Theoretical and functional complexity of white variegation of unripe fleshy fruits

Abstract

In many plant species, the bright colors of ripe fruit serve to attract frugivores to enable efficient seed dispersal. Here I show that the fleshy fruit of several dozens of species originating from Asia (southeastern, eastern and central), the Middle East, Africa, America (South, Central and North), Australia, Polynesia and Micronesia, with fruit usually larger than 1 cm, have white or light green spots while they are still unripe. In many of these species, while the spots are conspicuous, the unripe fruit is known to be poisonous, bitter or sour. I propose that this fruit syndrome may signal frugivores that the fruit is still unripe. Similarly to the succulent leaves of window-plants, these spots form windows that enable light to penetrate deeper into the photosynthetic layers in the developing fruit. This seems to be a solution to overcome the limitations of light harvest because of the high volume to surface ratio of developing fleshy fruits. The white or whitish variegation in these unripe fleshy fruits may serve at least five functions: 1) Windows for photosynthesis, 2) camouflage, 3) signaling to frugivores that they are not ripe (possibly sometimes a type of mutualism with frugivores), 4) signaling to frugivores that they are poisonous - aposematism, and 5) mimicking insect eggs to reduce egg laying. All these functions may be partly or fully simultaneous. Because these white spots appear in plants of diverse geographical and taxonomic origin, it is probably an old adaptation, and such a syndrome has appeared and been selected for many times.

Fruit colors have several functions: photosynthesis when young, defense in all stages and advertisement for dispersing animals when ripe.^1-4 Advertisement is an important aspect of plant/animal relationships and it was proposed long ago to attract seed dispersers that consume ripe fruits. The visual signals plants use to communicate with frugivores are usually the colors of the fruit itself,^1,2,4-6 although other plant parts (branches of the raceme, cyme or panicle, or leaves) may also participate in this visual signaling.^1,2,4,7 Repulsive fruit coloration in the form of animal mimicry,^8-12 Batesian mimicry^9-11 or chemically- and physically-defended based aposematism^3,11,13 has also been proposed.

Here I discuss several alternative but probably simultaneous and not mutually exclusive hypotheses (physiological and defensive) that may explain the common, but overlooked phenomenon of white variegation in unripe fleshy fruits.

While documenting white or light green variegation and other unexplained color patterns in plants I realized that many unripe fleshy fruits larger than several millimeters have white or light green variegation. These fleshy fruits belong to several dozen species originating from Asia (southeastern, eastern and central), the Middle East, Africa, America (South, Central and North), Australia, Polynesia and Micronesia. In spite of the numerous studies dealing with the ecology and evolution of fruit color, the generality of the phenomenon of white or light green spots on the surface of unripe fleshy fruit and the possible significance of this character had not been studied previously.

Data about the protective chemistry of the fruits was compiled from the literature. When not found, I tasted the unripe fruits in the field in order to distinguish between alkaloid-based bitterness, tannin-based bad taste and sour taste.

The many species described here (Fig 1)to have white spots on their unripe fleshy fruit reflect a very common phenomenon. For instance, the number of Ficus species described in Condit¹⁶ is about 10% of the global number of Ficus species, 50 of which have white or whitish spots on their unripe fleshy fruit. If the proportion of Ficus species with this mottled syndrome is the same in the remaining ca. 90% of the genus, several hundred more Ficus species have white or light green spots in their unripe fruit. Since I have examined only a small fraction of the world’s species with fleshy fruit, the number of species with this syndrome might be much higher.

Figure 1. (A) Fleshy fruit with white spots when green: Juglans regia. (B) Fleshy fruit with white spots when green: Ficus drupacea. (C) Fleshy fruit with white spots when green: Melia azedarach. (D) Fleshy fruit with white spots when green: Persea americana. (E) Fleshy fruit with white spots when green: Olea europaea.

The white or whitish variegation in these many unripe fleshy fruits may serve at least 5 functions: 1) Windows for photosynthesis, 2) camouflage, 3) signaling to frugivores that they are not ripe (possibly sometimes a type of mutualism with frugivores), 4) signaling to frugivores that they are poisonous - aposematism, and 5) mimicking insect eggs to reduce egg lying. All these functions may be partly or fully simultaneous. Because these white spots appear in plants of diverse geographical and taxonomic origin, I propose that this is an old adaptation, and that such a syndrome has appeared and been selected for many times.

Windows for Photosynthesis

Fleshy fruits are a widespread phenomenon in many plant families, and in plants of various growth habits and geographical origins. They cover a considerable size range from very small (only 2–3 mm in diameter) to very large (dozens of cm in diameter or length). Usually, they grow within a short time and result in a heavy physiological and sometimes also mechanical load on the limited resources and mechanical strength of the mother plant and of the individual branches carrying them. As a fleshy fruit increases in size, the ratio of the volume to surface increases drastically and the fruit’s photosynthetic potential per mass unit decreases. One of the proposed functions of fruits being green when immature is to add to the photosynthetic pool.²⁴ The contribution of the photosynthesis of the immature green fruit of non-tropical trees ranges from 2.3% in Quercus macrocarpa to 64.5% in Acer platanoides, but the common values range between ca. 10–35%.²⁵ Janzen²⁴ proposed that for tropical species this value might be even higher than 64.5%. Thus, in many cases, fruit photosynthesis contributes considerably to fruit development and maintenance not only by net CO₂ fixation, but probably also by refixation of CO₂ from mitochondrial respiration.^26,27

A number of leaf succulents that have a high volume-to-surface ratio have areas of a transparent epidermis and are therefore known as “window plants.” These clear areas (windows) are in the form of scattered dots, lines, wide streaks or even large portions of the leaves. These clear areas were proposed to function as windows for light penetration into deeper photosynthetic layers^28-31 although measurements in certain species and conditions failed to demonstrate a net increase in photosynthesis.^32,33

Clear windows that enable light penetration deeper into plant organs in leaf succulents^28-30 and in fleshy fruit as proposed here is not the only type of internal paths of illumination in plants. Both foliar sclereids and bundle sheath extensions in leaves, act as transparent windows that enrich the neighboring mesophyll areas with photosynthetically active radiation.^34-36 The windows in fleshy fruit, seen as white spots or flecks on the epidermis of unripe fruit, are therefore only a special case of a much more general phenomenon of structures that enrich internal layers in thick plant organs with photosynthetically active radiation. The fact that these spots are not found in very small fleshy fruits in which the surface/volume ratio is high, but are common in larger fleshy fruits where this ratio is small, also supports such an explanation.

Camouflage

The role of white variegation in plant camouflage sensu Givnish³⁷ and Allen and Knill³⁸ in leaves of understory plants discussed above seems to fit the possible role of white leaf variegation in unripe green fruits growing among the leaves in dense canopies. Givnish's³⁷ hypothesis has to be carefully tested before anything significant can be said about it.⁴ However, strong theoretical support emerges from zoology, since many animals inhabiting forest habitats use mottling as camouflage.^39-43

Signaling to Frugivores that the Fruit is Unripe

I also propose that the white spots might serve as a signal for frugivores that the fruit is unripe. The possible benefits for the signaler are that it 1) reduces damage to fruit that might later impede frugivory and seed dispersal since frugivores avoid eating damaged fruit (especially large fruit)^44-47; 2) reduces the consumption of immature seeds that will never germinate even if dispersed to a good niche; and 3) reduces abortion of damaged fruit before or after embryo maturation because fruit wounding can stimulate ethylene production and abscission.⁴⁸ The frugivores benefit from avoiding unripe fruit because 1) they avoid fruit at a poisonous or a bitter/sour phase,⁴⁹ and 2) those who are permanently or temporarily territorial refrain from spoiling future resources in their territory.⁵⁰

Aposematism

Proving that a certain fruit is poisonous or unpalatable is not easy. Certain frugivores can consume fruit that is poisonous to other animals⁵¹ and it is well known that specialist herbivores can consume plants that other herbivores cannot and that the specialists may even sequester their toxins.^52,53 Nerium oleander, milkweeds, Calotropis procera and many other highly toxic plants have their specialist herbivores, while they can kill many other herbivore species that make the mistake of consuming them. However, the unripe fruits of at least some of the species described above are known to be full of tannins (Juglans regia¹⁵), to be very poisonous (Melia azedarach, Bryonia syriaca,^19,21 Bryonia cretica²²) and/or bitter, or very sour (Averrhoa carambola, Ficus retusa, F. auriculata, Laurus nobilis, Juglans regia, Malus kirghisorum, Olea europaea, Pyrus korshinski, Pyrus salicifolia, Sabal adansonii) (I have tasted them in the field).

Since what is toxic for one animal might be harmless to another,⁵¹ and since it is practically impossible to identify all the historical selective agents that selected for the white variegated unripe fleshy fruit syndrome in most if not all plant species, it might be difficult to determine whether the white spots on unripe fleshy fruits serve as a reliable aposematic signal, or mimic such a signal. For an animal that is poisoned by the chemicals that are found in the unripe fruit, or that is sensitive to the bitter or sour taste, the spotted fruit is a reliable signal. If the animal is not sensitive to the chemical defense of the specific fruit, but avoids the fruit because the white variegation (the signal) is similar to unripe fruits of another, poisonous one, the spots may act as Batesian mimicry. The possibility of the existence of Müllerian and Batesian mimicry rings of species with defended and non-defended white-variegated unripe fleshy fruits should also be considered.

Mimicry of Insect Eggs

Insect egg mimicry is one of the first studied types of defensive mimicry in plants. The leaves of several Passiflora species have whitish-yellowish or yellow spots that have been suggested to reduce egg-laying by Heliconius butterflies, but the spots that reduced egg-laying seem to operate also for other plant and butterfly taxa.^4,54-56 Herbivorous insects tend to avoid ovipositing on plants with high egg loads to reduce competition with more mature and therefore more competitive insects.⁵⁷ I propose that such mimicry may be one of the functions of white and whitish spots as well as of colorful spots on unripe fleshy fruits, a parallel phenomenon not discussed here in depth.

White Spots on Unripe Green Fleshy Fruits: A General View

When discussing fruit traits that defend them from pre-dispersal predators and pathogens, the absolute and relative roles of unripe fruit characters against various types of enemies is not well known.⁵⁸ Since physiological aspects are also involved, the picture is even more complicated. Altogether, it is currently impossible to determine the role or roles of white spots on unripe green fleshy fruits. The proposed functions (windows for photosynthesis; camouflage; signaling to frugivores; mimicry of insect eggs) and possibly other unknown physiological and defensive functions may operate individually in certain species or developmental phases of the fruits, or it may be that some or even all these functions operate simultaneously. A similar complexity of mixed physiological and defense functions resulted hot debates concerning the functions of yellow and red autumn leaves and only when the simultaneous operation of adaptations to overcome the physiological and herbivory challenges was recognized the hot debate relaxed.^59,60 The case of unripe white-variegated fleshy fruits is a good demonstration of the functional and theoretical complexity of variegated phenomena.

A possible important issue is the possibility that different patterns and different sizes of white spots may sometimes have a different function. For instance, in Juglans regia, Persea americana, Passiflora edulis and Averrhoa carambola that have large fruits there are many small spots, almost evenly dispersed. In Melia azedarach, Olea europaea and Ficus spp, taxa with smaller fruits there are much less white spots on the fruits. In the toxic and terribly bitter Citrullus colocynthis, the large fruit has both small white spots and large white areas. There is a possibility that such differences reflect differences in function, when the contribution of camouflage, aposematism, insect egg mimicry, windows for photosynthesis, and possibly also mimicry of fungal attacks (in fruits of Citrullus colocynthis that also have large white areas) differ from species to species, from habitat to habitat, and possibly also differ with the ontogeny of the individual fruit. A good understanding of these differences among various species demands many years of comparative studies.

Testing the Hypotheses: A Very Complicated Issue

An important issue for hypothesis validity in general is the possibility of testing it. Intuitively it sometimes seems simple, but in reality it is very complicated to test defensive hypotheses in plants and there are contradicting theoretical considerations that I discuss below.

If white variegation has an aposematic role, then it should deter herbivores with previous foraging experience, but not naïve ones (unless the aversion has a genetic basis). However, some young and naïve herbivores learn from their parents to avoid certain dangerous plants.^61,62 When a defense system is strong enough to deter enemies (herbivores in this case) the lack, or small number of attacks may cause many to assume that if there are no attacks, there is no danger. This way of thinking is commonly wrong since it may just reflect that the defense operates. It may, however, cause statistical problems because the level of attacks is too low to allow statistical analysis. The problem is also that since aposematism is a very old and common phenomenon, there are no genetically naïve animals toward aposematism, but only inexperienced (usually young) individuals. When several defense mechanisms operate simultaneously it is very hard to distinguish between their relative defensive contributions. For instance, the possibility that thorny, spiny and prickly plants simultaneously use visual and olfactory aposematism and that some of these plants are also poisonous was never studied systematically, e.g,¹¹ and the same is true for pathogenic bacteria harboring spines.^63,64 Moreover, when non-defensive gains due to white variegation occur it may be difficult to distinguish them from direct defenses because better resource acquisition may allow a larger allocation to various other types of defense. All these mentioned factors and probably others that are still unknown complicate the analyses and evaluation of whatever results one gets.⁶⁵ All these complicated issues have not yet been studied in depth in even a single model species to give a realistic and balanced view. Table 1

Table 1. Description of the variegated white fruits.

Species	Family	Size at Maturity (cm)	Color at Maturity	Origin	Comments: Defense and Reference
Juglans regia	Juglandacea	5	Black	Southeastern Europe, Turkey	Tannins14,15
Ficus drupacea	Moraceae	3	Orange brown	India	Latex
Ficus retusa		0.5	purple	Tropical eastern Asia	Latex
Ficus auriculata (= roxburghii)		5–6	Reddish brown	India	Latex
Ficus carica		3–5	Yellow purple	Middle East	Latex, spots not in all genotypes
Fifty Ficus species of the 108 described by Condit16				Southeaster and easter Asia (24 species) Africa (10), America South, Central and North; (11) Australia (2)Polynesia (1) Micronesia (1) Papua (1)	latex17,18
Melia azedarach	Meliaceae	1–1.5	Yellow brownish	India	very poisonous when green19 and mature fruits eagerly eaten by birds and bats
Persea Americana (Avocado)	Lauraceae	7	Black	Central America
Laurus nobilis		2	black	Middle East	dots appear mostly in the distal part of green fruits
Olea europaea	Oleaceae	1–2	Yellow	Mediterranean	very bitter, tannins, oleuropreine, a glucosid and citric, malic and oxalic acids20
Malus kirghisorum	Rosaceae	1–3	yellow	central Asia
Crataegus pontica		1	yellow	Turkey, Iraq, Iran
Pyrus korshinski		2–3	yellow	Central Asia
Pyrus salicifolia		2–4	yellow	Caucasus
Bryonia syriaca	Cucurbitaceae	1	Orange red	Middle East	Poisonous glycoside21
Bryonia cretica		1	Orange red	Middle East	very poisonous22
Citrullus colocynthis		5–12	Yellow-orange	Middle East and Africa	Exceedingly bitter and several poisonous glycosides 23. The fruit has both white spots and large white areas
Cucurbita pepo		25		American tropical
Sabal adansonii ( = S. minor)	Arecaceae	2	Black	America
Argania sideroxylon	Sapotaceae	3	Brown	Morocco
Passiflora edulis	Passifloraceae	8	Crimson	Brazil	Whitish dots appear when the fruit is still small, on either on the proximal part or all over
Averrhoa carambola	Oxalidaceae	5	Yellow	Southeastern Asia	Very sour

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.