Constraints to growth of annual nettle (Urtica urens) in an elevated CO₂atmosphere: Decreased leaf area ratio and tissue N cannot be explained by ontogenetic drift or mineral N supply

The current literature indicates that the stimulation of relative growth rate (RGR) by an elevated atmospheric CO 2 concentration is transient. Urtica urens L. was exposed to an elevated atmospheric CO 2 concentration for 26 days to better understand the factors involved in this constraint to growth. Plants were grown hydroponically without nutrient limitation in controlled‐environment cabinets. Consistent with studies of other C3 species, the initial CO 2 stimulation of RGR of U. urens was not sustained and declined in the early stages of exposure. Whilst the decline in RGR was most strongly linked to a reduction in the CO 2 stimulation of net assimilation rate (NAR), its initial increase was constrained by an early and persistent reduction in leaf area ratio (LAR) due to a decreased specific leaf area (SLA). The decline in NAR could not be linked to any down‐regulation of photosynthetic capacity of individual leaves, despite an accumulation of soluble sugars in them. The reductions in LAR and SLA reflected an accumulation of structural weight in addition to an accumulation of total non‐structural carbohydrate (TNC). To account for the impact of ontogenetic drift on the partitioning of weight and leaf area, this study extends the usual allometric approach to include an analysis of effects on the vertical placement of regression lines (i.e. their elevations). Using this approach, we argue that CO 2 ‐induced reductions in LAR and SLA cannot be explained by ontogenetic drift. By monitoring the tissue N concentration, external N supply was shown unambiguously to be non‐limiting for growth at any plant size. Nevertheless, tissue N was consistently lower in elevated CO 2 , independent of both ontogeny and TNC accumulation, raising the possibility that the reductions in NAR, LAR and SLA are related to some internal constraint on N utilization.