidative stress of leaf tissues and damaged cell membrane integrity, finally resulting in reduced leaf photosynthetic efficiency. With increasing temperature, the antioxidant enzymes (SOD, APX and GR) showed a downward trend after initial increase, reflecting a different changing pappern with respect to leaf Pn and antioxidant abilities.3. A variety of exogenous antioxidants could improve the cell membrane antioxidant capacity and leaf photosynthetic efficiency, and reduce the superoxide anion productive rates of LOX activity. During a week of the dynamic changes, the leaf photosynthetic efficiency and antioxidative stress could be improved through raising the critical value of antioxidant enzymes (SOD, APX, GR) activity induced by exogenous antioxidants, among which SA was the most effective, followed by those in a descending order: CaCl2>AsA>VE.4. Under natural comditions, a great difference in photosynthetic capacity and antioxidative properties existed among leaves on different canopy exposures, as influenced by distinctive reception of solar radiation and other ecological parameters. Exterior leaves on southwest canopy received more light in the afternoon than those on other exposures, resulting in higher daily maximum leaf temperatures. Thus, it was refered that the exterior canopy on southwest exposure was where leaves most easily suffered from high temperature and intense light stresses.5. The tree canopy characteristics, photosynthetic rates and antioxidant properties varied with different seasons. In July, the tree was most vulnerable to suffering from high temperature and intense light stresses and the superoxide anion productive rates, anbient temperatures and PAR were 37.37%, 28.31% and 32.19% higher, reapectively, than the average of those from other four months.Therefore, it could be inferred that in sunny days, the pear trees were more easily stressed by high temperature and intense light in the afternoons in July. |
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