Phytohormonal changes in greening and senescing intact cotyledons of oilseed rape and pumpkin: influence of the growth retardant BAS 111 ˙˙ W

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Abstract
The triazole growth retardant BAS 111‥W delayed senescence in cotyledons of pumpkin (Cucurbita maxima L. cv. Gelbe genetzte Riesenmelone) and stimulated chlorophyll synthesis in greening cotyledons of oilseed rape (Brassica napus L. cv. Petranova) seedlings. In both cases, changes of phytohormone‐like substances in the cotyledons were analyzed on a fresh weight basis by immunoassay.After soil treatment with increasing retardant concentrations, a close correlation was observed in senescing cotyledons of pumpkin between a reduced loss in total chlorophyll and increasing levels of dihydrozeatin riboside (DZR) and trans‐zeatin riboside (ZR)‐type cytokinins. In contrast, the levels of isopentenyladenosine (IPA)‐type cytokinins, 3‐indoleacetic acid (IAA) and gibberellin (GA) did not change significantly. The levels of abscisic acid (ABA) were slightly elevated at low retardant concentrations but dropped considerably below those of controls at higher doses. Consequently, the molar ratio of total cytokinin to ABA content changed from approximately 1:40 in controls (50% of initial chlorophyll) to 1:3 in cotyledons treated with 3 mg BAS 111‥W plant−1 (85% of initial chlorophyll). These changes, together with the known reduction of ethylene production by plants treated with nitrogen‐heterocyclic retardants, can explain the delayed senescence in pumpkin cotyledons. Likewise, when etiolated, BAS 111‥W‐treated seedlings of oilseed rape were exposed to light, the stimulation of chlorophyll synthesis in the cotyledons was accompanied by an accumulation of DZR‐ and, particularly, ZR‐type cytokinins and IAA. In contrast, GA and ABA contents decreased slightly. We conclude that the influence of BAS 111‥W on cytokinin levels might be involved in the stimulation of greening.