Field studies were conducted in southern Idaho to evaluate the
possibility of using thermal infrared measurements of potato foliage to
detect soil water deficits. Concurrent measurements of foliage-air temperature
differences (Tf-Ta), leaf water potential (?leaf) and vapor pressure
deficit (VPD) were obtained from differentially-irrigated Russet Burbank
and Kennebec potatoes during the 1982 and 1983 growing seasons. Foliage-air
temperature differences for well-watered potatoes were linearly related
to VPD. Differences in Tf-Ta values between stressed and well-watered
potatoes were relatively small in the early morning when evaporative demand
was low. However, severe soil water deficits caused afternoon Tf-Ta
values to rise as much as 8.0 C above non-stressed levels under conditions of
high VPD.
Foliage-air temperature differences and VPD data were used to construct
a plant water stress index (PWSI) which reflected the rise in Tf-Ta,
above non-stressed levels at a given VPD. The PWSI was linearly related to
depressions in ?leaf caused by moderate to severe soil water deficits.
However, the PWSI did not increase significantly above non-stressed values
unless the soil matric potential ?soil fell below – 70 kPa (centibars). Since
potatoes are normally irrigated before ?soil falls below – 60 kPa, it appears
that foliage temperature measurements cannot be used to effectively
schedule irrigation for this crop
