Wheat (Triticum aestivum L.) cultivars with high canopy temperature depression (CTD) tend to have higher grain yield under dry, hot conditions. Therefore, CTD has been used as a selection criterion to improve adaptation to drought and heat. The CTD is a result of the leaf's energy balance, which includes terms determined by environment and physiological traits. We hypothesized that one or more of several physiological traits contributed to consistent CTD differences among three closely‐related winter wheat lines grown under dryland conditions. For three years we measured several leaf traits, including CTD, leaf dimension, gas exchange rates, and carbon‐13 isotope discrimination (Δ). Soil water content was also monitored. Data showed that daytime CTD was related to the leaf size in these wheat lines. The most drought‐tolerant line, TX86A8072, had consistently smaller and narrower leaves than TX86A5606, the least drought tolerant. For TX86A8072, dryland and irrigated average noon CTD was −0.8°C, and average flag leaf area (LA) 11 cm2, for TX86A5606, values were −1.7°C and 12.5 cm2, respectively. However, TX86A8072 also had higher CTD (i.e., lower temperatures) than TX86A5606 at night, despite a theoretically greater sensible heat transfer coefficient, suggesting greater nighttime transpiration (T). Implications of these traits on nighttime leaf energy balance and possible adaptive roles of nighttime T are discussed.

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climate-dependent threshold time, then an irrigation event of a fixed depth is scheduled (Fig. 1). Evett et al.The application of the temperature-time threshold (TTT) method (1996, 2000) demonstrated in drip-irrigated plots that of irrigation scheduling to self-propelled irrigations systems requires a method of estimating the diurnal canopy temperature dynamics automatic irrigation using the TTT method was more using only a one-time-of-day measurement. Other research efforts responsive to plant stress and showed the potential to such as the crop water stress index (CWSI) and field canopy temperaoutyield manual irrigation scheduling based on a 100% ture mapping using moving irrigation systems could also be served replenishment of crop water use as determined by neuby the use of this method. This was accomplished using a stationary tron probe soil water content measurements. It is desirreference measurement to capture the canopy temperature dynamics.able to apply the TTT method to moving irrigation sys-Two different methods were developed for estimating a temperature tems such as center pivots or linear-move systems. curve for a remote location from a one-time-of-day measurement at However, infrared thermometers (IRTs) mounted on that location. The first method (scaled method) uses the ratio between moving irrigation systems provide only one-time-of-day the reference temperature and the remote location temperature, refermeasurements. This necessitates a method of estimating enced to the predawn temperature, to scale the reference curve to yield the predicted curve. In the second method (Gaussian difference the diurnal canopy temperature dynamics using only a method), a three-parameter Gaussian equation was empirically fitted one-time-of-day canopy temperature measurement. to the temperature differences between the reference and the mea-Other canopy temperature-based crop stress indicasured remote canopy temperature curves. To test these two methods, tors such as the Crop Water Stress Index (CWSI; Jackcanopy temperature data, sensed using stationary infrared thermomeson, 1982) are sensitive to the time of day that the meaters, from three different crops {corn (Zea mays L.), cotton (Gossypsurements are taken (U.S. Water Conserv. Lab., 2004). ium hirsutum L.), and soybean [Glycine max (L.) Merr.]} were ana-There is also an increased interest in quantifying spalyzed. For a few hours after dawn and before sunset, the scaled method tially varying crop response to soil-, water-, disease-, or was generally more accurate while during the middle of the day, the

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