Effects of exogenous free oleic acid on the ultrastructure, 14C-acetate incorporation and respiration of Lemna minor fronds

Fronds of Lemna minor L. were grown hydroponically and aseptically for 1 or 7 days in the absence or presence of different concentrations of free oleic acid (18:1). Particularly after 7 days of growth in the presence of 1062 μM of exogenous 18:1, L. minor epidermal and mesophyll cells increased the starch grain content of their chloroplasts and they considerably accumulated, in their cytoplasm, structure resembling to oil bodies (oleosomes or oleosins) found in oily seeds. Increasing exogenous 18:1 concentrations induced a significant decrease in the rate of respiration of L. minor cells. These results could explain the increase in the percentage of dry matter relatively to fresh weight and the decrease in the growth and in O 2 evolution observed in L. minor fronds at higher concentrations of exogenous 18:1. Incorporation of [U- 14 C]acetate into total fatty acids of L. minor showed that absorption of exogenous 18:1 stimulated the de novo synthesis of palmitic acid and to a lesser extent that of stearic acid. The radioactivity incorporated into 18:1 was similar between treated and control plants, but the specific radioactivity was far lower in treated plants due to the accumulation of cold exogenous 18:1. Unexpectedly, these results seemed to show that the de novo synthesis of 18:1 was not inhibited by a high cellular content of 18:1, particularly in the form of free fatty acid.