Brassinolide amelioration of aluminum toxicity in mungbean seedling growth

Brassinolide (BL) is known to be essential for plant growth and development and regarded as a new class of plant hormones. The ameliorative effects of BL-treatment on stressed plants have been reported. However, little is known about the effects of BL on plant growth under aluminum (Al) stress. In the present study, mungbean (Phaseolus aureus Roxb.) seedlings were grown in a growth chamber in solution with combined concentrations of Al (0, 2, and 5mM) and BL (0, 0.1, 10, 1000, 100,000 ng L-1) under a randomized complete block design experiment for 2 weeks, to evaluate the ameliorative effects of BL on Al toxicity for the seedling growth under Al stress. Results showed that BL had promoted growth of mungbean seedlings under Al stress. In the absence of Al stress, BL had no significant effect in increasing epicotyl length of mungbean seedlings, while in the presence of Al stress, epicotyl length showed significant increase under Al stress in the presence of BL. A similar result was obtained in seedling height and root length under 2 and 5 mM Al stress. Brassinolide also significantly increased fresh weights of shoots and roots under Al stress. Interestingly, BL at 0.1 ng L-1 had significant differential response in dry weight distributed between shoots and roots under Al stress, with no effect in dry weight of shoots, but significant increase in root dry weight. Brassinolide also significantly increased chlorophyll content under Al stress. However, the most significant promotive effect of BL on morphological traits and chlorophyll content seems to depend on the addressed traits, BL concentration, and Al stress. The mechanisms for differential response in dry weight between shoots and roots by BL at low concentration and increase of chlorophyll content under Al stress needs to be addressed further.