The effect of a diurnal period of supra-optimal temperature on the seed vigour of sunflower

The daily maximum temperature in the upper 20 mm of sandy soils is often higher than 45°C on cloudless days during November, December and January in the summer rainfall region of South Africa. At these supra-optimal temperatures, sunflower seedling vigour declines, resulting in a low plant population and lack of uniformity of plant density. The aim of this experiment was to quantify the effect of peak daily temperatures on the emergence of sunflower in a controlled environment. A3×4×5×3 factorial design with three cultivars, four growth chamber temperature treatments of 40, 45, 50 and 55°C, applied daily for periods of 3, 4, 5, 6 and 7 hours and three replicates of 20 seeds, were used. Seeds were planted to a depth of 25 mm in sand moistened to field capacity and incubated at 30°C. Treatments commenced after 24 hours. Emerged seedlings were counted daily. An emergence index, which takes both the rate and homogeneity of emergence into account, was calculated. Cultivars differed in their heat tolerance. Thi...