| Verticillium wilt of alfalfa, caused by Verticillium albo-atrum, is found throughout the northern alfalfa growing region of the United States. Resistance is the only successful control method. Resistant cultivars are available, but resistance mechanisms are obscure. The effect of abiotic stress on resistance is also unknown. My objectives were to: (1) assess the impact of combined biotic and abiotic stress on the growth of resistant alfalfa and, (2) elucidate mechanisms of resistance.; To examine the effect of the abiotic stress, drought, on resistance to V. albo-atrum, a technique was developed to impose a gradually intensifying drought stress on greenhouse-grown alfalfa. The method was nonlabor intensive, effective, and the drought stress was reproducible and allowed plant acclimation.; Resistant alfalfa clones were subjected to drought and V. albo-atrum in a factorial experiment to determine whether the two stresses interacted to alter resistance. Drought stress reduced symptoms, and reduced V. albo-atrum's suppressive effect on stem dry weight. Verticillium albo-atrum had the same suppressive effect on height and flowering as it did under nondrought-stressed conditions. Resistance, therefore, was stable under drought stress.; Experiments were conducted to verify previous reports that V. albo-atrum caused growth suppression in resistant alfalfa cultivars. Resistant alfalfa clones infected with V. albo-atrum were shorter, had a lower percentage of plants flowering, and had reduced leaf, stem, and aerial biomass. Stomatal conductance was reduced during the afternoons, evidence that the infected plants were responding to pathogen-induced drought stress. Results confirmed previous reports of a dynamic host/pathogen interaction in infected, resistant alfalfa.; Research on net photosynthesis and in vivo and in vitro activity of ribulose 1,5 bisphosphate carboxylase/oxygenase (rubisco), in V. albo-atrum-infected clones detected a potential resistance mechanism. Net photosynthesis and rubisco activity were reduced in the susceptible, infected alfalfa clone, but not in the resistant clone, despite the presence of the pathogen in the leaves analyzed. In vitro assays indicated that activity was not altered but that there was more rubisco in the infected, resistant leaves.; The conclusions from this research were: (1) resistance to V. albo-atrum is stable under the additional abiotic stress of drought, (2) energy-consuming defense mechanisms are operating in infected, resistant alfalfa plants, (3) maintenance of photosynthetic rate may be a mechanism of resistance to V. albo-atrum. |
