Effect of Nitrogen Starvation on the Responses of Two Rice Cultivars to Nitrate Uptake and Utilization

Abstract Ammonium (NH + 4 ) is the main nitrogen (N) form for rice crops, while NH + 4 near the root surface can be oxidized to nitrate (NO − 3 ) by NH + 4 oxidizing bacteria. Nitrate can be accumulated within rice tissues and reused when N supply is insufficient. We compared the remobilization of NO − 3 stored in the tissue and vacuolar between two rice ( Oryza sativa L.) cultivars, Yangdao 6 (YD6, indica) with a high N use efficiency (NUE) and Wuyujing 3 (WYJ3, japonica) with a low NUE and measured the uptake of NO − 3 , expression of nitrate reductase (NR), NO − 3 transporter genes (NRTs), and NR activity after 4 d of N starvation following 7-d cultivation in a solution containing 2.86 mmol L −1 NO − 3 . The results showed that both tissue NO − 3 concentration and vacuolar NO − 3 activity were higher in YD6 than WYJ3 under N starvation. YD6 showed a 2- to 3-fold higher expression of OsNRT2.1 in roots on the 1st and 4th day of N starvation and had significantly higher values of NO − 3 uptake (maximum uptake velocity, V max ) than the cultivar WYJ3. Furthermore, YD6 had significantly higher leaf and root maximum NR activity (NRA max ) and actual NR activity (NRA act ) as well as stronger root expression of the two NR genes after the 1st day of N starvation. There were no significant differences in NRA max and NRA act between the two rice cultivars on the 4th day of N starvation. The results suggested that YD6 had stronger NRA under N starvation, which might result in better NO − 3 re-utilization from the vacuole, and higher capacity for NO − 3 uptake and use, potentially explaining the higher NUE of YD6 compared with WYJ3.