Phytochrome Action and Its Time Displays

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Abstract
Flowering is regulated by the far-red-absorbing form of phytochrome, Pfr, and the degree of regulation depends on the amount of phytochrome transformed to Pfr. The reaction can be started by irradiating the plant with red light, which converts phytochrome to active Pfr, and stopped by far red, which converts active Pfr to inactive Pr. The required period of operation of the enzymatic reaction for effective control of flowering was measured by this method for several plants. The periods ranged from less than one minute to more than one hour. In Chenopodium rubrum seedlings the period of 70 minutes of Pfr action to inhibit flowering, measured in the middle of 16-hour dark periods, was necessitated because rate of formation of substrate for Pfr action was limiting. Rate of supply of substrate depends on the time in the daily dark period it is measured and on light conditions during the preceding photoperiod. In chrysanthemum the necessarily long period of Pfr action resulted from inadequate levels of the enzyme converted by incandescent-filament light. Pfr undergoes reversion in darkness to Pr and the rate of reversion is linked to rate of enzymatic action. Thus, a limitation on substrate supply limits enzymatic action and thereby limits the rate of dark reversion. Physiological response provides reliable evidence of Pfr action but lack of response does not necessarily indicate lack of such action.