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Is Change in Electrical Potential or pH a Hatching Signal for Heterodera glycines?

First, second, sixth, and seventh authors: Department of Plant Microbiology and Pathology, University of Missouri, Columbia 65211; third author: Plant Physiology, Pennsylvania State University, University Park 16802; fourth and fifth authors: Department of Chemistry, Iowa State University, Ames 50011; and eighth author: Department of Crop Science, University of Illinois, Urbana 61801

This study explored the possibilities that changes in the egg shell/lipid layer electrical potential or pH communicate external hatching conditions to the Heterodera glycines second-stage juvenile (J2) within the mature egg and that electrophysiology could measure effects of chemicals on emergence. Potentials were measured following application of the emergence inducers (ZnSO₄ and ZnCl₂), ions that do not affect emergence, or synthetic emergence inhibitors. Results were compared with pH measurements and emergence bioassays. Healthy appearing eggs had negative resting potentials. Application of ZnSO₄ caused a smooth depolarization. However, eggs containing J2 and immature eggs depolarized to a similar degree when ZnSO₄ was added. In addition, ZnSO₄, synthetic emergence inhibitors, and CaCl₂ caused similar depolarization, and some depolarization was measured in dye-permeable eggs and empty shells. Results suggest that change in cation surface charge contributed to depolarization and that Cl¯ penetrated the egg shell/lipid layer without causing potential changes. In bioassays, zinc consistently stimulated emergence to a greater degree than H₂O, other cations, or buffers, and counteracted emergence inhibitors. Zinc-caused emergence stimulation was independent of pH. In summary, it is concluded that depolarization and pH are not emergence signals and electrophysiology is unlikely to measure effectiveness of emergence stimulators or inhibitors.

Additional keywords: cyst nematode, soybean.