THE RESPONSE OF PLANTAGO-MAJOR SSP PLEIOSPERMA TO ELEVATED CO2 IS MODULATED BY THE FORMATION OF SECONDARY SHOOTS

The effect of elevated CO2 on the relative growth rate (RGR) of Planta go major ssp. pleiosperma was studied during the vegetative stage, in relation to plant development, by growing plants at 350 mu l l(-1) or at 700 mu l l(-1) CO2 in non-limiting nutrient solution with nitrate. To minimize interference by the accumulation of non-structural carbohy drates in the interpretation of results, RGR was expressed on a f. wt basis (RGR(FW)), as were all plant weight ratios. Stimulation of the R GR(FW) Of the whole plant by elevated CO2 was transient, and did not l ast longer than 8 d. At the same time a transient increase in root wei ght ratio (RWR) was observed. In order to investigate whether the tran sient effect of elevated CO2 on RGR(FW) was size-dependent, the data w ere plotted versus total f. wt (log(e) transformed). The transient per iod of stimulation of RGR(FW) and of RWR by elevated CO2 was still fou nd, but in both CO2 treatments RGR(FW) decreased after a certain plant size had been reached. This size coincided with the stage at which se condary shoots started to develop, and was reached earlier in plants g rown at elevated CO2. The RGR of these secondary shoots (RGR(see)) was Still increased when the period of whole plant stimulation of RGR(FW) had ended, indicating that the development of these new sinks took pr iority over a continuation of the stimulation of RWR. It is hypothesiz ed that in this Plantago subspecies the response of the RGR(FW) of the whole plants to elevated CO2 is modulated by the formation of seconda ry shoots. Apparently, partitioning of the extra soluble carbohydrates at elevated CO2 to this tissue takes precedence over partitioning to the roots. resulting in a cessation of stimulation of plant RGR(FW) by elevated CO2.