IMPACT OF DROUGHT STRESS ON THE EXPRESSION OF RESISTANCE TO VERTICILLIUM-ALBO-ATRUM IN ALFALFA

Drought stress alters plant metabolism, causing changes in growth, hormone levels, photosynthesis, and nutrient uptake. Verticillium alboatrum reduces growth and stomatal conductance in resistant alfalfa clones in the absence of abiotic stress. The additional effect of drought stress on the growth of resistant alfalfa infected with V. albo-atrum is unknown; consequently, experiments were conducted to assess the stability of resistance to V. albo-atrum in alfalfa grown under the stress of drought. Two resistant alfalfa clones were grown in the greenhouse in 0.03-m3 cylinders that allowed the development of a gradually increasing drought stress. Plants were stubble-inoculated with V. albo-atrum and grew for 6 wk before drought stress was imposed. Effect of the combined stresses was measured over the next two 6-wk growing periods, with measurements taken at harvest during the second growth period and weekly during the third growth period. The experiment was repeated. Plant height, leaf and stem dry weight, and aerial biomass were significantly reduced by drought during both the second and third growth periods. V. albo-atrum-infected plants were shorter and had more stems, fewer internodes, and reduced stem and aerial biomass at the second harvest. Only height and flowering were reduced in infected plants during the third growth period. Significant pathogen X clone interactions indicated that the two clones differed in sensitivity to V. albo-atrum. Significant pathogen X water interactions in disease rating and stem dry weight demonstrated that stem dry weight was less affected by the pathogen, and fewer symptoms were present under drought stress than under nondrought-stressed conditions. V. albo-atrum had no consistent significant effect on stomatal conductance but did alter leaf water potential. The absence of pathogen X water interactions for most of the growth parameters and the decreased effect of the pathogen on stem dry weight under drought stress indicated that resistance to V. albo-atrum in alfalfa is stable under drought stress.