Transpiration Response of Warm-Season Turfgrasses to Drying Soil.

Water for irrigating recreational and home lawns in the southern U.S. is in increasing demand.  Understanding the physiological responses of warm season grasses to drying soil may aid in the selection of drought resistant material and provides a starting point for developing deficit irrigation protocols.  The objectives of this study was to determine the point at which the transpiration rate of ‘Argentine’ bahiagrass, ‘Floratam’ St. Augustinegrass, and ‘Empire’ zoysiagrass subjected to gradual soil drying deviated from that of well-watered plants and to evaluate turf quality as the drought stress developed.  Sod was established in pots under adequately watered greenhouse conditions until their roots fully-explored the pots.  Well-watered and gradually drying replicates of each grass were weighed daily as drought progressed to monitor water loss.  A normalized transpiration ratio (NTR) comparing the water loss of each drying plant to the average of well-watered plants of the same grass was calculated.  The daily fraction of transpirable soil water (FTSW) remaining in each drying pot was calculated, and NTR was plotted against FTSW for each pot.  The NTR deviated from one at 0.13 FTSW for ‘Argentine’ and ‘Floratam,’ and at 0.12 FTSW for ‘Empire.’  At the point when FTSW was zero, ‘Argentine’ and ‘Floratam’ were 70% fired, and ‘Empire’ was 50% fired.  After two additional days without water, ‘Argentine’ was 90% fired, ‘Floratam’ was 100% fired, and ‘Empire’ was 100% rolled but not fired.  Warm season turfgrasses growing in sandy soil can withstand severe drought conditions before stomatal closure occurs, and turf firing does not occur until after transpirable soil water has been fully-exhausted.  This initial data indicates that these grasses may be able to endure longer irrigation intervals than previously considered without permanent canopy damage, and that further work in the field to find these deficit limits is warranted.