GENETIC-REGULATION OF DEVELOPMENT IN SORGHUM-BICOLOR .10. GREATLY ATTENUATED PHOTOPERIOD SENSITIVITY IN A PHYTOCHROME-DEFICIENT SORGHUM POSSESSING A BIOLOGICAL CLOCK BUT LACKING A RED LIGHT-HIGH IRRADIANCE RESPONSE

The role of a light-stable, 123-kD phytochrome in the biological clock , in photoperiodic flowering and shoot growth in extended photoperiods , and in the red light-high irradiance response was studied in Sorghum bicolor using a phytochrome-deficient mutant, 58M (ma(3)(R) ma(3)(R)) , and a near-isogenic wild-type cultivar, 100M (Ma(3) Ma(3)). Since ch lorophyll a/b-binding protein mRNA and ribulose bisphosphate carboxyla se small subunit mRNA cycled in a circadian fashion in both 58M and 10 0M grown in constant light, the 123-kD phytochrome absent from 58M doe s not appear necessary for expression or entrainment of a functional b iological clock. Although 58M previously appeared photoperiod insensit ive in 12-h photoperiods, extending the photoperiod up to 24 h delayed floral initiation for up to 2 weeks but did not much affect shoot elo ngation. Thus, although 58M flowers early in intermediate photoperiods , a residual photoperiod sensitivity remains that presumably is not du e to the missing 123-kD phytochrome. Since rapid shoot elongation pers ists in 58M under extended photoperiods despite delayed floral initiat ion, long photoperiods uncouple those processes. The observed absence of a red light-high irradiance response in 58M, in contrast to the pre sence of the response in 100M, strengthens the suggestion that the 123 -kD phytochrome missing from 58M is a phyB.