The metabolism of triacylglycerols (TAG) and their conversion into new triglycerides and polar lipids such as linolenic acid take place during the first phase of development of Helianthus annuus L. seed after seeding. Oleate desaturase and linoleate desaturase are two major enzymes responsible for the synthesis of these compounds. To determine whether enzymatic mechanisms function the same in catabolizing seed store lipids of normal and mutant sunflower lines, modifications of fatty acids composition of seeds and the initial stages of growth (seeding-VE) cultivated under conditions of controlled temperature were assessed. The increase of linoleic acid content during all stages of development in a mutant line revealed a higher activity of oleic acid desaturase (∆12-desaturase) in transforming oleic acid to linoleic acid probably due to higher availability of the substrate of this enzyme in this line; however, its amount increased moderately in a normal line. The activity of FAD2-3 in developing sunflower seeds is recognized even two days after seeding (DAS) in both lines; however, linolenic acid accumulation starts seven DAS.