Abstract: Al sensitive soybean genotype BD2 was pretreated by soaking seeds with two kinds of brassinosteroids, 24-epibrass-
inosteroid (EBR) and long-effective brassinosteroid (coded as TS303), and then hydroponically cultured with 1/5 strength Hoagland solution with or without 1 mmol L^-1 Al. Root elongation, root tip Al and callose concentrations were determined to find the optimal applicative concentration of BRs. Effects and mechanisms of EBR and TS303 on photosynthetic characteristics were subsequently investigated at optimal applicative concentration. The results showed that EBR and TS303 enhanced soybean toler-ance to Al, as indicated by increased root elongation and in accordance with decreased Al and callose concentrations in root tips. The optimal effects occurred at concentration of 0.1 mg L^-1 for both EBR and TS303, and TS303 showed a better effect than EBR. The results also showed that 0.1 mg L^-1 of EBR and 0.1 mg L^-1 of TS303 both ameliorated the decrease in leaf area and net pho-tosynthetic rate caused by Al stress, indicating an enhancement in photosynthetic capacity. EBR and TS303 could counteract, at least in part, the decrease in chlorophyll content, maximal PS II efficiency (F_v/F_m), stomatal conductance, carbonic anhydrase activity, concentration and activity of RuBPCase caused by Al stress. The results suggested that, under Al stress, the increased net photosynthetic rate of soybean caused by EBR and TS303 was due to the increase in light energy harvest and transfer, and CO₂ transportation and fixation as well.

Key words: Brassinosteroid, Soybean, Al stress, Photosynthetic characteristics