Effect of drought and rehydration in vegetative growth period on nitrogen content
As shown in Fig 2, in the V3 stage, the total nitrogen content in the leaves and petioles of HN44 decreased under drought stress.
Fig 2: Effect of water change on total nitrogen content (mg/g) in vegetative growth period (V3) (Significance analysis was carried out between different degrees of stress in the same organ. P<0.05).
No significant differences were observed between the stems. After rehydration, the total nitrogen in the leaves under mild drought rehydration showed a significant compensatory effect, with an increase of 4.8% and the petioles, similarly, showed a compensatory effect. The total nitrogen content in the stems after rehydration was still lower than that of the control. Under drought stress at the V3 stage, the variable trend of the total nitrogen content of HN65 was similar to that of HN44. The change in nitrogen content in the stems was not significant, but in the leaves and petioles, it decreased significantly. The nitrogen content in the leaves of HN65 plants decreased significantly by 12% under mild drought. In comparison, the nitrogen content of HN44 was only reduced by 5% under the same circumstances, indicating that drought during this period had a greater impact on the nitrogen level of HN65. After rehydration, all organs of HN65 showed a compensatory effect compared to the control, but there was no significant difference under different degrees of drought rehydration treatment. The compensatory effect of HN65 after rehydration was higher than that of HN44, indicating that the compensatory effect was more significant for varieties that were greatly affected by drought.
A certain degree of abiotic stress can induce a compensatory effect in crops, which is reflected in their yield or growth state (Balducci et al., 2016).
Our study further confirms this view, because nitrogen also shows a compensating effect after drought rehydration. During the vegetative growth period, the total nitrogen content in stems did not change significantly, whereas that in leaves and petioles decreased, indicating that during drought stress, the nitrogen in leaves was transferred to roots through the phloem. This conclusion was supported by the results of Shi et al., (2020). Song et al., (2019)
showed that nitrogen content affects the drought tolerance of maize and on comparing this with our results, we concluded that nitrogen transport may be a mechanism for plants to adapt to drought. The findings of Parabha et al., (2018)
confirmed this conclusion.
Effect of drought and rehydration in parallel period of vegetative growth and reproductive growth on nitrogen content
Fig 3 shows that in the R2 stage, the total nitrogen content in all organs of HN44 showed a downward trend and the nitrogen in leaves decreased the most under mild stress by 22.7%.
Fig 3: Effect of water change on total nitrogen content (mg/g) in Parallel period of vegetative growth and reproductive growth (R2) (Significance analysis was carried out between different degrees of stress in the same organ. P<0.05).
After rehydration, the nitrogen content in all organs of HN44 under mild drought rehydration were lower than that of the control, but nitrogen loss was alleviated. A compensatory effect is evident after moderate and severe stress rehydration. The nitrogen content of HN65 also decreased after drought stress at the R2 stage, but the decrease was much smaller than that of HN44, which reflects the difference between the two varieties under drought stress. From the perspective of nitrogen content, HN65 performed better than HN44. After rehydration, the amplitude of compensatory mechanism gradually decreased with an increase in stress.
During the R2 stage, the nitrogen absorbed by the roots is transported to the leaves and then distributed to new leaves and flowers (Tegeder and Masclaux-Daubresse, 2018)
. Under drought stress, the nitrogen content decreased more than that observed in the V3 stage. Therefore, this served as a reflection of the significance of the water demand during this time. Previous studies have revealed that the plants during flowering period are water-sensitive (Faralli et al., 2017),
therefore, ensuring water availability during this time is crucial for production (Gol et al., 2021).
When the leaves were rehydrated, their nitrogen concentration increased in a manner reminiscent of the V3 stage. Because vegetative growth still occurs during this period, excessive accumulation of nitrogen occurs (El-Nakhlawy et al., 2018).
However it was still unable to reverse the damage to flowers during drought. Studies have shown that water stress during pollen development can inhibit meiosis and spore formation in microsporocytes (Lamin-Samu et al., 2021),
which is the basic reason for reduction in the yield when drought occurs during this period (Yu et al., 2019).
Concurrently, there was a decrease in nitrogen in the stems and petioles, which may be a unique change during this period, which we believe is unfavorable.
Effect of drought and rehydration in the reproductive growth phase on nitrogen content
A new sink organ-pod appeared in soybean during the reproductive growth phase. Therefore, drought in this period will have a more complex impact on plants. As shown in Fig 4, under drought stress, the total nitrogen content in the leaves and petioles of HN44 was not significantly affected and in stems it increased, whereas in pods it decreased.
Fig 4: Effect of water change on total nitrogen content (mg/g) in generative growth phase (R5) (Significance analysis was carried out between different degrees of stress in the same organ. P<0.05).
The nitrogen content decreased by up to 6.2% in pods under severe stress. This indicates that drought affects the transport of nitrogen. After rehydration, the nitrogen content in the leaves, stems and petioles decreased, whereas the nitrogen content in the pods increased.
In HN65, drought stress significantly decreased the nitrogen content in stems, petioles and pods. The nitrogen content of the pods gradually decreased with increasing drought stress. After rehydration, the nitrogen content in the leaves was lower than that in the control, whereas in the stems, it increased by 34.4% after severe stress rehydration.
After entering the reproductive growth stage, nutrients are transported to pods to form seeds. This period is critical for yield formation (Shao et al., 2021).
Water status plays an important role in nutrient transport. The main causes of yield loss during this period are premature senescence and lodging caused by water stress (Farooq et al., 2017),
which means that substances such as amino acid and carbohydrates in the leaves are decomposed and transported and thus leaf loses its biological function (Yu et al., 2022).
Crop yield is a continuous process that requires the continuous transport of nutrients to grains (Mitchell et al., 2020).
In this study, it was found that the normal redistribution of nitrogen was blocked under drought. After rehydration, the nitrogen transport disorder caused by water deficit was alleviated and nitrogen was supplied to the pods.
Nitrogen transport under water change in whole growth period
To show the change in nitrogen content in plants after drought and rehydration more clearly, a heat map was drawn (Fig 5).
Fig 5: Heat map based on percentage change in drought and rehydration compared to control. The map using severe drought and severe drought–rehydration data. The “*” represents a significant difference compared with the control. P<0.05.
It can be seen from the map that the drought at the V3 stage led to a decrease in nitrogen in the leaves and recovered after rehydration, indicating that the transportation of nitrogen is susceptible to water. The process of nutrient transport depends on the collection of xylem and leaf transpiration is the main driving force of the process (Kunrath et al., 2020).
As shown in figure 6A, drought stress affected nitrogen transport during vegetative growth. After recovery from stress, a compensatory effect was observed in the stems and leaves. Therefore, when the seedlings are in the vegetative growth stage, appropriate drought (not enough to kill the plants) followed by rehydration could have a positive effect on plant growth. A similar finding was reported by Poveda et al., (2018).
At the R2 stage, nitrogen was transported from the roots to the leaves and then redistributed to the flowers and new leaves. As shown in figure 6B, under drought stress, this period was similar to V3 performance, indicating that the nitrogen transport process was blocked. After rehydration, the nitrogen content in the leaves increased, indicating that the plants had increased the absorption and transport of nitrogen.
During the reproductive growth stage (R5), leaf redistribution is the main source of nitrogen in the pods (Taniguchi et al., 2018).
After drought treatment, the nitrogen content in the pods of HN44 decreased significantly, while the remaining parts did not change significantly, indicating that the nitrogen flow in the plant was stagnant and the process of leaf redistribution to the pod was significantly inhibited (Fig 6C).
Fig 6: Nitrogen transport diagram (A); Nitrogen Transport in Vegetative Growth Period (B); Nitrogen Transport in the Period of Vegetative Growth and Reproductive Growth (C); Nitrogen transport during reproductive growth Period. The dotted line in (A) represents the inference process.
This effect is relieved by rehydration.