Brassinolide, a growth‐promoting steroidal lactone: I. Activity in selected auxin bioassays

Abstract

Brassinolide (BR), a naturally‐occurring‐steroidal lactone from rape (Brassica napus L.) pollen, was compared with auxin for activity in a number of bioassay systems. Responses similar to IAA were elicited by BR in bioassays based upon bean hypocotyl hook opening, elongation of maize mesocotyl, pea epicotyl and azuki bean epicotyl sections, and fresh weight increase in Jerusalem artichoke (2,4‐D used) and pea epicotyl sections. The azuki bean and dwarf pea epicotyl bioassays were much more responsive to BR than IAA (at 10 μ M). Responses approximately two‐fold greater in magnitude were elicited by IAA in the maize mesocotyl, bean hypocotyl hook and Jerusalem artichoke bioassays. Little or no response was elicited by BR (0.01 to μ M) in the cress root or decapitated pea‐lateral bud bioassays. A powerful synergism between BR and IAA was observed in the azuki bean, pea epicotyl and bean hypocotyl hook bioassays. Although, as previously reported, other steroidal substances are active in some of the bioassay systems tested, none compared with BR in magnitude and diversity of elicited responses.