VERY-LOW FLUENCE AND LOW FLUENCE RESPONSE IN THE INDUCTION AND INHIBITION OF SEED-GERMINATION IN CELOSIA-ARGENTEA

Seeds of Celosia argentea L. displayed an absolute requirement for lig ht for germination. Germination could be induced by far-red light as w ell as red light, and therefore, the effect of red light was not compl etely reversible by far-red light. A considerable proportion of the se ed population was sensitive to red light within the range 0.01-0.1 mu mol m(-2), which suggested that phytochrome was operating in the very low fluence response mode. Seeds showed a gradual increase in germinat ion in response to red light with an increasing duration of previous d ark imbibition. interruption of the dark imbibition period with very l ow fluence (10(-4)-10(-1) mu mol m(-2)) or low fluence (1-10(3) mu mol m(-2)) red light prevented germination subsequent to a terminal, satu rating red light irradiation. Increasing responsiveness to red light w ith increase in imbibition temperature and inhibition of development o f photosensitivity by cycloheximide suggested that phytochrome synthes is occurred during the dark imbibition period. An absolute requirement for light for seed germination, inhibition of seed germination on int errupting the dark imbibition period with very low fluence or low flue nce red light, and the recovery from photoinhibition of seeds in darkn ess are consistent with the hypothesis that PhyA is the principal phyt ochrome involved in the photoregulation of seed germination in C. arge ntea.