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Effects of Moderately Low Temperature (20°C) on Phytochrome Responses During Preirradiation: Anthocyanin Synthesis in Sorghum bicolor at High‐ and Low‐Pfr/Ptot Ratios

Abstract— Effects on phytochrome‐mediated anthocyanin synthesis of moderately low temperature (MLT) given during the preirradiation culture period were studied with seedlings of broom sorghum (Sorghum bicolor Moench, cvs. Acme Broomcorn and Sekishokuzairai‐Fukuyama). Seedlings were grown in the dark at 20°C (MLT) and 24°C (control). The MLT treatment strikingly enhanced the action induced by a red light (R) pulse above ca 200 μmol m−2 and suppressed the action induced by an R pulse below ca 30 μmol m−2 and by a far‐red light (FR) pulse alone. We refer to these MLT‐affected distinct responses as “high‐Pfr/Ptot response” and “low‐Pfr/Ptot response,” as they have features different from the high‐irradiance and very‐low‐fluence responses, respectively. The destruction rate of spectroscopically detectable phytochrome (phyA) and the time course of escape of anthocyanin synthesis from FR reversibility did not match, and hence the possibility of phyA being involved in high‐Pfr/Ptot response was rejected, although it might be involved in low‐Pfr/Ptot response. Possible mechanisms for the two distinct phytochrome responses are discussed.