GIBBERELLIN AND PHYTOCHROME CONTROL SENESCENCE IN ALSTROEMERIA LEAVESINDEPENDENTLY

In alstroemeria (Alstroemeria hybrida). leaf senescence is effectively retarded by the application of gibberellins and by low fluences of re d light. In this study we examined the possible interaction of gibbere llins and red light in the regulation of senescence. Determination of endogenous gibberellins revealed that leaf senescence is accompanied b y significant changes in the concentrations of non-13-hydroxylated gib berellins, the onset of senescence coinciding with a dramatic drop in GA(4), 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-1 3-hydroxylated GAs were more active than the 13-hydroxylated GAs. It a ppeared that the effect of red light is additive to that of active GAs . We hypothesise that GA(4) and phytochrome control senescence in alst roemeria mainly through separate mechanisms and have independent effec ts and that the observed differences in gibberellin concentrations are a consequence of delayed leaf senescence rather than a cause for it.