Choice of Reference Surfaces to assess Plant Health through leaf scale temperature monitoring

We use infrared thermal imaging to remotely monitor the temperature of the leaves and plant which in turn is an indicative of their health and stress; in particular water stress. A series of experiments were conducted using Fluke TiX580 thermal imager on tomato plants to correlate the leaf temperature to different stages such as healthy, dying/wilted, and completely dead/dry. The leaf temperature was compared to a series of paper-based reference surface temperatures; these are of different colors and some are dry while some are wet. All the reference surfaces were insulated at the bottom ensuring the heat interaction between their top surfaces and ambient only. The surfaces were kept sufficiently far from one another. The healthy leaf temperature was found to be close to that of white dry and black wet reference surfaces whereas the wilted and dying leaves temperature were observed to range between yellow and red signifying that this temperature range can better predict the onset of water stress in the leaves. The completely dead/dry leaves were observed to range between green and blue dry surfaces, respectively. However, most of the dead leaf temperature data was found to be accumulated closer to the green surface signifying green dry surface can better indicate a dead leaf condition. The dying leaves were observed to exhibit 8-10 degree centigrade higher temperatures as compared to the healthy leaves in similar ambient conditions. Temperature-based health assessment provides us with a timely intervention to prevent leaf death compared to the optical monitoring since IR images revealed elevated leaf temperature 2-3 days before the optical unhealthiness (appearance of yellowness on the leaf) was noticed.