- Temperature and vapor pressure deficit (VPD) effects on turfgrass growth are almost always confounded in experiments because VPD commonly is substantially increased in elevated-temperature treatments. The objective of this study as to examine specifically the influence of VPD on transpiration response of four 'warm-season' (C4) and four 'cool-season' (C3) turfgrasses to increasing VPD at a stable temperature (29.3 1.5 C). Although transpiration rates were noticeably lower in C4 grasses, transpiration rates increased linearly in response to increasing VPD across the range of 0.8-3.0 kPa. In contrast, transpiration rates of C3 increased sharply with increasing VPD across the range of low VPDs, but became constrained at higher VPDs (>1.35 kPa). Restricted transpiration rate at elevated VPD was most evident in Agrostis palustris and Lolium perenne. Assuming restricted transpiration rates reflect a limitation on leaf CO2 uptake, these results indicate that the commonly observed decline in growth of C3 (and success of C4) grasses at elevated temperature may include a sensitivity to elevated VPD. 2009 Elsevier B.V. All rights reserved.