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Heat stress is one of the major limitations to crop productivity worldwide. Global warming effects are expected to increase the number of hot days and increase the probability and intensity of heat stress events. Short periods (3–5 days) of heat stress with maximum temperatures exceeding 35°C often occur during late spring and early summer in some pyrethrum growing regions of Australia. These heat stress events usually coincide with pyrethrum flowering period. Pyrethrum is a perennial herbaceous plant which is commercially grown for extraction of pyrethrins which accumulate in the achenes of the flowers and are used as a natural insecticide. This experiment was conducted to understand the effects of timing of short periods of heat stress on flower development and pyrethrum yield. Plants were subjected to short periods of high temperature treatments (12 hr at 35–40°C) for three consecutive days at three flower maturity stages (early, mid, late). Control plants were grown at ambient temperature (10–25°C) throughout the flowering period. Exposure of pyrethrum plants to short periods of high temperature during the flowering period caused a significant reduction in the flower and pyrethrin yield. This was associated with the reduction in flower size and accelerated flower senescence. Exposure of pyrethrum plants to heat stress significantly increased the rate of flower development resulting in a shorter flowering period. Overall, plants grown under control treatment showed slower rate of flower development and longer duration flowering period. This resulted in longer duration of pyrethrin accumulation and higher yield of pyrethrins per flower. Timing and duration of heat stress significantly influenced pyrethrin yield per flower. Heat stress caused more severe yield reductions at early flowering than later in the flowering period. Research focusing on agronomic strategies, phenology and breeding for tolerance to heat stress is therefore important to cope with future climate changes and to obtain maximum pyrethrin yield.

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