Carbon isotope discrimination characteristics oas an index of tall fescue–endophyte association response to light availability and defoliation

Authors: Belesky, David; Ruckle, Joyce; Halvorson, Jonathan

Submitted to: Environmental and Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2009
Publication Date: 3/15/2010
Citation: Belesky, D.P., Ruckle, J.M., Halvorson, J.J., Williams, D.G. 2010. Carbon isotope discrimination characteristics oas an index of tall fescue–endophyte association response to light availability and defoliation. Environmental and Experimental Botany. 67(3):515-521.

Interpretive Summary: Tall fescue is a perennial grass that grows on more than 30 million acres in the USA. The widespread distribution and use of tall fescue seems to occur in part because of a fungus that inhabits the plant enabling it to tolerate or avoid stresses associated with environment and management. Understanding how plants adapt to environmental stresses requires numerous, and time-consuming measurements made at fixed times during a growing season. We used a stable carbon isotope discrimination technique that measured differences between carbon dioxide in the atmosphere and in the plant leaf to define how the plant responded to long-term growing conditions. The carbon isotope discrimination technique showed the presence of the fungus was not detrimental to tall fescue, no distinction between a native and a modified strain that did not produce compounds harmful to grazing livestock. Patterns of carbon isotopes were influenced by the amount of light reaching the tall fescue plants. These patterns corresponded to those for dry weight and nutritive value of leaves. The technique offers a rapid way to gain some understanding of patterns of plant growth and composition in different growing conditions. Carbon isotope patterns might be a useful tool for identifying plants and plant management practices that sustain growth and persistence in a range of growing conditions.

Technical Abstract: Tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] is adapted to diverse growing conditions occurring in humid, temperate agro-ecosystems. A mutualistic endophytic symbiont (Neotyphodium coenophialum, [Morgan-Jones et Gams] Glenn, Bacon, and Hanlin) contributes to this resilience, but at the same time compromises grazing livestock health. Novel endophytes that do not produce ergo-alkaloids were developed to circumvent livestock health issues, but the response of novel associations to common microsite conditions and management practices, such as partial shade and repeated defoliation, is unclear. We conducted a field experiment to determine if carbon isotope discrimination could be used to characterize host-endophyte association responses to microsite and management. Plants infected with either the native or novel endophyte, or that were endophyte free were clipped to residual sward heights typical of pasture (5 cm) or silvopasture (10 cm) each time mean canopy height reached 20 cm. The carbon isotope discrimination was inversely related to light availability, used to define a microsite, with greater variation in carbon isotope discrimination occurring when plants were clipped to 5 rather than a 10-cm residue height. The trend in carbon isotope discrimination was similar whether expressed relative to potential evapotranspiration (ETo), leaf dry matter content (LDMC), or an index of herbage nutritive value (expressed as energy relative to crude protein). Host-endophyte association did not affect carbon isotope discrimination. This might be attributable to the relatively benign growing conditions occurring in the central Appalachian region of the eastern USA. The lack of response in carbon isotope discrimination might also reflect the minimal cost to the host of harboring endophyte when resources are plentiful. Carbon isotope discrimination might be useful to help identify shade tolerant plants and management practices for silvopasture applications.