As global temperatures rise, heatwaves are becoming more frequent and intense, presenting a serious threat to nature. While much attention has been focused on how these extreme temperatures affect human and animal life, the impact on plants has received less consideration. However, a new study recently published in AoB PLANTS found that rising temperatures pose a serious risk to flowering plants.
Reproduction in flowering plants is a delicate process, involving the precise timing and functioning of floral organs and gametes (sperm and egg cells). When temperatures increase, this finely tuned system starts to break down. The most vulnerable stages include pollen development, pollen germination, and the growth of the pollen tube that carries the sperm to the egg.
Previous research has shown that heat can damage these sensitive stages, but how exactly does it impact the overall success of plant reproduction? And does it affect cross-pollination and self-pollination differently? These questions are critical for understanding the full impact of heat on plant populations, both wild and cultivated.
To explore these issues, the research team from University of California first investigated how extreme heat affects pollen tube growth and seed set, two essential components of successful plant reproduction. They exposed developing flowers to either normal temperatures of 25°C during the day and 20°C at night, or a simulated heatwave with 35°C days and 20°C nights.
Following the temperature treatments, researchers hand-pollinated the flowers with either cross-pollen or self-pollen derived from plants exposed to the same temperature conditions. This approach allowed them to determine not only the direct effects of heat on pollen tube growth but also any interactive effects that might arise from the combination of heat and the type of pollen used.
Under the cooler control temperatures, the survival rate of pollen tubes from self-pollen was about 27% lower than that of pollen from cross-pollination. However, the real shock came with the heat-treated flowers. The survival rate of pollen tubes dropped by 71% in cross-pollinated flowers and 77% in self-pollinated ones, compared to their control groups. Interestingly, there was no significant difference between pollen tube survival between heat-treated, cross-pollinated and self-pollinated flowers, suggesting that extreme heat may similarly damage the viability and function of both pollen types.
The study didn’t stop at pollen tube survival. Researchers also looked at seed set—the number of seeds produced following pollination—as a measure of reproductive success. Here, the impact of heat was even more pronounced. Extreme heat reduced seed set by 87%, regardless of whether the pollen was from cross-pollination or self-pollination. This reduction suggests that heatwaves could cause widespread reproductive failure in flowering plants, potentially leading to serious impacts on plant populations and entire ecosystems.
In addition to reducing seed set, extreme heat also affected pollen production during flower development. Anthers from heat-treated flowers produced about 20% less pollen than those from flowers kept at control temperatures. This reduction in pollen quantity, combined with the diminished viability of the pollen that is produced, suggests that plants developing under heatwave conditions face a double threat: fewer pollen grains and less vigorous pollen.
Our results demonstrate that heat can strongly increase the risk of pollen limitation through its effects on pollen at multiple stages of the pollination process (pollen production, pollen tube survival, and seed set). As we predicted, heat decreased pollen vigour and pollen tube growth, reducing the number of pollen tubes that reached the base of the style.
The impact of these findings is quite significant. For wild plant species, heatwaves could severely restrict mating and reproduction, reducing genetic diversity and limiting the ability of populations to adapt to changing environments. In agricultural systems, the consequences could be equally serious. Many crops depend on successful pollination to produce the fruits and seeds we eat. If pollen becomes scarce or less effective, it could lead to lower crop yields, threatening food security.
As climate change continues, heatwaves are likely to become more frequent and intense. This study highlights how important it is to understand and address the effects of extreme heat on plant reproduction. If we don’t take action, the natural and agricultural landscapes we rely on could be dramatically altered by the invisible but significant impacts of heat on how flowering plants reproduce.
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Rosenberger N.M., Hemberger J.A. and Williams N.M. (2024) “Heatwaves exacerbate pollen limitation through reductions in pollen production and pollen vigour” AoB PLANTS. https://doi.org/10.1093/aobpla/plae045
