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Pedro Perdomo, James A. Murphy, and Gerald A. Berkowitz

Understanding the factors influencing the performance of Kentucky bluegrass (Poa pratensis L.) cultivars under summer stress is necessary for developing criteria for identifying resistant germplasm. The objectives of this study were to evaluate two Kentucky bluegrass cultivars for leaf water (ψl) and osmotic potential (ψπ), stomatal resistance (Rs), leaf: air temperature differential (ΔT) and determine the relationship of these parameters to drought and heat tolerance. Stress-resistant (`Midnight') and susceptible (`Nugget') cultivars were evaluated in a field study during 1993 and 1994 under moisture-limiting conditions. Leaf water potential for `Nugget' was higher than for `Midnight' in 1993 and similar in 1994. `Midnight' had lower ψπ than `Nugget' during the evaluation period in 1994. `Midnight' maintained more open stomata (lower Rs) and lower ΔT than `Nugget' at the end of the dry down period when `Nugget' was showing visual signs of stress. `Midnight' and `Nugget' had similar root weight at the 0- to 45-cm depth zone in 1994. Lower basal osmotic potential (i.e., higher solute concentration) may be the physiological mechanism allowing larger stomatal aperture in `Midnight'. Greater transpirational cooling in `Midnight' relative to `Nugget' was correlated with higher turf quality for `Midnight'.

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Thomas R. Clarke

Irrigation scheduling can be improved by directly monitoring plant water status rather than depending solely on soil water content measurements or modeled evapotranspiration estimates. Plants receiving sufficient water through their roots have cooler leaves than those that are water stressed, leading to the development of the crop water stress index, which uses hand-held infrared thermometers as tools for scheduling irrigations. However, substantial error can occur in partial canopies when a downward-pointing infrared thermometer measures leaf temperature and the temperature of exposed, hot soil. To overcome this weakness, red and near-infrared images were combined mathematically as a vegetation index, which was used to provide a crop-specific measure of vegetative cover. Coupling the vegetation index with the paired radiant surface temperature from a thermal image, a trapezoidal two-dimensional index was empirically derived capable of detecting water stress even with a low percentage of canopy cover. Images acquired with airborne sensors over subsurface drip-irrigated muskmelon (Cucumis melo L.) fields demonstrated the method's ability to detect areas with clogged emitters, insufficient irrigation rate, and system water leaks. Although the procedure needs to be automated for faster image processing, the approach is an advance in irrigation scheduling and water stress detection technology.

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Alexander X. Niemiera and Monika Goy

A study was conducted to determinethe feasibility of using crop water stress index (CWSI) to schedule irrigation of eight species of freeway landscape plants, Acacia redolens B.R. Maslin, Acacia salicina Lindl., Caesalpinia pulcherrima Sw., Cassia nemophila A. Cunn. ex Vogel, Cercidium floridum Benth., Eucalyptus microtheca F.J. Muell., Nerium oleander L., and Prosopis chilensis Mol. Nerium oleander and C. pulcherrima were suited to the use of the CWSI, tolerated repeated exposures to CWSI values of 0.6, and remained aesthetically acceptable. Irrigation of N. oleander via the CWSI resulted in a 19% reduction in water use, compared to the conventional method. CWSI data of other species were too variable, and, thus, irrigation could not be scheduled by CWSI values. Variability was attributed, in part, to lack of a dense canopy, which is necessary to fill the view of the infrared thermometer.

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D.M. Glen

This work focuses on recent developments and examples of irrigation scheduling that concern where in the root system and when in the plant's phenology water should be applied. Information is provided on using and measuring soil variability to help schedule irrigation. An irrigation model is described that emphasizes the soil water-holding capacity and root distribution in designing irrigation systems and scheduling water application. Recent research is reviewed on the subject of fruit crops that can tolerate severe water stress during specific growth periods of the fruit. Finally, a method of using infrared thermometers and canopy temperature data in cloudy, humid regions is presented that has the potential to extend the use of this technology.

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L. Lombardini and J.A. Flore

The recent development of small portable infrared thermometers has made canopy temperature an easily measured characteristc in the field. Our objective was to correlate a reduction of soil water with foliage temperature and to compare it with other indicators of plant stress (Pn, E, gs, leaf expansion, sap flow). During Summer 1998, we evaluated the responses of potted apple rootstocks (cultivars Budagowski 9, M9, and Mark) to soil water deficit. Irrigation was withheld for 7 days, and the canopy temperature (Tc) was measured daily with an infrared camera. Tc was always higher than air temperature (Ta). Tc between control and stress plants began to differentiate from day 3. In Mark, this difference was maintained until the end of the experiment. However, gas exchange in Mark seemed to be less affected by the stress than in the other two cultivars. At day 7, midday stomatal conductance (gs) was 38.0, 32.3, and 72.0 mmol·m–2·s–1 in Budagowski 9, M9, and Mark, respectively (control values varied between 161.6 and 164.3 mmol·m–2·s–1 for all the cultivars). Heat-pulse sapflow sensors installed on Mark indicated that the speed of the xylem sap was affected by the stress from day 4 (19-26 cm/h for the controls vs. 15–21 cm/h for the stressed plants). Specific details on the physiological data will be presented.

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Rohini Deshpande, D. P. Coyne, K. G. Hubbard, J. R. Steadman, E. P. Kerr, and Anne M. Parkhurst

The microclimate of Great Northern (GN) dry bean lines with diverse plant architecture was investigated in terms of white mold (WM) incidence and yield. A split-plot design was used with protected (3 weekly sprays of benomyl 0.9 KG HA-1 after flowering) and unprotected treatments as main-plots and GN lines as sub-plots in a WM nursery (1990, 1991). Canopy density, erectness, leaf area index, and plant characteristics were measured. `Starlight' (upright) and `Tara' (prostrate) were selected for detailed microclimate studies. An infrared thermometer, humidity sensor, and a thermistor were placed within the canopy at the advent of flowering. Leaf wetness and its duration were estimated by the leaf temperature in combination with air temperature and dewpoint temperature. `Starlight' showed later and shorter duration of leaf wetness, lower humidity, and WM and higher yield than `Tara'. Severe WM and reduced yields occurred also on all other susceptible entries with dense prostrate plant habits in the unprotected plots. Fractal analysis was done on the images of the canopy to quantify the light interception within the canopy.

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Christopher J. Currey and John E. Erwin

and 18 °C, respectively. The plant temperature was measured on the newest fully unfolded leaf pair from five plants of each species in each chamber using an infrared thermometer (Westward 1EZ22A Infrared Thermometer; Grainger Global Sourcing, Lake

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John Erwin, Ken Altman, and Fran Esqueda

four environmental growth chambers (over time) where air temperatures were managed in each to achieve a 10, 16, 22, or 28 °C plant temperature (mean of three different infrared thermometer brands across species). The uppermost “unfolded” (≥45° from the

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Joseph Masabni, Youping Sun, Genhua Niu, and Priscilla Del Valle

calculated as F v /F m = (F m − F 0 )/F m . Leaf temperatures were measured using an handheld infrared thermometer (OS530; Omega Engineering), five plants per treatment per species. The thermometer was pointed at a healthy and fully expanded leaf at a