Carbon Isitope Discrimination, Water Relations, and Photosynthesis in Tall Fescue

Abstract

Carbon isotope discrimination (Δ) shows promise for estimating water‐use efficiency (WUE) in crop species. Research was undertaken to examine Δ and water relations of tall rescue (Festuca arundinacea Schreb.) accessions and to determine if low Δ is associated with high gas exchange WUE as predicted by theory. The accession PI 438522 had lower Δ than the accession PI 231522 in both a dryland and an irrigated field environment and at two sampling dates. Correlations among Δ, water potential, solute potential, and turgor pressure were not significant under irrigation. Under dryland conditions, correla. tions among these factors were generally significant and positive for PI 438522, but not significant for PI 231561. In a greenhouse experiment, plants selected for low Δ from the irrigated field environment had lower A and internal leaf [COz] (Ci), and a higher ratio of CO2 assimilation rate (A) to transpiration than high‐A selections. This was true for both well‐watered plants and plants stressed with a ‐0.38 MPa polyethylene glycol solution. Stomatal conductance and A were positively correlated with Δ, suggesting that lower stomatal conductance was a factor leading to lower C1 and Δ. The results suggest that associations between water relations and Δ vary depending on accession and environment, but selecting tall rescue plants for low Δ does identify genotypes with low C1 and high WUE. Using Δ for germplasm enhancement of WUE in tall rescue appears promising.