In two field experiments with cotton (Gossypium), the relation between moderate soil water depletion and leafair temperature difference (ΔT) was investigated for feasibility of measurement and possible use as a guide to irrigation scheduling. Hourly temperatures of the upper leaves and of the air 1 m above the canopy were obtained for three cultivars of short‐staple cotton (G. hirsutum L.) and one of long‐staple (G. barbadense L.). In addition, hourly measurements were made of the vapor pressure 1 m above the crop. These data were taken during six irrigation cycles extending over two summers and encompassing a wide range of saturation deficit (SD). In contrast to previous experiments in which a severe drought had been developed deliberately to intensity plant response, the experimental plots were irrigated on a schedule to obtain a high lint yield. The temperature differences were measurable and predictable. When the SD remained steady from day to day, the mean daytime ΔT decreased 1 C on the day of irrigation and an additional degree the next day owing to leaf rehydration and consequent stomatal opening. Conversely, this 2 C decrease upon irrigation reflects the rise in ΔT that had occurred during the previous drying cycle. When the SD decreased appreciably, the stomatal opening caused by irrigation was masked. Because the SD seldom remains steady over long periods, a measurement of ΔT alone is not a sufficient indicator for irrigation. Thus, allowance was made for such changes in SD by calculating a regression of ΔT on the SD from data taken from a recently irrigated crop. Although the use of ΔT to schedule irrigations seems feasible, the method is restricted primarily to regions with sunny climates. Even here it requires accurate, wellreplicated leaf temperature measurements as well as measurements of air temperature and vapor pressure.

A relatively simple method for determining the infrared emittances of leaves gives reproducible results that are in good agreement with two other methods currently used to determine infrared emittances of soils. Measurements on a wide variety of plants indicate that significant differences exist among the various species. Emittances determined by infrared thermometers of different spectral sensitivity are not uniquely related, thereby emphasizing the importance of the emittance measurement.

Drought‐induced stomatal closure causes a rise in canopy temperature that can be detected remotely by infrared thermometers. Remote sensing of such a rise could lead to large scale irrigation scheduling and improvements in yield forecasts, provided that the increase in canopy temperature is a reliable indicator of plant water stress. Plant water stress can be measured reliably and simply by determining the plant water potential (Ψplant). This experiment was undertaken to demonstrate that crop canopy temperature responds specifically to changes in Ψplant and therefore can be used for long term crop monitoring by remote sensing techniques. The temperature difference between plant canopy and air (ΔT) was related to Ψplant, as shown by simultaneous, daily measurements of these parameters at 12 sites on six plots under different irrigation treatments. These data were obtained on clear days at predawn (just before sunrise) and 1400 hours. At predawn, Ψplant ranged from −2.0 bars in recently irrigated soil, in which the volumetric water content (θv) = 0.28, to −16 bars at the wilting point (θv = 0.15). At 1400 hours, Ψplant ranged from −15 bars when θv was 0.28 to −30 bars at the wilting point. At 1400 hours, ΔT was −3.8 C at full plant hydration (Ψplant = −15 bars). Increasing drought decreased Ψplant progressively and increased ΔT accordingly: when Ψplant decreased to −19 bars, ΔT was zero; when Ψplant was −48 bars, ΔT increased to 4.8 C. In demonstrating the relationship between ΔT and Ψplant these results support the validity of the temperature difference method for sensing plant response to drought.

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Chlorophyll fluorescence of mature lemon trees was measured with a Fraunhofer line discriminator (FLD). An increase in fluorescence was correlated with plant water stress as measured by stomatal resistance and twig water potential.