| MadSci Network: Botany |
Sun and shade leaves on the same plant will usually differ in several ways including leaf area, leaf thickness, cuticle thickness, chlorophyll content, and orientation of chloroplasts. Shade leaves are thinner with a single palisade layer, a thinner cuticle, a greater leaf area, higher chlorophyll content and lower protein content than sun leaves. Sun leaves often have multiple palisade layers. Phototaxis of chloroplasts orients them perpendicular to light in shade leaves to maximize light interception but vertical in sun leaves. In oaks and maples, sun leaves have deeper lobes to allow greater light penetration into the canopy. Petioles also move shade leaves to minimize shading of neighboring leaves and to maximize light interception. Sun leaves often show more shading of their neighbors because light is not so limiting. You might expect leaf petioles to be longer in shade leaves because elongation of plant parts is usually greater in the shade due to the lower red to far-red light ratio. It is a phtyochrome effect. I have not seen stomatal density mentioned as a factor that differs between sun and shade leaves in plant physiology texts, however, it could because stomatal density does vary depending on the environment in which the leaf develops as well as because of genetic factors. Because stomatal density is influenced by many factors, be cautious in concluding you have a cause and effect relationship even if you find a correlation between stomatal density and sun versus shade leaves. One reason it may not vary significantly is that stomatal aperture can vary from fully closed to wide open so a leaf can adjust gas exchange widely without altering stomatal density. You might expect to find a greater stomatal density in sun leaves where photosynthesis rates are higher, however, the greater need to minimize water stress in sun leaves may counter the need for a greater stomatal density. Be sure to measure replicate sun and shade leaves, such as ten or more of each, and take an average to increase the accuracy of your measurements. Pay attention to the variation among measurements of individual sun leaves and individual shade leaves to see if it is greater than the difference between the averages for sun and shade leaves. Plotting your data on a graph may help in your interpretation. Stomatal density has received a lot of attention as a potential method to investigate carbon dioxide levels in ancient times because stomatal density of fossilized leaves is easily determined. Stomatal density apparently decreased during the last 200 years in many species due to the increased carbon dioxide level in the atmosphere. Do a search for stomatal density on google.com to find a lot of info on that topic. References Comparing Sun Leaves To Shade Leaves Developmental Plasticity in Oak Leaves The response of leaf discs from sun and shade plants to green light Sun and Shade Leaves: The Same Leaf in Different Environments Carbon dioxide as a selective agent for stomatal density A subtilisin-like serine protease involved in the regulation of stomatal density and distribution Plants as keys to past climatatic conditions Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth. Woodward, F.I., 1987, Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature 327, 617-618.
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