Lowering soil temperatures improves creeping bentgrass growth under heat stress

High soil temperature is a major factor limiting growth of cool-season grasses, The objectives of this study with creeping bentgrass (Agrostis palustris Huds.) were to examine growth responses to lowering soil temperatures from the supraoptimal level under high air temperature conditions, and to determine the minimum reduction in soil temperature that could improve shoot and root growth. Two creeping bentgrass cultivars, L-93 and Penncross, were exposed to the following air and soil temperature regimes in growth chambers and water baths: (i) optimal air and soil temperatures (20/20degreesC, control); (ii) lowering soil temperature by 3, 6, and 11degreesC from 35degreesC at high air temperatures (35132, 35/29, 35/24degreesC); and (iii) high air and soil temperatures (35/35degreesC). Soil temperature was reduced from 35degreesC by circulating cool water (18degreesC) in water baths at variable rates. Turf quality, leaf chlorophyll content, shoot growth rate, and root/shoot ratio (R/S) ratio increased as soil temperature was reduced from 35 to 32degreesC to a greater extent for Penncross than for L-93. Significant increases in tiller density, clipping yield, root number, and fresh weight were not observed until soil temperature was reduced to 29degreesC. When soil temperature was reduced to 24degreesC, turf quality, shoot growth rate, and R/S were maintained at the same levels as in the control regime. These results suggested that reducing soil temperature by 3degreesC or more is effective in improving turf quality and shoot and root growth of creeping bentgrass exposed to high air temperatures.