PHYTOCHROME and THE DEVELOPMENT OF PHOTOPHOSPHORYLATION CAPACITY—AN APPRAISAL OF THE EXPERIMENTAL APPROACH*

Abstract

Abstract— Development of the capacity for photophosphorylation (= total capacity for light‐driven ATP formation) in the mustard (Sinapis alba L.) cotyledons is strongly influenced by a red light pulse pretreatment which operates through phytochrome. The present report deals with several objections raised against the in situ assay of the rate of photophosphorylation. Experimental evidence is given in support of the assumption that the linear increase of the ATP content of the cotyledons as measured over 1.5 min after the onset of saturating white light (370 Wm‐2) in fact represents the maximum rate of photophosphorylation (‘capacity’). Moreover, it is confirmed that control by phytochrome of the development of the photophosphorylation capacity and of the capacity for chlorophyll synthesis are unrelated phenomena. The failure of development of the capacity for photophosphorylation in isolated cotyledons from dark‐grown seedlings cannot be attributed to deficiencies of chlorophyll synthesis.It is concluded that the photophosphorylation response is particularly useful to study the mechanism of phytochrome (Pfr) action in case of a response which involves a threshold reaction and an interorgan (hook→cotyledon) cooperation.