3.1 Phosphorus significantly improves dry matter accumulation and yield of soybean plants in shading
Table 1 shows the yield and formation factors of each treatment. The yield of soybean increased significantly after phosphorus application under the same light conditions. Soybean yield significantly reduced under shading conditions, showed a trend of L1P1> L1P0> L2P1> L2P0.
The effects of phosphate fertilizer on soybean yield were different under ambient light (L1) and shade treatment (L2). Two-way ANOVA revealed a significant interaction effect between light intensity and phosphorus concentration on effective pods per plant, number of seeds per plant, and yield of soybean. Effective pods per plant and the number of seeds per plant were closely related to irradiance (P <0.01), phosphate fertilizer (P <0.01), and the interaction between the two (P <0.01), but the effect of 1000-grain weight is not affected by phosphate fertilizer. Compared to P0, the yield of P1 treatment is 27.1% and 36.7% increased under L1 and L2 treatment. It can be seen that under low light conditions, soybean yield increased significantly with the application of phosphate fertilizer, and its increase was significantly higher than that under full light conditions. Phosphate fertilizer can promote the flowering and pod formation of soybean, which increases soybean yield via promoting the number of effective pods per plant and the number of seeds per plant.
3.2 Phosphorus fertilizer increases chlorophyll content of plant leaves under low light
As shown in Figure 2, the content of chlorophyll a and b at each growth stage after anthesis increased significantly with the decrease of light intensity and the application of phosphate fertilizer. The shading is the principal reason for the significant increase in soybean chlorophyll content. Phosphorus fertilizers have different effects on the improvement of chlorophyll content under different light environments, compared to L1P0 treatment, the chlorophyll content of L1P1 treatment increased by 9.6% and 16.1%, respectively, which was significantly less than 13.8% and 21.8% under low light environment; however, chlorophyll b increased by 16.8%, 29.3% and 23.2%, 24.9% under L1 and L2 after application of phosphate fertilizer, during R3 and R5 growth stage, respectively. It can be seen that the application of phosphate fertilizer has a significant effect on the chlorophyll content under shading, especially the content of chlorophyll b was more notable.
3.3 Phosphate fertilizer improves the plant's light energy utilization capacity in shading
To explore the changes of photosynthetic rate under shading condition, light-saturated rate of photosynthesis (Asat) were determined. As shown in figure 3, the Asat in the R1 period after shading was significantly lower than that in L1 treatment. The Asat of L2P0 decreased by 38.3% and L2P1 decreased by 26.3% under the same phosphate fertilizer treatment. The CO2 assimilation rate of P1 treatment increased significantly in shading compared with P0 treatment as the processing time increases. After relighting in the R5 period, L1P1 was 4.6% higher than L2P1, the difference was not significant (P>0.05), indicating that phosphorus application under shading condition is beneficial to maintain a high rate of CO2 assimilation after re-lighting in the later stages of plant growth.
To further explore the reasons for the significant increase in the Asat of the L2P1 treatment during the R3 period, CO2 assimilation rates were determined as a function of light intensity. From these light response curves the CO2 assimilation rates were shown to be significantly lower in L2 treatment when compared to L1 treatment, showing a trend of L1P1> L1P0> L2P1> L2P0, but as the shading treatment time increased, the CO2 assimilation rate of the L2P1 treatment gradually increased, and showed to be L1P1> L2P1> L1P0 > L2P0 (Figure 4), and the CO2 assimilation rate of L2P1 remains at a high level after re-lighting.
We focused on the response of the CO2 assimilation rate to low light intensity (≤200 mmol m-2 s-1) to make a thorough probe of change in apparent quantum yield (PQY) during low light. The initial slope of the A / Q curve reflects the plant's weak light utilize the ability. The PQY of plants under weak light shows a significant upward trend, especially with L2P1 treatment. Figure 5 compares the photosynthetic capacity unit light intensity, the weak light (100 mmol m-2 s-1) utilization capacity of plants in a shading environment was significantly higher than that in the full light environment. As the adaptability to weak light improves, its high light intensity (800 mmol m-2 s-1) utilization capacity also increased synchronously. The low light utilization capacity of L2P1 plants after relighting maintaining high, indicating that phosphorus application under shading makes the plants quickly adapt to low light environments while ensuring the restoration of photosynthetic capacity of plants after re-lighting, ensure the accumulation of photosynthetic products, and significantly increase yield.
3.4 Phosphate fertilizer increases the rate of CO2 assimilation in shading
To explore the changes in the carboxylation system under different treatments, we also determined A as a function of internal CO2 concentration (Ci) in the same plants. The CO2 assimilation rate of L2P0 and L2P1 decreased significantly after shade treatment. As the plant's adaptability increased, the CO2 assimilation rate of L2P1 increased significantly during the R3 period, which maintaining a high level compared to L2P0 treatment when re-lighting at R5 (Figure 6).
Further analysis of the A/Ci curves using the equations published by von Caemmerer and Farquhar (1981) [23] illustrated that the light environment significantly affects the maximum rate of carboxylation by Rubisco (Vcmax) and maximum electron transport rates (Jmax). As shown in figure 7, Vcmax and Jmax in the natural light environment were significantly higher than in the shading environment, showing to be L1P1> L1P0> L2P1> L2P1, except for the R5 period. Vcmax and Jmax respond differently to phosphate fertilizer under two light conditions. The use of phosphate fertilizer has no significant effect on Vcmax in ambient light environment, L2P1 and L2P0 show significant difference only in the R3 period in shading. After applying phosphate fertilizer under different light environments, Jmax changed significantly. Compared with P0 treatment, Jmax increased by 8.1% and 16.9% in P1 treatment during R3 and R5 periods under L1 treatment, and 38.8% and 32.3% in L2 treatment, respectively. It can be seen that the application of phosphate fertilizer significantly improves the regeneration rate of RUBP in shading.
3.5 Chlorophyll fluorescence analysis reveals increased photosynthetic efficiency in plants in shading
Variable fluorescence curves and DVt [For ΔVt analysis (ΔVt=D(Ft–Fo)/ (Fm− Fo)) fluorescence in L1P0 treatment on each day of the experiment (was a used as a reference and) equaled 0.] were constructed to compare differences between treatments on each measuring day during the experiment (Figure 8). Compared with plants under ambient light, where the K and J-bands showed significantly higher values for both shading treatments, indicating that the PSII donor side and acceptor side were reduced to different degrees in shading and it has alleviated during the R3 period. J-bands were relatively low with phosphorus treatment, especially in the shade environment, which means phosphate fertilizer can alleviate the effect of low light on the performance of the leaf receptor.
Figure 9 shows the change of the relative variable fluorescence intensity at points K and J of chlorophyll a fluorescence transient (OJIP) curves. After shade treatment, Wk and Vj both increased, especially Vj, indicating that the performance of the donor side and the receptor side was reduced to varying degrees, and the receptor side was significantly affected by the low light. Wk and Vj of phosphorus-applied plants showed a downward trend in two light treatments. The effect of phosphate fertilizer on Wk was not significant, but Vj was significantly reduced. After re-lighting, the Vj of L2P1 was significantly lower than that of L2P0 treatment, but not significantly different from that of L1P1 treatment. The above results indicate that phosphorus application improves the electron transport performance on the receptor side in shading stress, and is beneficial to the retention of the receptor side performance after the plant was relighted.
The analysis of the relative variable fluorescence at points K and J of the OJIP curve found that both light and phosphate fertilizer had a significant effect on Vj. The change in the relative fluorescence at point J is closely related to the electron transport performance of the PSII acceptor side, so we studied the maximum quantum yield of PSII primary photochemistry (Fv/Fm), the probability that a trapped exciton moves an electron into the electron transport chain beyond QA- (yo), the quantum yield of electron transport (jEo) and performance index on absorption basis (PIABS).
Figure 10 shows that Fv/Fm, yo, jEo, and PIABS all decreased significantly after shading. With the improvement of the plant's low light adaptability, the photosynthetic mechanism changed adaptively. During the R3 period, P1 Fv/Fm, yo, jEo and PIABS increased by 0.9%、3.4%、9.9%, and 20.5% in L1 treatment compared to P0, and which were1.5%, 6.4%, 48.5%, and 69.7% under L2, respectively. Phosphate fertilizer can strengthen PSII performance, especially under low light. Compared with other treatments, L2P1 has the least decreased in the R5 period after re-lighting, which was not significantly different from the L1P1 treatment, indicating that phosphorus application benefits maintain the performance of the photosynthetic system after relighting, ensuring that the absorbed light energy is fully used to promote electron transfer, effectively improve the soybean photosynthetic capacity in the later period.
Table 2. Effects of different treatments on yield and its components of summer soybean
Treatment
|
1000-seeds weight (g)
|
Effective pods per plant
|
Seeds per plant
|
Yield (kg ha-1)
|
Light (L)
|
Phosphorus (P)
|
L1
|
P0
|
276.2 b
|
66.9 b
|
117.2 b
|
3883.8 b
|
P1
|
272.6 b
|
77.7 a
|
150.9 a
|
4936.7 a
|
L2
|
P0
|
298.1 a
|
39.2 d
|
73.0 d
|
2611.4 c
|
P1
|
301.2 a
|
45.9 c
|
98.8 c
|
3570.4 b
|
Significance
|
|
L
|
**
|
**
|
**
|
**
|
P
|
ns
|
**
|
**
|
**
|
L×P
|
**
|
**
|
**
|
*
|
L1: ambient sunlight; L2: 40% shade; P0: phosphate fertilizer application of 0 kg P2O5 ha-1; P1: phosphate fertilizer application of 180 kg P2O5 ha-1. L: light factors, P: phosphate fertilizer factors, L×P: interaction of light and phosphate fertilizer. Different lowercase letters in the same column indicate significant differences (P<0.05). *P<0.05, ** P<0.01, ns: not significant.