Assessing plant water status in a hedgerow olive orchard from thermography at plant level
Water scarcity is the most limiting factor in many irrigated areas of Mediterranean countries such as South Spain. Olive growing has been traditionally associated to rain-fed agriculture, although irrigation and practices related to intensive agriculture have been progressively introduced, requiring a more precise irrigation scheduling to save water. Thermal imaging is among the alternatives to assess the crop water status, especially when deficit irrigation (DI) strategies are applied. However, this technique requires of new advances to be more user friendly and robust for practical usage. The aims of this study were: i) to define threshold values of canopy temperature (TC), Crop Water Stress Index (CWSI) and the temperature difference between canopy and the surrounding air (ΔTcanopy-air) for the assessment of the olive water status when a DI strategy is applied; ii) to define the best time of the day and the best area of the canopy to carry out thermal measurements, and iii) to obtain relationships between thermal indicators and main physiological parameters useful to estimate the crop water status from thermal data. The trial was conducted during 2015, in a hedgerow olive orchard (SW Spain) with 8-year-old trees (Olea europaea L., cv. Arbequina), under three irrigation regimes: a full-irrigation treatment (FI) and two regulated deficit irrigation treatments aimed to supplying 45% of the irrigation needs. In one of them, irrigation was scheduled from leaf turgor pressure related measurements (45RDITP). In the other, the crop coefficient approach was used to schedule irrigation (45RDICC). Significant correlations between Tc versus stem water potential (Ψst) and leaf gas-exchange parameters (stomatal conductance to water vapour, gs; net CO2 assimilation, AN; transpiration, E) were obtained (p≤0.05), in particular from measurements taken at 10:30 GMT in the lower part of the sunlit side of the canopy. Moreover, the relationships between both ΔTcanopy-air and CWSI with the monitored physiological variables were very robust. We concluded that values of ΔTcanopy-air higher than 0°C and values of CWSI up to 0.2 reliably reflect the plant water stress. Our results, therefore, suggest that both ΔTcanopy-air and CWSI measured at midday provide reliable information on the tree water status and are useful to schedule irrigation in hedgerow olive orchards, especially under DI conditions.
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