Effects of nitrogen application rate on the photosynthetic pigment, leaf fluorescence characteristics, and yield of indica hybrid rice and their interrelations

Photosynthetic pigments in rice leaf blades

The contents of chlorophylls a, and b and carotenoids showed an upward trend with increasing nitrogen application rate. Pigments in the N4 treatment were significantly higher than those in the N1 treatment at the heading and maturity stages (Fig. 1).

Photochemical efficiency of rice leaf blades

Photochemical efficiency is calculated from to the number of photons of the product obtained by the effective photochemical reaction divided by the total quantum number of light absorbed. F_v/F_m represents the maximum photosynthetic quantum yield of the reaction center of PSII and reflects the internal light energy conversion efficiency or maximum light energy conversion efficiency of PSII; this characteristic may indicate the photosynthetic potential of crops²⁰. Φ_PSII represents the actual photosynthetic quantum yield of PSII and reflects the actual primary light energy capture efficiency of the reaction center of PSII when it is partially closed²¹. ETR reflects the actual electron transfer rate of PSII²¹.

As the nitrogen application rate increased, F_v/F_m first decreased and then increased at the booting and maturity stages but decreased at the heading stage. F_v/F_m in the N3 treatment was the highest and significantly higher than that in the N2 treatment at the booting stage. Moreover, F_v/F_m in the N0 treatment was the highest and significantly higher than those in the N2, N3, and N4 treatments at the heading and maturity stages (Fig. 2).

As the nitrogen application rate increased, Φ_PSII first decreased and then increased at the booting stage, decreased at the heading stage, and then increased at the maturity stage. Compared with those of other treatments, Φ_PSII in the N0 treatment was significantly higher than those in the N1, N2, and N4 treatments at the booting stage. Among the treatments studied, the N0 treatment showed the highest Φ_PSII at the heading stage, but no significant difference was observed among the five nitrogen application treatments. Φ_PSII in the N4 treatment was the highest and significantly higher than that in the N0 and N2 treatments at the maturity stage (Fig. 2).

ETR showed a downward trend followed by an upward trend at the booting stage. However, at the heading and maturity stages, ETR first increased and then decreased. The highest ETR were observed in the N3 treatment at the heading stage and in the N1 treatment at the maturity stage, although differences between the treatments were not significant. ETR in the N0 treatment was highest and significantly higher than that in the N2 treatment at the booting stage (Fig. 2).

Fluorescence quenching coefficient of rice leaf blades

Fluorescence quenching may be categorized as photochemical quenching and non-photochemical quenching. Photochemical quenching is represented by qP, which reflects the degree of reaction center opening in the redox state of the original electron acceptor QA of PSII²¹, while non-photochemical quenching is represented by qN, which reflects the part of light energy absorbed by pigments of the PSII antenna and dissipated in the form of heat instead of being used for photosynthetic electron transfer²².

Figure 3 demonstrates that the effect of nitrogen application rate on qP is not evident at the booting stage. However, qP increased at the heading stage, and first decreased and then increased at the maturity stage with increasing nitrogen application rate. Among the qPs obtained, that inthe qPs obtained, that in the N4 treatment was the highest. The qP in N4 was also significantly higher than that inin N1 and N2 at the heading stage. Similarly, the qP in the N4 treatment was the highest at the maturity stage, but no significant difference was found among treatments. qN first increased and then decreased at the booting, heading, and maturity stages with increasing nitrogen rate. Among the qNs obtained, that in the N2 treatment was the highest and significantly higher than that in N3 at the booting and maturity stages. Moreover, among the qNs obtained, the qN in the N3 treatment was the highest and significantly higher than that in the N1 treatment in the heading stage.

Non-photochemical quenching quantum yield of rice leaf blades

Y(NPQ) refers to the quantum yield of regulatory energy dissipation at PSII, which is an important index of light protection²⁰, while Y(NO) refers to the quantum yield of non-regulatory energy dissipation at PSII, which is an important index of light damage²⁰.

Y(NPQ) first increased and then decreased at the booting, heading, and maturity stages with increasing nitrogen application rate. The Y(NPQ) in the N2 treatment was the highest among the results obtained and significantly higher than that in the N3 treatment at the booting and maturity stages. However, at the heading stage, the highest Y(NPQ) was observed in the N1 treatment, although no significant difference occurred between the five nitrogen application rates. Y(NO) first decreased and then increased at the booting, heading, and maturity stages as the nitrogen rate increased (Fig. 4). The Y(NO) values in the N2 and N3 treatments were highest at the booting and heading stages, respectively, and no significant difference was observed among all N treatments. The Y(NO) in the N3 treatment was significantly higher than those in the N1, N2, and N4 treatments at the maturity stage (Fig. 4).

Yield and its components

The grain yield first increased and then decreased with increasing nitrogen application rate (Table 1), and the grain yield of the N2 treatment was the highest and significantly higher than those of the N0 and N4 treatments. The regression equation between grain yield (y) and nitrogen application rate (x) was y =  − 0.0413x² + 13.89x + 10,637 (R² = 0.9121, P < 0.01). The best nitrogen application rate of rice was 168.16 kg ha⁻¹, and the highest yield was 11,804.87 kg ha⁻¹. The effective panicle number (EPN), thousand-grain weight (TGW), and spikelet filling (SF) first decreased and then increased with increasing nitrogen application rate. In particular, the EPN of the N4 treatment was the highest and significantly higher than that of the N1 treatment. The TGW and SF of the N0 treatment were the highest and significantly higher than those of the N2, N3, and N4 treatments. The spikelets per panicle (SPP) first increased and then decreased with increasing nitrogen application rate. Here, the SPP of the N2 treatment was the highest, although no significant difference was observed among all treatments (Table 1).

Between the two rice cultivars studied, the EPN, SPP, and grain yield of V1 were higher than those of V2. However, the TGW and SF of the former cultivar were lower than those of the latter. In particular, the SPP and grain yield of V1 were significantly higher than those of V2 (Table 1).

The results of variance analysis showed that differences in the SPP, TGW, and grain yield between rice varieties were extremely significant, and SF displayed significantly different. Significant differences in TGW, SF, and grain yield were observed among the nitrogen application rates. The interactions of these characteristics between varieties and nitrogen application rates were not significant except for TGW and grain yield (Table 1).

Relationship between photosynthetic pigments and fluorescence parameters in rice leaf blades

At the booting stage, photosynthetic pigments in leaf blades were negatively correlated with Y(NPQ), qP, and qN but positively correlated with Fv/Fm and Y(NO); however, the correlations observed did not reach significant levels (P > 0.05). At the heading stage, photosynthetic pigments in leaf blades were negatively correlated with F_v/F_m, Φ_PSII, and ETR but positively correlated with Y(NO), qP, and qN. Chlorophylls a and b were significantly correlated with F_v/F_m, qP. At the maturity stage, leaf photosynthetic pigments were negatively correlated with F_v/F_m, ETR, Y(NPQ), qP, and qN but positively correlated with Φ_PSII; however, the correlations found did not reach significant levels (P > 0.05) (Table 2).

Relationships among rice yield and its components, photosynthetic pigments, and fluorescence parameters

At the booting stage, carotenoids had a significant positive correlation with EPN. At the heading stage, carotenoids had a significant negative correlation with SPP. At the maturity stage, chlorophylls a and b and carotenoids had significant positive correlations with EPN. However, chlorophyll a and carotenoids had a significant negative correlation with SF (Table 3).

At the booting stage, qP was negatively correlated with EPN at the 5% significance level, and the correlation coefficient was − 0.892. At the heading stage, F_v/F_m and Y(NO) were negatively correlated with EPN and SPP (P < 0.05) with correlation coefficients of − 0.913 and − 0.884, respectively, and F_v/F_m and Φ_PSII were positively correlated with SF (P < 0.05) with correlation coefficients of 0.895 and 0.901, respectively. Moreover, qP was positively correlated with EPN(P < 0.05) with a correlation coefficient of 0.953. At the maturity stage, F_v/F_m was negatively correlated with grain yield (P < 0.05) with a correlation coefficient of − 0.935 (Table 3).

Source: Ecology - nature.com