Triose phosphate utilization and beyond: from photosynthesis to end product synthesis

Abstract

During photosynthesis, plants fix CO2 from the atmosphere onto ribulose-bisphosphate, producing 3-phosphoglycerate, which is reduced to triose phosphates (TPs). The TPs are then converted into the end products of photosynthesis. When a plant is photosynthesizing very quickly, it may not be possible to commit photosynthate to end products as fast as it is produced, causing a decrease in available phosphate and limiting the rate of photosynthesis to the rate of triose phosphate utilization (TPU). The occurrence of an observable TPU limitation is highly variable based on species and especially growth conditions, with TPU capacity seemingly regulated to be in slight excess of typical photosynthetic rates the plant might experience. The physiological effects of TPU limitation are discussed with an emphasis on interactions between the Calvin–Benson cycle and the light reactions. Methods for detecting TPU-limited data from gas exchange data are detailed and the impact on modeling of some physiological effects are shown. Special consideration is given to common misconceptions about TPU.

Genotypic variation in source and sink traits affects the response of photosynthesis and growth to elevated atmospheric CO2

“…A negative and significant linear correlation ( R 2 = 0.90, p < .05) was observed between A max response to e‐CO 2 treatment ( A max ratio, A max800 / A max400 ) and local source–sink ratio (Figure ). Under the assumption that A max mainly reflects leaf TPU capacity (McClain & Sharkey, ), these results suggest that TPU capacity probably varied with genotypic, local C source–sink ratio. This was supported by a significant cultivar effect on the A max ratio ( p < .001, Table ), indicating a significant enhancement in photosynthetic capacity for LSS cultivars under e‐CO 2 .…”

“…A decrease in carboxylation rate ( V cmax ), stomatal conductance and other biochemical parameters may have involved as well (Shimono, Suzuki, Aoki, Hasegawa, & Okada, ; von Caemmerer & Farquhar, ; Zhu et al, ). Coadjustment between V cmax and TPU can occur (Fabre et al, ; McClain & Sharkey, ; Sage, Sharkey, & Seemann, ; Sharkey, Berry, & Sage, ). If TPU is too low, plant may be damaged by the excess at the photosystems level (Kiirats, Cruz, Edwards, & Kramer, ).…”

“…Often, the maximum photosynthetic rate is limited by the rate of conversion and use of triose phosphates into starch and sucrose (McClain & Sharkey, ; Sharkey et al, ). In EXP1 only, after the measurement for A 400 , the CO 2 concentration in the measurement cuvette was increased through the following step‐wise sequence: 900, 1,300 and 1,600 μmol mol −1 , and waiting for steady‐state photosynthesis at each step.…”

“…TPU has been less studied than other photosynthetic parameters such as maximum carboxylation rate of Rubsico ( V cmax ). It acts at the interface between the production and the consumption of assimilates (Yang, Preiser, Li, Weise, & Sharkey, ) and may have to be considered when predicting photosynthesis and crop performance under e‐CO 2 conditions (Rogers Alistair et al, ; Busch & Sage, ; Lombardozzi et al, ; Busch, Sage, & Farquhar, ; McClain & Sharkey, ).…”

Isoprene Suppression by CO2 Is Not Due to Triose Phosphate Utilization (TPU) Limitation

“…TPU limitation occurs when downstream sinks of photosynthetic carbon (such as sucrose, starch, and amino acid synthesis) are unable to keep up with the flux of triose phosphates produced by the Calvin-Benson cycle, which leads to feedback control of photosynthesis (Sharkey, 1985;McClain and Sharkey, 2019). Under TPU limitation, buildup of phosphorylated intermediates lowers the free phosphate in the chloroplast, slowing ATP synthase, which in turn lowers ATP levels and decreases the pH of the lumen, leading to a broad alteration in chloroplastic conditions.…”