The effect of CO₂ enrichment and irradiance on the growth, morphology and gas exchange of a C₃ (Panicum laxum) and a C₄ (Panicum antidotale) grass

The effect of CO₂ enrichment and irradiance on the growth and gas exchange of two tropical grasses, Panicum laxum (C₃) and Panicum antidotale (C₄) were investigated. The two species were grown at either 350 (low) or 700 (high) μL L^-1 CO₂ concentration, under 40% (low) or 100% (high) of direct sunlight and supplied with ample water and nutrition. Elevated CO₂ enhanced plant dry weight at both irradiances in the Cs species (1.41-fold and 1.71-fold increase at low and high light, respectively) but only at high light in the C₄ species (1.28 fold increase). CO₂ enrichment had no effect on the dry weight of P. antidotale, when stem development was suppressed by growth under artificial lighting. When measured at the CO₂ concentration at which they were grown, assimilation rates were similar in the low and high CO₂ grown plants, for both species. However, when measurements made at low CO₂ were compared, CO₂ assimilation rates of the high light, high CO₂ grown Cs and C₄ species were lower than those of their low CO₂ grown counterparts. High CO₂ strongly reduced the stomatal conductance of both species, while it affected the Rubisco content (30% decrease) of the high light Cs species only. This work shows clearly that C₄ species can respond to CO₂ enrichment under favourable growth conditions, and that acclimation to elevated CO₂ in pasture grasses does not necessarily involve accumulation of non-structural carbohydmtes or reduction of total N or soluble proteins in source leaves.