PHOTOSYNTHETIC PHOTON FLUX-DENSITY X-PATHOGEN INTERACTION IN GROWTH OF ALFALFA INFECTED WITH VERTICILLIUM-ALBO-ATRUM

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 X PPFD X week interactions were detected in disease rating, plant height, stem dry weight, and aerial biomass, and a pathogen X 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.