The physiological effects of heat stress on anthesis and pollination in domesticated sunflowers (Helianthus annuus L.)

Abstract

Anthesis and pollination are precisely timed events, which change with the surrounding environment depending on light, temperature, and humidity. Plants and pollinators may exhibit mismatched behaviors as a result of temperature changes, and relatively little information exists about how heat stress and heat waves affect anthesis and pollination in sunflowers (Helianthus Annuus L.). This study investigates the effect of heat stress on timing of pollen emergence, style elongation rate, pollinator visits, pollen viability and stigma receptivity. We sampled during the anthesis period in a field setting under different summer conditions, including sunny and overcast days each with artificially heated and unheated plants; and a natural heatwave. Sunflower insect visits were video imaged in all test conditions, and individual insects were counted. A variety of staining and imaging techniques were used to determine pollen fertility, style elongation, and stigma receptivity. Heating sunflower capitula to approximately 10°C above the natural temperature resulted in a faster rate of style elongation, earlier pollen emergence and reduced stigma receptivity. In both the sunny and overcast days, time-lapse images showed that pollen emergence was 15 minutes earlier on heated plants than unheated ones. Heatwaves, however, bring in significantly more early morning visitors than unheated sunny, which was not observed by artificial heating. At temperatures of 40°C on sunny days, stigma receptivity and pollen fertility appears to significantly decrease over time. Sunflowers are considered a robust and temperature hardy plant, which maintains yields even when subjected to harsh conditions. This is one of the first studies to assess the effect of heat stress on sunflower anthesis. Our findings suggest that even though sunflower anthesis and pollination can be negatively affected by the high temperatures associated with heatwave events, it is able to adapt and manage stress by changing timing of pollen emergence and insects visits to earlier in the day before the high temperatures can have their detrimental effect.