Nitrogen remobilization during the reproductive period in two Brazilian rice varieties

Two rice varieties, IAC-47 (improved) and Piaui (traditional), were grown in a greenhouse with and without foliar nitrogen (N) supplementation at 10 and 20 days after anthesis (DAA). Changes in N content in shoots (leaves+ sheaths+stalks) and grains were monitored at regular intervals from five days before anthesis (5 DBA) to grain maturity (28 DAA). In the controls of both varieties, the decrease of total N in the shoot was proportional td the increase in total N in the grains. At anthesis, variety IAC-47 exhibited a higher level of total N in the shoot than variety Piaui (9.32 mg N g(-1) versus 7.22 mgN g(-1)). At 28 DAA, IAC-47 exhibited a lower level of grain crude protein than Piaui (66.5 mg N g(-1) versus 74.7 mg N g(-1)). In Piaui, the highest level of protein N in the flag leaf occurred at the time(-1)of anthesis and decreased with grain maturity. In IAC-47, the protein N of the flag leaf increased parallel to grain N up to 10 DAA. The lower level of crude protein in the grains of IAC-47 was perhaps caused by the competition between the flag leaf and the grains for plant N. In the controls, the daily rate of N loss in the shoot of Piaui between anthesis and final harvesting was approximately twice that of IAC-47 (9.94 mg N per day per pot versus 4.66 mg N per day per pot). In the plants receiving supplemental N, the daily rate of N loss in the total shoot was lower in both varieties than in the controls, indicating that when an external source of N was available the plant reduced mobilization of N from its vegetative reserves to fill the grains. Variety IAC-47, by having been selected in the presence of an abundant supply of external N, has a lower ability to mobilize N from the shoot, resulting in lower N content in the grains than in variety Piaui. Therefore, it may be concluded that plants selected to depend less on external N, are perhaps more efficient in increasing grain protein.