VARIATION IN THE RATES OF SYNTHESIS AND DEGRADATION OF PHYTOCHROME IN COTYLEDONS OF CUCURBITA PEPO L. DURING SEEDLING DEVELOPMENT

Abstract— The time courses for P_r appearance, P_r disappearance and P_fr destruction have been analysed in cotyledons of Cucurbita pepo L. after different preirradiation programs. In etiolated seedlings the rate of P_r appearance is low in young seedlings reaching a maximum in 3.5–5 day old seedlings then decreasing rapidly with increasing age. The rate of P_fr destruction is very low in young seedlings, increases rapidly up to the 4th day and then remains almost constant. The disappearance of P_r becomes significant for seedlings older than 45 days. These reactions seem not to be influenced by short preirradiations. However, after prolonged preirradiation, a degree of control of P, appearance and/or disappearance by the "internal clock appears to be operative.