The Influence of Light Intensity on the Internodal Response of Intact Bean Plants to Brassins

The response of pinto bean (Phaseolus vulgaris L.) plants to a single application of brassins (10 μg) in the second-internode assay was determined under high-intensity (2,000 ft-c = 21.5 klux) and low-intensity (100 ft-c = 1.08 klux) cool-white fluorescent light in a controlled environment. Under high light, maximum elongation of the second internode was reached in 4 days in both the lanolin controls and the brassin-treated plants. After 4 days of high light, second internodes treated with brassins were five times longer than those of controls. Under low light, however, brassin-treated internodes were nearly 10 times longer than those of controls after the same length of time, and the controls and the treated internodes continued to elongate for 10 days. Under both low and high light, brassins induced not only elongation but also a marked thickening of the treated internode. Under low light, radial increase was caused predominantly by cell enlargement in the cortical and phloem tissues, whereas under high light, the increase in stem diameter was caused by cell division in the pith. These findings demonstrate that light plays a major role in determining the response of bean plants to brassin treatment.