Physiological Responses to Drought of Turfgrass Species Under Field Conditions

Drought stress during summer is common in the Intermountain West but turfgrasses differ in how they respond to this stress. Poa pratensis L. (Kentucky bluegrass) quickly enters summer dormancy while tall fescue [Schedonorus phoenix (Scop.) Holub] remains green far longer. In order to understand the different drought tolerance or avoidance mechanisms, our objective was to study the physiological responses of KBG and TF under a prolonged drought.Two irrigation treatments were applied in 2007 and 2008 to plots of ‘Newport’ and ‘Midnight’ Kentucky bluegrass and ‘Gazelle’ tall fescue in North Logan, UT. Each plot was 2 x 1.5 m in size. Irrigation treatments were (1)well-watered—irrigated 3 times a week with 1.4 cm depth  of water—and (2) plots that did not receive irrigation. Measurements were made twice each week in 2007 and once each week in 2008 to document the plants’ response to the irrigation treatments. Stomatal conductance and surface temperature were measured between 11 a.m. and 2 p.m. Predawn xylem water potential was measured with a pressure chamber. Soil water content at 10 cm intervals to 100 cm was measured everyday in 2008.Kentucky bluegrass decreased in stomatal conductance and xylem water potential faster and greater than TF in response to dry conditions and was in complete summer dormancy within 5 weeks in both years. In contrast, most of TF leaves were green throughout the drought periods in both years. This may be explained by the ability of TF to consume water from the deepest soil profiles in the non-watered plots (80 - 100 cm depth) while KBG used most of the water to the 50 – 70 cm depths. Stomatal conductance of KBG decreased sharply when it did not receive water for 3 weeks while xylem water potential gradually dropped until 5 weeks in year 2007. However in 2008, the water potential was not significantly different from the well-watered treatment until the sixth week when values immediately dropped in KBG from -0.5 MPa to -2.5 MPa and in TF from -0.3 MPa to -1.0 MPa. In late summer when plots were watered for recovery, KBG plots were mostly green within 3 weeks in both years. The surface temperature of grasses showed the same trend in both years where the well-water plots were cooler than the non-irrigated plots and TF showed lower temperature than KBG in both well-watered and no-watered plots.