Leaf senescence in alstroemeria : regulation by phytochrome gibberellins and cytokinins

Leaf senescence in plants is a regulated process influenced by light as well as phytohormones. In the present study the putative role of the phytohormones cytokinins and gibberellins as mediators for the light signal on leaf senescence in alstroemeria was studied. It was found that low photon fluences of red light ensured maximal delay of chlorophyll and protein breakdown. This effect of red light could be completely counteracted by a subsequent far red irradiation, indicating phytochrome involvement.Application studies with gibberellins showed that GA ₄ was most effective in delaying leaf senescence and it was proven that GA ₄ is not converted into GA ₁ but is biologically active by itself. A total of 11 gibberellins was detected to be endogenous in alstroemeria leaves. During senescence the relative concentration of precursors and active gibberellins decreased whereas that of inactivated gibberellins increased strongly. Although irradiation of the leaves with red light resulted in delayed senescence and a higher GA ₄ concentration compared to dark-incubated leaves, based on the obtained results, GAs are not considered to act as mediators for the transduction of the light signal.Alstroemeria leaves were found to contain isoprenoid-derived cytokinins and aromatic cytokinins. Irradiation of leaves with red light resulted in a transient increase in <em>meta</em> -topolin and <em>meta</em> -topolin riboside approximately one hour after the start of illumination. No light related changes in concentration were found for other cytokinins in these leaves.Although the visual effect of red light, cytokinins and gibberellins is similar, the mode of action of the regulators may be different. It was found that both red light and <em>meta</em> -topolin had a positive effect on chlorophyll biosynthetic reactions as well as on the rate of photosynthesis and expression of genes encoding for chlorophyll binding proteins ( <em>cab</em> ). GA ₄ did not positively affect these parameters. The chlorophyll catabolic reaction, determined as Mg-dechelatase activity was not differentially affected by either <em>meta</em> -topolin, GA ₄ or red light. From the results, it is suggested that aromatic cytokinins are primarily involved in regulation of leaf senescence and can function as a mediator for the transduction of the phytochrome signal.