13C Discrimination and Water Use Efficiency of Perennial Ryegrass Genotypes in Response to Wilt-Based Irrigation.

Turfgrass performance under moisture limiting conditions can be improved by selecting for low 13C isotope discrimination (CID). Wilt-based (WB) irrigation is a common practice in scheduling irrigation to turfgrass as an alternative to well-watered (WW). Low CID has been associated with higher water use efficiency (WUE) but this has not been investigated in response to WB irrigation. The objective of this study was to compare six perennial ryegrass (Lolium perenne L.) genotypes in response to two irrigation schedules (WW and WB). Clipping yield (dry matter accumulated above a 5-cm base mowing height), evapotranspiration (ET), WUE (clipping yield-to-ET ratio), CID (derived from leaf clippings) and ci/ca ratio (internal leaf CO2-to-ambient CO2 concentration) derived from CID values were measured from greenhouse samples grown in weighing lysimeters. Plant measurements in 2007 and 2008 were based on sampling conducted at the last cycle after 68 d of irrigation with 100% of ET applied at leaf-roll (WB) versus ET replacement every 4 d for 68 d (WW). Differences due to irrigation (WW and WB) were observed in both years for CID (and ci/ca ratio) with lower CID values observed under WB. Lower CID (ci/ca ratio) values suggest greater stomatal closure (water stress) and higher WUE. No difference among treatments was detected in either year with ET and no relationship between WUE with CID was observed, most likely due to violations in the assumptions of CID theory. Most of the genetic variation in WUE (85%) in 2007 and 2008 was due to yield (r=0.92, p=0.01). Genetic variation in 2007 for CID (ci/ca ratio) under WB was correlated with wilting tendency (r=-0.91, p=0.01) but no relationship was observed in 2008. Overall, CID (and ci/ca ratio) values were the only measurement that exhibited significant response to irrigation (WW and WB) in both years of study. CID integrates ci/ca over the life of the tissue and the low CV (1%) in this study suggests greater sensitivity to water stress than other measured parameters.