Temperature as an Indicator of Water Stress in Leaves and Plants: Leaf-Scale to Field-Scale

The water uptake status of a plant is directly linked to water stress levels in the leaves. It is important to identify this stress strategically, and if possible in advance. This study reviews current practices and highlights canopy/leaf temperature as a reliable indicator of plant water status and stress. Several indices were developed to quantify this stress out of which the crop water stress index (CWSI), based on temperature (of leaf and reference surfaces) measurement, is the most widely accepted technique despite challenges posed by the environmental variability. Further, experimental results are also presented, carried out using IR imaging to correlate the leaf temperature to different stages such as ‘healthy’, ‘dying/wilted’, and completely ‘dead/dry’. Healthy leaf temperatures are closely aligned with white dry and black wet reference surfaces. Dying leaves exhibited temperatures ranging between yellow and red dry surfaces, while dead leaves aligned between dry green and blue surfaces, with most data clustering near green, indicating its suitability for representing dead leaf conditions. Dying leaves were observed to be 8–10 °C warmer than healthy leaves under similar ambient conditions. These findings helped to formulate a refined and user-friendly CWSI scale using optimal combinations of dry and wet reference surfaces, enabling accurate detection of water stress stages. This work underscores the potential of temperature-based assessments for early stress detection and precision irrigation.