The effect of nitrogen concentration on nitrogen use efficiency and related parameters in cultivated rices (Oryza sativa L. subsp. indica and japonica and O. glaberrima Steud.) in hydroponics

Nitrogen use efficiency (NUE) in cultivated rice has become of utmost importance due to ecological and economical consequences of nitrogen (N) losses. However, detailed information on the components of NUE—physiological, absorption and agronomical NUEs—is lacking. The present study aimed to determine the components of NUE for three cultivars of Asian and African rices [IR64 (Oryza sativa indica), Azucena (O. sativa japonica) and TOG7105 (O. glaberrima)] exposed to a wide range of N supply, from excess to deficiency—from 4× to 1/8× of the standard Yoshida solution (1×). Increasing the N supply to 2× or 4× did not induce much change for most parameters, including tissue N concentrations and pNUE. However, aNUE and agNUE decreased gradually as the N supply increased from 1/2× to 4×. In contrast, lowering the N supply, particularly to 1/4× or 1/8×, induced a significant decrease in most measured parameters, except NUEs. The pNUE increased gradually when lowering the N supply from 1× to 1/8×, while aNUE and agNUE were maximal at 1/2 and/or 1/4× according to the cultivar. In contrast, the F_v/F_m and PSII values remained unchanged. Differences between cultivars were low. However, the O. glaberrima cultivar showed a significantly lower aNUE and agNUE than both O. sativa cultivars under low N supply (1/4 and 1/8×). These results demonstrate that all NUE components were sharply and differentially affected by low N supply, while the PSII remained unaffected. These results are important for determining the cultivar and N supply with the best balance between intensive and sustainable rice cultivation.