## 4 Discussion  
The reliable and early assessment of yield and overall crop performance is of paramount importance for breeding programs. Plant breeders need to be able to phenotype large numbers of lines rapidly and accurately identify the best progeny [9]. The performance of the breeding programs for crop yield and productivity must be evaluated under natural conditions [6, 7]. In this study, we demonstrate that high-resolution thermal and hyperspectral imagery obtained from imagers installed on-board aircraft can provide valuable information about yield and crop performance, with the resolution necessary for phenotyping studies. We have observed that some indices derived from the thermal and hyperspectral imagery described the variations in yield in bread and durum wheat varieties more accurately than the standard structural-derived indices such as NDVI, which demonstrated good results in previous studies [19, 49]. The relationship between the indices and the assimilation rate at the times of flights demonstrated that the results were robust and physiologically relevant. The thermal-based index CWSI and the fluorescence retrieval through the FLD method demonstrated their value for monitoring physiological processes. It has previously been observed in the context of water stress monitoring [23, 38]. On the other hand, the widely used structural-derived NDVI index was not suitable in the context of this study for successfully tracking rapid changes in water status and assimilation rate. Earlier studies support this conclusion [17]. The climatic conditions during this study were within average values for the site, with a relatively rainy winter and spring, followed by a hot and dry summer. The crop would have maintained its water status during the vegetative phase, and thus a relatively satisfactory rate of growth. However, the depletion of soil water reserves and the increase in temperature and evaporative demand in the spring and summer left the plants under terminal water stress, which is typical in this region under rainfed conditions. Terminal stress causes substantial losses of yield in wheat. This is mainly driven by early leaf senescence and thus a shortening of the green leaf area duration, reduced rates of assimilation and reduced grain set and development, among other effects [50]. The relationship between the indices and crop yield was only significant for the second flight, performed during grain filling, while no relationship was observed on DOY 76. Previous studies have demonstrated that yield can be estimated at earlier stages, such as stem elongation [49]. The contrasted and variable climate would explain the inability to estimate yield before grain filling, as this period is critical for yield formation. Under most conditions, 90%-95% of the carbohydrate in grain is derived from carbon dioxide fixation after anthesis. Thus, the post-anthesis period seems the most appropriate period for screening crop performance in arid environments, as has already been observed by other authors [18, 21]. The thermal-derived indices displayed the best relationship with yield. It was already proposed as a method to carry out high-throughput field phenotyping [24, 25, 51]. The close links between temperature changes, stomatal conductance, and finally, yield, are well known. The thermal-derived index CWSI showed a good relationship with assimilation rate (Figure 7a,b). [PERSON] _et al._[52] demonstrated that the increase in yield in eight cultivars over six years was correlated with the difference between canopy and air temperatures (known as canopy temperature depression). A similar conclusion was reached by [PERSON] _et al._[22], who suggested canopy temperature as a criterion for grain yield selection in wheat breeding in hot climates. [PERSON] _et al._[21] obtained similar results. In addition to thermal-derived indices, those related to chlorophyll fluorescence emissions are also candidates for screening of wheat yield under these conditions.