Triose phosphate limitation of photosynthesis: an unnecessary complexity in terrestrial biosphere models

Abstract:

Triose phosphates are the principal product of photosynthesis. They are used within the chloroplast for starch synthesis, or translocated to the cytosol where they fuel sucrose synthesis. Use of chloroplastic and cytosolic triose phosphate releases inorganic phosphate, and is under strict metabolic control that matches the supply of triose phosphate from the Calvin-Benson cycle to demand for carbon by sinks. However, a low rate of triose phosphate utilization (TPU) can limit photosynthesis when the rate of sucrose and starch synthesis is not sufficient to maintain a phosphate pool that is capable of meeting the demand for ATP synthesis and the subsequent use of that ATP by the Calvin-Benson cycle. Recent work has demonstrated the sensitivity of terrestrial biosphere model (TBM) output to model representation of TPU and showed limitation of photosynthesis by TPU under present day and future projected CO₂ concentrations, most consistently at high latitudes. However, a global scale analysis provided empirical evidence that TPU limitation rarely limits photosynthesis under growth conditions, even at low temperatures typical at high latitude. Additionally, new work has revealed an artifact in TBM model representation of photosynthesis that exaggerates the limitation of TPU on modelled CO₂ assimilation. We detail the parameterization and assumptions associated with the representation of TPU in TBMs and make the case for removing TPU from current TBMs. This work highlights the need for improved physiological understanding of the conditions under which TPU limitation might be important, and if retained, the need for improved representation of TPU in TBMs.