Nitrogen remobilization during the reproductive period in two Brazilian rice varieties

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Abstract
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 to 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 mg N 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.