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The nitrate reductase activity (NRA) was determined every 10-12 days throughout the growing season in young and mature leaves, stems, roots and tubers of potato cv. Uran, Narew and PB 2081 plants grown in pots and given 80 kg/ha NPK (20:20:20) in 3 split doses (50% at planting, 25% at flower bud formation, and 25% at tuberization). In another experiment, the influence of metribuzin on NRA was determined in field trials carried out over 3 years with 40 cultivars and in glasshouse experiments with 7 cultivars. There were peaks in NRA in June-July in all cultivars about 40 days after planting. NRA was lowest at midday, particularly on hot days, and was low also in the evenings. The most repeatable and highest NRAs were measured at 0900-1100 h. The upper leaves showed the highest NRA and all underground parts had low NRA. There was a linear correlation between nitrate reduction capacity and …