Reproductive growth and respiration in an annual under elevated CO ₂

We analyzed reproductive yield of an annual, Xanthium canadense, at elevated [CO₂] as the product of (1) the amount of plant photosynthesis, (2) relative allocation of photosynthates to reproduction and (3) photosynthate use efficiency in reproductive organs (reproductive yield per photosynthates allocated to the reproductive organ). Plants were grown under ambient and elevated [CO₂] (360 and 700 μmol mol^-1) and were harvested periodically during the growing period. The rate of respiration was determined for each organ and the amount of photosynthates allocated to organs was obtained by summing biomass increase and respiratory consumption. Reproductive yield increased with CO₂ enrichment by 9%, while plant dry mass increased by 16%. The amount of photosynthates assimilated in the growing period increased with CO₂ enrichment by 9%. However, because the relative allocation of photosynthates to the reproductive organ decreased with CO₂ enrichment by 7%, the increase in the amount of photosynthates allocated to the reproductive organ by elevated [CO₂] was small (+2%). Reproductive yield per photosynthates allocated to the reproductive organ was enhanced by elevated [CO₂] by 8%. These results indicate that respiratory consumption has a considerable effect on the response of reproductive yield to elevated [CO₂].