Detection of water deficit in greenhouse cucumber by infrared thermography and reference surfaces

Detection of water deficit in greenhouse cucumber (Cucumis sativits L.) by using the method of infrared thermography (IRT) and wet and dry reference surfaces was studied. The experiment was conducted under 18 and 22 h supplementary lighting in winter. Indices based on the temperatures of crop canopy and reference surfaces along the row were computed. As a result, the indices were related to the changes of stomatal conductance (g(s)) and water potential (Psi(m)) in growing medium. g(s) was clearly reduced when Psi(m) was lower than -10 to -20 kPa and fell to very low values at -80 kPa. Decline of Psi(m) below -40 to -60 kPa was detected by the indices under spatially heterogeneous radiation and a marked daytime trend of g(s). The sensitivity and stability of the indices at high g(s) are discussed. It is concluded that the method can detect water deficiency before any long-term damage to crop has occurred, but the warming signal is often masked by the spatial variability of radiation and the daytime trend of g(s) preventing the application of the method in its current form to commercial greenhouses. (c) 2005 Elsevier B.V. All rights reserved.