High temperature and drought stresses may reduce quality in cool-season turfgrasses during summer months in the transition zone. This growth chamber study was conducted to evaluate effects of high temperature and drought on physiology and growth of 'Apollo' Kentucky bluegrass (Poa pratensis L.) (KBG), 'Dynasty' tall fescue (Festuca arundincea Schreb.) (TF), and 'Thermal Blue', a hybrid (HBG) between KBG and Texas bluegrass (Poa arachnifera Torr.). Turfgrasses were exposed for 48 days to supra-optimal (high temperature; 35/25 o C, 14-h day/10-h night) and optimal (control; 22/15 o C, 14-h day/10-h night) temperatures under well-watered (100% evapotranspiration [ET] replacement) and deficit (60% ET replacement) irrigation. Heat resistance was greater in HBG, which had greater visual quality, gross photosynthesis (Pg), dry matter production, and lower electrolyte leakage and soil-surface temperatures than KBG and TF under high temperature. Cumulative Pg during the study was 16% and 24% greater in HBG than in KBG and TF, respectively. Green leaf area index (LAI) in HBG was not affected by high temperature, but LAI was reduced by 29 % in KBG and 38% in TF. Differences in drought resistance were negligible among species. The combination of high temperature and drought caused rapid declines in visual quality and dry matter production, but HBG generally performed better. Results indicated greater heat resistance, but not drought resistance, in HBG than in KBG or TF.1 High temperature and drought stresses are significant problems in cool-season turfgrasses during summer months in the U.S. transition zone, which covers 480 to 1120 km north to south between the northern regions where cool-season grasses are adapted and the southern regions where warm-season grasses are adapted (Dunn and Diesburg, 2004). High temperature and drought stresses often occur simultaneously during summer months and may limit growth and cause a severe decline in the visual quality of cool-season turfgrasses (Perdomo et al., 1996;Bonos and Murphy, 1999;Jiang and Huang, 2000;Wang and Huang, 2004). Recent increases in competition for water have resulted in restrictions in water use for irrigation of turfgrasses (EIFG, 2004), which further exacerbates the problem of drought stress in cool-season turfgrasses. Predictions of higher temperatures from global warming also suggest that heat stress in cool-season turfgrasses may become more common in some regions, including the transition zone (National Assessment Synthesis Team, 2000).Hybrid bluegrasses (HBG), which are genetic crosses between native Texas bluegrass and KBG, may have greater heat and drought resistance than other cool-season grasses. Hybrid bluegrasses have similar visual qualities to KBG, which is a fine-textured, cool-season turfgrass commonly used in lawns and golf courses in the U.S. (Read et al., 1999;Turgeon, 2002).Consequently, new cultivars of HBG are being investigated as potential water-saving, heatresistant alternatives to current cool-season turfgrasses. Abraham et al. (...

Turfgrass A gronomy J our n al • Volu me 10 0 , I s sue 4 • 2 0 0 8 949 Published in Agron. J. 100:949-956 (2008).

Normalized difference vegetation index (NDVI, computed as [near infrared (NIR) -Red)]/[NIR +Red]) may provide an objective means to evaluate visual quality of turfgrass. The NDVI is influenced by red (visible) and NIR reflectance (invisible), but each may respond differently to environmental factors; basic information is lacking about the two components in relation to turf quality. In this 3-yr study near Manhattan, KS, we examined relationships of NDVI and its component reflectances along with visual quality ratings in Kentucky bluegrass {Poa pratensis L., 'Apollo'), two Kentucky bluegrass x Texas bluegrass {Poa arachnifera Torr.) hybrids (Thermal Blue' and 'Reveille'), and tall fescue {Festuca arundinacea Schreb., 'Dynasty'). Percentage green cover was measured with digital image analysis and shoot density was estimated visually to evaluate their impacts on turf quality and reflectance. Differences in NDVI and red and NIR reflectances were observed among turfgrasses at each level of quality. Across the range of turf quality, NDVI was influenced more strongly by red than NIR reflectance. Red reflectance was strongly affected by density (r = 0.85) and green cover (r = 0.86); NIR reflectance was affected by density (r = 0.63) but negligibly by green cover. Results suggest other fundamental factors that are poorly understood may be affecting NIR reflectance and, hence, NDVI in turf. These factors may confound relationships between NDVI and turf quality and require further study.

Canopy spectral reflectance may provide an objective means to evaluate visual quality of turfgrass, but evaluations of quality may be confounded by cultural practices that affect reflectance, such as mowing height. In this 2‐yr study near Manhattan, KS, we examined effects of mowing height on relationships between normalized difference vegetation index (NDVI) and visual quality ratings in Kentucky bluegrass (KBG; Poa pratensis L., ‘Apollo’) and in a KBG × Texas bluegrass (Poa arachnifera Torr.) hybrid (HBG; ‘Thermal Blue’). Mowing heights were 7.62 cm (high) and 3.81 cm (low). The NDVI averaged 4.5 to 7% greater in high‐ than in low‐mown plots. Distinct regression models of visual quality were found at each mowing height and in each species (r2 from 0.40 to 0.81); separate relationships between NDVI and visual quality were also found between years in the same plots. Correlations between NDVI and visual quality were stronger at high than at low mowing heights, possibly because of greater green biomass at high mowing heights. The 95% confidence intervals surrounding predictions of visual quality from NDVI ranged from ±1.34 to 2.75 (on a 1‐to‐9 scale). Thus, lack of precision is a concern when using these models for detection of differences between treatments. Results indicate that when using NDVI to evaluate turfgrass quality, evaluations should be limited to plots maintained at the same mowing height and with the same species to reduce variability in NDVI.

Canopy spectrai reflectance may provide an objective means to evaluate visual quality of turfgrass, but evaluations of visual quality may be confounded by differences in reflectance among species or cultivars. In this 3-yr study near Manhattan, KS, we examined effects of species and cultivars on relationships between normalized difference vegetation index (NDVI) and visual quality ratings in Kentucky bluegrass {Poa pratensis L., 'Apollo'), two Kentucky bluegrass x Texas biuegrass {Poa arachnifera Ton.) hybrids (Thermal Blue' and 'Reveille'), and tall fescue {Festuca arundinacea Schreb., 'Dynasty'). A broad range of visual quality was imposed on all four grasses through deficit irrigation and NDVI was measured using broadband spectral radiometry across this range for each grass. Distinct linear regression models of visual quality were found for each grass, and models were also distinct among years in each grass. Relationships between NDVI and visual quality were stronger in the bluegrasses (r^ = 0.41 to 0.83) because they had a greater range in quality under deficit irrigation than tall fescue. The 95% confidence intervals surrounding predictions of visual quality from NDVI ranged from ± 1.25 to 2.10 (on a 1 to 9 scale). Results indicated that the requirement to develop separate models for each grass and in each year, combined with relatively wide confidence intervals, represents a practical limitation to predicting visual quality with NDVI.