NITRATE AND NITRITE CONTENTS IN SPRING CABBAGE AS RELATED TO NITROGEN FERTILIZER TYPE, METHOD OF FERTILIZER APPLICATION AND TO NITRATE AND NITRITE REDUCTASE ACTIVITY.

The effect of different forms of nitrogen fertilizer (NO₃, NH₄, urea), and different methods of application (broadcast and placement fertilization), on the content of nitrate, nitrite, and free amino acids, and on the activities of nitrate and nitrite reductase was investigated in spring cabbage at harvest and during short-term storage at low (5°C) and high (20°C) temperature. In freshly harvested heads the highest activity of both enzymes and the highest content of nitrate were found in cabbage treated with potassium nitrate, compared with ammonium sulphate and urea. By comparison, the highest levels of nitrite and free amino acids were determined in plants fertilized with ammonium sulphate. A considerable decrease in nitrate reductase activity and content of nitrite was observed during cabbage storage. No significant changes in nitrate in stored cabbage heads fertilized with potassium nitrate were found, whereas with the other N forms there was a systematic increase in its content.