In addition to pigmentation, there may be a second major component to leaf color, structural color. Leaves may appear lighter in color, i.e., gray, silver, white, blue, copper, or gold, so that the green is not easily observed. This modification of color is due primarily to structures formed on the leaf surface that increase reflectance. Either leaf surface may also be hidden or partially so by trichomes that are less reflective, darker, or pigmented.
To understand the evolutionary and ecological important of plant hairs, a long discussion would be required to discuss the many ways that hairs have been used by plants to deal with environmental parameters or animal visitors or both. At this site, we are primarily interested in nonliving trichomes as they affect leaf color, and why.
Most shifts in leaf coloration to silver and white are attributable to the presence of nonliving plant hairs (trichomes) covering both upper and lower leaf surfaces (desert brittlebush). Trichomes are outgrowths of the outer epidermal cells. Dry nonglandular trichomes tend to be the most reflectant, especially when they are very dense. The whitish, silvery, and other metallic finishes of leaves are general produced by these trichomes (Examples: Psathyrotes ramosissima, Tidestromia oblongifolia, and Dicoria canescens; for another type, upper leaves of Elaeagnus and lower leaves).
The most thoroughly studied case is the reflectant leaf type of desert brittlebush (Encelia farinosa), a desert perennial of western North America. Investigators have demonstrated that the silvery leaves of late spring and summer are several degrees cooler than the same leaf would be if it was lacking the silvery cover of trichomes (leaf comparison). This is because the trichomes reflect infrared radiation that causes heating of objects (just as it does in a microwave oven). Having a cooler leaf on a hot summer day in the desert permits the leaf to avoid reaching the high lethal temperature and, at the same time, reduces its loss of water vapor via transpiration because leaf temperature is lowered.
Although having reflectant leaf is a benefit for certain plant species living in hot summer habitats, there are some negatives as well. The same trichomes that reflect infrared radiation, lowering leaf temperature, also reflect away useful wavelengths of solar radiation that are required for photosynthesis. As a consequence, a reflectant leaf has a much lower photosynthetic capacity than would an equal leaf without the reflectant trichomes. This is a cost, or trade-off, for the benefit of avoiding overheating. This species produces nonreflectant green leaves during the cooler springtime, when it is beneficial to have a warmer leaf (closer to growth optimum) and at which time photosynthesis can yield high rates because visible light is not lost by being reflected from the leaf.
Do all leaves with white or silvery hairs behave the way as desert brittlebush? For example, does this explanation hold for a plant from a mediterranean-type climate (Example: lavender-cotton (Santolina) or dusty miller, Senecio cinerascens)? In general, what to look for are the same circumstances, i.e., a plant that retains its leaves into a hot and very bright season (very high albedo) and has reflectant trichomes on both leaf surfaces. One classic example of a silvery plant from high elevation is the Hawaiian silver sword (Argyroxiphium sandwicense), which grows on the barren slopes of Mauna Loa on Hawaii. Some likely habitats would be along sandy desert washes or bright beaches. Remarkably, most plant species growing in such habitats do not have silvery leaves, and the ones that do have generally not been studied.
A special case of reflective trichomes occurs in the genus Atriplex, saltbush. Saltbushes, superb halophytes, have on each leaf surface a continuous blanket of vesiculated trichomes involved in secreting salts. However, these trichomes also are highly reflective and can, like the ones previously mentioned, help to lower leaf temperature on a very hot summer day.
There are numerous plants with dense reflective trichomes occurring only on the lower leaf surface (Examples: ramie and a senecio). One hypothesis that needs testing is that the reflectance is needed on the lower surface, as it would if reflected from sand or rock. But in the lowland tropics, a number of pioneer trees that invade light gaps have highly reflective leaf undersides (neotropical species), and in particular the highly successful Cecropia in the New World and different genera in Asia. The key may be to understand temperature controls on these very broad leaves during the hottest days.
Having a dense cover of plant hairs was at one time considered to be an adaptation to reduce transpiration (loss of water vapor) by sheltering the surface (boundary layer effect), but direct reduction of transpiration is probably not the effect of trichomes, as currently modeled and understood. Dense, thick hair likely has different effects on plants growing in different climates. Very dense, thick hair can, in some circumstances, be an adaptation to insulate the plant from cold, on a cold night maintaining the leaf or shoot tip temperature a few degrees higher than if the hairs were not present. Hairs that function as a thermal blanket can be white, tan, or brown, and, in fact, dark hair may improve heat absorptance during the day and then have no disadvantage at nighttime. There are also situations where thick hairs seem to be involved in preventing insects from eating the leaf from the lower side or inserting sucking parts into veins. Many studies have shown that presence of stiff trichomes can be very important in preventing predation by selected insect larvae.
Gray and blue hues of leaves (Examples: Atlas blue cedar, Aloe plicatilis, a cultivated agave, and Faucaria) can be produced by the presence of thick wax on the leaf surface or also by the minute irregular contours of the epidermal cells (microrelief). Many species of Agave have an irregular or conical surface topography, which causes light to be reflected (conical projections, very thick cell walls). Plant cuticle is hard wax and generally transparent, yielding a dull or glossy finish to a leaf surface. However, many plants form a loose surface wax, called epicuticular wax, which is more reflective (a type of Mexican poppy, Argemone corymbosa and the stem of a Central American columnar cactus). When a leaf appears bluish or gray (glaucous), rather than bright green, epicuticular wax is generally the cause (Examples: Kleinia and cabbage). This can be demonstrated by rubbing the powdery wax from the surface to reveal the green leaf color beneath. In contrast, the bluish leaf hues of a desert mahonia, Berberis trifoliolata, has microscopic, nipple-like projections from the epidermal cells, called epidermal papillae.
Most people automatically assume that thick wax is an adaptation to reduce water loss, stopping water vapor from leaking through the outer surface of leaves. Although that may be true, wax can have other functions:
Although naturalists have observed that some plants, especially succulents, in deserts have thick gray or bluish photosynthetic organs, caused by thick wax buildup, species living in cooler and even wet climates may also have glaucous leaves and stems. In the wet tropics, some broad-leaved monocotyledons have grayish leaves from wax deposition on the upper side. For example, Calathea lutea has leaves that are almost white; close examination reveals that epicuticular wax coats the lower surface and makes the leaf highly reflective, as would white trichomes on nearby plants of Cecropia. The "why" has not been thoroughly studied to explain wax-induced gray and blue colors of leaves and stems.