Changes in endogenous levels of three cytokinins during development of excised watermelon cotyledons

We have determined by an immunological method the endogenous levels of three cytokinins: dihydrozeatin riboside (DHZR), transzeatin riboside (tZR) and isope-ntenyladenosine (IPA) in watermelon (Citrullus vulgaris Schrad., cv. Fairfax) cotyledons that were either attached to the seedling or excised from the seed after imbibition and then grown on water. Both seedlings and cotyledons were grown either for 5 days in continuous light or for 3 days in the dark and 2 days in light. Our aim was to verify whether endogenous cytokinin levels are lower in excised than in attached cotyledons as could be expected since excised cotyledons are much more sensitive to exogenous cytokinin application. The levels of the three cytokinins were very low immediately after imbibition, but gradually increased during the following days. They were higher in excised cotyledons after 5 days of culture in the dark than in cotyledons of the same age that had developed on the seedling. Dihydrozeatin riboside was by far the most abundant of the three cytokinins in cotyledons as well as in the hypocotyl and the root.
Irradiation reduced the level of DHZR, negating the concept that light promotes cotyledon development by increasing endogenous cytokinins. Transzeatin riboside when supplied exogenously, stimulated cotyledon development at a lower concentration than the other two cytokinins. Exogenous supply of ben-zyladenine (BA) induced a strong increase in endogenous tZR already after 24 h.