The response of Plantago major ssp. pleiosperma to elevated CO₂ is modulated by the formation of secondary shoots

summary The effect of elevated CO 2 on the relative growth rate (RGR) of Plantago major ssp. pleiosperma was studied during the vegetative stage, in relation to plant development, by growing plants at 350 μl 1 −1 or at 700 μl 1 −1 CO 2 in non‐limiting nutrient solution with nitrate. To minimize interference by the accumulation of non‐structural carbohydrates in the interpretation of results, RGR was expressed on a f. wt basis (RGR FW ), as were all plant weight ratios. Stimulation of the RGR FW of the whole plant by elevated CO 2 was transient, and did not last longer than 8 d. At the same time a transient increase in root weight ratio (RWR) was observed. In order to investigate whether the transient effect of elevated CO 2 on rgr fw was size‐dependent, the data were plotted versus total f. wt (log e , transformed). The transient period of stimulation of rgr fw and of rwr by elevated CO 2 was still found, but in both CO 2 treatments rgr fw decreased after a certain plant size had been reached. This size coincided with the stage at which secondary shoots started to develop, and was reached earlier in plants grown at elevated CO 2 . The rgr of these secondary shoots ( rgr set .) was still increased when the period of whole plant stimulation of rgr fw had ended, indicating that the development of these new sinks took priority over a continuation of the stimulation of rwr . It is hypothesized that in this Plantago subspecies the response of the rgr fw of the whole plants to elevated CO 2 is modulated by the formation of secondary shoots. Apparently, partitioning of the extra soluble carbohydrates at elevated CO 2 to this tissue takes precedence over partitioning to the roots, resulting in a cessation of stimulation of plant rgr fw by elevated CO 2 .