Anyone who has ever gone shopping for plants is likely familiar with how different botanical varieties prefer different amounts of sun. However if light is necessary for them to grow, why shouldn’t more sun automatically be better? As it turns out, there are a number of different lines of evidence that would indicate that it is indeed possible to stress plants by exposing them to excessive levels of light.
To begin, one must consider what it does for a plant. Light provides the initial energy that a plant needs to convert atmospheric carbon dioxide into basic carbohydrates. Plants are able to absorb light at different wavelengths, with the optimal absorption happening at 680 nm for Photosystem II and 700 nm for Photosystem I. While the majority of absorbable light is within the visible spectrum, it has also been shown that plants’ growth can also be affected by the absorption of ultraviolet light.
Perhaps most obviously, an increase in light often means an increase in exposure to heat. Not only can heat itself be damaging to a plant, but it can also dry up essential water resources. If heat levels rise in less dry environments, heat from the sun can also produce humidity that can slow plant growth by slowing the rate at which water can be released through the stomata—or pores in the bottom of leaves. This in turn affects the rate at which water and other nutrients are sucked up from the roots to the shoot system of the plant. In addition, excessive humidity promotes the growth of pathogenic fungi and bacteria that can further damage the already-stressed plant.
Furthermore, exposure to direct sunlight also increases exposure to ultraviolet rays. While some plants have been demonstrated to utilize ultraviolet light as a regulatory mechanism, as with the majority of organisms, UV rays have a deleterious effect on the plant’s health by damaging DNA and cellular components.
But in addition to heat and ultraviolet radiation, it turns out that light itself can be damaging. A 2001 review published in the journal for the American Society of Plant Physiologists concluded that “Most days plants encounter light intensities that exceed their photosynthetic capacity.” When too much is absorbed, the plant becomes much more likely to produce harmful substances such as reactive oxygen species, free radicals, and over-excited pigment states.
To limit the absorption of excess light, some plants have evolved the ability to cause slight re-positioning of their leaves and/or chloroplasts. Some species may also grow vertically to maximize sunlight exposure at dawn and at dusk, but limit sun exposure during midday. In addition, some have developed the ability to thermally dissipate the extra energy conferred from excessive light conditions. In fact, Dr. Donald Ort from the University of Illinois in Urbana-Champaign reports that even in healthy leaves, most plants use less than half of the light originally absorbed by photosystem II. The rest is diffused out of the system as heat.
But how much light creates enough excess as to stress plants? Naturally, the answer to this question varies depending on the species or variety being considered. For all but a few model organisms, this topic remains unexplored. Such information about light may in the future contribute towards optimizing greenhouse conditions, growing food in space, or breeding plants with increased vitality and production.
By Sarah Takushi