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).