Water Relations, Forage Production, and Photosynthesis in Tall Fescue Divergently Selected for Carbon Isotope Discrimination

Carbon isotope discrimination (Δ) has been correlated with the ratio of dry matter production to transpiration (water‐use efficiency, WUE) in C 3 plants and is potentially useful for breeding crops with improved WUE. Therefore, an assessment of the selection response of Δ and its relationship to plant water status and forage production is needed. Divergent selection for high Δ (low WUE expected) and low Δ (high WUE expected) was completed for two cycles from a ‘Kentucky 31’ tall fescue base ( Festuca arundinacea Schreb.) population (C o ). Water relations, forage production, and Δ were evaluated in C o and the selected populations in irrigated and dryland field environments in 1995 and 1996. Average realized heritability for Δ was 0.49, suggesting that Δ could be successfully manipulated in a breeding program. In 1995, leaf pressure potential (turgor) was higher in the populations selected for low Δ, but in 1996, no differences in water relations measurements were observed. High‐Δ populations always had lower forage production than observed in C o , but the low‐Δ populations never produced more than the C o population. In greenhouse‐grown plants, high‐Δ populations had higher internal substomatal [CO 2 ] than C o , linking Δ with mechanisms that cause lower WUE. However, the internal [CO 2 ] of the low‐Δ populations and C o did not differ, suggesting that selection for low Δ may not have increased WUE as expected. The results show that Δ is a heritable trait in tall fescue, but an absence of increased production in populations selected for low Δ may limit its utility in tall fescue breeding programs.