The influence of atmospheric carbon dioxide enrichment on belowground growth

Root growth under elevated atmospheric CO{dollar}\sb2{dollar} was examined with the intention of quantifying the effects of elevated CO{dollar}\sb2{dollar} on growth, cell wall characteristics and root carbon status of three herbaceous legumes: Phaseolus vulgaris L., Phaseolus acutifolius Gray and Pisum sativum L. Plants were grown hydroponically in 0.1 strength Hoagland's solution in the UNLV greenhouse at atmospheric CO{dollar}\sb2{dollar} concentrations of ambient (350 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}),{dollar} 1.5x ambient (550 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}){dollar} and 2x ambient (700 {dollar}\mu{dollar}mol mol{dollar}\sp{-1}).{dollar} Root biomass increased under elevated CO{dollar}\sb2{dollar} for P. acutifolius and P. sativum, but not for P. vulgaris. Phaseolus vulgaris roots grew faster during the day under elevated CO{dollar}\sb2{dollar}, P. acutifolius root growth rate increased at night, and P. sativum root growth rate did not change. In spite of these variable changes in growth rates, root:shoot ratios decreased in all three species studied. Measurements of biophysical growth parameters were inconclusive. Starch and ABA concentrations of P. acutifolius roots increased under 2x ambient CO{dollar}\sb2.{dollar} Increased carbon allocation to the roots presumably indicated a strong sink of rapidly elongating cells, possibly directed by ABA. An understanding of the effect of elevated CO{dollar}\sb2{dollar} on belowground growth processes will be necessary to predict the impact of increased industrialization on crop production and ecosystem functioning.