The promotive effect of brassinolide on lamina joint-cell elongation, germination and seedling growth under low-temperature stress in rice (Oryza sativa L.)

The cell elongation-promoting activities of brassinolide (BR) and indoleacetic acid (IAA) in the laminainclination test using etiolated rice seedlings were compared at a low temperature. IAA promoted the lamina joint-cell elongation at 30degreesC, but the effect was markedly lowered with decreasing temperature and reached null at 15 degreesC. On the other hand. BR and castasterone (CAS), a brassinosteroid, promoted this cell elongation even at 15degreesC. Homobrassinolide (HBR), a brassinosteroid, also promoted this cell elongation, although the activity was weak. The combination of BR and IAA synergistically promoted this cell elongation at a low temperature (15 degreesC). Immersion of the dry seeds in 2 X 10(-8) M BR for the first 24 h promoted the early growth of rice seedlings. This treatment also enhanced the germination rate and the growth after direct sowing in submerged paddy pots in a greenhouse or phytotron at a low temperature (15degreesC. Leaf spraying of BR on the rice seedlings at the 4th leaf stage increased plant height and the fresh weights of tops and roots even at a low temperature. BR sprayed on the completely expanded 4th leaves did not increase their blade length, but that sprayed on the expanding 5th and 6th leaves strikingly increased their blade length, and the effect tended to be stronger at a low temperature. These results suggest that BR promotes cell elongation in young rice seedlings under low-temperature stress, although IAA had no such effect, and that BR may promote germination and the early growth of rice seedlings at a low temperature in direct sowing in the submerged paddy field and in the rice nursery.