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Effect of Light on Bacteria-Induced Hypersensitivity in Soybean. L. R. Giddix, Jr., Graduate student, Department of Plant Pathology, The Pennsylvania State University, University Park 16802; F. L. Lukezic(2), and E. J. Pell(3). (2)(3)Professor, and associate professor, respectively, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Phytopathology 71:111-115. Accepted for publication 4 June 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-111.

Erwinia amylovora-infiltrated, intact primary soybean leaves, incubated in 24 hr of postinfiltration darkness, expressed more severe hypersensitive response (HR) symptoms than did leaves illuminated for 24 hr at 11.5 and 23 W/m2. HR symptom severity decreased as the 24-hr postinfiltration light intensity was increased from darkness to 11.5 W/m2. Variations in postinfiltration photoperiod at 11.5 and 23 W/m2 resulted in varied HR severity levels in primary leaves. With 11.5 W/m2 intensity, leaves initially exposed for ≤ 4 hr expressed a more intense symptom than did initially dark-incubated counterparts. No significant differences in percent panel necrosis were observed between initial light and dark exposures of 6 hr at that intensity. With an intensity of 23 W/m2, leaves exposed to ≤ 6 hr initial illumination expressed a more intense symptom than initially dark-incubated counterparts. With initial incubations exceeding 6 hr in both light regimes, dark-incubated leaves expressed the more intense symptom. Light increased the in vitro lag phase of E. amylovora whereas the lag phase was increased by 4 hr in the dark. The HR, as measured by electrolyte leakage from bacteria-infiltrated leaves, was photosensitive after 6 hr post infiltration and was more severe in dark-incubated tissue. Infiltration of bacterial suspensions in atrazine solutions into leaves reversed the effects of light after 24 hr and induced panel necrosis equal to that of leaves infiltrated with bacteria alone and incubated in the dark for 24 hr. Atrazine (5 105 M) did not affect the in vitro growth of E. amylovora.