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Photosynthetic Photon Flux Density Pathogen Interaction in Growth of Alfalfa Infected with Verticillium albo-atrum. B. W. Pennypacker, Department of Agronomy, The Pennsylvania State University, University Park 16802; D. P. Knievel(2), M. L. Risius(3), and K. T. Leath(4). (2)(3)Department of Agronomy, The Pennsylvania State University, University Park 16802; (4)USDA-ARS, Pasture Systems and Watershed Management Unit, University Park 16802. Phytopathology 84:1350-1358. Accepted for publication 10 August 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994. DOI: 10.1094/Phyto-84-1350.

Photosynthetic photon flux density (PPFD) was manipulated in greenhouse experiments to determine whether carbon assimilation regulated expression of resistance to Verticillium albo-atrum. Treatments were pathogen (V. albo-atrum or no V. albo-atrum), clone (resistant or susceptible), PPFD (100, 70, or 40% of ambient), and time (3 wk). Treatment effects on disease ratings, dry weight of plant parts, net photosynthesis, and stomatal conductance were evaluated weekly. Significant pathogen PPFD week interactions were detected in disease rating, plant height, stem dry weight, and aerial biomass, and a pathogen PPFD interaction was noted in leaf dry weight when data from the resistant clone were analyzed. In all cases, the interactions were caused by the loss of host resistance under 40% PPFD. V. albo-atrum did not affect net photosynthesis or stomatal conductance of the resistant clone, but these parameters were reduced by the 40 and 70% PPFD treatments. Carbon assimilation, therefore, was critical for expression of resistance. The susceptible clone failed to respond to PPFD levels when treated identically to the resistant clone. The inability of the susceptible clone to alter its response to V. albo-atrum is evidence that the defense mechanism under investigation is not simply a constitutive part of all alfalfa plants but is unique to the resistant clone.

Additional keywords: Medicago sativa, photosynthetic stress, Verticillium wilt.