Fil: Gundel, Pedro E. MTT Agrifood Research Finland. Department of Plant Production; Finland.Fil: Garibaldi, Lucas Alejandro. Universidad Nacional de Río Negro. Sede Andina; Argentina.Fil: Helander, Marjo. University of Turku. Department of Biology. Section of Ecology; Finland.Fil: Saikkonen, Kari. MTT Agrifood Research Finland. Department of Plant Production; Finland.Fil: Gundel, Pedro E. Instituto de Investigaciones Fisiológicas y Ecológicas vinculadas a la Agricultura (IFEVA)-CONICET; Argentina.Fil: Garibaldi, Lucas Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinatrueStudying the controls on biomass allocation trade-offs in plants are important since they affect harvestable product yields and are critical to understanding symbiotic interactions. Epichloae fungal endophytes associate with cool-season grasses, growing systemically within the plant inter-cellular spaces and are transmitted through seeds. We explore the endophytes influence on the relationship between the plant reproductive and vegetative aboveground biomass (reproductive effort: RE) and on the trade-off between two components of the reproductive biomass, number and weight of panicles (RPN), using tall fescue as a model system. Naturally endophyte-colonized, manipulatively endophyte-free, and naturally endophyte-free plants from Northern European wild-populations together with the cultivar Kentucky-31 were grown under different environmental conditions (nutrients x water). The endophyte had an effect on the RPN (E+: 6.19, ME-: 4.68 and E-: 4.40) which indicates how reproductive biomass is partitioned into number and mass of panicles, but not on RE (≈0.06). As expected, wild plants showed higher reproductive effort (≈0.06) compared to the cultivar KY-31 (0.05), irrespective of endophyte presence. Endophyte-colonized plants had lighter panicles than endophyte-free plants, a pattern that was clear among low-yielding plants. Similarly, the trade-off between RPN and RE was higher for endophyte-colonized plants. This was again evident among plants with low RE indicating that colonized plants split the yield into either greater number of panicles and/or lighter panicles. The effect of vertically transmitted endophytes has earlier been studied as ratios (e.g. RE); however, our study shows that this approach may hide size-dependent endophyte effects on these relationships. Our study reveals that Neotyphodium endophyte affects trade-offs in tall fescue plants in a complex manner, and is influenced by a number of biological and abiotic factors.