Simulation of potato gas exchange rates using SPUDSIM

SPUDSIM was developed from the model SIMPOTATO to incorporate mechanistic approaches for simulating photosynthesis and canopy growth and development needed to improve modeling accuracy for studies involving nutrient/water stress and climate change. Modifications included routines for simulating individual leaf appearance rates and leaf expansion as a function of leaf physiological age and plant assimilate status. Coupled sub-models for leaf-level photosynthesis, transpiration, and stomatal conductance were used to replace the older radiation use efficiency approach. A radiation transfer routine that estimated diffuse and direct-beam photosynthetically active radiation for sunlit and shaded leaves was also added. During each time increment, net photosynthetic rate was estimated for sunlit and shaded leaf area. Photosynthate was partitioned among leaves in the canopy according to leaf age, potential expansion, and plant assimilate status. Assimilate allocation to branches, roots, and tubers proceeded according to partitioning coefficients defined in the original model, SIMPOTATO. Remaining photosynthate was stored in the canopy and, when accumulated over a threshold amount, reduced leaf-level photosynthetic rate via feedback inhibition. Whole plant gas exchange and harvest data from SPAR (soil–plant–atmosphere research) chamber experiments conducted at USDA-ARS, Beltsville, MD were used to evaluate SPUDSIM predictions over a broad range of temperatures from 12.6 to 32.3 °C (24-h average basis). An additional independent SPAR chamber dataset was used to parameterize SPUDSIM crop coefficients. Root mean square error (RMSE) was less than 0.29 mol CO2 m−2 season−1 for seasonal daily net assimilation rates and indices of agreement (IA) were 0.80 and higher except at the 32.3 °C study (0.62). Comparison of canopy photosynthetic rates at four different days indicated the model slightly under-predicted leaf area early in the season and over-predicted later in the season. IA and RMSE for leaf-level photosynthetic rates were above 0.88 and less than 1.6 μmol CO2 m−2 s−1 respectively for all studies except the 32.3 °C (0.61 and 3.8 μmol CO2 m−2 s−1). Dry matter predictions fell within two standard deviations of measured values for most plant organs at harvest. Overall, these results indicated that SPUDSIM accurately captured potato growth and development responses over a wide-range of temperatures and will be suitable for a variety of applications involving complex soil–plant–atmosphere relationships.