Leaf Shapes Optimize Sunlight

olive trees in a field in front of pink and blue sky

Biological Strategy

Olive tree

AskNature Team

Image: Albrecht Fietz / Pixabay / Some rights reserved

Adapt Phenotype

Living systems evolve over time in response to selective pressures. Selective pressures are environmental factors that reduce the reproductive success of some individuals in a population. Genetic makeup (genotype) is one way in which living systems mitigate selective pressures, but another is adaptable phenotypes. A phenotype is an observable characteristic that can be thought of as the expression of the genotype, combined with modifications caused by the environment or developmental conditions. Individuals, populations, or ecosystems that have phenotypes capable of reducing the effects of selective pressures can survive. Some plants, for example, modify their leaf shape in response to changing environmental conditions. A single olive tree has variable leaf shapes on sunny compared to shady areas of the tree, yet the next year, those same buds may develop differently shaped leaves.

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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Distribute Energy

Many forms of energy are naturally available, including kinetic, potential, thermal, elastic, radiant, chemical, and more. All living systems require energy to carry out their many activities. Energy isn’t a physical object that can be held, so it also can’t be pushed, pulled, or carried. However, it can be transmitted or transformed. For example, the golden-scale snail has a tri-layered shell that dissipates, or transfers, forces from a predator biting down on it, thereby minimizing damage to its soft body inside the shell.

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Capture, Absorb, or Filter Energy

Energy is naturally available in many forms, including kinetic, potential, thermal, elastic, radiant, chemical, and more. All living systems require energy to carry out their many activities, and have developed strategies appropriate to one or more of those forms. For example, some plants maximize their surface area available for capturing radiant energy from the sun while others have strategies to focus scattered light onto photosynthesizing areas.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Leaves of olive trees optimize sunlight harvesting by differing in shape and being flexible to changing conditions

In some deciduous trees, the leaves on the exterior of the tree canopy differ from those inside the tree canopy. The exterior leaves are referred to as “sun leaves,” while the interior leaves are “shade leaves.” These leaves have differences in shape, internal anatomy, and chemistry that translate into specialized abilities to use different kinds of solar radiation effectively.

Sun leaves are typically smaller, more elongate, and thicker than shade leaves, with more layers of chlorophyll-containing tissues and more extensive internal vascular systems. It is thought that sun leaves are better adapted to capture and use direct solar radiation (when it isn’t too intense to cause heat and other stress-related damage). Their elongate shape is also correlated with higher levels of solar radiation reaching the inner canopy where the shade leaves are located. Shade leaves appear to effectively use diffuse solar radiation, which reaches the inner canopy after having been scattered by other objects, like the outer sun leaves, in the path of direct light. Shade leaves can also be found on the exterior canopy on the side that faces away from the prevailing sun.

It appears that exterior sun leaf characteristics can change with environmental conditions (they show plasticity), and that their shape in particular can influence the internal canopy environment that shade leaves experience. Plasticity in sun leaves seems to help stabilize inner canopy conditions, buffering it from stresses. Genetic variation and tree size also affect leaf characteristics, but it seems that whole-tree can be optimized by having sun and shade leaves respond differently to the environment. Sun and shade leaves occur in other species, too. In oaks, external leaves have narrower lobes while the lobes of shade leaves are broader.

Last Updated September 25, 2017

References

“1. Canopy plasticity, the expression of different leaf phenotypes within the crown of an individual tree has complex functional and evolutionary implications that remain to be thoroughly assessed. We hypothesized that it can lead to disparity in how leaves in different positions of the canopy change with allometric growth and population genetic structure.
2. Leaf phenotypes of the inner and outer canopy were estimated using eight morphological and physiological characters…With these data, we investigated the extent to which leaf phenotypes change with plant size, genetic processes and in = response to environmental conditions inside and outside the canopy.
3. The size of trees measured in the field was clearly associated with the phenotype of sun [leaves] but not to that of shade leaves. The phenotype of sun leaves depended on both direct and diffuse light, while that of shade leaves was found to correlate only with diffuse radiation. Additionally, light availability inside the canopy was conditioned by the shape of external leaves, and increasing elongation of sun leaves led to higher radiation in the inner canopy.
4. The field phenotypes of both inner and outer canopy leaves were correlated with genetic variation among populations. Conversely, in the common garden, the different genotypes expressed a homogeneous sun phenotype, while phenotypic differences among populations remained apparent in shade leaves.
5. We conclude that, in agreement with our working hypothesis, canopy plasticity is both cause and consequence of the environment experienced by the plant and might lead to the differential expression of genetic polymorphisms among leaves. Furthermore, we propose that it can contribute to buffer abiotic stress and to the partition of light use within the tree crown.” (de Casas et al. 2011:802)

“The diverging phenotypes of modules in different positions of the canopy might thus maximize overall performance, with sun leaves most active when conditions for light capture are optimal (i.e. during spring, and early morning and late afternoon in summer without drought (Diaz-Espejo, Nicolas & Fernandez 2007) and shade leaves ensuring a stable photosynthetic performance throughout the year.” (de Casas et al. 2011:810)

Journal article