Stomata are the microscopic pores that facilitate the movement of gasses into and out of leaves. Carbon dioxide goes into the leaf, while oxygen and water vapor go out. The opening and closing of stomata (stoma=singular) are mediated by the guard cells, which can expand and contract depending on their turgor pressure. Turgid guard cells open the pores, flaccid cells close them. Stomata are key to evapotranspiration and water and solute transport from roots, to shoots, to leaves. Coupled with other plant functional traits, stomata can indicate how a plant is interacting and coping with its environment. In an upcoming project I will being quantifying stomatal density with other traits, among different species along an environmental gradient.
From reading the literature, it is apparent that changing one variable in a leaf, such as stomatal density, can have a cascade of effects on other traits and photosynthetic rates. However, I cannot think about stomata density without first considering Woodward’s 1987 paper, ‘Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels.’ I really like this study because it used herbarium records dating back 200 years to show stomata densities have decreased over time. After growing plants in varying concentrations of carbon dioxide, Woodward was able to show that the decreases in stomata density of herbarium specimens likely resulted from increases in global CO2 concentration.
It is interesting to note that atmospheric CO2 concentrations were 340 µmol/mol at the time of Woodward’s study. We have just recently passed the grim 400 µmol/mol milestone, which is disconcerting in light of another observation Woodward made: Some plants did not change their stomatal density above ambient CO2 (340 µmol/mol) conditions. What does that mean for plant physiology? Rico et al. 2013 suggest plants may shift towards lower water requirements and greater xylem fortification – in other words – plants become more drought tolerant…if they can.
While I’m reading up on that, I will leave you with a 400x close up of the more stomata (below). In both top and bottom pictures, I have circled some of the stomata. The guard cells look like two touching crescents. The guard cells are mostly closed, because the leaves are dead (ultra-flaccid), plus the cells have probably contracted some. The black cone shapes are hairs. These images are nail polish molds of the the abaxial (bottom side) of Cananga odorata. The green color is an after-effect.
Rico, C., Pittermann, J., Polley, H. W., Aspinwall, M. J. and Fay, P. A. 2013. The effect of subambient to elevated atmospheric CO2 concentration on vascular function in Helianthus annuus: implications for plant response to climate change. New Phytologist 199: 956–965.
Woodward, F. I. 1987. Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature 327:617–618.