3.2 | The activity of enzymes related to nitrogen metabolism in the medium
Abiotic stress can significantly down-regulated S-NR activity in the culture medium, with single factors N and T stresses having the strongest inhibitory effect, while the combined effect was most severe in synergistic stresses of high-temperature and nitrogen deficiency (TN), synergistic stresses of nitrogen and phosphorus deficiency (NP), and DTN stresses (p <0.05). DN and TN stress up-regulated S-NiR activity (p <0.05), but the other factors did not show statistical significance. Moreover, N, T, D, synergistic stresses of high-temperature and drought (DT), DN, synergistic stresses of drought and phosphorus deficiency (DP), and TNP stresses all elevated S-UE activity in the culture medium, while synergistic stresses of high-temperature and phosphorus deficiency (TP), TN, NP, and DTN stresses significantly down-regulated S-UE activity in the culture medium (p <0.05) (Figure 2). These findings indicate that under abiotic stress, plants reduce the secretion of S-NR enzyme from their root system, which curtails denitrification and prevents nitrate-nitrogen loss. In response to N, T, D, DT, DN, DP, and TNP stresses, plants secrete S-UE to enhance organic nitrogen utilization.
Drought or high-temperature stress can significantly up-regulate the effect of nitrogen deficiency on the release of S-NiR from roots (e.g., DN and TN stresses) (p <0.05). Drought stress can significantly alleviate the effect of nitrogen deficiency on the release of S-NR from roots and simultaneously significantly promote the release of S-UE from roots (e.g., DN stress) (p <0.05). High-temperature or synergistic stresses of drought and high-temperature inhibit the effect of nitrogen deficit on the induction of S-UE release from roots (e.g., TN and DTN stresses) (p <0.05) (Figure 2-a). Moreover, drought stress can significantly enhance the effect of phosphorus deficiency on the release of S-UE from roots, while the high-temperature can alleviate the effect of phosphorus deficiency on the release of S-NR from roots and inhibit the release of S-UE from roots (e.g., DP and TP stresses) (p <0.05) (Figure 2-b). Furthermore, drought or high-temperature or the synergistic stresses of both can alleviate the effect of nitrogen and phosphorus deficiency on the inhibition of root secretion of S-UE and the synergistic stresses of high-temperature and drought can also significantly promote the release of S-NR from roots (e.g., DNP, TNP, and DTNP stresses) (p <0.05) (Figure 2-c). Finally, the synergistic stresses of drought and high-temperature can significantly reduce the effect of drought on the release of S-UE from roots (e.g., DT stress) (p <0.05) (Figure 2-d).