Plant Camouflage: Ecology, Evolution, and Implications

An elevational cline in leaf variegation: Testing anti-herbivory and abiotic heterogeneity hypotheses in maintaining a polymorphism

PREMISE

While some studies have found leaf variegation to reduce photosynthetic capacity, others showed that it can increase photosynthesis. Thus, what maintains variegation remains an open question. Two primary hypotheses-the anti-herbivory and abiotic heterogeneity hypotheses-have been posited, yet little empirical research explicitly investigates the maintenance of naturally occurring variegation.

METHODS

We used field surveys, image analysis, and climatic associations to explore the anti-herbivory and abiotic heterogeneity hypotheses in 21 populations of Hexastylis heterophylla and H. shuttleworthii, both polymorphic for leaf variegation. We measured the frequency of variegated individuals, variegation intensity, and herbivory for each morph, assessed abiotic correlates with variegation, and measured photosynthetic efficiency.

RESULTS

We found a strong elevational cline in leaf variegation strongly linked with abiotic heterogeneity; variegation was more common in lower-elevation populations characterized by higher temperatures, UV-B exposure, seasonal light change, and drier, more basic soils. Variegated and nonvariegated individuals experienced similar levels of herbivory. Morphs had similar photosynthetic quantum yields. However, nonvariegated leaves experienced more nonphotochemical quenching, an indication of photoinhibition, and had higher surface temperatures under high light.

CONCLUSIONS

Our results suggest that variegation may serve as an adaptation to high temperatures and light conditions and can reduce photoinhibition in certain environmental contexts. Thus, abiotic factors can maintain variegation in wild populations and shape geographic clines in variegation.

Pollinator shift ensures reproductive success in a camouflaged alpine plant

BACKGROUND AND AIMS

There are intrinsic conflicts between signalling to mutualists and concealing (camouflaging) from antagonists. Like animals, plants also use camouflage as a defence against herbivores. However, this can potentially reduce their attractiveness to pollinators.

METHODS

Using Fritillaria delavayi, an alpine camouflaged plant with inter-population floral colour divergence, we tested the influence of floral trait differences on reproduction. We conducted pollination experiments, measured floral morphological characteristics, estimated floral colours perceived by pollinators, analysed floral scent and investigated reproductive success in five populations.

KEY RESULTS

We found that the reproduction of F. delavayi depends on pollinators. Under natural conditions, a flower-camouflaged population had 100 % fruit set and similar seed set to three out of four yellow-flowered populations. Bumblebees are important pollinators in the visually conspicuous yellow-flowered populations, whereas flies are the only pollinator in the flower-camouflaged population, visiting flowers more frequently than bumblebees. The camouflaged flowers cannot be discriminated from the rock background as perceived by pollinators, but may be located by flies through olfactory cues.

CONCLUSIONS

Collectively, our results demonstrate that the flower-camouflaged population has different reproductive traits from the visually conspicuous yellow-flowered populations. A pollinator shift from bumblebees to flies, combined with high visitation frequency, compensates for the attractiveness disadvantage in camouflaged plants.

The Key Role of Plant Hormone Signaling Transduction and Flavonoid Biosynthesis Pathways in the Response of Chinese Pine (Pinus tabuliformis) to Feeding Stimulation by Pine Caterpillar (Dendrolimus tabulaeformis)

In nature, plants have developed a series of resistance mechanisms to face various external stresses. As understanding of the molecular mechanisms underlying plant resistance continues to deepen, exploring endogenous resistance in plants has become a hot topic in this field. Despite the multitude of studies on plant-induced resistance, how plants respond to stress under natural conditions remains relatively unclear. To address this gap, we investigated Chinese pine (Pinus tabuliformis) using pine caterpillar (Dendrolimus tabulaeformis) under natural conditions. Healthy Chinese pine trees, approximately 10 years old, were selected for studying induced resistance in Huangtuliangzi Forestry, Pingquan City, Chengde City, Hebei Province, China. Pine needles were collected at 2 h and 8 h after feeding stimulation (FS) via 10 pine caterpillars and leaf clipping control (LCC), to simulate mechanical damage caused by insect chewing for the quantification of plant hormones and transcriptome and metabolome assays. The results show that the different modes of treatments significantly influence the contents of JA and SA in time following treatment. Three types of differentially accumulated metabolites (DAMs) were found to be involved in the initial response, namely phenolic acids, lipids, and flavonoids. Weighted gene co-expression network analysis indicated that 722 differentially expressed genes (DEGs) are positively related to feeding stimulation and the specific enriched pathways are plant hormone signal transduction and flavonoid biosynthesis, among others. Two TIFY transcription factors (PtTIFY54 and PtTIFY22) and a MYB transcription factor (PtMYB26) were found to be involved in the interaction between plant hormones, mainly in the context of JA signal transduction and flavonoid biosynthesis. The results of this study provide an insight into how JA activates, serving as a reference for understanding the molecular mechanisms of resistance formation in conifers responding to mandibulate insects.

Camouflaged plants are shorter than non-camouflaged plants in the alpine zone

Camouflage has been reported as a defensive strategy in plants, while our understanding of the evolution of such defensive coloration is still limited. In the present study, we tested the hypothesis that camouflaged plants are shorter than non-camouflaged ones in the same habitat. Based on a species list from the subnival zone from the Hengduan Mountains, SW China and the herbarium collection, we measured the plant heights of 2915 individuals from 621 species (either camouflaged or not), with elevation information as a reference. We show that camouflaged plants were significantly shorter than non-camouflaged ones, though the effects of phylogeny and elevation were considered. Interestingly, a negative correlation between plant height and elevation was found in non-camouflaged plants, but not in camouflaged ones. These results revealed the correlation between defensive coloration and plant height. Camouflage may have evolved from shorter ancestors because they may suffer stronger selection and provide a more efficient defence.

Heterogeneity of interaction strengths and its consequences on ecological systems

Ecosystems are formed by networks of species and their interactions. Traditional models of such interactions assume a constant interaction strength between a given pair of species. However, there is often significant trait variation among individual organisms even within the same species, causing heterogeneity in their interaction strengths with other species. The consequences of such heterogeneous interactions for the ecosystem have not been studied systematically. As a theoretical exploration, we analyze a simple ecosystem with trophic interactions between two predators and a shared prey, which would exhibit competitive exclusion in models with homogeneous interactions. We consider several scenarios where individuals of the prey species differentiate into subpopulations with different interaction strengths. We show that in all these cases, whether the heterogeneity is inherent, reversible, or adaptive, the ecosystem can stabilize at a new equilibrium where all three species coexist. Moreover, the prey population that has heterogeneous interactions with its predators reaches a higher density than it would without heterogeneity, and can even reach a higher density in the presence of two predators than with just one. Our results suggest that heterogeneity may be a naturally selected feature of ecological interactions that have important consequences for the stability and diversity of ecosystems.

The phenomenon of red and yellow autumn leaves: Hypotheses, agreements and disagreements

Yellow and red autumn leaves are typical of many temperate/boreal woody plants. Since the 19th century, it has been either considered the non-functional outcome of chlorophyll degradation that unmasks the pre-existing yellow and red pigments or that the de novo synthesis of red anthocyanins in autumn leaves indicated that it should have a physiological function, although it was commonly ignored. Defending free amino acids and various other resources released especially following the breakdown of the photosynthetic system, and mobilizing them for storage in other organs before leaf fall, is the cornerstone of both the physiological and anti-herbivory hypotheses about the functions of yellow and red autumn leaf colouration. The complicated phenomenon of conspicuous autumn leaf colouration has received significant attention since the year 2000, especially because ecologists started paying attention to its anti-herbivory potential. The obvious imperfection of the hypotheses put forth in several papers stimulated many other scientists. Hot debates among physiologists, among ecologists, and between physiologists and ecologists have been common since the year 2000, first because the various functions of yellow and red autumn leaf colouration are non-exclusive, and second because many scientists were trained to focus on a single subject. Here, I will review the debates, especially between the photoprotective and the anti-herbivory hypotheses, and describe both the progress in their understanding and the required progress.

Predator responses to prey camouflage strategies: a meta-analysis

Although numerous studies about camouflage have been conducted in the last few decades, there is still a significant gap in our knowledge about the magnitude of protective value of different camouflage strategies in prey detection and survival. Furthermore, the functional significance of several camouflage strategies remains controversial. Here we carried out a comprehensive meta-analysis including comparisons of different camouflage strategies as well as predator and prey types, considering two response variables: mean predator search time (ST) (63 studies) and predator attack rate (AR) of camouflaged prey (28 studies). Overall, camouflage increased the predator ST by 62.56% and decreased the AR of prey by 27.34%. Masquerade was the camouflage strategy that most increased predator ST (295.43%). Background matching and disruptive coloration did not differ from each other. Motion camouflage did not increase ST but decreases AR on prey. We found no evidence that eyespot increases ST and decreases AR by predators. The different types of predators did not differ from each other, but caterpillars were the type of prey that most influenced the magnitude of camouflage's effect. We highlight the potential evolutionary mechanisms that led camouflage to be a highly effective anti-predatory adaptation, as well as potential discrepancies or redundancies among strategies, predator and prey types.

Understanding interdisciplinary perspectives of plant intelligence: Is it a matter of science, language, or subjectivity?

BACKGROUND

Evidence suggests that plants can behave intelligently by exhibiting the ability to learn, make associations between environmental cues, engage in complex decisions about resource acquisition, memorize, and adapt in flexible ways. However, plant intelligence is a disputed concept in the scientific community. Reasons for lack of consensus can be traced back to the history of Western philosophy, interpretation of terminology, and due to plants lacking neurons and a central nervous system. Plant intelligence thus constitutes a novel paradigm in the plant sciences. Therefore, the perspectives of scientists in plant-related disciplines need to be investigated in order to gain insight into the current state and future development of this concept.

METHODS

This study analyzed opinions of plant intelligence held by scientists from different plant-related disciplines, including ethnobiology and other biological sciences, through an online questionnaire.

RESULTS

Our findings show that respondents' personal belief systems and the frequency of taking into account other types of knowledge, such as traditional knowledge, in their own field(s) of study, were associated with their opinions of plant intelligence. Meanwhile, respondents' professional expertise, background (discipline), or familiarity with evidence provided on plant intelligence did not affect their opinions.

CONCLUSIONS

This study emphasizes the influential role of scientists' own subjective beliefs. In response, two approaches could facilitate transdisciplinary understanding among scientists: (1) effective communication designed to foster change in agreement based on presented information; and (2) holding space for an interdisciplinary dialogue where scientists can express their own subjectivities and open new opportunities for collaboration.

Morphological and anatomical traits during development: Highlighting extrafloral nectaries in Passiflora organensis

Passiflora organensis is a small herbaceous vine with characteristic morphological variations throughout its development. The plant bears button-shaped extrafloral nectaries exclusively in adult leaves. Extrafloral nectaries are structures that secrete nectar and play an important role in plant-animal interactions as a strategy for protecting plants against herbivory. In this work, we performed anatomical and ultrastructural studies to characterize P. organensis extrafloral nectaries during their secretory phase. We showed extrafloral nectaries in Passiflora organensis are composed of three distinct regions: nectary epidermis, nectariferous parenchyma, and subnectariferous parenchyma. Our data suggests that all nectary regions constitute a functional unit involved in nectar production and release. The high metabolic activity in the nectary cells is characterized by the juxtaposition of organelles such as mitochondria and plastids together plasmalemma. In addition, calcium oxalate crystals are frequently associated to the nectaries. An increasing concentration of calcium during leaf development and nectary differentiation was observed, corresponding to the calcium deposition as calcium oxalate crystals. This is the first description of extrafloral nectaries in Passiflora organensis that is a promising tropical model species for several studies.

Avoiding rather than resisting herbivore attacks is often the first line of plant defence

A common idea is that resisting or blocking herbivore attacks by structural, chemical and molecular means after they have commenced is the first line of plant defence. However, these are all secondary defences, operating only when all the various methods of avoiding attack have failed. The real first line of plant defence from herbivory and herbivore-transmitted pathogens is avoiding such attacks altogether. Several visual, chemical and ‘statistical’ methods (and commonly their combined effects) have been proposed to allow avoidance of herbivore attacks. The visual types are camouflage, masquerade, aposematic coloration of toxic or physically defended plants (including Müllerian/Batesian mimicry), undermining herbivorous insect camouflage, delayed greening, dazzle and trickery coloration, heterophylly that undermines host identification, leaf movements, and signalling that colourful autumn leaves are soon to be shed. The mimicry types include: herbivore damage, insects and other animals, fungal infestation, dead/dry leaves or branches, animal droppings, and stones and soil. Olfactory-based tactics include odour aposematism by poisonous plants, various repelling volatiles, mimicry of faeces and carrion odours, and mimicry of aphid alarm pheromones. The ‘statistical’ methods are mast fruiting, flowering only once in many years and being rare. In addition to the theoretical aspects, understanding these mechanisms may have considerable potential for agricultural or forestry applications.

Consciousness and cognition in plants

Unlike animal behavior, behavior in plants is traditionally assumed to be completely determined either genetically or environmentally. Under this assumption, plants are usually considered to be noncognitive organisms. This view nonetheless clashes with a growing body of empirical research that shows that many sophisticated cognitive capabilities traditionally assumed to be exclusive to animals are exhibited by plants too. Yet, if plants can be considered cognitive, even in a minimal sense, can they also be considered conscious? Some authors defend that the quest for plant consciousness is worth pursuing, under the premise that sentience can play a role in facilitating plant's sophisticated behavior. The goal of this article is not to provide a positive argument for plant cognition and consciousness, but to invite a constructive, empirically informed debate about it. After reviewing the empirical literature concerning plant cognition, we introduce the reader to the emerging field of plant neurobiology. Research on plant electrical and chemical signaling can help shed light into the biological bases for plant sentience. To conclude, we shall present a series of approaches to scientifically investigate plant consciousness. In sum, we invite the reader to consider the idea that if consciousness boils down to some form of biological adaptation, we should not exclude a priori the possibility that plants have evolved their own phenomenal experience of the world. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Philosophy > Consciousness Neuroscience > Cognition.

Commercial Harvesting Has Driven the Evolution of Camouflage in an Alpine Plant

Color in nature mediates numerous among and within species interactions,¹ and anthropogenic impacts have long had major influences on the color evolution of wild animals.² An under-explored area is commercial harvesting, which in animals can exert a strong selection pressure on various traits, sometimes greater even than natural selection or other human activities.^3,4 Natural populations of plants that are used by humans have likely also suffered strong pressure from harvesting, yet the potential for evolutionary change induced by humans has received surprisingly little attention.⁵ Here, we show that the leaf coloration of a herb used in traditional Chinese medicine (Fritillaria delavayi) varies among populations, with leaves matching their local backgrounds most closely. The degree of background matching correlates with estimates of harvest pressure, with plants being more cryptic in heavily collected populations. In a human search experiment, the time it took participants to find plants was greatly influenced by target concealment. These results point to humans as driving the evolution of camouflage in populations of this species through commercial harvesting, changing the phenotype of wild plants in an unexpected and dramatic way.

Proportional fitness loss and the timing of defensive investment: a cohesive framework across animals and plants

The risk of consumption is a pervasive aspect of ecology and recent work has focused on synthesis of consumer-resource interactions (e.g., enemy-victim ecology). Despite this, theories pertaining to the timing and magnitude of defenses in animals and plants have largely developed independently. However, both animals and plants share the common dilemma of uncertainty of attack, can gather information from the environment to predict future attacks and alter their defensive investment accordingly. Here, we present a novel, unifying framework based on the way an organism's ability to defend itself during an attack can shape their pre-attack investment in defense. This framework provides a useful perspective on the nature of information use and variation in defensive investment across the sequence of attack-related events, both within and among species. It predicts that organisms with greater proportional fitness loss if attacked will gather and respond to risk information earlier in the attack sequence, while those that have lower proportional fitness loss may wait until attack is underway. This framework offers a common platform to compare and discuss consumer effects and provides novel insights into the way risk information can propagate through populations, communities, and ecosystems.

Protective Role of Leaf Variegation in Pittosporum tobira under Low Temperature: Insights into the Physio-Biochemical and Molecular Mechanisms

Leaf variegation has been demonstrated to have adaptive functions such as cold tolerance. Pittosporum tobira is an ornamental plant with natural leaf variegated cultivars grown in temperate regions. Herein, we investigated the role of leaf variegation in low temperature responses by comparing variegated “Variegatum” and non-variegated “Green Pittosporum” cultivars. We found that leaf variegation is associated with impaired chloroplast development in the yellow sector, reduced chlorophyll content, strong accumulation of carotenoids and high levels of ROS. However, the photosynthetic efficiency was not obviously impaired in the variegated leaves. Also, leaf variegation plays low temperature protective function since “Variegatum” displayed strong and efficient ROS-scavenging enzymatic systems to buffer cold (10 °C)-induced damages. Transcriptome analysis under cold conditions revealed 309 differentially expressed genes between both cultivars. Distinctly, the strong cold response observed in “Variegatum” was essentially attributed to the up-regulation of HSP70/90 genes involved in cellular homeostasis; up-regulation of POD genes responsible for cell detoxification and up-regulation of FAD2 genes and subsequent down-regulation of GDSL genes leading to high accumulation of polyunsaturated fatty acids for cell membrane fluidity. Overall, our results indicated that leaf variegation is associated with changes in physiological, biochemical and molecular components playing low temperature protective function in P. tobira.

Colour change and behavioural choice facilitate chameleon prawn camouflage against different seaweed backgrounds

Camouflage is driven by matching the visual environment, yet natural habitats are rarely uniform and comprise many backgrounds. Therefore, species often exhibit adaptive traits to maintain crypsis, including colour change and behavioural choice of substrates. However, previous work largely considered these solutions in isolation, whereas many species may use a combination of behaviour and appearance to facilitate concealment. Here we show that green and red chameleon prawns (Hippolyte varians) closely resemble their associated seaweed substrates to the vision of predatory fish, and that they can change colour to effectively match new backgrounds. Prawns also select colour-matching substrates when offered a choice. However, colour change occurs over weeks, consistent with seasonal changes in algal cover, whereas behavioural choice of matching substrates occurs in the short-term, facilitating matches within heterogeneous environments. We demonstrate how colour change and behaviour combine to facilitate camouflage against different substrates in environments varying spatially and temporally.

Thermal Benefits From White Variegation of Silybum marianum Leaves

Leaves of the spiny winter annual Silybum marianum express white patches (variegation) that can cover significant surface areas, the outcome of air spaces formed between the epidermis and the green chlorenchyma. We asked: (1) what characterizes the white patches in S. marianum and what differs them from green patches? (2) Do white patches differ from green patches in photosynthetic efficiency under lower temperatures? We predicted that the air spaces in white patches have physiological benefits, elevating photosynthetic rates under low temperatures. To test our hypotheses we used both a variegated wild type and entirely green mutants. We grew the plants under moderate temperatures (20°C/10°C d/n) and compared them to plants grown under lower temperatures (15°C/5°C d/n). The developed plants were exposed to different temperatures for 1 h and their photosynthetic activity was measured. In addition, we compared in green vs. white patches, the reflectance spectra, patch structure, chlorophyll and dehydrin content, stomatal structure, plant growth, and leaf temperature. White patches were not significantly different from green patches in their biochemistry and photosynthesis. However, under lower temperatures, variegated wild-type leaves were significantly warmer than all-green mutants - possible explanations for that are discussed These findings support our hypothesis, that white variegation of S. marianum leaves has a physiological role, elevating leaf temperature during cold winter days.

Background matching and disruptive coloration as habitat-specific strategies for camouflage

Camouflage is a key defence across taxa and frequently critical to survival. A common strategy is background matching, resembling the colour and pattern of the environment. This approach, however, may be ineffective in complex habitats where matching one patch may lead to increased visibility in other patches. In contrast, disruptive coloration, which disguises body outlines, may be effective against complex backgrounds. These ideas have rarely been tested and previous work focuses on artificial systems. Here, we test the camouflage strategies of the shore crab (Carcinus maenas) in two habitats, being a species that is highly variable, capable of plastic changes in appearance, and lives in multiple environments. Using predator (bird and fish) vision modelling and image analysis, we quantified background matching and disruption in crabs from rock pools and mudflats, predicting that disruption would dominate in visually complex rock pools but background matching in more uniform mudflats. As expected, rock pool individuals had significantly higher edge disruption than mudflat crabs, whereas mudflat crabs more closely matched the substrate than rock pool crabs for colour, luminance, and pattern. Our study demonstrates facultative expression of camouflage strategies dependent on the visual environment, with implications for the evolution and interrelatedness of defensive strategies.

Understanding insect foraging in complex habitats by comparing trophic levels: insights from specialist host-parasitoid-hyperparasitoid systems

Insects typically forage in complex habitats in which their resources are surrounded by non-resources. For herbivores, pollinators, parasitoids, and higher level predators research has focused on how specific trophic levels filter and integrate information from cues in their habitat to locate resources. However, these insights frequently build specific theory per trophic level and seldom across trophic levels. Here, we synthesize advances in understanding of insect foraging behavior in complex habitats by comparing trophic levels in specialist host-parasitoid-hyperparasitoid systems. We argue that resources may become less apparent to foraging insects when they are member of higher trophic levels and hypothesize that higher trophic level organisms require a larger number of steps in their foraging decisions. We identify important knowledge gaps of information integration strategies by insects that belong to higher trophic levels.