October and November are the months of year for pumpkins; they’re used in many activities from Halloween pumpkin carving to baking Thanksgiving pumpkin pie. However, less well-known events include the annual giant pumpkin festivals that take place all around the world. Farmers carefully nurture pumpkins for months, producing truly amazing results – the world record for the heaviest pumpkin weighed in at 2624.6 pounds at a competition in 2016 . In comparison, the world’s largest watermelon weighed 350.5 pounds and the largest lemon weighed in at 11.6 pounds . The history of giant pumpkin breeding is a fascinating science, with growers experimenting with different germination conditions, fertilizers, and horticultural oils to promote growth and deter pests. Every year, growers have bred bigger and bigger pumpkins with great success, consistently breeding pumpkins far larger than the 1857 record of 245 pounds .
So how is it that pumpkins can grow to such massive sizes? Pumpkins are a type of squash, a member of the Cucurbitacae family. The pumpkins bred for their size are of a different variety than your average Halloween pumpkin. Many science students will be familiar with the artificial selection process that bred modern day maize from the grass teosinte. Similarly, the giant pumpkin variety, known as the Atlantic Giant, is the result of years of crossing Mammoth pumpkins descending from the species Cucurbita maxima . Atlantic Giants have been artificially selected for plants that allocate more resources to fruit and seed production.
Artificial selection aside, we know rather little about the physiological reasons of how giant pumpkins come to be. Plants transport sugar from the source tissue where it produced to sink tissue where it is needed via the phloem. It is unknown, however, precisely how this sugar is allocated . Two major hypotheses in this area suggest fruit growth is limited by either sink activity or source activity. Another suggestion emphasizes the importance of vascular transport, the plant’s ability to move carbon and nutrients into the fruit . A study by Savage et al., compared the Atlantic Giants, the Mammoths, and the Hubbard squash (an ancestral pumpkin species) to shed light on this issue. They found similar photosynthetic rates between the three species, although Mammoths and Atlantic Giants allocated more of their carbon product to fruit production. They also found the giant pumpkins’ phloem structure is not significantly different, although they do have more phloem highways (as measured by cross-sectional area), which may result in greater volumes of carbon flux. Atlantic Giants and Mammoths also have longer maturation periods (up to 140 days) while Hubbard squash mature in about 100 days . Giant pumpkins have stronger outer tissue layers, which facilitate their rapid expansion.
However, the process of growing giant pumpkins is quite tedious. There are many environmental factors that contribute to a pumpkin’s size, such as temperature, rainfall, and fertilizer in the soil, but chance also plays a role. Giant pumpkins grow into lopsided mounds rather than into spheres due to the compressive forces from their own weight. One storm too many or a period of particularly low temperatures could ruin the crop by causing cracks or sudden growth that put excessive mechanical stress on the pumpkin . This can cause Atlantic Giants to literally explode.
Atlantic Giants are an amazing example of what biology and artificial selection in particular can do. Unfortunately, giant pumpkins probably aren’t economically sustainable, so you won’t be buying slices of giant pumpkin at the supermarket anytime soon. They consume massive amounts of resources for a single fruit per plant . They consist of up to 98% water and relatively little sugar and starch, making them unpalatable . Instead, competition-size pumpkins are often used for entertainment purposes, carved into boats or massive jack-o-lanterns. Now that would be a sight to see!