34.10: Light Acquisition

In order to produce glucose, plants need to capture sufficient light energy. Many modern plants have evolved leaves specialized for light acquisition. Leaves can be only millimeters in width or tens of meters wide, depending on the environment. Due to competition for sunlight, evolution has driven the evolution of increasingly larger leaves and taller plants, to avoid shading by their neighbors with contaminant elaboration of root architecture and mechanisms to transport water and nutrients.

Because larger leaves are more susceptible to water loss, the biggest leaves are typically found in plants where rainfall is plentiful. In the driest environments, chloroplasts of succulents are located in the stem of the plant, minimizing evaporation. The orientation of leaves to the sun can also influence light acquisition. In exceptionally sunny environments, horizontally oriented leaves are susceptible to excessive dehydration. In these environments, like those of grasslands, leaves may be oriented vertically to capture light when the sun is low in the sky, thereby reducing sun damage.

Light capture can also be optimized by the positioning of plant leaves with respect to the stem; the arrangement of leaves on a stem is called phyllotaxy. Alternate phyllotaxy describes the scenario in which a single leaf emerges from a single position on the stem. Some plants demonstrate opposite phyllotaxy, in which two leaves emerge in opposing directions from the same location. Whorled phyllotaxy is when several leaves emerge from the same point on a stem. The plant hormone auxin controls the pattern in which leaves emerge from the plant stem.

The Leaf Area Index (LAI) is a representation of light capture efficiency. By measuring the one-sided, horizontal surface area of the leaves on a plant and dividing that by the horizontal ground area the plant covers, a ratio is generated. Typically, a higher LAI indicates more efficient light capture. However, over a LAI greater than seven appears to cause shading and pruning of lower leaves, having no additional effect on light acquisition. In practice, measurement of LAI is often accomplished via satellite imaging and is used to measure the productivity of an ecosystem.

Plants exhibit striking diversity in leaf shape, size, and orientation. The leaves of ornamental rhubarb Gunnera manicata grow horizontally, spanning up to 2.5 meters by 4 meters, while some grasses grow short blades vertical to the ground. Why is there such variety in leaf architecture?

Sunlight is essential for the formation of glucose during photosynthesis. Hence, plants need to efficiently capture sunlight to thrive. Leaves are specialized plant structures, packed with many chloroplast-containing cells that capture sunlight.  

In general, larger leaves on taller plants capture more light energy. However, large leaves lose more water to evaporation and increase overall water demands. While taller plants face less competition for light, they require more substantial roots and adaptations to transport water over long distances.

Large leaves are common in wet environments; for example, the giant rhubarb is native to Brazil’s mountainous rainforests. In contrast, plants living in arid environments have evolved strategies to reduce their water loss as much as possible. This is why the leaves of desert plants such as creosote are relatively small, presenting a smaller surface area.

Phyllotaxy, or arrangement of leaves on a plant stem, helps to optimize light capture. In angiosperms, leaves are often arranged in one of three ways:  whorled, alternate, or opposite. 

The efficiency of light capture can be estimated by determining the leaf area index.  The leaf area index is the surface area of all leaves in relation to the ground area under the plant. Plants with leaf area indices of about 7 demonstrate efficient light capture; additional leaves and higher indices result in shading and pruning of lower leaves. 

Leaf morphology and size are shaped by the selection pressures imposed by the environment and by the plant species’ unique evolutionary history, resulting in the diversity of leaf structures that efficiently capture sunlight and conduct photosynthesis.