We can define respiration as the release of CO₂ (or energy) during heterotrophic metabolism. That CO₂ (and energy) release is related to growth, maintenance, and transport processes within crops (there could be a wastage component of respiration, but it is to-date unquantified). The efficiency of respiration is defined in different ways (e.g., number of ADP phosphorylated per carbon atom processed by the TCA cycle), with the most encompassing definition called carbon-use efficiency(CUE): the amount of carbon incorporated into new phytomass per unit carbon assimilated in photosynthesis (net of photorespiration). An analogous energy-use efficiency (EUE) exists; it exceeds CUE because phytomass is more reduced than photosynthate. Theory of respiration associated with crop growth and transport processes is well developed, with associated data more limited, while maintenance remains somewhat elusive in theory and practice.

It has been suggested that a biochemically and ecologically "allowable" range in higher-plant CUE is about 0.2–0.7, integrated over a growing season or a plant’s lifetime. Theory and data indicating that crop CUE leans toward the higher end of this range is discussed, along with evidence that some other vegetation types are skewed toward the lower end. CUE is expected to be most efficient when crops synthesize "inexpensive" cell walls (more cellulose, less lignin) and generate large fractions of storage compounds (related to high harvest index), especially carbohydrates. This is related to both increased efficiency of growth and reduced maintenance needs because (a) cellulose and storage carbohydrates can be synthesized with high efficiency and (b) most classes of storage compounds (and cell wall structural compounds) can be maintained with minimal metabolic expenditures. It implies that crop CUE may be greatest during grain (or tuber) filling. CUE might also be high for crops growing in cooler temperatures (as long as growth is not impaired), because of slower maintenance.

I propose a path forward to better quantify CUE in the field, test theory with field data, and compare genotypes with the aim of improving yield. It is to revive ¹⁴C-labeling of photosynthate to determine the fraction of assimilated carbon remaining in a crop over time, as well as the chemical forms and locations of retained carbon. The approach was well developed in the 1970s, involving relatively high concentrations of ¹⁴C, but little used since then. Today’s technology allows the use of trace amounts of ¹⁴C, and also the use of ¹³C as tracer. I suggest that quantification of labeled-photosynthate retention--a direct measure of CUE--could become a valuable tool for crop genetic improvement. Analysis of biochemical pathways of labeled-C flow might isolate targets for selection of improved germplasm.