APPARENT INCREASE IN PHOTODETECTABLE PHYTOCHROME IN THE PRESENCE OF CALCIUM CARBONATE*

Abstract

Abstract— –Using squash seedling extracts (Cucurbita pepo L.), we describe an apparent increase in photodetectable phytochrome content that is depended upon the use of either CaCO3 or starch as a scattering agent and is also a function of preirradiation with red light of either intact tissue or crude extracts prior to sample preparation for spectrophotometric assay. This apparent increase in photodetectable phytochrome content requires about 1 h for full expression after addition of the scattering agent. If irradiations are given in vitro, the increase is partially reversed by far red light when using squash extracts, and is fully reversed with Aoena and Zea extracts. For squash extracts the magnitude of this increase. which is typically between 20 and 40%. is quantitatively correlated with both the amount of Pfr produced by a brief red irradiation and with the proportion of the total phytochrome pool that is pelletable upon centrifugation at 20,000 g. The correlation with pelletable phytochrome does not hold for Auena and Zea, at least when irradiations are gwen in uitro. The increase in photodetectable phytochrome may result from changed phytochrome extinction, changed phototransformation quantum yields, and/or specific trapping of phytochrome by the scattering agent as it settles. An important consequence of these data is that they indicate a need for caution when using a scattering agent during spectrophotometric assay of phytochrome.