Biologist Jessica Savage answers a few of our questions about her research on the physiology behind giant pumpkin size.

In October 2014, a giant pumpkin grown by Beni Meier of Switzerland tipped the scales at 1056 kilograms (2323 pounds) and set a new world record for the heaviest pumpkin ever weighed. Modern competitive pumpkin growers have been imposing very strong selection on pumpkin size for decades. Pumpkin fruit size keeps climbing, and old records are broken every year or two (Savage et al. 2015).

The giant pumpkins breaking records are probably not the same kinds of squashes that are destined to be Halloween jack-o-lanterns or pumpkin pie. The behemoths are primarily fruit specimens of the Atlantic Giant variety of the species Cucurbita maxima (Savage et al. 2015). C. maxima varieties are collectively known as the hubbard squashes (see our post on squash diversity for information on the several squash species that earn the name “pumpkin”). Many hubbard squash varieties are grown as food and have been under selection for delicious, sweet, firm fruit. If you buy canned “pumpkin” for pie, you’re probably eating one of those sublime hubbard varieties.

The edible hubbard varieties do produce impressively big fruit, but they are nowhere near the size of the Atlantic Giants (Savage et al. 2015). “These fruits really present an interesting study system because they have been under strong selection from humans for years for their size,” says Jessica Savage, a biologist who studies the evolution and physiology of giant pumpkins. Savage continues: “While it is desirable to breed many fruits to be large, in the case of edible fruits, there are other things to consider including taste. This is not a concern for the giant pumpkin growers. In fact, giant pumpkins tend to be fibrous and are not considered palatable to many. All that matters is size.”

So, how did giant pumpkins get to be so large? What physiologically separates the giant pumpkin varieties from their brethren destined for pumpkin spice lattes? The short answer is that giant pumpkin plants produce more phloem than do plants of other squash varieties (Savage et al. 2015). Phloem is the component of a plant’s vascular system that moves solutes, mostly sugar, around the plant. Xylem is the other main component of plant vasculature, and its primary job is to move water and nutrients from the soil through the plant. We discussed xylem function in our post on maple syrup. Jessica Savage and her colleagues studied the anatomy and physiology of both phloem and xylem and a variety of other components of squash plants’ carbon acquisition and allocation in an effort to determine how Atlantic Giant squashes achieve giant size (Savage et al. 2015).

Savage and her colleagues grew an edible hubbard squash variety and Atlantic Giants under identical conditions in a greenhouse and measured several aspects of the plants’ carbon (sugar) supply chain, from the carbon source in the leaves, through the vascular transport system, to the carbon sink: the fruit. Squashes and other species in the squash family (Cucurbitaceae) are particularly suited to studies of plant vasculature, as Jessica Savage explains below in a special interview post to explain phloem function and how it helps us understand the evolution of size in giant pumpkins.

Q&A with Jessica Savage about phloem and giant pumpkin physiology

Question 1: How do you like to explain phloem and its function to a general audience? “With pictures!” is a good beginning to an answer here.

Jessica Savage: “Plants are able to “feed themselves” by producing sugar in their leaves through photosynthesis but they need to be able to move this sugar to different parts of their body to sustain growth in various organs like roots and fruits/flowers. They achieve this using part of their vascular tissue, the phloem, which is specialized in transporting sugars. However, plants, different than animals do not have a heart to drive circulation of fluid in their vascular system, instead, they rely on mostly passive processes.”

“In the phloem, transport is driven by the movement of water into and out of the tissue in different parts of the plant. In their leaves, which we refer to as “source tissue” because it is the source of sugar, the phloem contains a high amount of sugar. This sugar draws extra water into the cells making them full and swollen. Meanwhile, other locations in the plant act as “sink tissues” (for example the roots or fruits) because they pull sugar out of the phloem to use for growth and other processes. In these locations, sugar concentrations are low inside the phloem. This causes water to leave the cells making them more limp. Because the source and sink tissue are connected, water moves from the full cells in the source to the more limp cells in the sink tissue. This “pressure-driven” pump allows for plants to move sugars from where it is plentiful to where it is more scarce.”

Question 2: Why do pumpkins (and other cucurbits) lend themselves to the study of plant vasculature?

Jessica Savage: “Pumpkins and cucurbits are great for studying sugar transport because the cells in their phloem are wide and have large pores connecting them. This makes them easy to see using a microscope. They also have some great properties which help us study their physiology, for example, the cells in their leaves are very connected. This seemingly simple characteristic is very important because if we want to put dye into the phloem to watch phloem transport in living plants, we can put the dye in any of the cells in the leaves and it will eventually get into the phloem.”

Question 3: On the other side of that coin, how does understanding phloem function and its evolution help us understand how giant pumpkins got so giant?

Jessica Savage: “The size of giant pumpkins to me is intriguing because it presents what I often refer to as a “traffic control problem”. In normal sized pumpkins, there are many small fruit and sugar is transport in different directions to feed them all. This is similar to traffic in the country where everyone is going to work in different directions and there is not too much traffic on any one road. However, in giant pumpkins a large amount of sugar is going into one fruit. This is more like the situation where you have a big city and everyone is going into this city for work in the morning. As a result, there needs to be either more roads or roads with a higher capacity to support the high amount of traffic entering the city. The same is true for giant pumpkins, either they need more phloem or phloem with a higher transport capacity to sustain these large fruits, especially considering that giant pumpkins can move a couple pounds of sugar into their fruit in a day.”

“When we look at giant pumpkins, we find that the solution that pumpkins have to this problem is producing more roads, or more phloem. When you compare different varieties of Cucurbita maxima, the giant varieties that have been bred for pumpkin competitions all produce more phloem. This allows them to move more carbon into individual organs than smaller fruited varieties. This changes is likely important in supporting the rapid growth rates we see in these fruits.”

Figuring out the mystery of how Atlantic Giants produce massive fruit

To figure out how giant pumpkin size was achieved, Savage and her colleagues traced the anatomy of sugar production, transport and consumption from leaf to fruit. In their greenhouse study, Savage and her colleagues (2015) reported that the leaves from both varieties photosynthesized at about the same rate, so each square centimeter of leaf brought similar amounts of sugar into the plant in a given amount of time. And the hubbards actually produced slightly more leaf area than did the Atlantic Giants, so the total sugar inputs were similar or tipped slightly in favor of hubbards. The vascular system of both types had similar amounts of xylem (the amount of area in cross section occupied by xylem). Both the aboveground and belowground vegetative parts of the plants grew at roughly the same rate. The Atlantic Giant, though, made more flowers, and their fruits grew much faster than the hubbards. How? The vascular transport system—composed of stems, the petioles connecting the leaf blades to the stem, and the pedicels connecting the fruit to the stem (the “handle” of a ripe pumpkin)–of Atlantic Giants contained much more phloem than did the petiole-stem-pedicel pipe system of hubbards (Savage et al. 2015). The structure and function of the phloem itself was very similar for the different squash varieties. Only the amount of phloem changed, providing the capacity to transport the high volume of sugar needed to grow the giant fruit.

Additionally, Atlantic Giants seem to take a substantially longer time to develop the ovary (the immature fruit prior to pollination, as well explained in Katherine’s post about watermelon, a close relative of pumpkin) and mature the fruit, with the longer maturation time translating to larger overall size (Savage et al. 2015). Further, Atlantic Giants have a thinner, softer outer fruit wall (exocarp) than do edible hubbards, which may increase the ability of Atlantic Giants to expand to large size (Savage et al. 2015). I imagine the softer, more elastic outer fruit wall would decrease its storage capacity, but that’s not important for these competitively grown pumpkins, unlike in the hubbards you expect to store for months to eat through the winter and spring.

Understanding how squashes have achieved the feat of large fruit size is interesting in and by itself, but the work of Savage and her colleagues also has strong implications for understanding the limits of agricultural yield and fruit and seed production strategies of wild plants (Savage et al. 2015).

Many thanks to Jessica Savage for helping us understand phloem function and her outstanding research on the physiology and evolution of giant pumpkins. We wish everyone a happy Halloween!

References

Savage, J. A., D. F. Haines, and N. M. Holbrook. 2015. The making of giant pumpkins: how selective breeding changed the phloem of Cucurbita maxima from source to sink. Plant, Cell and Environment 38:1543-1554.