Germplasm of Solanum tuberosum and Solanum phureja exhibit a wide (over 20-fold) variation in tuber carotenoid content. The levels of carotenoids during tuber development and storage were compared in a high carotenoid-accumulating S. phureja accession (DB375\1) with two S. tuberosum cultivars (Pentland Javelin and Desiree) that accumulate lower levels of tuber carotenoid. On a dry weight basis, total carotenoid levels were at a maximum early in tuber development. However, in the S. phureja accession, carotenoid levels remained at a high level throughout tuber development, whereas in the S. tuberosum accessions, carotenoid content decreased as dry weight increased. The carotenoid profiles of tissues during tuber development were analysed in greater detail by reverse phase HPLC. In S. phureja tubers at maturity the major carotenoids were zeaxanthin, antheraxanthin, and violaxanthin. Following 9 months storage at 4 degrees C the levels of zeaxanthin and antheraxanthin decreased, whereas the level of lutein increased; overall, however, there was only a small decrease in total carotenoid content. In order to explore the reasons for the wide variation in tuber carotenoid content, the expression patterns of the major genes encoding the enzymes of the carotenoid biosynthetic pathway were compared. Significant differences in the profiles were detected, suggesting that transcriptional control or mRNA stability gives rise to the large differences in tuber carotenoid content. In particular, there was an inverse trend between the level of zeaxanthin epoxidase transcript level and tuber carotenoid content in a range of potato germplasm, giving rise to an hypothesis for the regulation of carotenogenesis in potato tubers.
