| Tomato is a kind of light loving vegetable, and currently weak light stress has become a main limit in its protected production. In order to investigate the significance of exogenous plant growth regulators on improving weak light resistance in tomato seedlings, two genotypes of tomato'zhongza 9'tolerance and'zhongshu 6'sensitive to weak light were used as materials in this experiment, and at the phase of three fully expanded leaf, exogenous epibrassinolide (0.1mg/kg EBR,0.01mg/kg EBR,0.001mg/kg EBR) and Spermidine (1mg/kg Spd,0.1mg/kg Spd,0.01mg/kg Spd) were applied to the foliage of tomato seedlings, then the seedlings were raised under 150umol·m-2·s-1 weak light, and the plants sprayed with distilled water under 150umol·m-2·s-1, 450umol·m-2·s-1 light intensity were used as weak light control and normal light control, individually. Plant photosynthesis, chlorophyll fluorescence parameters, activities of protective enzymes and reactive oxygen species (ROS) content were tested on the 0,3rd,5th,7th day of weak light stress, the results were as follows:1. Exogenous EBR and Spd improved leaf photosynthesis in tomato seedlings under weak light stress. Whether'zhongza 9'tolerance to weak light or'zhongshu 6'sensitive to weak light, compared to normal light control, leaf Pn and maximum photochemical efficiency of PSⅡ(Fv/Fm) remarkably decreased, and initial fluorescence (Fo) obviously increased under weak light stress, furthermore Pn and Fv/Fm decresed gradually with the extension of stress time, however Pn and Fv/Fm of'zhongza 9'were higher than those of'zhongshu 6', and Fo was lower, which indicated that weak light stress caused some damage to leaf photosystemⅡof two cultivars, and the damage to'zhongshu 6'sensitive to weak light was more serious. However exogenous EBR and Spd alleviated the decrease of leaf Pn and Fv/Fm, and the increase of leaf Fo under weak light, thus maintained higher photosynthetic metabolism level to improve plant resistance to weak light stress. The optimal concentration for'zhongshu 6'was 0.1mg/kgEBR and 1mg/kgSpd, which improved Pn by 18.8% and 20.3% compared with weak light control on the 5th day of stress, individually, simultaneously its Fv/Fm was obviously higher, and Fo was obviously lower than those of weak light control. However optimal concentration for'zhongza 9'was 0.01mg/kgEBR and 0.1mg/kgSpd, which also significantly improved Pn and Fv/Fm, and decreased Fo compared with weak light control on the 5th day of stress, and Pn was increased by 21.7% and 18.1%, individually.2. Exogenous EBR and Spd induced the improvement of plant adaptive response to weak light stress. Under weak light stress, along with the decrease of Pn and Fv/Fm and increase of Fo, tomato seedlings adapted stress by reducing light compensation point (LCP) and dark respiration rate (Rd) to increase the use of weak light and decrease the consumption to assimilates; by improving chlorophyll especially chlb content, reducing chla/chlb to increase light harvesting, transportation and transmission in chloroplast. Simultaneously exogenous EBR and Spd induced the further decline of leaf LCP and Rd, further increase of chlb, and further decrease of chla/chlb, thus induced the improvement of this adaptive response. The optimal concentrations 0.1mg/kgEBR and 1mg/kgSpd in'zhongshu 6'remarkablely increased chlb content by 10.6% and 13.1%, respectively; obvious decreased chla/chlb by 10.5% and 12.8%; decreased LCP by 28.6% and 35.6%, and decreased Rd by 39.0% and 37.9% on the 5th day of weak light stress. The optimal concentrations 0.01mg/kgEBR and 0.1mg/kgSpd in'zhongza 9'obviously increased chlb content by 25.9% and 19.3%; and significantly decreased chla/chlb by 0.5% and 12.8%, decreased LCP by 28.6% and 35.6%, and decreased Rd by 39.0% and 37.9% compared to weak light control, respectively.3. Exogenous EBR and Spd induced the increase of protective enzyme activities and the decrease of membrane lipid peroxidation. Weak light stress obviously increased leaf O2.- producing rate, MDA content and POD activity, and significantly decreased SOD and CAT activity, at the same time the increasing extent of O2.- producing rate and MDA content in'zhongshu 6'sensitive to weak light was bigger, which indicated that weak light caused membrane lipid peroxidation in tomato seedlings of two cultivars, and peroxidative extent in'zhongshu 6'was greater. Simultaneously exogenous EBR and Spd alleviated the increase of leaf O2.- producing rate and MDA content, the decrease of SOD and CAT activity, and induced the further increase of POD activity under weak light stress, which suggested that exogenous EBR and Spd could inhibit membrane lipid peroxidation to some extent by improving the activity of protective enzymes under weak light, thus weakened the hurt from weak light. The optimal concentration in'zhongshu 6'was 0.1mg/kgEBR and 1mg/kgSpd, and compared with weak light stress control on the 5th day of stress, they obviously reduced O2.- producing rate by 22.1% and 28.9% respectively, obviously reduced MDA content by 16.1% and 13.0%, remarkably increased SOD activity by 9.6% and 12.5%, POD activity by 59.3% and 49.6%, and CAT activity by 68.2% and 59.5%. And optimal concentrations in'zhongza 9'were 0.01mg/kgEBR and 0.1mg/kgSpd, which induced obvious decrease of O2.- producing rate and MDA content, and obvious increase of SOD, POD, CAT activity on the 5th day of stress, compared to weak light control, O2.- producing rate decreased by 23.0% and 33.8%, MDA content decreased by 15.2% and 25.1%, and their SOD, POD, CAT activity increased by 24.1%, 25.6%; 82.9%, 49.6%, and 47.2%, 33.5% respectively.4. According to principle component analysis, Fo, Rd, chla/chlb, Fv/Fm, chl content, LCP, MDA content and CAT activity were the main indexes in'zhongshu 6', and O2.- producing rate, SOD activity, chla/chlb, Fv/Fm, chl content, Rd, LCP, Pn and MDA content were the main indexes in'zhongza 9'induced by EBR and Spd to improve plant resistance to weak light. And according to the factor scores, 0.1mg/kg EBR and 1mg/kg Spd showed the best induction effects in'zhongshu 6', however in'zhongza 9'the optimal concentrations were 0.01mg/kg EBR and 0.1mg/kg Spd.
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