## 4 Discussion  
Water management practices are an important factor for GPP gain or loss in productive systems, particularly in crops grown under rainfed conditions, where there are inherent limitations related to drought-induced GPP losses [40]. Our results show that GPP in Irrigated was 337.5% higher than in Rainfed (with a difference of 803.7 g C m\({}^{-2}\)), and the sum of NEE was negative (sink) in Irrigated and positive (source) in Rainfed, which evidences the influence of soil water conditions on carbon dynamics [41; 42]. In Irrigated, the SWC was kept close to field capacity, while in Rainfed, the SWC was, most of the time, below the easily available water and even below the wilting point. The conditions described for Rainfed are related to agricultural drought [43], which is compatible with a higher VPD max, a higher T mean, and a higher T max than in Irrigated. In general, half-hourly GPPs were intricately linked to the carbon demand in each of the growth stages, reaching maximums of daily sums of GPP in the tuberization stage, around 90 DPP for both, Irrigated and Rainfed. For each of the growth stages, averaged half-hourly GPPs were significantly greater compared to Rainfed (\(p<0.05\)), which agrees with drought-induced GPP losses reported in potato crops by [44; 45] and in other species [40; 41; 46; 47; 48; 49; 50; 51; 52; 53]. Half-hourly GPP variation throughout the day is associated with incident PAR [44; 54; 55; 56]; such variation has also been reported for potatoes [54; 57]. In this study, an asymptotic exponential curve of GPP Vs. PAR was fitted on each growth stage, which shows that carbon flux at PAR saturation (95% of asymptote) was greater in Irrigated than in Rainfed (\(p<0.05\)), with PAR values around 1450 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) in Irrigated and 250 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) in Rainfed. The mean PAR in Irrigated was higher throughout the crop growth than in the rainfed site. However, although there is a lower PAR in rainfed, this condition does not affect the response of the GPP, due to saturation of the photosynthesis transduction phase occurring at low PAR values. This results in a restricted carbon fixing, which may be attributed to the fact that in a highly water-restricted scenario, non-stomatal restrictions depend on the severity of the water stress [58], which are related to reduced mesophyll conductance, and photochemical and enzymatic constraints [59]. At this point, the injury of the photosynthetic apparatus, destruction of chlorophyll components, disorganization of chloroplast's ultrastructure and enzyme inactivation, and photo-inhibition [60; 61] cause a permanent decline in carbon assimilation [58; 61]. These non-stomatal limitations were also reported for potato crops in [62] at the Lonzeee Terrestrial Observatory. Variations in GPP with respect to LAI were evident along growth stages related to LAI evolution and canopy formation. The highest GPP data dispersion was observed in the vegetative stage, when there was a progressive canopy growth, with the presence of leaves of different ages, and therefore, with differences in light use efficiency. Conversely, in the tuberization stage, when the maximum LAI is reached, the canopy is homogeneous and fully formed, which reflects in the lower data dispersion. Several authors have reported LAI as one of the main causes of daily GPP variation [63; 64; 65]. In Irrigated, daily GPP dynamics showed the same trend as LAI, evidencing that there exists an efficient feedback regulation mechanism between GPP and LAI, where carbon fluxes are mainly destined to GPP, which guarantees canopy growth and expansion. The canopy, in turn, is highly functional to photosynthetic processes increasing GPP since, in addition to a larger leaf area, the leaves do not show irreversible limitations for carbon assimilation, have a high response to PAR, and greater duration of the leaf area. Conversely, In Rainfed, there was no correspondence between GPP and LAI trends. In this case, the available leaf