Constraints to growth of annual nettle (Urtica urens) in an elevated CO2 atmosphere: 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 ra te (RGR) by an elevated atmospheric CO2 concentration is transient. Urtica urens L. was exposed to an elevated atmospheric CO2 concentration for 26 da ys 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 C-3 species, the ini tial CO2 stimulation of RGR of Li. ur cns 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 CO2 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 he linked to any down-regulation of photosynthetic capacity of individual leaves, despite an accumulation of soluble sugars i n them, The reductions in LAR and SLA reflected an accumulation of structur al weight in addition to an accumulation of total non-structural carbohydra te (TNC), To account for the impart of ontogenetic drift on the partitionin g 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 CO2-induc ed reductions in LAR and SLA cannot be explained by ontogenetic drift. By m onitoring the tissue N concentration, external N supply was shown unambiguo usly to be non-limiting for growth at any plant size. Nevertheless, tissue N was consistently lower in elevated CO2, independent of both ontogeny and TNC accumulation, raising the possibility that the reductions in NAR, LAR a nd SLA are related to some internal constraint on N utilization.