3.1 Changes in the N content and N accumulation in key functional leaves of maize under different N application rates
Figure 1 shows the changes in the N content and N accumulation in key functional leaves of Jiyu 5817 during the growth period. Overall, with the decrease in the N application rate, both metrics showed a downward trend with the development of growth period. There was no significant difference in the N content or N accumulation in key functional leaves when N was applied at 120 kg/hm2 and 180 kg/hm2 (N2 and N3 treatments), and the recommended N application rate of 180 kg/hm2 (N3 treatment) resulted in the highest N content and N accumulation at the ripening stage (R6). When the N application rate was less than 120 kg/hm2, N accumulation decreased significantly with decreasing N application. Both the N content and N accumulation showed significant variation among the different treatments at the V12 stage, which is also the most important N requirement period and the key period for topdressing.
Figure 2 shows the changes in the N content and N accumulation in key functional leaves of Zhengdan 958 during the growth period, and the overall change trend was consistent with that of Jiyu 5817. There was also no significant difference in the N content in key functional leaves between the N2 and N3 treatments, and the difference in N accumulation was significant among the different N treatments at the silking stage (R1). At the ripening stage (R6), the recommended N application rate of 180 kg/hm2 (N3) resulted in the highest N content and N accumulation. At the R5-R6 stage, except for the group without N application (N0), there was no significant difference in the N content among the other three treatment groups, and there was no significant difference in leaf N accumulation between N3 and N2; however, when the N application rate was less than 120 kg/hm2, the N accumulation decreased significantly with increasing N stress. Comprehensively, considering both the N content and N accumulation, the period of Zhengdan 958 sensitivity to N stress was from the V12 to R1 stages, among which R1 was the most informative because the N accumulation during this stage was the most sensitive to N stress.
3.2 Yield analysis of different maize varieties
As shown in Table 3, the yield was relatively sensitive to different N stresses, N deficiency significantly reduced the yield, and the average yield of Jiyu 5817 and Zhengdan 958 reached the maximum values under the N3 treatment. Compared with that under the N3 treatment, the average yield of Jiyu 5817 under the N0, N1 and N2 treatments was decreased by 24.04%, 7.76% and 3.11%, respectively, and the N2 treatment presented no significant difference from the N3 treatment. The average yield of Zhengdan 958 under the N0, N1 and N2 treatments decreased by 22.26%, 13.91% and 7.66%, respectively, and the average yield showed a significant difference under the different N treatments. According to the yield response to the different N application rates, 180 kg/hm2 (N3) was recommended as the optimum N application rate for the two maize varieties to achieve the highest yield. The changes in the yields of Jiyu 5817 and Zhengdan 958 were basically consistent with the change in the N in the ear leaf, especially at maturity. In Zhengdan 958, there was no difference in the N content or accumulation between N2 and N3, but the yield under N3 was significantly higher than that under N2; in Jiyu 5817, there were no significant differences between N2 and N3 in the N content, N accumulation or yield.
Table 3
Yields of Jiyu 5817 and Zhengdan 958 under different N rates
Treatment
|
JY5817 (kg/hm2)
|
Standard error
|
ZD958 (kg/hm2)
|
Standard error
|
N0
|
11397 c
|
268
|
12002 d
|
151
|
N1
|
13839 b
|
260
|
13290 c
|
288
|
N2
|
14536 ab
|
298
|
14255 b
|
321
|
N3
|
15003 a
|
420
|
15439 a
|
197
|
Note: Different lowercase letters in the table indicate a significant difference of 0.05 (p<0.05).
3.3 Correlations between N accumulation and spectral reflectance in key functional leaves of different maize varieties
The correlation between N accumulation and spectral reflectance in key functional leaves of different maize varieties was analyzed. Fig. 3 (left) shows that the correlation between N accumulation and spectral reflectance in key functional leaves of Jiyu 5817 reached a significant correlation at 532–565 nm, 700–716 nm and 1406–1485 nm at the 6-leaf stage (V6) and at 525–576 nm, 706–721 nm, 756–955 nm, 1421–1506 nm and 2018–2398 nm at the 12-leaf stage (V12). There was a significant correlation at 705–733 nm and 785–1138 nm and an extremely significant correlation at 1397–1519 nm, 1848–1889 nm and 2000–2430 nm at the ripening stage (R6). Figure 3 (right) shows the correlation between N accumulation and spectral reflectance in key functional leaves of Zhengdan 958. At the V6 stage, there was a significant negative correlation at 508–724 nm and 1972–2100 nm and an extremely significant correlation at 509–597 nm and 697–724 nm. A significant negative correlation was observed at 760–142 nm at the silking stage (R1) and at 712–724 nm at the ripening stage.
3.1.3 Establishment of prediction models of maize leaf N content based on partial least squares regression (PLSR)
The N spectral prediction models of the two varieties were established using a partial least squares regression (PLSR) method. Figure 4 shows the changes in Y-variance with the number of principal components, which was determined to be 15 for Jiyu 5817, 14 for Zhengdan 958 and 15 for the combined dataset. Figures 5 and 6 are the regression coefficient diagrams (left) and prediction evaluation diagrams (right) of the models in Jiyu 5817 (sample size n = 165) and Zhengdan 958 (sample size n = 216), respectively. In Jiyu 5817, as shown in Figure 5, the root mean square error (RMSE) and the coefficient of determination (R2) of the calibration set were 0.122 and 0.935, respectively, and the RMSE and R2 of the validation set were 0.174 and 0.860, respectively; the top 10 central wavelengths that contribute greatly to the model are 521 nm, 689 nm, 1110 nm, 1188 nm, 1323 nm, 1421 nm, 1508 nm, 1875 nm, 2100 nm and 2200 nm. In Zhengdan 958, the RMSE and R2 of the calibration set were 0.135 and 0.883, respectively, and the RMSE and R2 of the validation set were 0.145 and 0.878, respectively; the top 10 central wavelengths that contributed greatly to the model were 518 nm, 559 nm, 689 nm, 1420 nm, 1585 nm, 1833 nm, 1875 nm, 2020 nm, 2109 nm and 2200 nm.
The data from the two varieties were mixed for further analysis. Fig. 7 shows the regression coefficient diagram and prediction evaluation diagram of the spectral prediction model of leaf N content in the key functional leaves of two maize varieties (sample size n = 381). The RMSE of the verification set model was 0.204, the R2 was 0.794, and the number of principal components was 15. The top 10 central wavelengths were 518 nm, 559 nm, 689 nm, 1110 nm, 1420 nm, 1513 nm, 1585 nm, 1875 nm, 2103 nm and 2200 nm.
Figures 4–6 show that most of the positions of the central bands with greater contributions in the three models were essentially similar. Compared with that of the integrated model of the two varieties (not considering the varietal differences), the prediction accuracy of the model with variety classification was improved; the accuracy of the calibration and validation sets of the combined varieties was lower than that of the individual varieties, but the model was also validated to be statistically accepted. Therefore, to simplify the prediction of leaf N content, a general model can be applied for different varieties. However, due to the different responses to N stress by the two varieties (3.1), the effects of varietal differences should be considered in the application of the model.
3.1.4 External test of the prediction models
In Unscrambler, the above models are called to be tested using external samples. A total of 25 external samples were used for the models of Jiyu 5817 and Zhengdan 958 alone, and a total of 50 samples were used to test the integrated model. In this paper, we tested the model in three ways. First, a model of the same variety, i.e., twenty-five external samples of Jiyu 5817, was used to test the model constructed for Jiyu 5817. Second, a model for the opposite variety, i.e., the Zhengdan 958 samples, was used to test the model constructed for Jiyu 5817; in turn, the Jiyu 5817 samples were used to test the model constructed for Zhengdan 958. Third, an integrated model was tested using different samples from different varieties, i.e., samples of Jiyu 5817 or Zhengdan 958 were used to test the integrated model (Table 4). The test results show that the determination coefficient R2 of all the evaluation results was greater than 0.803 and the relative error was less than 8.98%. The order of test accuracy from high to low was found for the model of the same variety, the integrated model validation and the model of the opposite variety. Although the determination coefficients and errors of the prediction model and its validation results were not particularly desirable, as a rapid, real-time, and nondestructive N nutrition diagnostic technique that can be applied under field conditions, it is sufficient to provide a reference basis for N regulation and management during the growing period of maize.
Table 4
Test results of the prediction model based on external samples
Number
|
Sample source for testing
|
Tested model
|
Sample number
|
Coefficient of determination (R2)
|
Average relative error (RE %)
|
1
|
JY5817
|
M1
|
25
|
0.873
|
5.63
|
2
|
ZD958
|
M2
|
25
|
0.861
|
6.21
|
3
|
JY517+ZD958
|
M3
|
50
|
0.856
|
6.74
|
4
|
JY5817
|
M2
|
25
|
0.821
|
8.01
|
5
|
ZD958
|
M1
|
25
|
0.803
|
8.98
|
Note: M1 represents the PLSR prediction model of Jiyu 5817, M2 represents the PLSR prediction model of Zhengdan 958, and M3 represents the PLSR prediction model of the two varieties combined. For example, Line 1 shows that Model 1 (M1) is tested using 25 unknown samples of Jiyu 5817, while Line 4 shows that Model 2 (M2) is tested using 25 unknown samples of Jiyu 5817. (Samples of Jiyu 5817 were used to test the Zhengdan 958 model). The explanations for the other lines are similar.