A comparative analysis of four models of photosynthesis for 11 plant species in the Loess Plateau

We compared performance and behavior of two suites of models of photosynthesis based on field data of 11 plant species in the semiarid Loess Plateau in northern China. Diurnal net photosynthesis rates and stomatal conductance of two C3 trees of Robinia pseudoacacia L. and Malus pumila Mill., two C3 shrubs of Caragana korshinskii Kom. and Hippophae rhamnoides L., a C3 subshrub of Lespedeza davurica (Laxm.) Schindl., two C3 forbs of Artemisia gmelinii Web. Ex Stechm. and A. giraldii Pamp., two C4 grasses of Panicum virgatum and Bothriochloa ischaemum (L.) Keng, and two C4 crops of Zea mays L. and Setaria italica (L.), were observed in field in three months of May, July and September in 2002. Net photosynthesis rates of the 11 species were then fitted with Farquharʹs biochemical model of photosynthesis for C3 species, Berry and Farquharʹs intercellular transport model for C4 species, and modified Thornley and Johsonʹs leaf photosynthesis models for C3 and C4 species, and the observed stomatal conductance was fitted with a stomatal conductance model, by means of nonlinear least square regression. The four photosynthesis models were then coupled with the stomatal conductance model to predict net assimilation rates of these plants under extrapolated micro-environmental conditions. The results of nonlinear least-square regression showed that the biochemical models explained on average 66% and 82% of variations in observed net photosynthesis rates for the C3 and C4 plants, respectively, in comparison with the explanations of 72% and 76% by the leaf photosynthesis models. The more mechanistic, detailed treatment of biochemical processes in the biochemical photosynthesis models did not offer significant advantage over the simpler leaf photosynthesis models in the explanation of the field data, but tended to under predict net assimilation rates at high ranges for most C3 species. The subsequent extrapolation analysis indicated that net assimilation rates calculated with the leaf models are more strongly affected by stomatal conductance and moisture conditions, than the biochemical models.