The membranes of mitochondria and chloroplasts contain a number of pigments that can act as endogenous sensitizers to produce activated oxygen species, most efficiently in blue light, which, in turn, attack functional targets in membranes. Therefore, intense blue light from the sun can exert various adverse effects on the functional and structural integrity of the membranes. One of the biochemical events of these negative effects could be the oxidative degradation of the unsaturated fatty acid constituents of membrane polar lipid, resulting in a decrease in the double bond index (DBI) of the lipid. The results obtained in the present work indicate that DBI of the lipid from mitochondria as well as from chloroplasts was considerably decreased by the periodical exposure (3 hrs per day) of crop plants to blue light from the sun, but not by the yellow/red light exposure. Also measured was the susceptibility of mitochondria) and chloroplast membranes to light-induced peroxidation, demonstrating that blue light is more effective than yellow/red light in causing the membrane peroxidation.