| Tomato is a kind of warm season vegetable, its suitable temperature for growth is from 22°C to 28°C. When the air temperature exceeds 30°C, the assimilation drops notability. When the air temperature exceeds 35°C, the procreated growth will get destroyed. And even at the short time in 45°C, it will produce physiological molestation, such as the fell off the flower and fruit and no developed fruit. With the speedy increasing the area of vegetable protected cultivation, the protected production of tomato also had unprecedented development, especially because of the popularization of Sun-Light Greenhouse, the whole year production of tomato became true. And because of the increasing of protected cultivation and the greenhouse effect on the earth, the adversity of high temperature had produced much bad influence in summer and autumn, which became more and more noticeable. So heat stress in summer and autumn becomes a serious problem in tomato production.The damage of heat stress to crops has brought deeply attention among many researchers. So many researchers are looking for different methods and different approaches to alleviate the damage, which caused by heat stress. And it has got great progress now. Some researchers found that salicylic acid and epi-BR can effectively improve the heat resistance in some crops, and made these crops grew naturally in high temperature environment. Although there were some reports in improving heat resistance in some other crops by salicylic acid, there was rare report about improving heat resistance in tomato, and there were some dispute at the enzyme of protected membrane in different crops and different concentration and different treats; As a new phytohormone, there were a lot of researches about improving environment stress by epi-BR, which mostly focused on cold resistance and drought resistance and salt resistance and disease resistance in crops. There was few research in vegetables, and much less in high temperature, there is no research about heat resistance in tomato seedlings. So at first, through tested MDA content, Pro content and the relative conductance , this study made sure that these two exogenous materials could induce heat resistance by spraying on tomato seedlings leaves, and each choose the best concentration in inducing heat resistance. And second ,in order to provide furthermore theory for raising seedlings and cultivation, this study investigated the physiological mechanism of heat resistance ,which induced by spraying these two exogenous materials on tomato seedlings leaves.The results are as follow:1. In experiment concentration scope, under the condition of 40°C, 20-120 mg·L-1 salicylic acid and 0.01～1.5 mg·L-1 epi-BR, which sprayed on tomato seedlings leaves at the period of four leaves, enhanced proline content, improved the heat resistance. The salicylic acid 60 mg·L-1 and the epi-BR 0.5 mg·L-1 each was the best concentration in inducing heat resistance.2. In experiment concentration scope, under the condition of 40°C, 20120 mg·L-1 salicylic acid and 0.01～1.5 mg·L-1 epi-BR, which sprayed on tomato seedlings leaves at the period of four leaves, reduced the relative conductance, improved the heat resistance. The salicylic acid 60 mg-L"1 and the epi-BR 0.5 mg·L-1 each was the best concentration in inducing heat resistance.3. The right concentration of salicylic acid and epi-BR could effectively reduce the oxidative stress under high temperature stress and enhance the heat resistance. Under the condition of 40 degrees, 20～100 mg·L-1 salicylic acid and 0.01～1.0 mg·L-1 epi-BR, which sprayed on tomato seedlings leaves at the period of four leaves, reduced MDA content, 120 mg-L'1 salicylic acid and 1.5 mg·L-1 epi-BR increased the MDA content. The salicylic acid 60 mg-L'1 and the epi-BR 0.5 mg·L-1 each was the best concentration in inducing heat resistance.4. There is the similar physiological mechanism in inducing heat resistance in tomato seedlings by salicylic acid and epi-BR. The results are as follow: under heat stress, the salicylic and the epi-BR sprayed on the leaves surface could maintain high activity of SOD,POD,APX,which helped to reduce the producing and accumulating of active oxygen and reduce the oxidative stress, enhance the stabilization of the cell membrane and improve the heat resistance in tomato seedlings. In the role of improving heat resistance in tomato seedlings, the activity of CAT is an aspect of high temperature stress, but there was no relationship with the activity of CAT in inducing heat resistance by salicylic acid and epi-BR.
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