The increasing intensity and frequency of extreme heat events threaten crop productivity globally. Certain phases of plant reproduction necessary for fertilization are highly sensitive to extreme heat, particularly during pollen development, germination, and tube elongation. However, few studies have assessed the effects of extreme heat on pollen performance in perennial crop plants. To understand how northern highbush blueberry pollen responds to high temperatures, we quantified pollen germination and pollen tube growth in vitro using four commercially relevant cultivars (Bluecrop, Elliott, Jersey, and Liberty) in climate-controlled chambers. We also tested recovery from high heat in ‘Bluecrop’ to determine whether pollen tubes can still germinate and grow after short bursts of extreme heat. We found the highest proportion of germinated pollen tetrads and the greatest pollen tube growth at 20 and 30 °C, and the lowest levels at 10 and 40 °C, with nearly complete inhibition at 40 °C. Exposure to between 30 and 40 °C revealed significant reduction in pollen germination and tube growth above 35 °C across all cultivars and assessment times. Exposure to 37.5 °C for only 4 hours resulted in substantial reductions in pollen germination and pollen tube growth, even after pollen was moved to optimal conditions of 25 °C. Extreme heat exposure, even for a short duration, significantly limits blueberry pollen germination and tube development. This is expected to have cascading effects on fruit set and crop yield. The nonreversibility of the effects on pollen highlights the need to prevent fields reaching damaging temperatures by developing crop monitoring and management strategies to protect crops during bloom.
To maximize yield of pollination-dependent agricultural crops, farmers must ensure that sufficient pollinators are present when flowers are open and viable. We characterized and compared the lower development threshold temperature, bloom phenology, and flower viability of five common cultivars of highbush blueberry (Vaccinium corymbosum L.) to enable prediction of when flowers would be available for pollination. Threshold temperatures of all cultivars were found to be very similar and range between 7 and 8 °C. Logistic regression was used to characterize bloom phenology for all cultivars under field and greenhouse conditions. Bloom phenology under greenhouse conditions was delayed ≈100 growing degree-days when compared with field conditions. Average flower viability was determined daily from first flower opening until 5 days after flower opening for each cultivar. Results indicated declining flower viability with increasing flower age with most flowers unsuitable for pollination more than 4 days after opening. Implications of these results for planning pollination of highbush blueberry fields are discussed.