Extreme heat poses a major threat to flowering plants and bee pollinators, yet the direct and indirect consequences of heat stress are not well understood, particularly for spring-blooming crops and native solitary bees. Pollen germination and tube growth are highly sensitive to extreme heat, yet few studies have determined the effects of extreme heat on pollen performance of spring-blooming perennial crop plants. To understand how northern highbush blueberry (Vaccinium corymbosum L.) pollen responds to different temperatures, pollen germination and tube growth were measured in vitro using four cultivars (‘Bluecrop’, ‘Elliott’, ‘Jersey’, and ‘Liberty’) at temperatures between 10-40°C and 30-40°C. Recovery from high heat was also tested in ‘Bluecrop’ pollen to determine whether pollen tubes can still germinate and grow after short bursts of extreme heat. Across all tested cultivars, the greatest germination success and longest pollen tubes occurred at 20 and 30°C and the lowest at 10 and 40°C, with nearly complete inhibition at 40°C. Significant reduction in pollen germination and tube growth occurred at temperatures at and 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 tube growth, even after pollen was moved to optimal conditions of 25°C. These results demonstrate that exposure to extreme heat, even for a short duration, can significantly inhibit pollen germination and tube growth and may have cascading effects on fruit set and crop yields. To determine whether extreme heat affects blueberry fruit set and berry parameters, ‘Bluecrop’ bushes at several floral bud development stages (tight bud, bud swell, bud break, early pink bud, late pink bud) were exposed to heat stress (37.5°C) or normal (25°C) conditions for 4 hours and flowers were hand-pollinated at anthesis. Fruit set was significantly lower when heat treatment was applied at bud swell, but there were no clear patterns for pre-bloom heat stress affecting fruit quality. Given the negative consequences of extreme heat on blueberry pollen, I investigated whether extreme heat exposure to host plants also affects the behavior, fecundity, development, or survival of Osmia lignaria, a native solitary bee pollinator of blueberry. A no-choice semi-field cage experiment was conducted to provide female O. lignaria bees with host plants (blueberry, lacy phacelia, white clover) previously exposed to extreme heat (37.5°C) or normal temperatures (25°C) for 4 hours at 25% bloom. Despite a similar number of open flowers and floral visitation between the two temperature treatments, female bees provided heat stressed plants laid significantly fewer eggs. Progeny were provided similar quantities of pollen provisions between the two temperature treatments, yet larvae consuming pollen from heat stressed plants had significantly lower survival as larvae and adults. Delayed emergence and reduced adult longevity were observed when larvae consumed heat stressed pollen. This study is the first to document how short, field-realistic bursts of extreme heat exposure to flowering host plants can indirectly affect bee pollinators and their offspring, with important implications for crop pollination and native, solitary bee populations. Given the consequences of extreme heat on northern highbush blueberry pollen performance and O. lignaria bee fecundity and survival, poor pollen nutrition is a potential mechanism driving these responses. To determine whether extreme heat affects the nutritional content of blueberry pollen, ‘Bluecrop’ pollen was collected from bushes exposed to extreme heat (37.5°C) or normal temperatures (25°C) for 4 hours at five development stages. Exposure to extreme heat had variable consequences for pollen nutrients across floral development stages. Pollen starch content was significantly reduced following heat stress at the tight bud stage. No significant differences were observed for glucose or sucrose. These results suggest that exposure to heat had stronger effects on pollen starch content and may have disrupted sugar metabolism, utilization, or transport, with potential negative consequences for pollen performance and bee nutrition. Pollen protein was significantly reduced at bud swell following heat exposure, but no significant effects were observed at other development stages. Total amino acid content was significantly lower at bud swell following heat stress, driven by reductions in aspartate, glutamate, methionine, proline, and lysine, potentially explaining the observed negative consequences for pollen performance, fertilization, bee fecundity, and brood development. When comparing essential and non-essential amino acids, exposure to extreme heat had no significant effects on essential amino acids yet it significantly reduced non-essential amino acids at bud swell. Some essential amino acids were increased in response to heat stress, with potentially conflicting effects on plant production and bee nutrition. Overall, these findings suggest the greatest heat sensitivity in blueberries occurs at bud swell, with negative consequences of brief extreme heat stress on pollen performance, fruit production, and bee health. This research provides novel insight into the effects of extreme heat in blueberry pollination systems and suggests that studies evaluating the direct effects of heat stress on bees or plants may have underestimated the consequences for plants, bees, and their interactions.
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