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.
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.
Please frugivore me for blushing
It is reasonable to guess that seed dispersal is the underlying selection force in the evolution of fleshy fruits attractive to vertebrate frugivores (2). The fruit pulp is the animal’s reward for ingesting the seeds within and depositing them unharmed (and with a fertilizer pile) away from the parent plant (see our pomegranate essay for more details on the various structures plants use to entice herbivores that are colloquially referred to as “fruit”). To do so, however, the would-be frugivore must first locate the fruit. The plant only benefits when a frugivore consumes ripe fruit containing viable, mature seeds, so it behooves the plant to ensure that the traits attracting seed dispersers also indicate ripeness. Being frugivores ourselves, we understand that a ripe fruit appeals to all of our senses. The same is true for other vertebrates, who cue in on fruit characteristics that change over the course of ripening: aroma, size, texture, chemical composition (Sugar! Fat! Um, antioxidants! No, seriously. See Ref. 3), and, of course, color (2–7).
Of all the contrivances a fruit might use to attract an herbivore, color has perhaps most strongly captured at least scientific interest. In The origin of species, Charles Darwin (8) speculated:
“…that a ripe strawberry or cherry is as pleasing to the eye as to the palate (…) will be admitted by everyone. But this beauty serves merely as a guide to birds and beasts, in order that the fruit may be devoured and the matured seeds disseminated.”
The typical color progression during fruit ripening is a shift from green (due to chlorophyll) to variously yellow, orange, or red (from carotenoids), or reddish-purplish, blue or black (from anthocyanins, although you can read our pigments essay about how betalains create this color spectrum in the taxonomic order Caryophyllales instead of anthocyanins). Day-time frugivores demonstrably use visual cues to find fruits, and frugivore species can have distinct color preferences, so it’s not wrong to assume that attracting helpful animals is likely a major function of these pigments. The evolution of fruit color and aroma in different fig species, for example, has clearly been strongly impacted by frugivory by bats or birds, which prefer different suites of fruit characteristics (called fruit syndromes) (2, 7).
It is possible, though, that we’ve overestimated the importance of herbivores in shaping fruit color, at least in plant taxa for which birds are not the primary seed disperser (3, 7, 9). I say this for two reasons. First, for a given fruit-frugivore pair, fruit traits other than color may be more important frugivore cues. This may be the case for Actinidia.
The Chinese word for kiwifruit is mihoutao, which means “monkey peach,” a reference to the monkeys which are the primary consumer and seed disperser of Actinidia throughout Asia (10), although a huge diversity of other mammals and birds contribute as well (11–14), including the Chinese ferret-badger, illustrated above eating Actindia fruit. I particularly like the name monkey peach because peaches are a model system for fruit trichomes that lent insight toward our understanding of kiwifruit fuzziness. Primates (including monkeys) like big, sweet fruit, and a lot of their fruit is colored green, brown, or yellowish-orange (6, 11, 15, 16). Actinidia comfortably fit in this range.
It seems less likely that primates prefer this drab color palette than they simply don’t care what color their fruit is. Anthropologists used to think that trichromatic vision in primates evolved primarily as a means to distinguish colorful ripe fruit from a background of green leaves. It now seems more likely, however, that primates use their trichromatic vision to distinguish immature leaves, which are often colored yellow or red (17–19), from green mature leaves (6).
If primates are visually detecting fruit using color as a cue, as long as the green of the ripe fruit is distinct from leaves, primates should be able to see it. Many primates are also fully capable of locating fruit by its aroma (6).
In the fig example I mentioned above, the bird-dispersed fig species produced fruits that were colorful and not particularly aromatic, whereas fruits dispersed by nocturnal bats were whitish-green and highly aromatic (see them pictured below; 7). It would seem that when it comes to primates or nocturnal seed dispersers, plants either don’t bother to expend resources producing pigments specifically to catch a frugivore’s eye, or their ability to do so was lost to no effect, and thus the loss persisted.
Second, pigments serve a variety of important physiological functions within plant tissue. If plant survival or reproduction is strongly impacted by the physiological role of pigments aside from their interaction with frugivores, then there’s an argument to be made that physiology, not just seed dispersal, exerts selection pressure on the evolution of fruit color.
More than just a pretty face
Fruit color might do more than just indicate ripeness or catch the eye of a would-be animal dispersal vector. Carotenoid pigments—the yellow, orange, and red pigments that include carotenes and lycopene—protect leaves and young fruit from ultraviolet radiation damage and serve as antioxidants (20). So do bluish-purplish-reddish flavonoid anthocyanins (21), which also might protect ripe fruit from fungal infection (3). Additionally, anthocyanins and carotenoids may contribute to a plant’s response to wounding or resistance to drought or freezing (3). Apple and pear growers, for example, know that those fruits need adequate light regimes and possibly cool temperatures in order for red anthocyanin pigments to fully develop in ripe fruit. This is likely because anthocyanins play a photoprotective and temperature response role and are only expressed in high-light conditions (3).
Anthocyanins and carotenoids may be present and functional in a green leaf or fruit even if we don’t see them because they are masked by chlorophyll.
Not easy being green
Chlorophyll is essential to photosynthesis. Its job is to absorb light energy and transfer that energy to the rest of the photosynthetic apparatus within the chloroplasts. Chlorophyll appears green because it preferentially absorbs the blue and red wavelengths of visible light and reflects back the remainder, which is mostly green. The series of chemical reactions involved in relieving the chlorophyll of its absorbed energy, by the way, include those that split water into its component hydrogen and oxygen and release the oxygen into the air (as O2). This evolutionary innovation in chemical engineering precipitated the evolution of all organisms that breathe oxygen.
And speaking of evolution and photosynthesis, the carpels that comprise a flower’s ovary evolved from photosynthetic leaves (22–24). The ovary starts green and photosynthetic and remains so through at least the early stages of fruit development in most species. The photosynthesis performed by the immature ovary can substantially offset the large carbon cost to the plant of making fruit and seeds (25–30), by as much as 65% (31). Species that remain green when ripe may maintain photosynthesis (30). This is the case with avocado (32), although I’m surprised that light penetrates the tough peel at all. The photosynthetic apparatus inside of green fruits might be more active with recycling internally respired carbon than fixing new atmospheric carbon, but I don’t know how that interacts with the chlorophyll pigment. With all green-ripe fruits, including kiwifruit and avocado, chlorophyll concentration is highest near the skin, within the range of light transmittance (29, 30, 33, 34).
Photosynthesis typically eventually ceases as fruits mature. The plant enzymatically breaks down the chlorophyll in the fruit, and green gives way to the colors indicative of ripe fruit. Upon chlorophyll degradation, other pigments that may have been present all along but masked by the chlorophyll come to the fore, especially those that provided a photoprotective or antioxidant role for the photosynthetic apparatus within chloroplasts (35). That is the primary mechanism responsible for colorful autumn foliage displays (20, 36).
Additionally, new fruit (37, 38) and leaf (20) pigments may be synthesized as these structures mature and age. The chloroplasts in which photosynthesis occurred may differentiate into chromoplasts, in which old and new carotenoid pigments accumulate (39).
How to stay green
The mechanisms responsible for green color retention have only been investigated for a few species. We already mentioned that avocado maintains photosynthesis. Other species fail to degrade chlorophyll because of mutations in the genes responsible for that process, whether photosynthesis is maintained or not. You may have seen heirloom tomato and bell pepper varieties that ripen to a chocolate brown color. In both of these nightshades, a mutation stops the function of a chlorophyll degradation enzyme, while the synthesis of carotenoids, including red lycopene, proceeds apace (40, 41). As the preschool-age painter in my house can gleefully attest, mixing red and green makes brown.
In a few other green fruits, chlorophyll is partially or fully degraded, but mutations prevent the synthesis of additional pigments. This has been shown for white/green grapes (Vitis) and honeydew melon (Cucumis), which fail synthesize anthocyanins or carotenoids, respectively (42, 43).
Neither of these scenarios is the case for kiwifruit. All A. chinensis fruit flesh starts out green. Golden kiwifruit does the usual fruit ripening thing, and chlorophyll degradation occurs. As with brilliant yellow autumn leaves, yellow-to-orange carotenoids are present behind all that chlorophyll and are apparent after the chlorophyll breaks down (44). The fuzzy, green A. chinensis only degrades a small amount of its chlorophyll (44, 45), so the bright green color blocks the yellow carotenoids throughout ripening. Having said that, though, golden kiwifruit actually does have slightly higher concentrations of carotenoids than does green kiwifruit (46), so they’ll be more yellow, anyway, and some carotenoids are synthesized only in the later stages of fruit ripening (37).
Green A.chinensis kiwifruit actually has functional genes for making all of the enzymes needed for chlorophyll degradation, but their expression is essentially blocked (44). We don’t know for sure exactly how or why green kiwifruit fails to degrade chlorophyll like the golden representatives of the A. chinensis species complex. Scientists suspect, however, that it has something to do with a group of plant hormones called cytokinins (44).
Kiwifruits and a photosynthetic fountain of youth
Cytokinins are involved in many transitions in plant development and growth (44). Important for our purposes here, they are likely instrumental in orchestrating senescence, which is a euphemistic term for the chemical equivalent of an estate sale. Senescence means the degradation of a particular structure. It starts when large molecules, such as chlorophyll, are broken down, and the plant repurposes the constituent molecules (47) or resorbs whichever of its component parts it can, especially limiting nutrients, like nitrogen and phosphorus (48). Senescence may then escalate to the level of organelle, cell, or organ, such as those autumn leaves losing their chlorophyll and eventually falling from the tree. Cytokinins actually delay or even reverse senescence as it relates to parts of the photosynthetic machinery (44). In short, then, chlorophyll senescence starts when cytokinins stop. Think of it as a kind of fruit menopause.
Green-fleshed A. chinensis produces more and different cytokinins than does yellow-fleshed A. chinensis (44). The high levels of cytokinins in green kiwifruit may have been due in part to its relatively low production of a set of compounds that moderate cytokinin (44). Think of this as leaving the cytokinin faucet running in the sink. What we know, then, is that the chlorophyll and chloroplasts in green-fleshed kiwifruit are maintained while awash in cytokinins, like some sort of photosynthetic fountain of youth.
As I mentioned previously (and at length in the kiwifruit fuzziness essay), Actinidia is prone to polyploidy. Polyploidy is created by genome duplication events. As the word “duplication” implies, these events introduce a huge volume of new genetic material into a population. Mutations occurring in the duplicated genetic material may provide mechanisms for adaptation to unique environments or, more likely, introduce benign variation into the population (49). We don’t know for sure why the green, fuzzy A. chinensis fails to degrade chlorophyll and maintains copious volumes of cytokinins, but it would be interesting to investigate if this departure from its golden, mild-climate-loving relatives was due to evolution following polyploidy.
On the other hand, smooth-skinned green A. arguta tends to share milder climates with golden A. chinensis, and it is also green, and we don’t know at this time if cytokinin dynamics are correlated with a failure to degrade chlorophyll in that species.
Genomically blundering your way to nutrition
Green color retention, then, in various kiwifruits may have arisen by genomic accident, and it may have persisted simply because it doesn’t need another color to attract seed dispersers, photosynthetic benefits or not.
That same “no harm, no foul” principle behind high chlorophyll expression in ripe green kiwifruit might also apply to other feats at which kiwifruits excel. For example, in our pineapple essay, we noted that kiwifruits, like pineapple, produce copious volumes of proteases, which are enzymes that break down protein. They attempt to do so with the proteins in your mouth or hands, which can feel weird or uncomfortable. We also noted that nobody seems to know why these fruits are so very prepared for protein degradation, and I don’t have an update for you here except for a hypothesis that it just might be another genomic accident that isn’t costly enough to warrant evolutionary extinction.
Similarly, if you Google kiwifruits, you’ll quickly ascertain from a bevy of enthusiastic internet nutritionists that kiwifruits are just exceptionally (!) chock full of vitamin C (ascorbic acid). All organisms need ascorbic acid, and it does have multiple physiological roles. In fruit, ascorbic acid is an antioxidant that protects photosynthesis in both leaves and fruit, and it is a precursor for the biosynthesis of other compounds. I asked William Laing, a Fellow at The New Zealand Institute for Plant & Food Research who has long studied ascorbic acid biosynthesis in Actinidia, why he thinks kiwifruits have such high constitutive concentrations of the stuff. He couldn’t give me a good reason, he told me, because science has not studied selective pressures affecting ascorbic acid concentrations in fruits. Well, they have in the sense that we know it would be fatal for a plant to produce none at all, but more or less? We don’t know the answer for vitamin C, much less the pharmacopeia of other organic acids that actually comprise more of kiwifruit biomass than does ascorbic acid. Dr. Laing shared my observation that ascorbic acid concentration varies widely among fruit species, and we know the key genes and enzymes involved, but we don’t know why.
We do know that genome duplication events are not a new thing for Actinidia. The researchers who sequenced the genome of A. chinensis determined that the genes responsible for the high levels of ascorbic acid in kiwifruit were among those originating with an ancestral genome duplication (49). At this point in time, then, Actinidia is genetically obligated to produce a lot of vitamin C as a result of a genomic blunder. All that ascorbic acid might be helpful, adaptive in some way that we don’t understand yet. Species occupying sunny or cold habitats, for example, Dr. Laing notes, might especially benefit from high antioxidant capacity, including ascorbic acid. Sunny and cold certainly describes the high altitude and arid habitats occupied by some Actinidia species, especially polyploid varieties (1), so there might be an adaptive story to be told with ascorbic acid in Actinidia. Then again it might be somewhat harmful to produce an unnecessarily large amount of ascorbic acid, a drain on resources. This seems unlikely to Dr. Laing, however, as ascorbic acid never comprises more than 1% of fruit biomass and is made from sugar, which fleshy fruits have in abundance. High ascorbic acid concentrations might also just be a benign artifact of evolutionary history.
I personally find this explanation of randomness to be the most satisfying because it mirrors the random genetic mutation in a human ancestor that knocked out our ability to synthesize our own ascorbic acid. The gene encoding the last enzyme in the vitamin C biosynthesis pathway is broken (50). We have to get it from our food.
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