Thermal imaging to detect spatial and temporal variation in the water status of grapevine (Vitis vinifera L.)

Thermal imaging can detect variation in stomata! conductance and therefore spatial variation in the water status of grapevine. Temporal variation can also be assessed, using indices that relate canopy temperature to reference temperatures, but, as yet, there is no standard approach to obtain these reference values. Also, the potential of above-canopy imaging is uncertain for vine rows with mainly vertically oriented leaves. Thermal images of the side (vertical leaves) and top (horizontal leaves) of 'Tempranillo' (Vitis vinifera L.) grapevine rows were captured on different dates and in different plots in a vineyard in the Rioja, Spain. Reference temperatures were determined separately (i) as the temperature of artificial leaves placed in the grapevine canopy, or (ii) by calculation from the leaf energy budget. With respect to imaging the side of grapevine rows, the two approaches exhibited similar potential to indicate variation in stomatal conductance or stem water potential within a single date and time of day. Over different dates (implying temporal as well as spatial variability) and both sides of the grapevines, the use of the artificial surfaces resulted in stronger correlations between thermal indices and physiological variables than the application of energy budget models. The use of such reference surfaces would allow a standardised approach to thermography. Imaging the top of rows produced some highly significant correlations between thermal indices and physiological variables. Modified artificial leaves could be developed to standardise the determination of reference temperatures, at the same spatial resolution as the leaves of interest. That imaging the top of grapevine canopies provides useful information on water status suggests that aerial imaging could be used to assess spatial variation in water status across entire vineyards.