Ion Relations, Chlorophyll Synthesis and the Question of ‘Bulk’ Phytochrome in Pisum sativum

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Abstract
Lack of correlation between spectrophotometric phytochrome assays and physiological experiments has led to the concept of a ‘bulk’ phytochrome that is measurable but inactive, in contrast to an active form that is not measured in the standard assay. Because of suggestions that phytochrome action involves changes in membrane permeability, particularly to potassium ions, we investigated potassium and sodium uptake and efflux in Pisum sativum epicotyl sections which were equally de‐etiolated yet differed three‐fold in their phytochrome contents. Although dependent on the metabolic integrity of cell membranes (as shown by respiratory inhibitors) the selective uptake of potassium in this tissue is not under phytochrome control. Experiments were also conducted to see whether a three‐fold difference in phytochrome content affects chlorophyll synthesis in Pisum. Differences in chlorophyll synthesis observed can be related to the effectiveness of the pretreatments in eliminating the lag phase, not to their differences in phytochrome; Pfr decay rate has no effect on chlorophyll synthesis. The failure to find any correlation with the amount or state of ‘bulk’ phytochrome in these and previous experiments indicates that no function of this material is yet known.