Effects Of Exogenous SA Pretreatments On Thermotolerance And Photosynthetic Characteristics In Pakchoi Under Heat Stress

Pakchoi (Brassica campestris L. ssp. chinensis (L.) Makino var. communis Tsen et Lee), origintated in China, grows well in the suitable environment where the temperature ranges from15to25℃. However, with the expansion of the greenhouse effect, heat stress (40℃) is becoming one of the main factors to limit the growth and development of pakchoi. How to alleviate the damage of high temperature and induce thermotolerance is becoming an important subject for us to study the physiological resistance to stress in pakchoi recently.Salicylic acid (SA), was regarded as a new plant hormone, which had been proved to improve most plant resistance to abiotic stresses such as drought, chilling, salinity stress and heavy metal tolerance. Most researchers focus on the thermotolerance induced by SA in many vegetables recently. Followed by some similar evidences on mustard seedlings, potatoes, peas and cucumbers. But the reports that SA could induce thermotolerance to heat in pakchoi seedlings were very rareTherefore, two pakchoi varieties, heat tolerant cultivar’Xia-di’and heat susceptible cultivar’Suzhou-qing’were used as materials in this paper. The effects of exogenous salicylic acid pretreatments on thermotolerance and photosynthetic characteristics were studied by spraying different SA under heat stress. We expect that the best concentration SA could be found and the mechanism of thermotolerance induced by SA could be explained by further study.The main results were concluded as follows:1. Under heat stress, proline content decreased in’Xia-di’ seedlings, but increased in ’Suzhou-qing’seedlings. Solube suger content increased at the beginning, then decreased again in two pakchoi varieties, but solube protein content increased all the time. The ability of asmotic regulation was enhangced by reducing proline content and increasing the content of soluble sugar and protein after spraying SA in pakchoi seedlings under heat stress.2. Under heat stress, the electrolyte leakage and MDA content increased at the beginning, then decreased again in two pakchoi varieties. Lipid peroxidation caused by heat was prevented by reducing the electrolyte leakage and MDA content after spraying SA in pakchoi seedlings under heat stress.3. Under heat stress, SOD, POD and CAT activities increased in’Xia-di’ seedlings. SOD and POD activities increased in’Suzhou-qing’seedlings, however, CAT activity went up at the beginning, then decreased again. The ability of scavenging reactive oxygen species was increased through enhancing SOD and POD activities after the application of SA, compared to the untreated control under heat stress. Meanwhile, a decline in CAT activity by the application of SA was observed, which maintained the H2O2content in dynamic balance.4. The heat stress decreased the net photo synthetic rate, internal CO2concentration and stomatal conductance at first, then increased at last in’Xia-di’seedlings, but transpiration rate was contrary to them. The some results were observed in’Suzhou-qing’seedlings about the net photosynthetic rate and internal CO2concentration. And transpiration rate was also contrary to them. However, heat stress decreased stomatal conductance all the time in’Suzhou-qing’seedlings. Exogenous application of SA was found to enhance stomatal conductance and transpiration rate, with a concomitant decrease in the net photosynthetic rate and internal CO2concentration in two pakchoi varieties under heat stress.5. Taking all the physiological and photosynthetic index into consideration, we found that100μmol·L-1was the best concentration to alleviate the damaging effect of high temperature stress on pakchoi seedlings. As compared with heat tolerant cultivar’Xia-di’, the effects of SA pretreatment on heat susceptible cultivar’Suzhou-qing’were more significant under heat stress.