# Upscaling Gross Primary Production from Leaf to Canopy for Potato Crop (_Solanum tuberosum_ L.)
## 4 Discussion  
The measured GPP has a close relationship with the carbon requirements of each crop growth stage. The maximum daily GPP and half-hourly GPP occur in the tuberization stage and maximum LAI (close to 90 DPP). The behavior of modeled data is consistent with our hypothesis: the big-leaf models overestimate the GPP, specifically during the period of maximum LAI and tuberization stage. Even though multilayer models tend to underestimate the GPP, they better described the daily GPP behavior, and their maximum values were close to the daily GPP and half-hourly GPP values determined by the EC station. Previous studies have shown that the big leaf approach overestimates EC measurements [24]. In particular, [PERSON] et al. [24] concludes that the performance of the big leaf model is strongly affected by the LAI and reports an overestimation of 50% in the GPP during the maximum LAI. Big leaf assumes that the canopy has the same photosynthetic behavior. It has an inability to consider the variation in the CO\({}_{2}\) assimilation of the leaves along the canopy, especially the shaded leaves, which have differences in photosynthetic capacity. This model assumes that the entire canopy is a single unshaded leaf located at the canopy top (with an evident greater photosynthetic capacity) [36]. Therefore, the increase in the amount of foliage with these characteristics resulted in overestimations of the maximum LAI. In contrast, with the ML approach, the canopy is separated into layers with different light environments and photosynthetic capacities. In the maximum LAI, the topmost unshaded layer remains constant, and the remaining LAI is distributed between the intermediate and lower layers. Our results indicate that multilayer models performed better than big-leaf models at simulating GPP (as evidenced by ML RMSE being lower than BL RMSE). Some other researchers have also found better performance of ML models compared to BL models in maize and forests [1; 34]. However, including several canopy layers for studying light interactions through the canopy requires more detailed input data. In our study for the big-leaf models, we used just 30 min data of PPFD and the zenith angle, and the extinction coefficient was used to define the PPFD attenuation through the canopy in a single LAI layer. In the multilayer models, in addition to daily data of the zenith angle and incident PPFD, transmitted and absorbed radiation in each canopy layer were computed, as well as  
Figure 8: One-to-one comparison of the measured gross primary production (GPP) and the GPP simulated by (**a**) multilayer Mitscherlich ([PERSON]), (**b**) multilayer Michaelis–Menten ([PERSON]), (**c**) big-leaf Michaelis–Menten ([PERSON]), and (**d**) big-leaf Michaelis–Menten ([PERSON]) approaches. Equations, root mean squared error (RMSE), determination and correlation coefficients (R\({}^{2}\)) are inserted inside graphs. the evolution of \(A_{Smax}\), quantum yield of assimilation (\(\Phi\)), and LAI in three different layers during growth was determined. The main difference is the treatment of the LAI in each approach. In potatoes, [PERSON] et al. [8] demonstrated that the evolution of daily GPP is linked to the behavior of the LAI. Furthermore, a direct effect of changes in canopy architecture on simulated productivity has been shown [36]. This effect is not only linked to the number of photosynthetic leaves but also to the variability in the photosynthetic capacity throughout the canopy. Not all the leaves along the canopy behave like those of the upper stratum. There was a gradient in the photosynthetic capacity within the canopy depending on the age of the leaf, its position in the canopy, and its light environment. In this study, the photosynthetic behavior through the canopy showed that the leaf photosynthesis responses to PPFD, the light-saturated net photosynthetic rate (\(A_{gmax}\)), and the quantum yield of assimilation (\(\Phi\)) of the middle and bottom layers of the canopy were highly different from the upper unshaded leaves.