| The symbiotic association of tall fescue (Festuca arudinacea Schreb.) with a non-parasitic fungal endophyte (Acremonium coenophialum Morgan-Jones and Gams) has been described as mutualistic, because endophyte-infected plants often have superior tolerance of environmental stress compared to uninfected plants. A plant feature frequently related to endophyte-infection is improved drought tolerance; however, drought tolerance mechanisms are poorly defined. Genetically identical endophyte-infected and uninfected plant materials were subjected to water stress in a series of controlled-environment studies designed to investigate drought tolerance mechanisms of tall fescue, including root growth, stomatal conductance, osmotic adjustment, and carbohydrate accumulation. Root growth of endophyte-infected plants was greater than uninfected plants when water was limited, but was not affected under well-watered conditions. Stomatal conductance was greater in infected plants of genotype CB1 under intermediate water stress, resulting in increased photosynthetic rates compared to uninfected plants; however, neither photosynthesis nor stomatal conductance was affected in genotype DN7 at intermediate stress. Increased osmotic adjustment of immature and mature leaf blades was observed in infected plants of genotype CB1, but osmotic adjustment of genotype DN7 was not affected by endophyte. Increased carbohydrate accumulation in infected plants of genotype CB1 suggested that greater soluble carbohydrate concentrations in infected plants was the basis of osmotic adjustment. However, increased soluble carbohydrate could not account completely for increased osmotic adjustment. The soluble carbohydrate pool of tall fescue was composed of simple carbohydrates, sugar alcohols, and fructans. The sugar alcohols mannitol and arabitol were exclusive to endophyte-infected plant tissues, but the amounts were insignificant relative to total plant osmotic potential. The largest carbohydrate variation existed in the fructan pool, as infected plants of one genotype had significant increases in fructan compared to uninfected plants at all water stress levels. Fructan concentrations decreased under increasing water stress and simple sugars increased in all genotypes, suggesting that these carbohydrates are depolymerized to simple carbohydrates which provide a more effective osmoticum. Therefore, infected plants with increased fructan released larger amounts of simple carbohydrates as stress increased. Increased carbohydrates in endophyte-infected plants represents a capacity for increased root growth and osmotic adjustment and, subsequently, greater drought tolerance. |
