INFLUENCE OF GROWTH SEASON ON THE CONTENT OF NITRATE, VITAMIN-C, BETA-CAROTIN, AND SUGAR OF HEAD LETTUCE UNDER GREENHOUSE CONDITIONS

The influence of growth season -January till November- on the content of vitamin C, beta-carotene, reducing sugars, and nitrate was investig ated for three butterhead lettuce cultivars ('Soraya, 'Vivaldi', 'Rave l'). The content of beta-carotene, sugar, and nitrate showed seasonal variations, which was also found for vitamin C, but at a lower level. beta-carotene, vitamin C, and nitrate decreased wi with increasing rad iation, while the content of sugar increased. The influence of radiati on on the contents was modified by head weight because beta-carotene, nitrate, and vitamin C decreased and sugar increased with enlarging he ad weight. A close connection could be established between sugar and n itrate concentration. High sugar content correlated with low nitrate c ontent. Significant differences in contents between cultivars were not found. Only the higher weight cultivar 'Soraya' contained a lower nit rate content from July to September.