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Zinc (Zn) deficiency and heat stress affect the productivity of cereal crops in many semi arid regions of the world. Zinc nutrition affects membrane integrity and activity of superoxide dismutase (SOD) enzymes which are also important to a plant’s ability to cope with high temperatures. Two experiments were conducted to examine the interaction between Zn nutrition and heat stress at two growth stages. Wheat was grown at deficient and adequate levels of Zn at 22/16A degrees C before being exposed to a short period of high temperature (40/20A degrees C). One experiment examined the response to a 3 day heat stress treatment at 10 days after anthesis and the second experiment examined the response to 6 days of heat stress in 30-day old seedlings. Varieties differing in thermotolerance and sensitivity to Zn deficiency were compared. Zinc deficiency and heat stress reduced kernel growth rates, kernel weight and grain yield and the greatest reductions were measured in Zn-deficient, heat stressed plants. The greater reduction in kernel growth rates from heat stress in Zn deficient plants occurred in both thermotolerant and sensitive varieties and was negatively associated with grain Zn concentration. Genetic differences in tolerance to Zn deficiency did not have consistent effects on the response to heat stress. Chlorophyll fluorescence and chlorophyll content were reduced and chloroplast ultrastructure was disrupted by heat stress and the effect was exacerbated by low supplies of Zn. While the responses to heat stress and low Zn were largely additive effects, the experiments demonstrated that low levels of Zn nutrition can exacerbate the adverse effects of short periods of heat stress on kernel growth and chloroplast function.