Light-Dependent Modifications in the Metabolic Responses of Squash Seedlings

Light-induced modifications in lipoxidase metabolism and chlorophyll formation in the cotyledon of squash (Cucurbita moscata) seedlings were determined. The enzyme activity decreased as light intensity increased, but chlorophyll continued to accumulate long after lipoxidase activity had virtually disappeared. Considering the differences in the levels of irradiance required to manifest the optimal responses, and also from the results obtained with ultraviolet and red, far-red light treatments, any causal relationship between lipoxidase disappearance and chlorophyll synthesis was ruled out.The observed light-saturation phase in the chlorophyll synthesis, indicated that chlorophyll formation was initially controlled by the phytochrome system. No similar saturation stage for the enzyme responses was observed.The sensitivity of lipoxidase to prolonged light exposures suggested a strong correlation with the known photoreactions presumed to be controlled by the high energy reactive-phytochrome system. Lipoxidase metabolism is, therefore, suggested as a biochemical index for the photomorphogenic reactions similar to the ones induced by the high energy reaction.