Gibberellin and phytochrome control senescence in alstroemeria leaves independently

In alstroemeria ( Alstroemeria hybrida ), leaf senescence is effectively retarded by the application of gibberellins and by low fluences of red light. In this study we examined the possible interaction of gibberellins and red light in the regulation of senescence. Determination of endogenous gibberellins revealed that leaf senescence is accompanied by significant changes in the concentrations of non‐ 13‐hydroxylated gibberellins, the onset of senescence coinciding with a dramatic drop in GA₄, whereas concentrations of 13‐hydroxylated gibberellins are far less influenced. However, no direct effect of red light on a specific GA‐metabolic step could be determined. When exogenously applied, non‐13‐hydroxylated GAs were more active than the 13‐hydroxylated GAs. It appeared that the effect of red light is additive to that of active GAs. We hypothesise that GA₄ and phytochrome control senescence in alstroemeria mainly through separate mechanisms and have independent effects and that the observed differences in gibberellin concentrations are a consequence of delayed leaf senescence rather than a cause for it.