Improvement of cold resistance in rice seedlings by 5-aminolevulinic acid

5-Aminolevulinic acid (ALA) at low concentrations increased cold resistance in rice seedlings. The pretreatment of rice seedlings by root-soaking with ALA solution at 0.1-1 ppm reduced the ratio of leaf rolling and electrolyte leakage from leaf tissue after cold treatment. Thirty days after cold treatment at 5 degrees C for 5 days, the plants treated with 1 ppm ALA resulted in 85% survival ratio, 6.1 leaves per plant and 111.8 mg dry weight per aerial part of seedling, while 65%, 5.9 and 65.0 mg respectively in the control plants. The dry weight of seedlings treated with ALA increased 1.7 folds as compared to the control plants. These results are the first evidence that ALA has protective effects against cold stress in rice seedlings. Abscisic acid (ABA) and brassinolide (BR) also increased the cold resistance in our bioassay system. Protective effect of BR at 0.001 ppm against cold stress was similar to that of ALA, However, protective effect of ABA was different from that of ALA in terms of the ratio of leaf rolling after cold stress. ABA protected young leaves rather than old ones, while ALA and BR were more effective on the protection of old leaves.