A plant-based thermal optinnum approach to irrigation scheduling provides potential benefits in that water applications are scheduled on the basis of plant response to vi/ater status. Such irrigation systems require a defined thermal optimum for the crop and while such optimum values have been identified for cotton {Gossyplum hirsutum L.) cultivars in the United States, tinere is no information of this type for cultivars common in Australian production. This paper outlines a threefold approach to determining the optimum temperature (T^^p,) of the commercial Australian cotton cultivar Sicot 70BRF in an Australian production system. It combines the use of a laboratorybased fluorescence assay, field-based net C assimilation rate (A) and stomatal conductance to water vapor (g J, and canopy temperature {TJ-yield relations. The fluorescence assay showed a T^pt between 28 and 30°C while leaf gas exchange rates peaked at a leaf temperature (T,) of 29°C. The T^-yield relations peaked at 26°C, with yield reductions observed when T^ > 28°C. We conclude the T^p, of the Australian upland cotton cultivar Sicot 70BRF to be 28 ± 2°C. This 1^ J will provide valuable information for use in thermal optimum irrigation scheduling systems.

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