Antagonistic action of blue and red light on shoot elongation in petunia depends on gibberellin, but the effects on flowering are not generally linked to gibberellin

Plant morphology and flowering are influenced by light quality, but responses vary between species. Here we investigated effects of irradiance (70 or 150 μmol m−2 s−1) of red (R) and blue (B) compared to white (W) light, provided by light emitting diodes, on growth and flowering in petunia. R light inhibited shoot elongation, but opposite to commonly observed, B light greatly enhanced shoot elongation. Consistent with this, bioactive gibberellins (GA1, GA4) showed higher and lower levels under B and R light, respectively, and GA3 application induced elongation in uniconazole-treated plants, although delayed in R. Inhibited shoot elongation under RB treatment in spite of higher GA level compared to R only, supports that R light inhibits elongation growth, and indicates negative effect of R light on GA signaling. Floral bud formation and flowering occurred earlier under B compared to W light. Whereas no floral buds were observed under low R irradiance, high R irradiance and temporal switching to B light during long-term low R irradiance induced floral development. Except slight trends of promoting effect of the highest GA3 level and delay in uniconazole-response under B light, lack of flowering under low R irradiance was not significantly affected by uniconazole or GA3 application. In conclusion, B and R light are strong signals enhancing and inhibiting shoot elongation, respectively, through modulation of GA content. B light is a strong signal in floral bud formation, whereas effect of R light depends on irradiance, indicating existence of an energy/photosynthesis-related floral pathway in petunia. Although light quality affects flowering and main shoot elongation, these responses do probably not correlate with each other through GA synthesis.

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