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.