Fig. 5.

Effects of NO⁻₃ (left) and NH₄⁺ (right) utilization as the sole nitrogen (N) source on root respiration and N acquisition in wetland plants. The downward (⇩) and upward (⇧) arrows indicate the decreasing and increasing responses, respectively. NO⁻₃ utilization results in a low root to shoot weight (S/R) ratio, which is unfavorable for O₂ supply. As the N uptake rate per root weight (NNUR) per root respiration rate decreases when the wetland plants utilize NO⁻₃, they develop the roots for N acquisition, consequently increasing the respiration of the whole roots. Therefore, NO⁻₃ utilization requires high O₂ supply to maintain productivity. In contrast, NH₄⁺ utilization results in a high S/R ratio, which is favorable for O₂ supply, and high NNUR per root respiration. Moreover, when NH₄⁺ concentrations increase, the wetland plants may assimilate NH₄⁺ in their shoots instead of their roots. These traits contribute to a decrease in the respiration of the whole root, and thus wetland plants can ensure NH₄⁺ utilization even under low O₂ supply. Photograph of Carex lyngbyei grown in 200 µM NO₃⁻ and NH₄⁺ treatments under hypoxic hydroponic culture for 1 month. Bar 5 cm