CAPITULUM POSITION IN SUNFLOWER AFFECTS GRAIN TEMPERATURE AND DURATION OF GRAIN FILLING

Effects of sunflower capitulum position on the thermal regime of grain s and receptacle, and on the duration and rate of grain filling, were explored using a sunflower cultivar that normally hides its head withi n the upper leaves of the canopy shortly after anthesis. The thermal r egime was manipulated by restraining the head in a vertical position ( V heads), and by using black (Vw heads) or reflective (Vc heads) envel opes around vertically restrained heads. Capitulum temperature dynamic s and grain growth were followed in heads in the natural position (H h eads) and in the vertically restrained ones. Once the H heads had beco me hidden in the canopy, large differences in daytime temperature regi mes developed (38 degrees C maximum temperature in V grains, 10 degree s C higher than H grains and air temperature at the same time on sunny days). Vw heads were exposed to higher temperatures than V heads, and Vc heads were slightly warmer than H heads. Duration of grain filling in V heads was reduced by 2 to 6 days for grains at the periphery of the capitulum (P<0.05). Individual grain weight was reduced up to 21% (P<0.05), and final grain yield per head by 10%. Most of this effect w as attributable to shortening of the grain-filling period, as the rate of grain growth was not affected. Cultivars with horizontal heads wou ld have an advantage in yield potential with respect to cultivars that maintain the capitulum in a vertical position during grain filling.