APPEARANCE OF PHOTOPHOSPHORYLATION CAPACITY: THRESHOLD vs GRADED CONTROL BY PHYTOCHROME

— It is shown that in attached mustard cotyledons graded control of chlorophyll synthesis by physiologically active phytochrome (P fr ) and threshold control by P fr of the ‘potential capacity’ to photophosphorylate are totally different phytochrome actions even though both controls are essential for the build‐up of the same functional complex, the machinery for photophosphorylation. The essential findings are as follows: The action of P fr (made by a 1 min red light pulse) on the capacity and efficiency of photophosphorylation is rapid—detectable after 15 min and completed after 30 min—whereas the action of P fr on chlorophyll formation is slower—only detectable 45 min after the original red light pulse (R). Detailed escape studies (loss of full reversibility of the inductive effect of a R pulse by far‐red) show that the effect of a R pulse on chlorophyll synthesis remains fully reversible for 45 min whereas the action of P fr on the capacity for photophosphorylation is very fast (occurring within 2 min). Control of capacity for photophosphorylation is a threshold response (whereby the threshold value is approximately 1.25% P fr based on total phytochrome at 36 h = 100%) whereas control by P fr of chlorophyll synthesis is graded. Control of capacity for photophosphorylation by P fr only operates if the hypocotyl hook is connected to the cotyledons for at least 2 min after the inductive R pulse, i.e. until full escape from reversibility has occurred, whereas chlorophyll formation in the cotyledons is not affected by the separation of hook and cotyledons.