By introducing a reference dry leaf (a leaf without transpiration), a formerly proposed plant transpiration transfer coefficient (h_at) was applied to detect environmental stress caused by water shortage and high temperature on melon, tomato and lettuce plants under various conditions. Results showed that there were obvious differences between leaf temperature, dry reference leaf temperature and air temperature. The proposed coefficient h_at could integrate the three temperatures and quantitatively evaluate the environmental stress of plants. Experimental results showed that the water stress of melon plants under two irrigation treatments was clearly distinguished by using the coefficient. The water stress of a tomato plant as the soil dried under a controlled environmental condition was sensitively detected by using h_at. A linear relationship between h_at and conventional crop water stress index was revealed with a regression determination coefficient R² = 0.97. Further, h_at was used to detect the heat stress of lettuce plants under high air temperature conditions (28.7°C) with three root temperature treatments (21.5, 25.9 and 29.5°C). The canopy temperature under these treatments was respectively 26.44, 27.15 and 27.46°C and the corresponding h_at value was –1.11, –0.74 and –0.59. Heat stress was also sensitively detected using h_at. The main advantage of h_at is its simplicity for use in infrared applications.