COMMENTARY

Why is latex usually white and only sometimes yellow, orangeor red? Simultaneous visual and chemical plant defense

Simcha Lev-Yadun

Received: 20 May 2014 / Accepted: 10 June 2014

� Springer Basel 2014

Abstract Latex is a toxic and sticky defense substance

produced by about 20,000 plant species and secreted fol-

lowing wounding. It defends the wounded plants by both

chemical and physical components. A striking fact about

latex is that the majority of plants produce it white,

although yellow, orange, and red latex can sometimes be

found. Theoretically, it is possible that there is no function

or importance to latex’s color, but it seems unlikely. It is

also possible that there are chemical constraints that dictate

the production of white latex, but the various colors indi-

cate that this is not a valid explanation. However, since

white latex evolved independently many times in various

plant taxa, there should have been a strong selection for

latex being white, a possibility that has never been

addressed, to my knowledge. I propose that latex is gen-

erally white because white is the best solution for visual

aposematism under the lighted conditions within and under

plant canopies, on the background of typical leaf and bark

color, even towards color-blind animals. Yellow, orange,

and red latex types should naturally also be considered as

aposematic because these are typical colors of toxic and

aposematic organisms. In addition, latex-exuding plants

simultaneously use a chemically based, non-visual olfac-

tory aposematism because wounding perse also causes the

emission of non-latex defensive volatiles and because latex

contains various but little-studied defensive volatiles. The

volatile defensive aspect of latex exudation may also

operate against nocturnal or blind herbivores.

Keywords Aposematic � Chemical defense � Herbivory �Insects � Latex � Mullerian mimicry rings � Volatiles

Introduction

Latex is a common chemical and physical plant defense

that exudates following wounding from canals, named

laticifers. It has evolved repeatedly, and is known from

over 20,000 species belonging to more than 40 families

(Agrawal and Konno 2009). Latex protects plants not only

from herbivores, especially insects, but also from fungi,

bacteria and even vertebrate herbivores, as well as it

physically seals wounds (Fahn 1979; Konno 2011). Latex

contains bioactive compounds including alkaloids, cardiac

glycosides, terpenes, digestive proteins, sugar-mimicking

substances, and many other components (Agrawal and

Konno 2009; Konno 2011; Mithofer and Boland 2012), but

the full repertoire of the compounds that repel or damage

herbivores and the identity of these herbivores is beyond

the scope of this essay. While these substances repel both

invertebrates and vertebrates, certain specialized herbi-

vores (especially various arthropods), may not be deterred,

but rather attracted to them (e.g., Zalucki et al. 1990;

Agrawal et al. 2012). Some even sequester them (Nishida

2002; Opitz and Muller 2009) for their own defense.

Moreover, latex commonly sticks insects to the plant and

by this causes their death, or glues their mouthparts, pre-

venting them from feeding (Dussourd 1995; Konno 2011).

The fact that various insect types sever and drain latex

before they consume plant tissues (e.g., Dussourd and

Eisner 1987) is an excellent demonstration of its defensive

role (Mithofer and Boland 2012). Latex exudes from plants

only after wounding, a situation well known to involve the

Handling Editor: Michael Heethoff.

S. Lev-Yadun (&)

Department of Biology and Environment, Faculty of Natural

Sciences, University of Haifa-Oranim, 36006 Tivon, Israel

e-mail: levyadun@research.haifa.ac.il

Chemoecology

DOI 10.1007/s00049-014-0160-7 CHEMOECOLOGY

123

simultaneous emitting of defensive volatiles, including

ones that attract predators and parasitoids (e.g., De Moraes

et al. 1998; Kessler and Baldwin 2001) and volatiles that

repel various herbivores (Birkett et al. 2000; De Moraes

et al. 2001). Altogether, the combination of toxicity and

mechanical defense causes many plants that produce latex

to be considered toxic or otherwise non-palatable or even

deadly to many insects, and when highly toxic (e.g., Ne-

rium oleander) even to various large vertebrate herbivores

(Knight and Walter 2001).

A striking fact about latex is that the majority of plants

produce it white, e.g., Papaver spp., Ficus elastica,

Euphorbia spp., Calotropis procera (Konno 2011; Mitho-

fer and Boland 2012), although yellow and orange, e.g.,

Nerium indicum, and red latex, e.g., Cannabis spp., Croton

lechleri also exist (Langenheim 2003; Agrawal and Konno

2009; Konno 2011). The white color is the outcome of the

rubber particles dispersed in the latex (Agrawal and Konno

2009), rubber being an important component of latex

stickiness. There are three theoretical options concerning

the function of white latex: (1) that there is no function or

importance to its color, (2) that there are chemical con-

straints for the production of latex in other colors, and (3)

that the white color has a visual advantage, a specific

function. The fact that latex can be yellow, orange or red

indicates that there is no inherent reason to produce it

almost only in white. Since white is its dominant color and

since white latex has evolved independently many times in

various plant lineages (e.g., Hagel et al. 2008), there should

have been a strong selection for white latex. This possi-

bility has never been addressed, to the best of my

knowledge. Moreover, because latex (or resin) color was

never considered to have a significant if any function, there

are no data on the relative frequency of various latex colors

or about the ecological affinities of these colors.

Here, I propose that latex is generally white because

white is the best solution for visual aposematism under the

lighted conditions, within and under plant canopies or

towards color-blind animals. White will also stand out

visually against the background of any typical color of

leaves, unripe fruit, branches, and stems. I also propose that

because latex signals both chemical and physical defenses,

many Mullerian mimicry rings of plants with white latex

probably exist, and that yellow, orange and red latex types

should also be considered to be visually aposematic,

especially in well-illuminated habitats. Latex, in all colors,

has a chemical-based potential because of its olfactory

component (see below). With the current low level of

understanding of plant aposematism compared to animal

aposematism, it is impossible to relate specific visual

(specific colors that are not white) not withstanding specific

chemical aposematic signaling to specific environments

and examining such possible relationships is an open

question.

Visual aposematism in plants

Aposematic (warning) coloration is a biological phenom-

enon in which poisonous, spiny, dangerous, unpalatable, or

otherwise unprofitable organisms visually advertise these

qualities to animals. The evolution of aposematic traits is

based on the ability of target enemies to associate the

visual or olfactory signal (or other types of signals) with

the risk, damage, or non-profitable handling, and later to

avoid such organisms as prey. Typical colors of aposematic

animals are yellow, orange, red, purple, black, white,

brown, and combinations of these (Cott 1940; Wickler

1968; Ruxton et al. 2004) and a large array of chemicals of

various types (Weldon 2013), and the same is true of

visually aposematic spiny and toxic plants (Lev-Yadun

2001, 2009a; Schaefer and Ruxton 2011). In addition to

learning, an innate component of aversion from certain

colors (e.g., yellow, black, red) and their combinations also

exists (Ruxton et al. 2004, and citations therein). In plants,

chemical-based olfactory aposematism of chemically

defended aposematism may operate independently or

simultaneously with the visual one (see below).

The high visibility of white markings

A white signal, when contrasted with a darker tissue, may

have a visual advantage over a colorful one, because even

color-blind animals can see it (Givnish 1990). In addition,

it is still visible even under low light and under various

spectra regimes, e.g., close to sunrise, near sunset (e.g.,

Troscianko et al. 2009), at the bottom of a dense forest or

within a dense canopy, and under very cloudy conditions

(Givnish 1990; Midgley 2004; Lev-Yadun 2009a, 2013). It

is therefore no surprise that many road markings are white

(yellow road markings—a typical aposematic color—are

also common), as the penalty for road mistakes by

wounding, jailing, financial losses, and death (for both

drivers and pedestrians) can be very high. In this respect,

signaling in white may be beneficial since its visibility is

not affected by color blindness.

Chemically based olfactory aposematism in plants

Olfactory aposematism, whereby poisonous plants deter

mammalian or insect herbivores, received the early atten-

tion of students of plant aposematism (e.g., Eisner 1964;

S. Lev-Yadun

123

Eisner and Grant 1981; Rothschild 1986; Rothschild and

Moore 1987; Moore et al. 1990; Launchbaugh and Prov-

enza 1993; Provenza et al. 2000; Massei et al. 2007; Lev-

Yadun 2009a). In the case of the very spiny zebra-like

rosette annual Silybum marianum (Asteraceae), which was

proposed to use visual aposematic markings by white

stripes (Lev-Yadun 2003), Rothschild and Moore (1987)

proposed that it uses olfactory aposematism via pyrazine. It

is likely that both types of aposematism (chemical-based

and spine-based) operates simultaneously in the Silybum

case, possibly towards different types of herbivores. Since

latex is exuded only following wounding, is toxic and

visually conspicuous, and has an olfactory component, it is

a classic case for simultaneous visual and olfactory

aposematism. The olfactory component of latex-related

aposematism is probably of significant importance against

nocturnal or blind herbivores.

Sticky latex as an indirect aposematic plant defense,

a case of an extended phenotype

Lev-Yadun (2014a) proposed that in addition to the well-

known direct physical/chemical defenses by trichomes,

resins and latex, insect carrion and trapped live insects that

are attached to the surfaces of many sticky plant species via

trichomes, latex or resin may defend plants by being apo-

sematic. This type of indirect aposematism (an extended

phenotype) is based on cueing visually to other herbivores

that the plants are already occupied, and on cueing and

signaling both visually and by rotting carrion or stress

volatiles emitted from trapped insects that such plants are

dangerous or even deadly.

Do members of the genus Euphorbia mimic oozing latex

as defense?

Mimicry of chemically defended butterflies by non-defen-

ded ones was the first type of proposed defensive mimicry

(Bates 1862), known as Batesian mimicry (Wickler 1968).

Since mimicry of defended organisms or defensive mech-

anisms is so common in animals (Wickler 1968; Cott 1940;

Ruxton et al. 2004) and is also found in plants (Lev-Yadun

2009a), theoretically, latex mimicry is expected to occur. It

seems that a number of succulent species belonging to the

genus Euphorbia (e.g., E. buruana, E. dauana, E. fluminis,

E. horwoodii, E. knobelii) (Sajeva and Costanzo 1994), a

genus typically defended by white latex, express conspic-

uous variegation via white pigmentation of parts of their

green tissues that look as if white latex was oozing there.

Conclusions

I propose that latex is predominantly white because the

white color increases its visibility under the typical light

conditions within plant canopies, in the forest under-story

and on the typical green color of leaves, young stems or

unripe fruits, and on the typical gray–brown colors of

mature barks. Thus the white color of plant latex allows it

to serve as a visual aposematic signal about the defensive

qualities of such plants. The yellow, orange, and red latex

types should also be considered as visually aposematic

because these are typical colors of toxic and aposematic

organisms. Since there are so many common plants that

produce white latex as defense, I propose considering the

ones that overlap in geographical distribution and are

attacked by the same generalist herbivores as Mullerian

mimicry rings. When there is a partial geographical overlap

of plants from one Mullerian mimicry ring with plants of

other mimicry rings, a chain of Mullerian mimicry rings of

such defended plants may exist. Such Mullerian mimicry

rings concerning defended spiny plants marked with white

or other types of defensive coloration were proposed

recently (Lev-Yadun 2009b, 2009c, 2014b). Because var-

ious plant taxa produce latex of different levels of toxicity

and because of the variable sensitivity of herbivores

towards the latex, there are probably complex networks of

regional mosaics of quasi-Mullerian or quasi-Batesian

mimicry rings concerning the chemical (olfactory) and

visual aposematic defense by latex.

Since latex contains so many types of toxic secondary

metabolites, including various volatiles (e.g., Oliveira et al.

2010) and because latex is exuded following wounding that

by itself involves the emission of various additional

defensive volatiles, there is a very strong direct and indirect

chemical aspect in defense by latex. The potential for a

chemically based olfactory aposematism role of such vol-

atiles, especially towards nocturnal or blind herbivores, is

an open area of research.

Acknowledgments I thank Paul J. Weldon for his inspiring

suggestions.

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