Estimation of stomatal conductance by infra-red thermometry in citrus trees cultivated under regulated deficit irrigation and reclaimed water.

The aim of this study was to evaluate the robustness of infra-red thermometry to estimate stomatal conductance (g s) in grapefruit trees. For this purpose, the ability of several thermal indices to determine g s was tested for different: (i) irrigation strategies: full irrigation (FI) and regulated deficit irrigation (RDI), (ii) water sources: transfer water (TW) -optimal for agricultural uses- and saline reclaimed water (RW) and (iii) phenological stages: flowering-sprouting (FS) and fruit growth (FG). During two growing seasons, measurements of g s and canopy temperature (T c) were taken in 14-year-old ´Star Ruby´ grapefruit trees (Citrus paradisi Macf.,). Air temperature (T a) was also recorded and then the derived thermal indices: T c -T a , and CWSI (crop water stress index) were calculated. Subsequently, thermal-g s correlations were established at different averaged thermal data intervals, and VPD (vapour pressure deficit) and PAR (photosynthetically active radiation) conditions. The results indicated that both, water quality and deficit irrigation, affected negatively g s and T c during FG. In addition, the highest r2 values in thermal indices-g s correlations were found when i) the thermal data was averaged 60 min before the g s measurement, ii) g s was measured over a wide range of VPD (between 0 and 3.5 kPa), and iii) PAR was 1200 µmol m−2 s−1. The ability of the thermal indices to estimate g s improved under RDI and RW conditions, when compared to the control treatment (TW-FI). Moreover, the thermal indices that more accurately estimated g s were T c -T a and CWSI in FS (P < 0.001), and the absolute values of T c in FG (P < 0.001). Overall, thermal indices obtained by infra-red sensors could be a useful tool to estimate g s in grapefruit trees. However, irrigation treatment, water quality, tree phenological stage and environmental conditions (VPD and PAR) must be considered when interpreting thermal indices-g s correlations. • Infra-red sensors allowed for estimating the stomatal conductance of grapefruit trees. • Environmental conditions affected the estimation of stomatal conductance. • Crop phenological stage affected thermal indices-stomatal conductance correlations. [ABSTRACT FROM AUTHOR]