Water shortage is becoming a severe problem in arid and semi-arid regions worldwide, reducing the availability of agricultural land and water resources. Deficit irrigation strategies can improve water-use efficiency and the sustainability of agro-ecosystems, although it is important to model the effects on yield loss due to irrigation water restrictions. This work estimates the water production function in citrus trees, determining the relationship between plant water stress and yield depression, as well as establishing a mathematical model for each phenological stage considered (flowering, fruit growth and ripening), and for the entire productive process. For three consecutive years (2006)(2007)(2008), four regulated deficit irrigation treatments plus a control (100% crop water evapotranspiration (ET C )) were implemented in 13-year-old citrus trees (Citrus sinensis L. Osb. cv. Navelina). Different water production functions were determined for each phenological stage, establishing the relationship between the irrigation water stress and crop yield. Our results show that the fruit growth and flowering stages were the most sensitive periods in relation to irrigation water deficit and yield loss. Water stress close to 50% of ET C during the flowering stage would impose a yield loss of up to 20%, whereas this same water stress level during the fruit growth or ripening stages would result in yield losses of nearly 10% and 6%, respectively. The adjustment with cross terms (r 2 =0.87) estimated the yield loss with good accuracy, being very similar to data measured in each study season. Consequently, the combined effect of deficit irrigation in different stages would be an additive-multiplicative model, considering that the effect of water stress in previous periods determined the crop yield response. Our model indicated that the crop water production function under deficit irrigation programmes would have a quasi-linear relation for water deficits below to 40% ET C . The previous model functions did not enable us to establish an accurate relationship when the water stress was applied in different phenological stages. Thus, this new interpretation is valuable to improve our knowledge and predict the impact of regulated deficit irrigation and have potential application in precision water stress and sustainable irrigation scheduling for citrus.

-Introduction. Optimising agricultural water use implies the combination of physiological, technological and engineering techniques, especially those for continuously monitoring the water status of plants subjected to deficit irrigation. A methodology to estimate water stress of young almond trees from thermal images was developed based on assessing the physiological status of almond crops under limited water-supply conditions. Materials and methods. Two irrigation treatments were tested during the maximum evapotranspirative demand period (214th to the 243rd day of the year) in an experimental almond [Prunus dulcis (Mill) D.A. Webb, cv. Guara] orchard: a low-frequency deficit irrigation (LFDI) treatment, irrigated according to the plant-water status, and a fully irrigated treatment (C 100 ) at 100% of crop evapotranspiration. Daily canopy temperature at midday (T C ) was measured with an infrared camera, together with standard measurements of stem-water potential (Ψ Stem ) and stomatal conductance (g S ). The time course of these parameters and their relationships were analysed. Results and discussion. The time course of the parameters studied showed highly significant correlations among the differentials of canopy-air temperature (ΔT), Ψ Stem and g S . The methodological protocol for analysing thermal images allowed a time saving in processing information and additionally offered the possibility of estimating the Ψ Stem and g S values. Conclusion. Our results confirm that infrared thermography is a suitable technique for assessing the crop-water status and can be used as an important step towards automated plant-water stress management in almond orchards.Spain / Prunus dulcis / canopy / temperature / infrared thermography / water requirements / soil water deficit Approche pour évaluer l'imagerie thermique infrarouge d'amandiers en conditions de stress hydrique.Résumé -Introduction. Optimiser l'utilisation de l'eau en agriculture implique de combiner des techniques physiologiques, technologiques et d'ingénierie, en particulier celles qui permettent de surveiller en permanence l'état hydrique de plantes soumises à un déficit d'irrigation. Une méthodologie pour estimer le stress hydrique de jeunes amandiers à partir d'images thermiques a été développée sur la base de l'évaluation de l'état physiologique d'arbres placés en conditions d'alimentation en eau limitée. Matériel et méthodes. Deux traitements d'irrigation ont été testés au cours de la période de demande evapotranspirative maximale (214e au 243e jour de l'année) dans un dispositif expérimental en verger d'amandiers [Prunus dulcis (Mill.) D.A. Webb., cv. Guara] : un traitement avec une irrigation déficitaire à basse fré-quence (LFDI), irrigué en fonction du statut hydrique des plants, et un traitement irrigué à 100 % de l'éva-potranspiration des cultures (C 100 ). La température quotidienne du couvert à midi (T C ) a été mesurée à l'aide d'une caméra infrarouge, ainsi que des mesures standards du potentiel hydrique de la tige (Ψ Stem ) et de la con...

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