Influence of spectral quality on the growth response of intact bean plants to brassinosteroid, a growth-promoting steroidal lactone. I. Stem elongation and morphogenesis

The growth response of bean (Phaseolus vulgaris L. ‘Pinto′) plants treated with 5 μg of brassinosteroid (BR) in the bean second-internode assay was measured in a controlled environment under 3 radiation sources: cool white fluorescent (CWF), far-red (FR) fluorescent and incandescent (INC) lamps. Growth comparisons were made under equal levels (90 μmol s-1 m-2) of photosynthetic photon flux density provided by CWF or INC lamps and equal levels of far-red (28 W m-2, 700–800 nm) radiation provided by the same INC or FR lamps. Treatment of the second internode with BR produced a sequential increase in elongation, curvature, and swelling under normal bioassay conditions (CWF lamps), as observed previously with brassins. In addition, BR induced marked splitting of the treated internode provided that ample photosynthate was available. Spectral quality had a differential effect on internodal elongation. Under CWF lamps, internodes, 6 days after BR-treatment, were 2–3 times longer than those of controls; under INC or FR lamps they were 30–60% shorter than those of controls. In all cases, BR-treatment greatly stimulated accumulation of photosynthate in the treated internode, as indicated by fresh and dry weights and stem diameter measurements. This suggests a possible mobilization role for BR in the intact plant. Brassinosteroid also partially overcame the natural inhibitory effects of CWF radiation on stem elongation.