EFFECT OF ELEVATED CO2 ON STOMATAL DENSITY AND DISTRIBUTION IN A C-4 GRASS AND A C-3 FORB UNDER FIELD CONDITIONS

Two common tallgrass prairie species, Andropogon gerardii, the dominan t C-4 grass in this North American grassland, and Salvia pitcheri, a C -3 forb, were exposed to ambient and elevated (twice ambient) CO2 with in open-top chambers throughout the 1993 growing season. After full ca nopy development, stomatal density on abaxial and adaxial surfaces, gu ard cell length and specific leaf mass (SLM; mg cm(-2) were determined for plants in the chambers as well as in adjacent unchambered plots. Record high rainfall amounts during the 1993 growing season minimized water stress in these plants (leaf xylem pressure potential was usuall y > -1.5 MPa in A. gerardii) and also minimized differences in water s tatus among treatments. In A. gerardii, stomatal density was significa ntly higher (190 +/- 7 mm(-2); mean +/- s.e.) in plants grown outside of the chambers compared to plants that developed inside the ambient C O2 chambers (161 +/- 5 mm(-2)). Thus, there was a significant 'chamber effect' on stomatal density. At elevated levels of CO2, stomatal dens ity was even lower (P < 0.05; 121 +/- 5 mm(-2)). Most stomata were on abaxial leaf surfaces in this grass, but the ratio of adaxial to abaxi al stomatal density was greater at elevated levels of CO2. In S. pitch eri, stomatal density was also significantly lower when plants were gr own in the open-top chambers (235 +/- 10 mm(-2) outside vs. 140 +/- 6 mm(-2) in the ambient CO2 chamber). However, stomatal density was grea ter at elevated CO2 (218 +/- 12 mm(-2)) compared to plants from the am bient CO2 chamber. The ratio of stomata on adaxial vs. abaxial surface s did not vary significantly in this herb. Guard cell lengths were not significantly affected by growth in the chambers or by elevated CO2 f or either species. Growth within the chambers resulted in lower SLM in S. pitcheri, but CO2 concentration had no effect. In A. gerardii, SLM was lower at elevated CO2. These results indicate that stomatal and l eaf responses to elevated CO2 are species specific, and reinforce the need to assess chamber effects along with treatment effects (CO2) when using open-top chambers.