Germination in Photosensitive Seeds: Does Phytochrome Stimulate Metabolism of GA19 and GA20 to GA1?

Abstract

The effect of light on gibberellic acid (GA) metabolism was investigated in Asteraceae seeds. White light (80 mmol m^-2 s^-1 PAR) increased seed germination in Craspedia sp., Erymophyllum ramosum and Rhodanthe chlorocephala subsp. splendida compared with darkness. In these taxa, red light (R, 640 nm, 8 mmol m^-2 s^-1) stimulated and far red light (FR, 720 nm, 4 mmol m^-2 s^-1) inhibited germination, indicating the involvement of phytochrome. Paclobutrazol, a GA biosynthesis inhibitor, reduced light-stimulated germination and this was overcome by exogenous GA₃. GA₃ and GA₁ promoted germination in the dark, but GA₁₉ and GA₂₀, which are precursors to GA₁, generally did not. GAs were applied to paclobutrazol-treated seeds which were incubated under either R or FR. Very few paclobutrazol-treated seeds germinated without GAs under FR, and exposure to R only marginally improved germination. Application of GA₁₉ or GA₂₀ to paclobutrazol-treated seeds increased germination in Rhodanthe chlorocephala subsp. splendida under R, with no germination under FR. This trend was observed in Craspedia sp., but not in Erymophyllum ramosum or Rhodanthe floribunda. CGA 163′935, a 3b-hydroxylation inhibitor, and R and FR were used to investigate phytochrome-stimulated conversion of GA₁₉ and GA₂₀ to GA₁. It could not be shown that R was required for 3b-hydroxylation in light stimulated germination of these species.