Turf to a Degree

As most turf managers know, warm-season turfgrasses grow best at temperatures between 80 degrees Fahrenheit and 95 degrees F, whereas cool-season grasses are better adapted to cooler temperatures ranging from 60 degrees F to 75 degrees F (Beard, 1973). High temperature stress has therefore long been considered the primary factor limiting the use of cool-season grasses in transition and warm climatic regions.

One confounding factor that's often 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. VPD represents the mathematical difference between the amount of water vapor actually present and the maximal amount that could exist without condensation at the same temperature. Since the capacity of the atmosphere to hold water increases greatly with temperature, the actual amount of water present at a specific relative humidity varies with changes in temperature. Therefore, VPD provides a very direct indication of the atmospheric moisture conditions independent of temperature (Anderson, 1936).

An experimental chamber system is used for imposing vapor pressure deficit treatments on turfgrass.

Atmospheric evaporative demand and plant transpiration increase with increasing atmospheric VPD (Sinclair and Bennett, 1998). However, stomatal conductance begins to decrease between VPDs of 1.0 and 2.5 kPa (kilopascals) (Bunce, 1981) in some plant species. A limited maximum transpiration rate that is reached when atmospheric VPD exceeds about 2 kPa has been associated with afternoon depression of photosynthesis (Hirasawa and Hsiao, 1999), restriction of evaporative cooling of leaves and increased leaf temperatures (Isoda and Wang, 2007) in many plant species.

In one of the few studies examining turfgrass sensitivity to changes in VPD, Sinclair et al. (2007) discovered that growth of tall fescue (a cool-season turfgrass), while expected to decline with rising temperatures over the range of 65 degrees to 81 degrees, actually increased markedly with increasing temperature so long as VPD was held constant. In contrast, growth declined in experiments where tall fescue was exposed to increasing VPD and temperature was held constant at 70 degrees. The authors concluded that decline of cool-season grasses at elevated temperatures may involve responses to high VPD.

The objective of this current study was to observe the transpiration response of some cool-season and warm-season turfgrasses to increasing VPD under stable temperature. We hypothesized that limitations on gas exchange at high VPD may contribute to the poor performance of cool-season turfgrasses under elevated temperatures.

An experimental system was developed (Fletcher et al., 2007), and turfgrass water loss was measured gravimetrically over 90-minute periods at VPD's ranging from 0 to 3 kPa. The system allowed for measurement of 12 pots at one time so that water loss from four grass species could be measured on three replicate pots. Pots of established turfgrass (81 cm2 surface area) were maintained in the greenhouse under optimal conditions and fully watered prior to each experiment.

To impose VPD treatments, a small chamber system was attached to each of the pots at the time of measuremen. High humidity was achieved by flowing air into chambers at 5 liters per minute using two pumps with their intake tubes inside an atomizing humidifier. Medium humidity was achieved by flowing ambient greenhouse air into chambers at a rate of 25 liters per minute from two compressors. Lower humidity treatments were obtained by flowing air into chambers at a rate of 10 to 30 liters per minute from two compressors through PVC tubes of 32-millimeter diameter filled with six-mesh Drierite desiccant. Each chamber was equipped with a 12-volt, 76-millimeter diameter computer box fan to mix air in chamber.

Relative humidity and temperature inside the chambers was recorded three times over each 90-minute experiment using a pocket humidity/temperature pen mounted through the sidewall of each container. At each setting, the humidity levels varied considerably among chambers, even within species, so actual measured humidity in each chamber was used to calculate VPD and means were not calculated for the replicates.

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