4.1 | Root secretion and soil nitrate cycling and
utilization
Plants require nitrogen to form new cells (Ke et al., 2020), and
nitrogen deficiency is a limiting factor in crop productivity (Zhao et
al., 2005). Soil nitrogen exists in organic and inorganic forms, with
nitrate and ammonium being the plant-available types (Xu et al., 2012).
Root secretion of soil nitrogen cycle enzymes influences nitrogen use
(Dong et al., 2021). This experiment revealed that S-NR activity
secreted by S. miltiorrhiza roots was significantly
down-regulated under 15 different abiotic stress conditions. This
suggests that under stress, S. miltiorrhiza can maintain the
supply of nitrate-nitrogen in the root environment by down-regulating
the activity of root-secreted S-NR and reducing its nitrate-nitrogen
reduction ability (Coskun et al., 2017). This is consistent with
nitrogen stress down-regulating NR secretion from maize roots (Qiang et
al., 2021) and phosphorus stress down-regulating NR activity from cowpea
roots (Qi bing-lin et al., 2010). The suggestion is that reducing
nitrogen loss is a strategy for plants to cope with abiotic stress.
However, the results of this study differ from the drought stress
induced upregulation of NR activity in wheat roots (Hosseini et al.,
2022) and the improvement of S-NR enzyme activity in soil by nutrient
stress treatment (Meng et al., 2021). The involvement of microorganisms
in root exudation activities, and the increased activity of denitrifying
bacteria, may contribute to the differences observed in nitrate
reductase activity in the soil.
This experiment shows that S. miltiorrhiza often up-regulates the
activity of root-secreted S-NiR under most stress conditions, which is
consistent with the fact that drought stress significantly increases the
activity of S-NiR in the rhizosphere of maize (Lin et al., 2021) but
different from the fact that low nitrogen reduces the activity of NiR in
the rhizosphere of tobacco (Xihuan et al., 2020). Notably, S.
miltiorrhiza significantly increased the activity of S-NiR in both DN
and TN stresses, indicating improved denitrification, which may be
related to the high-temperature causing soil oxygen overflow (Guan
hui-lin et al., 2010). It shows that after stress, plants can reduce the
damage of nitrite salt in the rhizosphere environment by up-regulating
the activity of S-NiR secreted from the root system.
S-UE plays a critical role in converting organic nitrogen in soil into
ammonium nitrogen available for plants. This study shows that S.
miltiorrhiza can significantly up-regulate the activity of S-UE
secreted by roots under N, D, T, DT, DN, DP, and TNP stresses,
consistent with the results that increasing temperature increases the
activity of S-UE in soil (Bai et al., 2017). Moreover, nitrogen
deficiency and its synergistic stress with drought can significantly
up-regulate the activity of root-secreting S-UE, but the synergistic
stress of high-temperature or synergistic stresses of drought and
high-temperature can significantly down-regulate the activity of
root-secreting S-UE (Figure 2-a) (e.g., DN, TN, and DTN stresses). In
addition, nitrogen deficiency, phosphorus deficiency and their
synergistic stress with drought or high-temperature or drought and
high-temperature can significantly up-regulate the activity of
root-secreting S-UE (Figure 2-c) (e.g., DNP, TNP, and DTNP stresses).
These findings suggest that plants demonstrate unique responses to
different synergistic stress. Under abiotic environmental stress, plants
can reduce the reduction of nitrate nitrogen and accelerate the
transformation of organic nitrogen by altering the activities of enzymes
involved in root secretion and soil nitrogen cycling to maintain
nitrogen available to plants and improve the reduction of nitrite to
reduce its harmfulness, and then improve nitrogen conversion and use
efficiency.