Senescence and Rejuvenation in Intact Cotyledons of Cucurbita

Growth parameters and total pigment content (chlorophyll a+b and carotenoids) were investigated through postgermination, greening, natural senescence and subsequent rejuvenation of intact cotyledons of Cucurbita pepo L. (zucchini). Dark-induced senescence and recovery of the cotyledons after 2-day darkening of a whole plant were also studied. Results showed significant increase in the fresh weight of cotyledons accompanied by a reciprocal decrease in dry matter during the first two weeks of their postgerminative growth. During the period of greening cotyledons rapidly accumulated chlorophylls and carotenoids in a synchronous manner. Maximum pigment contents were reached by days 7-8 after the onset of germination, followed by a steady state by day 14-15, and a gradual decline thereafter during the period of natural senescence. Cotyledons intensively yellowed, lost their turgor and died about 25 days after seed germination. Epicotyl decapitation at the stage of visible yellowing resulted in a recovery of both chlorophyll and carotenoids amounts during the next 15 days of rejuvenation period. When 8-day-old seedlings were transferred to the dark for 2 days, a progressive reduction in fresh and dry weight as well as in the chlorophyll and carotenoids content and net photosynthetic rate was registered. The pigment losses and CO2 assimilation as well as the growth reduction of darktreated cotyledons were completely reversed after the return of the plants to normal light regime. Eleven days after ceasing the dark treatment chlorophyll content of the recovering plants was even increased. In contrast, fiveday dark treatment of entire plants could not be overcome by returning to the light.