The sensitivity of yield and quality parameters to carbon dioxide concentration [CO 2 ] was determined for individual lines of hard-red spring wheat released in 1903, 1921, 1965 and 1996. All cultivars were evaluated with respect to growth and vegetative characteristics, grain yield and nutritional quality in response to [CO 2 ] increases that corresponded roughly to the CO 2 concentrations at the beginning of the 20th century, the current [CO 2 ], and the future projected [CO 2 ] for the end of the 21st century, respectively. Leaf area ratio (cm 2 g À1 ) declined and net assimilation rate (g m 2 day À1 ) increased in response to increasing [CO 2 ] for all cultivars during early vegetative growth. By maturity, vegetative growth of all cultivars significantly increased with the increase in [CO 2 ]. Seed yield increased significantly as [CO 2 ] increased, with yield sensitivity to rising [CO 2 ] inversely proportional to the year of cultivar release. Greater [CO 2 ] yield sensitivity in older cultivars was associated with whole-plant characteristics such as increased tillering and panicle formation. Grain and flour protein, however, declined significantly with increasing [CO 2 ] and with year of release for all cultivars, although absolute values were higher for the older cultivars. Overall, these data indicate that yield response at the whole-plant level to recent and projected increases in [CO 2 ] has declined with the release of newer cultivars, as has protein content of grain and flour. However, if agronomic practice can be adapted to maximize individual plant performance, [CO 2 ] responsive characteristics of older cultivars could, potentially, be incorporated as factors in future wheat selection.

To examine the impact of climate change on vegetative productivity, we exposed fallow agricultural soil to an in situ temperature and CO 2 gradient between urban, suburban and rural areas in 2002. Along the gradient, average daytime CO 2 concentration increased by 21% and maximum (daytime) and minimum (nighttime) daily temperatures increased by 1.6 and 3.3°C, respectively in an urban relative to a rural location. Consistent location differences in soil temperature were also ascertained. No other consistent differences in meteorological variables (e.g. wind speed, humidity, PAR, tropospheric ozone) as a function of urbanization were documented. The urban-induced environmental changes that were observed were consistent with most short-term (~50 year) global change scenarios regarding CO 2 concentration and air temperature. Productivity, determined as final above-ground biomass, and maximum plant height were positively affected by daytime and soil temperatures as well as enhanced [CO 2 ], increasing 60 and 115% for the suburban and urban sites, respectively, relative to the rural site. While long-term data are needed, these initial results suggest that urban environments may act as a reasonable surrogate for investigating future climatic change in vegetative communities.

Although rising atmospheric carbon dioxide (CO 2 ) is known to stimulate the growth of agronomic weeds, the impact of increasing CO 2 on herbicide efficacy has not been elucidated for field-grown crops. Genetically modified soybean [Glycine max (L.) Merr.] (i.e., Round-up Ready soybean) was grown over a 2-yr period at ambient and projected levels of atmospheric carbon dioxide (CO 2 , 250 mmol mol 21 above ambient), with and without application of the herbicide, glyphosate [N-(phosphonomethyl)glycine], to assess the impact of rising atmospheric carbon dioxide concentration [CO 2 ] on chemical efficacy of weed control. For both years, soybean showed a significant vegetative response to elevated [CO 2 ], but no consistent effect on seed yield. For 2003, weed populations for all treatments consisted entirely of C 4 grasses, with no [CO 2 ] effects on weed biomass (unsprayed plots) or glyphosate efficacy (sprayed plots). However, in 2004, weed populations were mixed and included C 3 and C 4 broadleaves as well as C 4 grasses. In this same year, a significant increase in both C 3 broadleaf populations and total weed biomass was observed as a function of [CO 2 ] (unsprayed plots). In addition, a [CO 2 ] by glyphosate interaction was observed with significant C 3 broadleaf weed biomass remaining after glyphosate application. Overall, these data emphasize the potential consequences for CO 2 -induced changes in weed populations, biomass, and subsequent glyphosate efficacy in Round-up Ready soybean.