Abstract
Brassinolide (BR), a naturally‐occurring steroidal lactone was compared with GA3 and kinetin for activity in a number of bioassays. BR was highly active in elongation bioassays employing the dwarf pea epicotyl and etiolated bean hypocotyl that are responsive to GA3 but not IAA (at 0.01 to 10 μM). The elongation of the cucumber hypocotyl elicited by BR and GA3 was reduced far more than that elicited by IAA, following removal of the cotyledons. BR, like GA3 also inhibited betacyanin accumulation in Amaranthus seedlings and prevented adventitious root initiation in hypocotyls of mung bean, dwarf bean and cucumber. Application of BR promoted rather than retarded senescence of Rumex leaves.While almost half as effective as kinetin in promoting expansion of cucumber cotyledons, BR was ineffective in cytokinin bioassays involving expansion of dwarf pea epicotyl hooks, dark synthesis of betacyanin in Amaranthus and retardation of Xanthium leaf disc senescence. Unlike the case with IAA, BR does not interact synergistically with GA3. BR has proven active in promoting stem elongation in plant bioassays responsive to either IAA or GA3, or to both phytohormones. This effect, coupled to its effect on adventitious root formation, qualifies BR as a unique plant growth substance.
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