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

Photosynthetic photon flux density (PPFD) was manipulated in greenhous e experiments to determine whether carbon assimilation regulated expre ssion of resistance to Verticillium albo-atrum. Treatments were pathog en (V. albo-atrum or no V. albo-atrum), clone (resistant or susceptibl e), PPFD (100, 70, or 40% of ambient), and time (3 wk). Treatment effe cts 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 heig ht, stem dry weight, and aerial biomass, and a pathogen X PPFD interac tion was noted in leaf dry weight when data from the resistant clone w ere analyzed. In all cases, the interactions were caused by the loss o f host resistance under 40% PPFD. V. albo-atrum did not affect net pho tosynthesis or stomatal conductance of the resistant clone, but these parameters were reduced by the 40 and 70% PPFD treatments. Carbon assi milation, therefore, was critical for expression of resistance. The su sceptible clone failed to respond to PPFD levels when treated identica lly to the resistant clone. The inability of the susceptible clone to alter its response to V. albo-atrum is evidence that the defense mecha nism under investigation is not simply a constitutive part of all alfa lfa plants but is unique to the resistant clone.