3 . 2 . Cultural practices
Augustin (1975) reported a decrease in AA content of some cultivars of potatoes with increas- ing amounts of nitrogen fertilizer used (Table 3). Lisiewska and Kmiecik (1996) reported that in- creasing the amount of nitrogen fertilizer from 80 to 120 kg ha−1 decreased the vitamin C content by 7% in cauliflower. Reduced levels of vitamin C in juices of oranges, lemons, grapefruits, and mandarins resulted from the application of high levels of nitrogen fertilizer to those crops, while increased potassium fertilization increased AA content (Nagy, 1980). Based on these reports, nitrogen fertilizers, es- pecially at high rates, seem to decrease the con- centration of vitamin C in many fruits and vegetables. Plant growth is generally enhanced by the nitrogen fertilization so that a relative dilution effect may occur in the plant tissues. Nitrogen fertilizers are also known to increase plant foliage and thus may reduce the light intensity and accu- mulation of AA in shaded parts. Since excess use of nitrogen fertilizers increases the concentration of NO3 and simultaneously decreases that of AA, it may have a double negative effect on the qual- ity of plant foods (Mozafar, 1993). Although vitamin C concentration has been found to be positively correlated with the nitrogen supply in butterhead lettuce (Muller and Hippe, 1987), it is inversely correlated with the nitrogen