Observations on spectrophotometrically assayable phytochromein vivo in etiolatedPisum seedlings

Abstract

Using 2-filter difference spectrophotometry, thein vivo phytochrome of etiolated pea seedlings was measured. On a fresh weight basis, the reversible optical density difference, Δ(Δ O.D.), at 660 and 730 mμ was highest in growing regions such as the epicotyl hook or the root tip, and decreased along the axis of shoot and root. The aging of the seedling did not affect the pattern of phytochrome distribution in the epicotyl, but Δ(Δ O.D.) per unit weight declined slowly in root-tips of intact seedlings and much more rapidly in those of excised root cultures. Although apparent amounts of phytochrome vary in each tissue, per cent conversion of PR to PFR was simply dependent upon total irradiation energy; that is, any tissue obtained from etiolated peas showed approximately 50% conversion of PR to PFR by 3 minutes exposure to red light of about 1.8 kiloergs x cm−2 x min−1. After conversion of PR to PFR by a brief red irradiation, dark transformation was observed in intact and excised tissues of stems and roots; after 4 hours at 26°C, most of the PFR disappeared optically, and total Δ(Δ O.D.) decreased to 1/2∼1/3 of the original level. No significant difference in rate was observed between intact and excised stem samples, or intact root samples, but the rate in excised roots was slower than in the intact. Contrary to previous reports, dark transformation occurs at 0°C, though at roughly 1/60th the rate observed at 26°C.