Warming Has Limited Effects on Carbon-Use Efficiency of Wheat and Upland Rice Plants-Evidence From a 13CO2 Labelling Study.

Plant carbon-use efficiency (CUE) is a key parameter quantifying the net carbon gain with respect to gross carbon fixation. Although it is critical for carbon cycle models, it is often assigned an empirical value due to measurement difficulty. Uncertainty remains whether CUE is affected by warming. We quantified CUE in wheat (Triticum aestivum) and rice (Oryza sativa) in a warming experiment (+ 4°C) using growth chambers. Pulse labelling using 13C enriched CO2 was performed to estimate photosynthetic carbon gain and the allocation of carbon to plant organs. We found physiological acclimation to warming in both species. Both foliar maximum carboxylation rate and specific leaf area increased while basal respiration decreased under warming. CUE was not significantly different between control (0.59) and warming-treated plants (0.54) during the vegetative stage. 13C tracer analysis showed that warming did not affect the turn-over of carbon pools used for respiration and growth. Plant-level CUE correlated positively with the proportion of assimilated carbon allocated to shoot growth (R2 = 0.91), but not with gross primary production. Our results indicate that plant-level CUE remains resilient to moderate warming due to a coordinated acclimation of photosynthesis and respiration, underpinned by consistent patterns in carbon allocation and carbon pool turnover.