The Evaluation of Continuous Thermistor-based Leaf Temperature

This project proposed to study the potential of using a novel method of measuring leaf temperature as an indicator of vine water stress. As vines undergo water stress, the stomata in the leaf close, reducing transpiration and subsequently raising the temperature of the leaf blade as compared to a well-watered leaf. In this study, thermistors (inexpensive miniature temperature sensors) were used to simultaneously measure leaf blade temperature and air temperature at 15-minute intervals, on vines at four different levels of water stress. Measurements were made on Cabernet Sauvignon vines in the Paso Robles area; the vines received four different irrigation treatments during the entire study period, equaling 50%, 75%, 100% and 125% of the irrigation amounts delivered to the control (100%) vines. In each irrigation treatment, three adjacent vines were monitored, with five basal leaves on each vine equipped with leaf blade temperature sensors. A single air temperature sensor accompanied each treatment. Leaf blade temperatures and air temperatures were measured simultaneously every 15 minutes during the summer study period (Aug. 1 – Sept. 16) using precision data loggers with an accuracy of 0.2º Celsius. During the nighttime, leaf blade temperatures and air temperatures were nearly identical, indicating very low transpiration rates. Soon after sunrise, differentiation in leaf temperature between treatments was noticed, with the wetter irrigation treatments having lower leaf temperatures as expected. For an initial comparison value between treatments, daily average leaf blade and air temperatures for the daytime hours (7 am – 7pm) were used. For most of the evaluation period, the daily average leaf temperature was inversely correlated with irrigation amount; in other words, the wetter irrigation treatments had lower leaf temperatures than the drier treatments. However, these differences were not statistically significant except for the two extreme treatments (50% and 125% irrigation). This lack of statistical significance may be influenced by the low number (three) of replicates per treatment. It is likely also influenced by the 12-hour average comparison value; a comparison period that targets the period of maximum temperature divergence may be more useful in this regard.

The implications of this study so far are:

  1. This method appears to measure the temperature differences between vines at four different levels of water stress, but not at a statistically significant level in the experimental design used;
  2. This method, with further testing, might prove to be an inexpensive, practical method for monitoring vine water stress;
  3. This method offers the advantage of automated, continuous data collection of a vine stress parameter, whereas most common vine water stress measurement methods currently in use are based on manual measurements.