Precise mechanical strength determination in sugarcane stalks
The mechanical strength of a stalk plays an important role in the growth and development of crops [9]. In this study, the Instron Universal Testing Machine was used to determine the rind penetrometer resistance (RPR) of the sugarcane stalk (Fig. 1B). As sugarcane stalks have multiple internodes (Fig. 1A), we compared the RPR of sugarcane stalks from the different internodes of selected genotypes that had contrasted higher and lower RPR. It was observed that RPR increased dramatically from the 3rd internode to the 5th internode (Fig. 1C). It is noteworthy, however, that no significant change was observed from the 7th internode to the 23rd internode (Fig. 1C). Additionally, the internode RPR showed similar variation patterns between genotypes (Fig. 1C). Therefore, we calculated the differences in RPR between genotypes within the same internode. From the 5th to the 23rd internode, stable differences were observed between materials with high and low RPR (Annex 1A). Furthermore, the results of the multiple comparison analysis of RPR between different internodes revealed that none of them showed significant differences, with the exception of the third internode (Annex 1B). It was suggested that any internode except the third and fourth one can be used as a representative internode for measuring RPR. As a means of verification, we measured the RPR of representative materials at the 12th internode in 2019 and 2020, respectively. Notably, significant differences were detected stably between high and low RPR materials, and no detectable difference was observed within high or low RPR materials between the two years (Fig. 1D). Ultimately, these results validated the reliability of this approach, confirming that the method could be effectively used to analyze RPR in sugarcane stalks in an accurate and suitable manner.
Greensnap is another important problem in multi-internode agriculture crop, influencing the net production [35]. Based on phenotypic observations in the field, we observed that greensnap occurred only in the younger node (the 3rd node) (Annex 2). To examine this phenomenon in more detail, we determined the breaking force across a large number of sugarcane genotypes (Fig. 1E). Several sugarcane varieties with high and low breaking forces were selected in order to determine their breaking force in different environments. Accordingly, there was no significant difference in the breaking force of a given variety in different environments, but there was a significant difference between those materials with a higher and a lower breaking force (Fig. 1F). Considering these results, it was concluded that breaking force is under strong genetic control, hence selection against this trait is possible.
Diverse mechanical strength in collected sugarcane samples
In the present study, RPR data were recorded on 270 and 256 sugarcane genotypes in 2019 and 2020, respectively (Fig. 2A). For comparison, RPR data were recorded on the same 46 genotypes in both 2019 and 2020. Further, the breaking force was measured on 440 sugarcane genotypes, of which 245 samples were common to the RPR and breaking force datasets (Fig. 2A). As a result, we observed a wide variation in RPR in different sugarcane genotypes in 2019 and 2020 (Fig. 2B). In detail, the RPR was ranged from 23.5 to 79.7 N mm− 2 in 2019 while in 2020 it ranged from 22.8 to 59.7 N mm− 2 (Annex 3). Besides, the RPR of samples collected in 2019 and 2020 showed an excellent normal distribution (Fig. 2B). Similarly, large variations were observed for breaking force in sugarcane genotypes as well, which presented a normal distribution of breaking forces ranging between 6.6 N and 32.8 N (Fig. 2C; Annex 3). Furthermore, a correlation analysis of common samples of RPR in two years revealed a significant correlation, indicating that RPR is an inherited characteristic (Fig. 2D). Remarkably, these two types of force trait (RPR and breaking force) exhibited a significant correlation coefficient of 0.338 at p < 0.01 level, indicating an important relationship between them (Fig. 2E).
NIRS data characterization in collected sugarcane stalks
The collected population of sugarcane with various genotypes was used for near-infrared spectroscopy modeling. In order to perform an online NIRS assay, samples were crashed and a near-infrared spectrum for each genotype was acquired within one minute. During the shredding process, none of the sugarcane stalk components were lost, and the moisture in the shredded bagasse was retained. For NIRS calibration, the OPUS software automatically averaged the collected near-infrared spectral reflectance values. As a result, the near-infrared reflectance values of all samples fluctuated within the normal range, indicating that sugarcane samples exhibit a wide range of characteristics (Fig. 3A&D). In near-infrared spectral data analysis, principal component analysis (PCA) has some advantages, such as characterization of spectral structure of populations [36]. A two-dimensional observation of the sample distribution was conducted using the first three principal components. Despite the fact that samples were collected from different years (2019 and 2020) for RPR determination, no significant discrimination were observed between the spectra (Fig. 3B&C). Observations of the spectra of these common samples revealed a smaller global distance (GH), suggesting a high level of similarity between them (Fig. 3B&C). In the case of these samples used for breaking force measurements, the first three principal components accounted for 98.6% of the total and displayed a continuous distribution (Fig. 3E&F), suggesting that these samples can be incorporated into a global calibration population for NIRS.
Determination of calibration and external validation sets
A calibration equation is typically evaluated by means of calibration and external validation. For RPR modeling, a total of 68 samples were randomly divided into external validation sets, and the remaining 458 samples formed the calibration set (Table 1). In the case of NIRS modeling of breaking force, 440 samples were used: 90 samples for external validation and 350 samples for calibration (Table 1). An analysis of descriptive statistics was conducted in order to compare the calibration and external validation sets. It is important to note that the minimum and maximum values at both ends of the external validation set were included in the calibration set to ensure that the model is both accurate and practical (Table 1). Additionally, RPR and breaking force displayed normal distributions for both calibration and external validation sets (Annex 4). All statistical distributions across calibration and external validation were comparable, suggesting that it is feasible to obtain accurate predictive equations.
Stalk mechanical strength modeling and evaluation
Using the OPUS software, we performed a linear fitting analysis on RPR and near-infrared spectral reflectance values based on the partial least squares (PLS) method. During PLS analysis, multiple parameters are combined based on the wavelength range and the pretreated spectrum to derive calibration equations [37]. Following this, the performance of the calibration equation was evaluated using the cross-validation and external validation parameters.
We applied NIRS modeling independently to two different types of mechanical strength indicators (RPR and breaking force) of sugarcane stalks. According to RPR calibration, we observed that the R2 was reaching 1.0, the RPD value was reaching 19.60, as well as a relatively low RMSEC value at 0.40 N (Table 2). In terms of NIRS calibration for breaking force, although the modeling parameters were not as good as for RPR, they still demonstrated excellent fitting with R2, RPD and RMSEC values of 0.88, 2.88 and 2.15 N, respectively (Table 2). These results indicated that based on the calibration results, both the RPR equation and the breaking force equation exhibited excellent application potential.
Further, internal cross validation was conducted to assess these obtained equations. During internal cross validation, the samples were divided into various groups, some of which were chosen at random from the calibration sets for cross-validation, which provides the root mean square error of cross validation (RMSECV) and coefficient determination (R2cv), respectively, for equation evaluation. According to our results, a high R2cv (0.99), RPD (10.30) value and a relatively low RMSECV (0.74 N) were observed for the equation of RPR prediction. Likewise, the R2cv value was 0.83, RPD was 2.42, and RMSECV was 2.51 N for the equation of RPR prediction (Table 2). In this case, the RPR model showed better predictive performance than the breaking force model, which was consistent with the calibration results.
Additionally, the equations were subjected to an external validation as an independent test to assess their performance. In a similar manner, for equation evaluation, root mean square error of external validation (RMSEP), coefficient determination (R2ev) and ratio of prediction to deviation (RPD) were calculated. It was found that, in this context, all equations for RPR and breaking force showed R2ev values of above 0.85 and RPD values well above 2.5 (Table 2). A notable feature of the equation for RPR was that the coefficient of determination and ratio of prediction to deviation remained constant at 0.99 and 10.20, respectively (Table 2), in accordance with the excellent performance observed during calibration and internal cross validation, suggesting their excellent prediction performance.
Global modeling of the stalk mechanical strength
We then combined the external validation set with the calibration set to form an integrated calibration set to perform an integrative calibration analysis to gain higher performance model predictions. The results showed that the parameters of the new RPR equation did not significantly improve, but the prediction performance remained extremely high (Fig. 4A&B; Annex 5). A slightly improved R2cv (0.84) and RPD (2.51) values were found for the breaking force equation (Table 2; Fig. 4C&D). Despite the high correlation between the true value and the fit (predicted) value (Fig. 4C&D), it is evident that the obtained breaking force equation can provide reliable predictions. Overall, all these newly generated equations for two kinds of force traits performed excellent in terms of R2, R2cv, and RPD, as well as relatively low RMSEC and RMSECV values, suggesting their ability to provide precise and consistent predictions.