Effects of Different Nitrogen Concentration and Form on Nitrate Accumulation in Leaf Radish

Biomass, nitrate content, and nitrate reductase activity (NRA) of hydroponically cultured radish, 'Dahongyanzhi', 'Maer', 'Qingyedongchunwang', and 'Liushizao' were studied in relation to N level (1, 2, 3, 4, 5 mM), N supply method (continual or reduced N supply), and N source (nitrate, urea, or both but in different ratios). Typically, yield increased with N concentration to gradually reach a plateau and then, sometimes, decrease. In contrast, nitrate content increased continuously with N rate. Different nitrate contents were found in different organs: contents in roots and petioles were considerably higher than those in leaves. Nitrate accumulation in roots was lower than in leaves or petioles. With the increase of nitrogen rate, the nitrate storage function of roots was gradually transferred to leaves with no change in petioles. The treatments being deprived of N for 3 days before harvest showed significant decrease of nitrate content, especially on the first day. Reductions in leaves and roots were more important than in shoots and petioles. With the decrease of nitrate to urea ratio, nitrate content, activity of nitrate reductase and biomass production all declined. The greatest reduction of nitrate accumulation was observed when the ratio of nitrate to urea in the nutrient solution changed from 25:75 to 0:100.