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Benjamin Wherley and Thomas R. Sinclair

Resolution of the effects of trinexapac-ethyl and nitrogen (N) application rate on evapotranspiration of low-cut, prostrate turfgrass species such as creeping bentgrass (Agrostis stolonifera L.) and hybrid bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt-Davy] has not been explored. This study sought to examine the integrated growth and evapotranspiration responses of these two turfgrass species to two N rates and application of trinexapac-ethyl, a commonly used growth-regulating compound for suppressing turfgrass shoot growth. ‘Penncross’ creeping bentgrass and ‘Tifdwarf’ bermudagrass were studied. We hypothesized that application of trinexapac-ethyl and/or lower N application rates might result in lower rates of turfgrass evapotranspiration. Two greenhouse studies were conducted over 6-week periods in 2008. A completely randomized design was used to support a two species × two N rate × two trinexapac-ethyl rate factorial. Shoot and root growth and evapotranspiration were determined from the two grass species when maintained in well-watered conditions and grown in pots. The treatments were either a high N (1.2 g N/m2/week) or a low N (0.3 g N/m2/week) application and either 0 or 28 mg·m−2 a.i. trinexapac-ethyl. Application of trinexapac-ethyl and use of the low N rate each significantly reduced shoot growth in both species; however, neither factor caused a reduction in turfgrass water use. The high N rate promoted increased root growth in both species during the spring, but there was no effect of either factor on rooting during the summer study. These results indicate that although decreasing N application rates and applying trinexapac-ethyl effectively suppress shoot growth, neither is likely to have any impact on overall water requirements for these species when maintained in well-watered conditions.

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Thomas R. Sinclair, Andrew Schreffler, Benjamin Wherley and Michael D. Dukes

Although root development is critical in the establishment of turfgrass sod, there appears to be no information on the response of root development during sod establishment to the frequency and amount of irrigation. Two alternate hypotheses for the root development response are that 1) frequent and high amounts of irrigation are needed to support sod growth and root development; and 2) deficit irrigation encourages more rapid and deeper rooting. The objective of this study was to observe root development of four warm-season turfgrasses subjected to various frequencies and amounts of irrigation. Root extension of the grasses was observed directly in soil contained in 90-cm tall, clear acrylic columns. No difference in root development was observed for any of the grasses among irrigation frequency treatments of daily, twice weekly, and once weekly. There were differences in response to the amount of irrigation. Zoysiagrass root development was maximal at the full amount of irrigation (35 mm per week). On the other hand, St. Augustinegrass, bermudagrass, and bahiagrass required deficit irrigation of only 13 mm water per week to achieve full root development. The results of this study showed that each of the two hypotheses were appropriate depending on the specific species.

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Georgene L. Johnson, Thomas R. Sinclair and Kevin Kenworthy

A consistent response has been observed among many plant species in their leaf gas exchange as soils are subjected to a drying cycle; except for one case, these studies have not included turfgrass species. The current study reports the change in transpiration rate of six genotypes of seashore paspalum (Paspalum vaginatum Swartz) during soil drying when grown on either an organic soil or sand. On organic soil, the response was consistent with results with other plant species in that there was no decline in transpiration rate until the fraction of transpirable soil water (FTSW) reached the range of 0.25 to 0.31. The decline in transpiration rate when plants were grown on sand occurred in the FTSW range of 0.10 to 0.17, which was also consistent with reports for other species when grown on sand. The lower FTSW for the decline in transpiration rate on sand appears to be a result of the greater retention of water in fully wetted sand in pot experiments as compared with field conditions. Because the decline in transpiration occurred at a higher FTSW in ‘SeaIsle Supreme’, ‘Aloha’, and ‘SeaIsle 1’ than in ‘SeaIsle 2000’, this is an indication that these genotypes are water-conserving and may be better suited to water-deficit conditions. Quality changes in these grasses were monitored daily during the drying cycle by reflectance measurements of their normalized difference vegetation index (NDVI). No change in NDVI was observed for grasses grown on either soil type until the soil had become very dry and transpiration had decreased to low rates.

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Sarah E. Cathey, Jason K. Kruse, Thomas R. Sinclair and Michael D. Dukes

Water management and turfgrass breeding efforts focused on water conservation can benefit from a better understanding of drought stress physiology because it relates to visual quality. In a repeated study under controlled conditions, ‘Argentine’ bahiagrass (Paspalum notatum Flugge), ‘Floratam’ st. augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze], and ‘Empire’ zoysiagrass (Zoysia japonica Steud.) were subjected to drought stress as defined by the normalized transpiration ratio (NTR) of drying to well-watered plants. Differences in total water extracted from the soil as the soil dried to stomatal closure were not different among grasses; however, zoysiagrass had the slowest water use rate and less firing under increasing drought stress than the other grasses. Optical sensing of the normalized difference vegetation index from the turf canopies was not an effective predictor of drought stress for either study. In both studies, severe wilting and some firing occurred in bahiagrass and st. augustinegrass when NTR was 0.3. Zoysiagrass was not severely wilted until 0.1 NTR and exhibited little firing even after drying had continued for an additional 7 days past 0.1 NTR. After 7 days at well-watered status after drought stress to a severity of 0.1 NTR, all grasses were able to recover to an acceptable visual quality rating. This recovery from severe wilt and some canopy firing (except for zoysiagrass), indicating that a return to well-watered soil after severe stress, can result in acceptable turf recovery.

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Carlene A. Chase, Thomas R. Sinclair, Daniel O. Chellemi, Stephen M. Olson, James P. Gilreath and Salvadore J. Locascio

Although summer soil solarization is a viable technique for the control of soilborne pests, periods of cloudy skies and high rainfall typical of the southeastern United States limit the heating of soils. This study was devised to evaluate whether polyethylene films designed for improved heat conservation could increase soil solarization temperatures under humid environmental conditions. Soil solarization was conducted in Summer 1996 at Quincy, Gainesville, and Bradenton: located in north, north central, and west central Florida, respectively. Temperatures at soil depths of 5, 10, and 25 cm were higher under clear solarization films than under black polyethylene. A clear, thermal-infrared absorbing film (TIR) was consistently more effective in increasing soil temperature than was a double-layered, clear bubble film or a 30 μm clear, low-density polyethylene film. Soil temperatures under all film types were reduced by rainfall, but remained highest under TIR film. On cloudy days with light rainfall, temperatures under TIR film exceeded 45 °C at 5 cm depth. Soil temperatures rose rapidly when rainy weather was followed by a clear day. Cumulative exposure to temperatures ≥45 and 50 °C was greater with the TIR film than with the other films, indicating that it has the greatest potential for soil solarization in humid climates.