Triose phosphate utilization limitation: an unnecessary complexity in terrestrial biosphere model representation of photosynthesis

Triose phosphates are the principal product of photosynthesis. They are used within the chloroplast for starch synthesis, or translocated to the cytosol where they are used to fuel sucrose synthesis. Use of triose phosphate releases inorganic phosphate, and is under strict metabolic control that matches the supply of triose phosphate from the Calvin–Benson cycle to the demand for carbon by sinks (Heldt & Piechulla, 2011; McClain & Sharkey, 2019). However, a low rate of triose phosphate utilization (TPU) can deplete the phosphate pool, restrict adenosine triphosphate (ATP) synthesis and reduce the availability of ATP to power the Calvin–Benson cycle, thereby limiting photosynthesis (Sharkey, 1985).

Recent work has demonstrated the sensitivity of terrestrial biosphere models (TBMs) to TPU (Lombardozzi et al., 2018) and showed that models predict limitation of photosynthesis by TPU most consistently at high latitudes and future elevated CO₂ concentration ([CO₂]). However, a global scale analysis provided empirical evidence that TPU limitation rarely limits photosynthesis under present day growth conditions and is unlikely to limit photosynthesis at elevated [CO₂], even at the low temperatures typical at high latitudes (Kumarathunge et al., 2019a). Additionally, Walker et al. (2020) revealed an artifact in TBM representation of photosynthesis that exaggerates the limitation of TPU on modeled CO₂ assimilation. This artifact arises from quadratic smoothing of the transition among the potential limiting processes governing photosynthesis and is closely associated with TBM representation of TPU limitation. Collectively, these recent advances, highlight the need for an examination of the representation of TPU in TBMs.