Why Nitrogen Use Efficiency Decreases Under High Nitrogen Supply in Rice (Oryza sativa L.) Seedlings

Hydroponic experiments were conducted in a greenhouse to examine the effects of different nitrogen (N) supply (low, 20 mg L⁻¹; intermediate, 40 mg L⁻¹; and high, 100 mg L⁻¹) on the growth, nitrogen use efficiency, and photosynthetic characteristics of rice seedlings (Oryza sativa L., cv. “Shanyou 63” hybrid indica. China). Leaf gas exchange was conducted to identify the photosynthetic-limiting factors in plants with high N supply. The results showed that (1) shoot biomass, leaf area, and tiller numbers per plant under low N were lower than under intermediate and high N supplies. No significant differences were observed between plants supplied with intermediate and high N. (2) About a 35% increase in leaf N content in plants fed by high N resulted in about a 15% increase in carboxylation efficiency (CE) and photosynthetic rate. (3) The noncorresponding increases in photosynthetic rate in rice seedlings fed by high N relative to low N resulted from Rubisco activity and/or CE. (4) The decreased Rubisco activity was induced by a relatively insufficient CO₂ supply under high N supply. These results indicated that insufficient CO₂ supply under high N supply accounted for the decreased Rubisco activity and the noncorresponding increases in photosynthetic rate to leaf N content, and as a result, decreased (photosynthetic) nitrogen use efficiency.