Light intensity, gibberellin content and the resolution of soot growth in Brassica

The role of gibberellins (GAs) in the regulation of shoot elongation is well established but the phytohormonal control of dry-matter production is poorly understood. In the present study, shoot elongation and dry-matter production were resolved by growing Brassica napus L. seedlings under five light intensities (photon flux densities) ranging from 25 to 500 micromole m-2 s-1. Under low light, plants were tall but produced little dry weight; as light intensity was increased, plants were progressively shorter but had increasing dry weights. Endogenous GAs in stems of 16- and 17-d-old plants were analyzed by gas chromatography-selected ion monitoring with [2H2] internal standards. The contents of GAs increased dramatically with decreasing light intensity: GA1, GA3, GA8 and GA20 were 62, 15, 16 and 32 times higher, respectively, under the lowest versus highest light intensities. Gibberellin A19 was not measured at 25 micromole m-2 s-1 but was 9 times greater in the 75 compared to 500 micromole m-2 s-1 treatment. Shoot and hypocotyl lengths were closely positively correlated with (log) GA concentration (for example: r2 = 0.93 for GA1 and hypocotyl length) but shoot dry matter was negatively correlated with GA concentration. The application of gibberellic acid (GA3) produced elongation of plants grown under high light, indication that their low level of endogenous GA was limiting shoot elongation. Although endogenous GA20 showed the greatest influence of light treatment, metabolism of [3H]GA20 and of [3H]GA1 was only slightly influenced by light intensity, suggesting that neither 2beta-nor 3beta-hydroxylation were points of metabolic regulation. The results of this study indicate that GAs control shoot elongation but are not directly involved in the regulation of shoot dry weight in Brassica. The study also suggests a role of GAs in photomorphogenesis, serving as an intermediate between light condition and shoot elongation response.