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.