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CROP BREEDING, GENETICS & CYTOLOGY

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

^a USDA-ARS, Box 646402, Washington State Univ., Pullman, WA 99164 USA
^b Institute of Soil and Water Conservation, Academia Sinica, Yangling, Shaanxi 712100, China

rcjohnson{at}wsu.edu

Carbon isotope discrimination () has been correlated with the ratio of dry matter production to transpiration (water-use efficiency, WUE) in C₃ plants and is potentially useful for breeding crops with improved WUE. Therefore, an assessment of the selection response of and its relationship to plant water status and forage production is needed. Divergent selection for high (low WUE expected) and low (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 were evaluated in C_o and the selected populations in irrigated and dryland field environments in 1995 and 1996. Average realized heritability for was 0.49, suggesting that could be successfully manipulated in a breeding program. In 1995, leaf pressure potential (turgor) was higher in the populations selected for low , but in 1996, no differences in water relations measurements were observed. High- populations always had lower forage production than observed in C_o, but the low- populations never produced more than the C_o population. In greenhouse-grown plants, high- populations had higher internal substomatal [CO₂] than C_o, linking with mechanisms that cause lower WUE. However, the internal [CO₂] of the low- populations and C_o did not differ, suggesting that selection for low may not have increased WUE as expected. The results show that is a heritable trait in tall fescue, but an absence of increased production in populations selected for low may limit its utility in tall fescue breeding programs.

Abbreviations: A_a, photosynthetic CO₂ exchange rate per leaf area • A_d, photosynthetic CO₂ exchange rate per g leaf • g_s, stomatal conductance to CO₂ • PEG, polyethylene glycol • SLA, specific leaf area • WUE, water-use efficiency • , carbon isotope discrimination

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