Phytochrome action in Oryza sativa L. : II. The spectrophotometric versus the physiological status of phytochrome in coleoptiles.

In-vivo phytochrome determinations in totally etiolated rice seedlings with a dual-wavelength spectrophotometer showed that on a fresh weight basis phytochrome concentration was highest in the coleoptile apex (0.175 of mean) Δ(Δ O.D.) g(-1) (fresh weight). The age of the seedlings had little effect on the pattern of phytochrome distribution in the coleoptiles.The extent of growth inhibition observed 2 days after the irradiations was proportional to the logarithm of P fr amount in the coleoptiles at the time of initial exposure to either red or blue light. Ultraviolet irradiation, however, did not induce either reversible growth inhibition or optically detectable phytochrome changes in vivo.After the conversion of P r to P fr bya brief red irradiation, non-photochemical transformation of phytochrome was observed in intact coleoptile tissues. Most of the optically measurable P fr disappeared within 6 hours at 27°, when the total Δ(Δ O.D.) decreased to about one fifth of the original level. The optical data did not agree with the fact that 50% of the initial physiological reversibility was still observed 9 hours later. No significant difference in dark transformation rate was seen between intact and excised coleoptile tissues.