Atmospheric CO₂ mole fraction affects stand‐scale carbon use efficiency of sunflower by stimulating respiration in light

Abstract

Plant carbon-use-efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO . Sunflower stands were grown at low (200 μmol mol ) or high CO (1000 μmol mol ) in controlled environment mesocosms. CUE of stands was measured by dynamic stand-scale C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO (compared with low CO ) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand-scale respiratory metabolism at high CO . Two main processes contributed to the reduction of CUE at high CO : a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.

Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

“…abrupt change of δ of CO 2 fed to a leaf), with the δ ‐value of the photorespired CO 2 (flux 0.5 V o ) following with only a short delay (half‐life in the order of a few minutes; Ludwig & Canvin, ). By contrast, the δ ‐value of respired CO 2 responds rather slowly (half‐life in the order of 1 d to a few days; Schnyder et al ., ; Lehmeier et al ., ; Tcherkez et al ., ; Gong et al ., ). This approach requires two sets of online ∆ measurements on similar leaves (or the very same leaves, as in this study), so as to examine the isotopic mass balance at the photosynthetic steady state (Gong et al ., ).…”

“…Thus knowledge of the drivers and controls of leaf respiration is essential for understanding plant physiology and the global carbon budget, and that knowledge is required for improving the representation of leaf respiration in climate‐vegetation models (Atkin et al ., ; Heskel et al ., ; Tcherkez et al ., ). The fact that leaf respiration rate is lower in the light ( R L , also termed day respiration) than in the dark ( R Dk ) – when normalized to the same temperature – has long been recognized and demonstrated in leaf‐ (Brooks & Farquhar, ; Atkin et al ., ; Gong et al ., ), stand‐ (Schnyder et al ., ; Gong et al ., ), and ecosystem‐scale (Wehr et al ., ) studies. The inhibition of respiration by light is underpinned by the light‐induced down‐regulation of the activity of several enzymes of respiratory metabolism (Tcherkez et al ., , ).…”

“…Second, the measurement must be performed at very low CO 2 and nonsaturating irradiance, which generally contrasts with growth conditions (Villar et al ., ; Yin et al ., ). Experimental evidence has indicated a CO 2 effect on respiration rate in the light (Gong et al ., ) and on the abundance of transcripts encoding enzymes of the respiratory pathway in both long‐term (Leakey et al ., ) and short‐term (Li et al ., ) treatments. These observations raise the concern that R L measured by the Laisk method might differ from actual R L under growth conditions.…”

“…The principle of deconvoluting CO 2 flux components by artificially created isotopic disequilibrium (i.e. labelling) has been widely explored for quantification of, for example, photorespiration rate (Ludwig & Canvin, ) or stand‐ (Schnyder et al ., ; Gong et al ., ) or ecosystem‐scale (Ostler et al ., ) autotrophic respiration rate. This type of labelling method exploits the fact that CO 2 flux components have distinct dynamics of tracer incorporation during the labelling.…”

“…This method has the following advantages: R L measurements can be performed under any environmental condition, including growth conditions (e.g. stand‐scale R L was determined at low and high CO 2 using the same principles of CO 2 flux partitioning; Gong et al ., ); it can measure R L at the photosynthetic steady state, that is, without manipulation of the photosynthesis rate; and it simultaneously provides a reliable measurement of mesophyll conductance to CO 2 ( g m ), another important gas exchange parameter. As it relies on the measurements of δ ‐values and CO 2 exchange rates, diffusive leaks across the gasket of the leaf cuvette must be minimized (Gong et al ., ) or accounted for (Gong et al ., ).…”