Differential Response of Warm and Cool Season Turfgrass Species to Increasing Atmospheric Vapor Pressure Deficit.

Elevated temperature is often observed to be deleterious to C3 grasses.  One confounding factor that is commonly overlooked is that vapor pressure deficit (VPD) of the atmosphere almost certainly has increased in an exponential fashion as a result of the elevated temperature.  To resolve the possible impact of VPD on grasses, a system was developed specially to subject plants to various atmospheric VPD while holding temperature relatively stable (~28-32 °C).  The water loss rate of eight turfgrass species was measured during 90-min exposure periods at VPDs ranging from ~1 to 3 kPa.  Major differences were observed between C3 and C4 species in the response of water loss rates to VPD.  In C3 grasses ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.), perennial ryegrass (Lolium perenne L.), annual bluegrass (Poa annua L.), and ‘Penncross’ creeping bentgrass (Agrostis palustris Huds.), water loss appears to be constrained to a maximum value (~1 to 1.3 mg s^-1) at VPDs above ~1.5 kPa.  Conversely, C4 grasses ‘Argentine’ bahiagrass (Paspalum notatum Flugg.), ‘Tifway’ bermudagrass [Cynodon transvaalensis Burtt-Davy x C. dactylon (L.) Pers.], ‘Empire’ zoysiagrass (Zoysia japonica Steud.), and ‘Floratam’ St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] all exhibit a linear increase in water loss rate over the entire range of VPD, and only begin to approach water loss rates of ~0.7 mg s^-1 at 3 kPa.  These results have profound implications for cool-season grasses in particular.  The poor performance of these cool-season grasses under elevated temperature may, in fact, not be solely a response to atmospheric temperature.  Rather, a limited ability to maintain gas exchange at high VPD, which is associated with high temperature, would result in a substantial loss in photosynthetic activity and growth potential, which could negatively affect the survival of the cool-season grasses.  Sensitivity to high VPD may be a critical trait in understanding and improving cool-season grass performance under elevated temperature.