Spectrophotometric measurements of phytochrome in vivo and their correlation with photomorphogenic responses of Phaseolus

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 phytochrome 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/cm(2) and for inactivation at 30 mj/cm(2). The rate of hook opening and the physiological response measured 20 h after induction are both shown to be directly proportional to the initial amount of PFR present spectrophotometrially. 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(deltaOD)/g fresh wt. If the total reversible phytochrome concentration is reduced by exposure to red light and PRF is allowed to decay out of the system, the remaining sensitivity of the tissue is shown to be directly correlated with the amount of PR remitining in the tissue. PFR reverts in the system to PR. At 40 degrees Celsius PFR does not disappear measurably up to 1 h and is nearly totally reversible to PR. (AS)