USE OF INFRARED THERMAL IMAGING FOR ESTIMATING CANOPY TEMPERATURE IN WHEAT AND MAIZE
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Wheat (Triticum aestivum L.) and maize (Zea mays L.) are among the most important cereal crops around the world and as a result there is increasing demand for their production. Drought is limiting the production of wheat and maize in most parts of the world. It is important to develop a crop production system that can perform better under water limited conditions and can sustain the increasing population. Understanding the physiological basis of drought tolerance is necessary to improve genetic ability of crop for higher yield and water use efficiency. Canopy temperature has been used as one of the traits for identifying drought tolerant cultivars because it shows the relationship between plants, soil and atmosphere and has been recognized as an indicator of plant water status. Several remote sensing approaches have been developed to study stomatal conductance and determine water stress in plants. Among them, thermal imaging has been used to measure the canopy temperature and study plant water relationships. This study investigates the potential use of infrared thermal imaging for calculating crop canopy temperature and determining relationship between canopy temperature and yield. Furthermore, the genetic variation among wheat genotypes and maize hybrids in terms of canopy temperature under different water regimes was studied. Thermal images were acquired on several dates from the field of 20 different wheat genotypes grown under dryland and irrigated conditions in 2014/2015 wheat growing season at Bushland, Texas. Moreover, images were also taken from the field experiment of maize where five different hybrids were grown under two (I50 and I100) irrigation regimes in 2015 maize growing season at Bushland, Texas. A handheld thermal camera was used to acquire thermal images and the images were processed using IR Crop Stress Image Processor Software. The software filters out the background soil from thermal image of the wheat and maize plots and gives the mean canopy temperature of the selected area in the image. Thus obtained canopy temperature was recorded and further analyzed. In the wheat study, a significant difference (P < 0.05) in canopy temperature among wheat genotypes grown under dryland condition was found several times when measurements were taken. Similarly, a significant difference in canopy temperature among the maize hybrids was found under I50 irrigation regime. However, in both the crops, consistent canopy temperature differences in wheat genotypes and maize hybrids were not observed under irrigated condition when the plants were having sufficient soil moisture to maintain transpiration. Canopy temperature of maize hybrids measured under I50 irrigation regime was higher than in I100 irrigation regime which gives an indirect indication of water status in soil. A strong negative correlation (P < 0.05) was found between canopy temperature and above ground biomass across the wheat genotypes under dryland condition. Also, a similar relationship was observed between canopy temperature and grain yield across maize hybrids. Moreover, in the maize study, the hybrid with lower canopy temperature (averaged across all the measurement dates) had relatively higher yield under I50 water regime. However, under fully irrigated condition, consistent correlation between canopy temperature and grain yield was not found. These results indicated that the genotypes that can maintain cooler canopies during water stress can produce higher yield. It is concluded that canopy temperature can be a good indicator of crop water status and may be used as a selection criterion in identifying drought tolerant genotypes under water-limited conditions. Infrared thermal imaging showed a potentially promising technique in studying the genotypic variation among wheat genotypes and maize hybrids thereby can be helpful to enhance breeding programs for drought tolerance of wheat and maize.