Carbon use efficiency depends on growth respiration, maintenance respiration, and relative growth rate. A case study with lettuce

ABSTRACT Carbon use efficiency ( C UE , the ratio between the amount of carbon incorporated into dry matter to the amount of carbon fixed in gross photosynthesis) is an important parameter in estimating growth rate from photosynthesis data or models. It previously has been found to be relatively constant among species and under different environmental conditions. Here it is shown that C UE can be expressed as a function of the relative growth rate ( r GR ) and the growth ( g r ) and maintenance respiration coefficients ( m r ): 1/ C UE = 1 + g r + m r / r GR . Net daily carbon gain ( C dg ), r GR , and C UE were estimated from whole‐plant gas exchange measurements on lettuce ( Lactuca sativa L.) ranging from 24 to 66 d old. Carbon use efficiency decreased from 0.6 to 0.2 with increasing dry mass, but there was no correlation between C UE and C dg . The decrease in C UE with increasing dry mass was correlated with a simultaneous decrease in r GR . From the above equation, g r and m r were estimated to be 0.48 mol mol −1 and 0.039 g glucose g –1 dry matter d −1 , respectively. Based on the g r estimate, the theoretical upper limit for C UE of these plants was 0.68. The importance of maintenance respiration in the carbon balance of the plants increased with increasing plant size. Maintenance accounted for 25% of total respiration in small plants and 90% in large plants.