Category Archives: Fruit

Kiwifruit 2: Why are they green?

Why are some kiwifruits green when they are ripe? Or avocados or honeydew melons? The answer involves genetic accidents, photosynthesis, hidden pigments, and the words “monkey peach.”

In our kiwifruit fuzziness essay we described how the type and density of trichomes—hairlike projections from the epidermis—on the skin of kiwifruits in the Actinidia chinensis species complex is correlated with the habitat in China to which a particular population is adapted and the ploidy level of its genome. Only polyploid (having multiple genome copies) Actinidia chinensis occupy the harshest environments—the high, arid reaches of western China—and have the highest trichome density and the longest trichomes. And those fuzzy, resilient, polyploid kiwifruits are all green on the inside (1). They are the plant kingdom’s version of an unshaved vegan after backcountry skiing for a week. The hardy plant had no trouble growing outside its plateau of origin and became the most common commercial kiwifruit in the world (A. chinensis var. deliciosa), followed closely by yellow-fleshed (“golden”), less fuzzy variants of the same species (A. chinensis var. chinensis).

An expanded view of the dozens of Actinidia species reveals orange, red, and purplish pigments that color fruits in the genus. While beautiful, this warm palette strikes me as noteworthy only in contrast to the bright green displayed by the fuzzy A. chinensis var. deliciosa that initially grabbed my attention, and, later, in green kiwiberries (A. arguta). A non-green (for lack of better terminology, “colorful”) ripe fruit, after all, is a common end point for species with fleshy fruit.

Fig. 1 from Crowhurst et al. (2008) of some fruit diversity in the kiwifruit genus Actinidia. We describe the botany and anatomy of kiwifruits in our kiwifruit fuzziness essay.

It is not difficult, however, to bring to mind other examples of species with green-ripe fruit: avocado, green grapes, some citrus, honeydew melon (I’m specifically thinking here of the pericarp or mesocarp tissue under the skin and exclude from this discussion immature fruits that lose their greenness when fully ripe, like green beans and olives). Green ripe fruit, then, in Actinidia and other taxa, seems to me to be something to explain. What, if any, function might it serve, and what are the mechanisms responsible?

While the literature on the subject is far from exhaustive, there is a fairly pedestrian explanation at least for the mechanism, if not any adaptive function, of unusually green fruit flesh outside of Actinidia: fruits start green, and straightforward mutations in a few key genes cause them to remain so. Like that intrepid, hirsute montane vegan, though, Actinidia performs the task a little differently, and it is a bit of a mystery. To understand why that is, we need some backstory on pigments in fruit and how and why they change as fruit ripens, with a focus on Actinidia. Continue reading

Kiwifruit 1: Why are they so fuzzy?

Kiwifruit is not covered in hairs. It’s covered in trichomes. And only if you’re talking about green Actinidia chinensis var. deliciosa. But, why? One answer is: pretty much to keep it from drying out. Another is: because it’s a polyploid from western China and was kind of chosen at random to be the most commonly grown kiwifruit, and they’re not all fuzzy. Those aren’t mutually exclusive answers. Put on your ecophysiology hats and grab a paring knife.

Think of fruit growth as a balancing act between ingoing and outgoing fluxes. When the balance is positive, fruits grow. When it is negative, they shrink—or shrivel. The main fluxes in question are carbon and water, which enter the fruit from the xylem and phloem of the plant vascular system. Water is lost mainly to the atmosphere via transpiration (evaporative water lost through stomata and other pores and from the skin surface). Keeping the ledger positive isn’t an easy job for a fruit. Hot, dry, and windy weather encourages transpiration and thereby increases the odds that a fruit will experience water stress. Excessive sunlight may cause sunburn. Fruits also need to avoid attack from pathogens and herbivores before the seeds within mature. A fruit’s skin—its cuticle and epidermis—is its first line of defense against abiotic and biotic threats. Some fruits resort to creative coverings to get the job done.

Here I’ll take a close look at the skin of kiwifruits. Why, exactly, are they so fuzzy?

A heart-shaped green kiwifruit (Actinidia chinensis var. deliciosa), covered in fuzzy trichomes

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Pirates of the Carob Bean

Maybe the name takes you back to gentler days of Moosewood Cookbook and the dusty spicy local co-op. Or maybe you were a kid back then and fell for a chocolate bait-and-switch. Whether you are sweetly nostalgic or wary and resentful, it’s worth giving carob another chance. Katherine argues that it’s time to pull this earthy crunchy 70’s food into the superfood age. She offers foraging tips and recipes to help you get to know carob on its own terms.

From November through January, the carob trees in my neighborhood dangle hard, lumpy, dark brown fruits resembling lacquered cat turds. They are delicious and nutritious and of course I collect them. I am, without apology, a pod plundering, legume looting, pirate of the carob bean. CarobPiratesIf you seek adventure and happen to live in California, Arizona, or on the Mediterranean coast, you can probably pilfer some carob fruits yourself and play with them in your kitchen. If you lack local trees or the pirate spirit, you can order carob powder and even whole carob beans with one simple click.

Although plundering season begins just as the year is ending, I always wait until January to gather carob fruits for two reasons. First, carob functions mainly as a healthful chocolate substitute, and during the holiday season, fake chocolate just seems sad. In January, however, eating locally foraged carob feels virtuous and resourceful. Second, November and December are when my local carobs make the flowers that will produce the next year’s crop, and those flowers smell like a pirate’s nether parts after a shore leave. Or so I imagine, and not without precedent. A man who should have been inured to such salty smells, Pliny the Elder, natural historian and commander in the Roman Imperial Navy, described the flowers as having “a very powerful odor.” It’s not clear why these flowers have a sort of seaman scent, since the main volatiles wafting from the flowers – linalools and farnesene – smell like lilies and gardenias (Custódio et al., 2006). In any case, I keep my distance until the flowers have finished mating season.

Carob trees

Despite their stinky flowers, carobs make great street trees and produce a valuable crop in many Mediterranean-type climates. They are beautiful, tolerant of dry and poor soils, pest resistant, and tidy. Carobs are legumes – like familiar peas and beans – but they belong to a different branch of the legume family (Caesalpinioideae), one that contains mostly trees and woody shrubs with tough inedible fruit (Legume Phylogeny Working Group, 2017). Carob pods look about as edible as Jack Sparrow’s boots, and the species’ scientific name, Ceratonia siliqua, means “horny long pod,” which well captures the intimidating nature of their leathery fruit. But as you will see below, the fruits are easy to harvest and process, and their sweet pulp is worth seeking out. Continue reading

A holiday pineapple for the table

This deep dive into pineapple anatomy is our contribution this year to the very fun Advent Botany essay collection, a celebration of plants that are at least somewhat tangentially connected to the winter holidays. In previous years we’ve contributed essays on figs, peppermint, and sugar.

December is the time to bring out the fancy Christmas china, polish the silver pitchers, and . . . bedeck your best bromeliads. In 2017, as in 1700, no proper hostess can be without a pineapple for her centerpiece. Here we unpack the botany of pineapple, which is as complicated and fabulous as its cultural history. A proper hostess, after all, should also be able to dazzle her guests with tales of tropical fruit morphology. Continue reading

Botany Lab of the Month: Jack-O-Lantern

Happy National Pumpkin Day! Turn carving your Halloween Jack-O-Lantern into a plant dissection exercise.

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The first Jack-O-Lanterns were carved out of turnips in 17th-century Ireland. While the large, starchy hypocotyls (fused stem and taproot) of cruciferous vegetables are anatomically fascinating, this post will be about the stuff you are more likely cutting through to make a modern Jack-O-Lantern out of squash. Continue reading

Preserving diversity with some peach-mint jam

We are knee deep in peach season, and now is the time to gather the most diverse array of peaches you can find and unite them in jam. Katherine reports on some new discoveries about the genetics behind peach diversity and argues for minting up your peach jam.

Jam inspiration

Fresh peaches at their peak are fuzzy little miracles, glorious just as they are. But peaches cooked into jam and spread across rough toast lose their buttery mouthfeel and dripping juice. To compensate for textural changes, processed peaches need a bit more adornment to heighten their flavor, even if it’s only a sprinkling of sugar. Normally I am not tempted to meddle with perfection by adding ginger or lavender or other flavors to peach jam. This year, however, as I plotted my jam strategy, the unusual juxtaposition of peach and mint found its way into my imagination over and over again, like the insistent echo of radio news playing in the background. Peach and mint, peach and mint, peach and mint – almost becoming a single word. To quiet the voice in my head I had to make some peach-mint jam. The odd combination turned out to be wonderful, and I’m now ready to submit the recipe to a candid world. As we will see below, it’s not without precedent. Mmmmmmpeachmint jam. Continue reading

Buddha’s hand citrons and a wish for peace on earth in 2017

Winter is the season for citrus fruit, and January is the month for breaking out of old routines, so stop staring at your navel and learn about one of the weirder citrus varieties.

I’ll never forget the day one of my general botany students brought to class a Buddha’s hand citron, pulled from a tree right outside our classroom. I had only recently moved to northern California from Indiana, and I’d never seen anything like it: it was a monstrous mass of a dozen pointed twisted fingers splayed irregularly from a stout base. It had the firm heft and girth of a grapefruit and the unmistakable pebbled skin of a citrus fruit, so I wondered whether my student had found a grossly deformed grapefruit; but the oil in the peel smelled heavenly and not at all like a grapefruit. In class we cut through a big finger and found no juicy segments, just white citrus pith all the way through.

Immature Buddha's hand on the tree

Immature Buddha’s hand on the tree

We eventually discovered that this fascinating fruit was a Buddha’s hand citron, Citrus medica variety sarcodactylis, meaning fleshy (sarco-) fingered (-dactyl) citron. Since that day many years ago I’ve become an unapologetic (if surreptitious) collector of the fruits from that same campus tree. The citrons do not drop from the tree on their own, yet I often find one or two lying nearby, probably torn off by a curious tourist or student and then abandoned. Obviously these fruits need a good home, and where better than the window sill in my office?

The first time I left one closed up in my office over a weekend, I opened the door on Monday morning to a waft of fruity floral aroma. It turns out that many people in China and India use the fruit to scent the air, although in west Asia and Europe the fleshy fingers are more often candied or used to flavor alcohol. I do both: the fruits make my office smell nice until they are fully yellow, and then I cook them.

It can be difficult or expensive to get your own hands on a fingered citron, but it’s easy to find a navel orange almost any time of the year. Fortunately, the patterns underlying the morphology of the fingered fruit can also be seen in an everyday navel orange. Between our photos of Buddha’s hands and your own navel orange, you should be able to follow along at home. Continue reading

How giant pumpkins got so big: A Q&A with Jessica Savage

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).

Beni Meier with his 2014 record-winning 2323-pound pumpkin, presumably a specimen of the Atlantic Giant variety of Cucurbita maxima. Photo from here.

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Triple threat watermelon

Will seedless watermelons make us superhuman or turn our children into giants?  Hardly, but they do give home cooks the power to count chromosomes without a microscope.   Just a knife or a hard thunk on the sidewalk are enough to get a watermelon to spill its genetic guts.

If you were reading a Hearst Corporation newspaper in late 1937, you might have thought humanity would eventually be swallowed up by giant carnivorous plants, unwittingly unleashed by uncontrolled biotechnology.  The San Francisco Examiner reported on November 21st of that year that the discovery of an “elixir of growth,” meant that “…science may at last have a grip on the steering wheel of evolution, and be able to produce at will almost any kind of species…”  including “…a plague of man-eating ones.”  In 1937 Americans had much more important things to worry about, just as we do now.  Still, that discovery may in fact have threatened one cherished aspect of the American way of life by triggering the slow demise of late summer state fair watermelon seed spitting contests.  It doubtlessly paved the way for seedless watermelons, and in 2014 the total harvest of seedless watermelons on American farms – nearly 700 thousand tons – outweighed the seeded watermelon harvest more than 13 to 1 (USDA National Watermelon Report). A similar pattern is emerging this year.  Is there no stopping the attack of the seedless watermelons?

Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

CLICK to read. Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

And more important, how is it even possible to get seedless fruit from an annual plant?  From a plant whose only mode of reproduction is through those very seeds?  From a plant that cannot make suckers as bananas do and cannot be perpetuated endlessly through grafts like fruit trees and vines?   Such is the challenge posed every single year by watermelons, but thanks to the “elixir of growth” discovered by Albert Blakeslee and subsequent work by Hitoshi Kihara, one of the most prominent agricultural geneticists of the 20th century, the world has an elegant solution. Breeders continually improve the varieties available, and consumer demand keeps growing, yet seedless watermelon production methods have remained essentially unchanged for three quarters of a century. Continue reading

The new apples: an explosion of crisp pink honey sweet snow white candy crunch

What’s in a name?  An apple with an old fashioned name could taste as sweet, but it might not sell.  The most sought after branded varieties reveal what people look for in an apple: sweet and crunchy and bright white inside.  Do the fruits live up to their names?  Are Honeycrisp apples crunchier than others?  Do Arctics actually stay white?  We zoom in on the cells to find out.

Some of you will remember the era when the Superbowl halftime show repeatedly featured Up With People.  That was around the time when Granny Smiths arrived in our supermarkets and finally gave Americans a third apple, a tart and crunchy alternative to red and golden delicious.  Those were simple days.  Continue reading