Corn leaf cuticle and wax
Leaf epidermis cells provide important functions beyond providing a tight layer of cell walls surrounding the inner mesophyll cells of the leaf. Epidermal cells also produce a polysaccharide layer outside the outer cell walls and a fatty acid layer of wax further outside. Synthesis of these cuticle and wax substances begins in plastids within the cytoplasm of the epidermal cells. These newly manufactured compounds are moved via the endoplasmic reticulum eventually being deposited on the outer surface of the epidermal cell walls. The fatty acid wax is moved further outside forming a waxy surface to the cuticle.
Multiple genes are involved in production of these complex molecules as synthesis requires linking simple products of photosynthesis (glucose) with inorganic materials to form new compounds. The basic process is common to all land plants as they adapted to life outside of the aqua environment of algae. Further selection for adaptation to varying corn environments allowed for selection of genetics affecting responses to environmental stress.
Outer wax causes water to run off the surface, taking pathogen spores with it. Chemicals applied by growers usually include a surfactant to overcome the water resistance by breaking the tendency of the water molecules to form drops, thus reducing this feature of wax. Pathogenic leaf fungi enter the leaf either by establishing a ‘drilling station’ on the surface from which hyphae extension (appresorium) is pushed through the wax and cuticle layers on the epidermal cells. Other fungi and some bacteria, unable to penetrate the wax and cuticle, avoid the problem by entering through the stomatal openings.
Wax also prevents water loss. Corn genotypes vary in this response to dry environments, some making thicker layers of wax than others when in a dry environment. Leaf surfaces of hybrids grown in the less humid environments of western corn belt have a different texture than the same hybrid grown in the more humid eastern US corn belt. Wax production differences among varieties is probably one of the components to more drought resistance.
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The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.