Leaves and the Effects of Elevated Carbon Dioxide Levels

Since the onset of the Industrial Revolution, the burning of fossil fuel and the clearing of land has led to dramatic rises in the concentration of CO(2) in the atmosphere. Leaves concentrate highly dilute CO(2) from the atmosphere and transform organic carbon compounds into useful forms (food and fiber). In light of the fact that leaves, and leaves only, serve this important purpose, we provide this short overview. Crops grown under high CO(2) usually exhibit increased mass due to more photosynthetic capacity and enhanced water use efficiency but, differences exist among plants. C4 plants can exhibit growth stimulation primarily due to increase water use efficiency. The C3 plant can benefit from both increased photosynthesis and water use efficiency. Enhanced photosynthesis can lead to the accumulation of non-structural carbohydrates in leaves which has been related to a decline in photosynthesis over time caused in part by source-sink imbalances. Increases in carbohydrates may repress photosynthetic gene expression. Leaf structural changes may help interpret divergent findings with regard to photosynthetic acclimation. Under high CO(2), starch accumulation may change chloroplast structure and function. Both photosynthetic and assimilate transport capacity may also be altered by CO(2)-induced shifts in mesophyll and vascular tissue. Changes in the leaf surface could affect water relations, susceptibility to pests and diseases, and surface properties which are important in chemical protectant application. Herbicide efficacy can vary by weed species under elevated CO(2) which is important for future weed management, including control of invasive species, and also raises questions about herbicide efficacy on cover crops. Although highly variable, increases in cell expansion may contribute to larger leaf size more than increased cell division. Increased expansion appears to result from greater cell wall relaxation and/or greater cell turgor. Under high CO(2), plants often exhibit increased area per leaf, greater leaf thickness, more leaves per plant, and higher total leaf area per plant, but effects on leaf area index are variable. Increases in leaf thickness is often the result of altered anatomy or starch accumulation. CO(2)-induced increases in leaf production and changes in leaf composition can significantly affect other trophic level organisms which utilize leaf tissue as a food substrate; this could impact pest management decisions and leaf litter decomposition rates. Leaf response to elevated atmospheric CO(2) will need to be further elucidated, if we are to accurately understand and predict the development and growth of crops under future environmental conditions.