THE EFFECTS OF BRASSINOSTEROIDS ON SEQUENTIAL LEAF SENESCENCE OCCURRING IN GLYCINE MAX L

Brassinosteroids (BRs) are steroidal plant hormones that influence varied growth and development processes such as germination of seeds, flowering, senescence and abscission. The effect of epibrassinolide (eBL), one of the most active brassinosteroids, on senescencing cotyledons of Glycine max L. seedlings, which represent epigeic germination, has been investigated in this study. The process of senescence was examined by spraying eBL solutions of 10-11 M and 10-9 M and 2,3,5-triiodobenzoic acid (TIBA), an inhibitor of auxin transport, to the 12 days-old seedlings. eBL (especially 10-9 M) was observed to have induced senescence, while it has delayed senescence when applied with TIBA together. These observations were confirmed with the measurements of total chlorophyll and protein amounts, and rate of peroxidase activity. One has discussed that combined brassinosteroid and TIBA, the former being known to work with auxin synergistically and the latter being an inhibitor of auxin transport, do not affect senescence but brassinosteroids accelerate senescence in presence of auxin. As a result, one may think that eBL and auxin together function as a senescence signal.