A plant-based thermal optimum approach to irrigation scheduling provides potential benefits in that water applications are scheduled on the basis of plant response to water status. Such irrigation systems require a defined thermal optimum for the crop and while such optimum values have been identified for cotton (Gossypium hirsutum L.) cultivars in the United States, there is no information of this type for cultivars common in Australian production. This paper outlines a threefold approach to determining the optimum temperature (T-opt) of the commercial Australian cotton cultivar Sicot 70BRF in an Australian production system. It combines the use of a laboratory-based fluorescence assay, field-based net C assimilation rate (A) and stomatal conductance to water vapor (g(s)), and canopy temperature (T-c)-yield relations. The fluorescence assay showed a T-opt between 28 and 30 degrees C while leaf gas exchange rates peaked at a leaf temperature (T l) of 29 degrees C. The T-c-yield relations peaked at 26 degrees C, with yield reductions observed when T-c > 28 degrees C. We conclude the T-opt of the Australian upland cotton cultivar Sicot 70BRF to be 28 +/- 2 degrees C. This T-opt will provide valuable information for use in thermal optimum irrigation scheduling systems.
