Spectrophotometric Measurements of Phytochrome in vivo and Their Correlation with Photomorphogenic Responses of Phaseolus

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Abstract
Direct in vivo measurements of phytochrome were made in Phaseolus vulgaris by 2-filter difference spectrophotometry (Ratiospect). All measurements were made at 730 versus 800 nm and it is assumed that the [DELTA]([DELTA]OD) is directly proportional to the PFR concentration of photochrome present. Dose response curves were determined for both physiological and spectrophotometric responses for red induction and far-red photoinactivation. For induction, saturation occurs at 100 mj/cm2 and for inactivation at 30 mj/cm2. The rate of hook opening and the physiological response measured 20 hr. after induction are directly proportional to the initial amount of PFR spectrophotometrically. The sensitivity of the tissue correlates well with the absolute amount of phytochrome present, the inner portion of the hook having the maximum concentration of 0.042 [DELTA]([DELTA]OD)/g fresh weight. If the total reversible phytochrome concentration is reduced by exposure to red light and allowing PFR to decay out of the system, the remaining sensitivity of the tissue is then directly correlated with the amount of PR remaining in the tissue. PFR disappears rapidly in the dark at 25[degree], andis not detectable after 6 hr. There is no indication that PFR reverts in the system to PR. At 4[degree] PFR does not disappear measurably up to 1 hr. and is nearly totally reversible to PR.

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