The current literature indicates that the stimulation of relative growth rate (RGR) by an elevated atmospheric CO2 concentration (Ca) is transient. The effects of a twice-ambient Ca on the understand the mechanisms behind the growth response. Plants were grown hydroponically without nutrient limitation in controlled-environment cabinets. Consistent with studies of other C3 species , the initial COi-stimulation of RGR of U. urens was not sustained, declining within days of exposure to elevated Ca. This decline in RGR was caused mainly by a decreased leaf area ratio (LAR) due to a decreased specific leaf area
(SLA); a reduction in the COi-stimulation of net assimilation rate (NAR) made a
relatively small contribution. The effects of elevated Ca on RGR, LAR, SLA and NAR were not attributable to interactions between accelerated plant size and ontogenetic drift or the earlier onset of environmental constraints. The early changes in NAR and LAR could not be explained by reductions in tissue N concentration or by current models of plant growth which propose a central role for soluble sugars in down-regulating growth by signalling the inability of sinks to use increased assimilate supply. A direct effect of elevated Ca on respiration was found, the causes and consequences of which were not clear. It was suggested that the mechanisms behind growth responses to elevated Ca are
still poorly understood, but might be improved if future research concentrated more on morphological responses.