Effect of elevated carbon dioxide on seedling morphology and anatomy in seven herbaceous species

The effects of carbon dioxide on stomatal density, stomatal index, stomatal size, and epidermal cell density were examined in seven genera (Sinapis alba L., Medicago sativa L., Celosia cristata L. 'plumosus', Dianthus caryophyllus L., Cucumis sativus L., Beta vulgaris L. var. cicla L., and Raphanus sativus L.) using scanning electron microscopy. In the case of Sinapis only, stem diameter, and the length and width of cortical parenchyma cells were examined using light microscopy. Plants were grown in growth chambers maintained at 20C and at one of three levels of carbon dioxide (350, 700, and 1400 $\pm$ 50 ppm). Seedlings were provided with enough light to induce the unfolding of the cotyledons but not enough to result in any significant photosynthesis (PAR = 45 $\mu$mol $\rm m\sp{-2}\ s\sp{-1}$ for 10 minutes per day). Stem elongation in Sinapis and Medicago, lengths of both guard and subsidiary-cell in Sinapis, and lengths of parenchyma cells were all significantly increased by elevated carbon dioxide. In contrast, stomatal density and epidermal cell density in Sinapis and Medicago were decreased for both cotyledon surfaces. Stomatal density on the lower surfaces of Celosia and Beta was decreased. Level of carbon dioxide had no effect on stem diameter, diameter of parenchyma cells or stomatal index in any of the species. Level of carbon dioxide did not significantly affect any of the measured parameters in either Dianthus or Raphanus. The results suggest that elevated carbon dioxide increases longitudinal cell expansion and that this has various consequences. Furthermore, the effect of carbon dioxide on cell expansion is species specific and independent of its effects on photosynthesis. (Abstract shortened by UMI.)