Diurnal patterns of nitrate assimilation in Kentucky bluegrass

Kentucky bluegrass (Poa pratensis L.) is a major C-3-type forage and turfgr ass, but it is less efficient than many grasses in utilizing nitrogen(N). T o determine how this grass can accommodate its greater N need, diurnal patt erns of nitrate reductase activity (NRA) and nitrite reductase activity (Ni RA) in its leaves and roots were examined and compared with those in barley (Hordeum vulgare L.). Plants were grown under greenhouse or growth room co nditions and assayed for NRA and NiRA by optimized in vivo methods. The diu rnal patterns of NRA and NiRA indicated that Kentucky bluegrass could assim ilate nitrate during the night at rates greater than or similar to those du ring the day. Leaf NRA of Kentucky bluegrass was minimal approximately 4 an d 10 h after illumination commenced and increased at night. The diurnal pat tern of leaf NRA among Kentucky bluegrass cultivars did not differ signific antly. In roots, NRA of Kentucky bluegrass was high in the morning and decr eased sharply during the afternoon and evening, but increased again late at night. Unlike Kentucky bluegrass, barley exhibited greater leaf NRA during the day than during the night and exhibited the greatest activity 6 or 10 h after illumination commenced. In both species, the equilibrium leaf nitra te pool was 20 to 30 times larger than the ammonium pool and 3,000 to 13,00 0 times larger than the nitrite pool. Leaf nitrate pool size showed a diurn al pattern complementary to that of leaf NRA. Our results suggest that a ni ghttime N use strategy might exist in Kentucky bluegrass.