Plant carbon-use efficiency under warming: insights from a ¹³CO2 pulse-chase experiment

Carbon use efficiency (CUE), defined as the ratio of net primary production (NPP) to gross primary production (GPP), reflects the efficiency of carbon conversion into plant biomass after accounting for respiratory losses. As a key parameter in plant carbon budgeting and terrestrial carbon sequestration assessments, CUE is difficult to measure directly due to the challenges in quantifying gross CO₂ fixation and total respiration. Thus, many carbon cycle models rely on simplified empirical values (e.g., 0.5). Although climate warming may influence CUE due to the widely observed temperature‐sensitivity of respiration, the responses of CUE to warming remain unclear.

In this study, we grew wheat (T. aestivum) and upland rice (O. sativa) in controlled chambers under two temperatures: 25°C (control) and 29°C (+4°C warming). We took advantage of a gas exchange and ¹³C-labelling facility to estimate the gross photosynthetic rate of individual plants and trace the allocation of fixed carbon to shoot and root growth. A compartment model was fit to the data of tracer dynamic during the chase period to analyze the turnover features of carbon pools.

Both species exhibited physiological acclimation to warming: increased leaf‐level maximum carboxylation rate and specific leaf area, but decreased basal respiration rate. Consequently, whole‐plant CUE did not differ significantly between temperature treatments. ¹³C dynamics further revealed that warming did not alter the turnover rates of carbon pools supporting respiration and growth. These results indicate that +4°C warming did not affect CUE in wheat or upland rice, demonstrating a coordinated acclimation of photosynthesis and respiration to elevated temperature.