What was done
The authors grew seven Brassica species of various sink strengths at 350 or 1000 ppm atmospheric CO2 for six to twelve weeks to study the effect of source:sink ratio on plant response to elevated CO2.  Among the species tested, broccoli and cauliflower possess large carbon sinks in reproductive structures, Chinese broccoli and marrow stem kale have carbon sinks in stems, while turnip has them in roots.  On the other hand, rape and white mustard lack well developed carbon sinks.

What was learned
After two weeks, elevated CO2 had positively impacted all young seedlings by increasing their total dry weights.  Likewise, after four weeks growth in elevated CO2, significantly greater dry weights were measured in all species regardless of sink strength or location.  However, when final growth data were obtained at either six or twelve weeks, depending on species, sink strength was found to have influenced the CO2 growth response of species within the Brassica genus.  All of the species with developed carbon sinks (except turnip) exhibited significantly greater dry weights when grown in elevated as opposed to ambient CO2, while the two species lacking significant sink strength displayed dry weights that were similar, regardless of CO2 growth concentration.

What it means
As the CO2 content of the air rises, young Brassica species should positively respond by increasing their biomass.  As these species develop and mature, however, those possessing large carbon sinks, regardless of tissue location, will likely exhibit greater final CO2-induced growth increases than those species that do not have well-developed sinks.  It is important, therefore, for agricultural crop developers to select for varieties containing large carbon sinks in order to maximize the potential CO2-induced growth increase that should result from the rising CO2 content of the air.