Abstracts

Ajayi, A.E. and Olufayo, A.A. 2004, 'Evaluation of Two Temperature Stress Indices to Estimate Grain Sorghum Yield and Evapotranspiration', Agronomy Journal, vol. 96, pp. 1282-1287.

This study was performed to examine the relationship between canopy temperature (T_c)-based stress indices and grain yield and evapotranspiration (ET) of sorghum [Sorghum bicolor (L.) Moench]. The crop was subjected to 14 differentially irrigated treatments of which two were controls maintained at well-watered and dry conditions during three consecutive years. Soil water content and T_c were measured, and relationships between T_c-based stress indices [stress degree day (SDD), temperature stress day (TSD), and crop water stressed index (CWSI)] and yield as well as ET were examined. The T_c-air temperature (T_a) difference varied from -2 to +8°C in the stressed treatments and maintained a negative value for most of the time in the well-watered treatment. The relationship between T_c - T_a and vapor pressure deficit, commonly referred to as baseline in the determination of CWSI, was examined on function of wind speed and global solar radiation. Although observations showed that T_c can be influenced by climatic condition, this study confirmed that it can serve as a useful indicator of water stress in the case of sorghum. High correlation found between T_c-based stress indices TSD, SDD, and CWSI and ET as well as grain yield suggest the possibility of using these relationships for predictive purposes.

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American Society for Testing and Materials 2001, 'Standard Test Methods for Radiation Thermometers (Single Waveband Type). Designation: E 1256 - 95 (Reapproved 2001)', in 2001 Annual Book of ASTM Standards, eds, American Society for Testing and Materials,, West Conshohocken.

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Baker, J.T., Gitz, D.C., et al. 2007, 'Using Leaf Gas Exchange to Quantify Drought in Cotton Irrigated Based on Canopy Temperature Measurements', Agronomy Journal, vol. 99, pp. 637-644.

Plant gas exchange provides a highly sensitive measure of the degree of drought stress to which a crop is exposed. However, equipment costs and time requirements for gas exchange measurements are major obstacles to the use of gas exchange measurements in real-time irrigation scheduling systems. Canopy temperature (T_c) provides a much easier to acquire indication of crop water deficit that has been used in irrigation scheduling systems, but interpretation of this measurement has proven difficult. Our goal was to test the ability of T_c to quantify the degree of crop water deficit by comparing T_c with simultaneous measurements of leaf-level gas exchange parameters, which were viewed as alternative indicators of water deficit. To provide a wide range of plant water deficit conditions for the comparison of T_c with leaf-level gas exchange parameters, cotton (Gossypium hirsutum L.) was subsurface drip irrigated using Tc according to the stress-time index method of irrigation scheduling during two growing seasons. Comparisons between T_c and leaf-level gas exchange were accomplished by measuring T_c diurnally with hand-held infrared thermometers and controlling cuvette leaf temperature (T_L) equal to T_c and then measuring leaf level net assimilation (A) and stomatal conductance (g) at a photosynthetically active radiation (PAR) level of 1500 µmol (photons) m^-2 s^-1. In general, as plant water deficit became more severe, leaf level gas exchange tended to decline with rising T_L. However, we found that A and g could vary by more than twofold at a given T_L, indicating that T_c was not a particularly robust indicator of the degree of drought stress. Furthermore, we found the leaf minus air temperature differential (T_L - T_a) and vapor pressure deficit calculated based on leaf temperature (VPDL) were better predictors of the degree of drought stress, as indicated by gas exchange parameters, than TL alone. Regression of A and g against (T_L - T_a) and VPDL indicated that the combination of these two variables accounted for >79% of the variability in A and g. We conclude that the term (T_c - T_a) either alone or in combination with VPD should provide a better predictor of the degree of drought stress in cotton than T_c alone.

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Garrot, D.J., Gibson Jr, R.D., et al. 1998, 'The Response of Table Grape Growth, Production, and Ripening to Water Stress', in 1998 Citrus and Deciduous Fruit and Nut Research Report, eds G. Wright and M. Kilby, University of Arizona, Tuscon.

Four year old 'Flame Seedless' grapevines located in a commercial vineyard subjected to increased water stress levels based on infrared canopy temperatures and the Crop Water Stress Index (CWSI) for two years. CWSI levels were approximately .18, .30 and .33 for the wet, medium and dry treatments. In the first year there were no significant differences in yield however, there was a significant reduction in the amount of water applied in both the medium and dry treatments when compared to the wet treatment. In addition, the wet treatment had significantly greater growth during the first growing season when comparing pruning weights.

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Hattendorf, M.J., Carlson, R.E., et al. 1988, 'Crop Water Stress Index and Yield of Water-Deficit-Stressed Alfalfa', Agronomy Journal, vol. 80, pp. 871-875.

The yield relationship of water-deficit-stressed alfalfa (Medicago sativa L.) with the Crop Water Stress Index (CWSI) may be dynamic over the alfalfa harvest period. This investigation was conducted to define the alfalfa yield-CWSI relationship(s), to test possible advantages of using the CWSI over the canopy-minus-air temperature (Tc-Ta) differential, and to combine the yield-CWSI relationship with a growth function. 'Apollo II' alfalfa was grown for 2 yr in 100-L containers set into the ground and protected from rain by a movable shelter. The soil was a Nicollet silt loam topsoil (fine-loamy, mixed, mesic Aquic Hapludoll). Treatments were irrigation levels of 112, 100, 88, 77, and 65% field capacity. Five harvests were taken at 7-d intervals beginning 21 d after an initial clipping. Canopy temperatures were taken daily with an infrared thermometer. A series of exponential yield-CWSI curves resulted from the sequential harvests. Yield reductions of about 10 and 20% resulted from mean CWSI of 0.05 and 0.10, respectively, over 42-d periods. The Gompertz growth function was modified for water-deficit-stress by combining it with the exponential yield-CWSI relationship. Vapor-pressure deficit (VPD) effectively normalized the Tc-Ta data, allowing yield data of the 2 yr to be combined in the yield-mean CWSI relationship. Yield and Tc-Ta data not normalized for VPD showed clear separation by year for slight VPD differences. Alfalfa yield response to water-deficit-stress over time appeared to be adequately described by the combined Gompertz function and the yield-CWSI (Tc-Ta normalized for VPD) curves.

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Irmak, S., Haman, D.Z., et al. 2000, 'Determination of Crop Water Stress Index for Irrigation Timing and Yield Estimation of Corn', Agronomy Journal, vol. 92, pp. 1221-1227.

Corn (Zea mays L.) grown under a Mediterranean semiarid climate requires supplemental irrigation to maximize the grain yield. Since the cost of irrigation application has been increasing, elimination of unnecessary irrigation applications would improve economics of corn production. There has been much interest in the crop water stress index (CWSI) as a potential tool for irrigation scheduling and yield estimation. An experiment was conducted to monitor and quantify water stress, and to develop parameters for irrigation scheduling and grain yield of summer-grown corn as a function of CWSI under Mediterranean semiarid cropping conditions. Three irrigation treatments were based on replenishing the 0.9-m deep root zone to field capacity when the soil water level dropped to 25, 50, and 75% of available water holding capacity (AWHC). A dryland treatment was also included. The lower (nonstressed) and upper (stressed) baselines were measured to calculate CWSI. An equation that can be used to calculate the yield potential of summer-grown corn under a Mediterranean climate was developed using the relationship between the corn grain yield and the seasonal mean CWSI. Permitting the seasonal average CWSI value to exceed more than 0.22 resulted in decreased corn grain yield. The CWSI behaved as expected, dropping to near zero following an irrigation and increasing gradually as corn plants depleted soil water reserves. We concluded that CWSI is a useful tool to monitor and quantify the water stress of corn under a Mediterranean climate.

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Jalali-Farahani, H.R., Slack, D.C., et al. 1993, 'Crop Water Stress Index Models for Bermudagrass Turf: A Comparison', Agronomy Journal, vol. 85, pp. 1210-1217.

A field study assessed the accuracy and consistency of crop water stress index (CWSI) estimates determined by two empirical models and one theoretical model for evaluation of bermudagrass [Cynodon dactylon (L.) Pers. cv. Midiron] water status. The simplest empirical model, describing canopy temperature minus air temperature (T_c - T_a) of well-watered turf as a linear function of air vapor pressure deficit, was inadequate for estimating CWSI, since seasonal averages of CWSI for well-watered, moderately stressed, and stressed turf had corresponding coefficients of variation of 1170, 160, and 86, respectively. The CWSI values were also highly influenced by net radiation, as indicated by coefficients of determination for regressions of calculated CWSI on net radiation of 0.68, 0.76, and 0.33 for the three levels of irrigation treatments, respectively. The second empirical model included net radiation in the regression analysis of T^c - T_a. The inclusion of net radiation in the equation for the unstressed baseline increased the coefficient of determination from 0.76 to 0.90 as compared with Model 1. The coefficient of determination for the upper baseline (which included net radiation) was 0.872, indicating a strong dependency of CWSI on net radiation. The theoretical CWSI model yielded the most accurate estimates of turfgrass water stress, as indicated by coefficients of variation for seasonal averages of CWSI for well-watered, moderately stressed, and stressed turf of 76, 3, and 86, respectively. Midday estimates of CWSI were related to percent available extractable water, with coefficients of determination for a regression of midday CWSI on percent available extractable water of 0.988, 0.989, and 0.991 for the empirical, modified empirical, and theoretical models, respectively. The theoretical CWSI appears to be the most promising approach for a turfgrass crop water stress index for irrigation scheduling.

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Sadler, E.J., Bauer, P.J., et al. 2000, 'Site-Specific Analysis of a Droughted Corn Crop: II. Water Use and Stress', Agronomy Journal, vol. 92, pp. 403-410.

In the southeastern USA Coastal Plain, spatial variation in soils causes extreme spatial variation in grain yield, as seen in yield maps. Corn (Zea mays L.) appears to be particularly susceptible to soil variation, especially during periods of drought. Our objectives were to compare variation in water use and stress of corn within and among soil map units. In one field, at two sites in each of four map units, we measured site-specific effects of soil variation on crop water use from 40 d after planting until after maturity using a time-domain reflectometer (TDR). On 4 d during vegetative growth, drought stress was evaluated on eight transects using infrared thermometer (IRT) measurements of canopy temperature (T_c). During the most severe drought, visibly stressed areas had canopy-air temperature differences (T_c - T_a) > 10°C, yet other areas remained <2°C. Two days after a 46-mm rain, T_c - T_a was near zero over the whole field, indicating little water stress. The time series of TDR measurements produced estimates of daily evapotranspiration, runoff, and infiltration; site-to-site differences in these dominated the water balance. Water stress, inferred from water use, matched that inferred earlier from yield components. In sum, corn at the eight sites arrived at final water use via fundamentally different paths. Further, variation between sites within soils was significant, indicating that soil map units are not homogenous with respect to water relations. These results underscore the need for within-season observations of crop water use and stress to augment interpretation of site-specific yield maps.

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