Light is a fundamental component of plant life, providing the energy required for photosynthesis. This energy source exists in a delicate balance, as light is a form of electromagnetic radiation that can be both nourishing and destructive. The portion of the spectrum plants primarily use for growth is Photosynthetically Active Radiation (PAR), spanning wavelengths between 400 and 700 nanometers (nm). While PAR is necessary for growth, other wavelengths and excessive intensity can cause severe and irreversible damage to plant tissue.
The Role of Harmful Wavelengths
The most damaging wavelengths of light fall into the Ultraviolet (UV) spectrum, specifically the UV-B and UV-C regions. UV-B radiation (280–315 nm) is highly energetic and is the primary cause of direct cellular damage. When UV-B photons are absorbed, they modify the plant’s DNA, leading to the formation of photoproducts. This genetic damage disrupts the cell’s ability to replicate and repair itself, impairing growth and reducing photosynthetic efficiency.
Excessive UV-B also inactivates enzymes and alters proteins, directly affecting biological processes. UV-A radiation (315–400 nm) is generally less damaging but can still contribute to stress by generating Reactive Oxygen Species (ROS) that harm cell structures. Plants have defense mechanisms, such as producing protective phenolic compounds, but sudden or intense exposure can overwhelm these defenses.
Damage from Excessive Intensity
Beyond the harmful UV wavelengths, even the visible light spectrum (PAR) can become damaging when its intensity exceeds the plant’s capacity to use it. This overload is known as photoinhibition, where the light energy absorbed is greater than the plant can process through photosynthesis. The primary target of this excessive light is Photosystem II (PSII), a protein complex in the chloroplasts responsible for the initial light-dependent reactions. When PSII is overloaded, its electron transport chain becomes clogged, leading to the formation of Reactive Oxygen Species (ROS).
These free radicals attack and damage lipids, proteins, and pigments, notably the D1 protein within the PSII reaction center. The resulting damage manifests as bleaching or “sunburn,” where chlorophyll is destroyed and the leaf tissue turns white or pale yellow. This physical damage forces the plant to shut down energy production to survive.
Thermal Damage from Light Sources
A separate mechanism of light damage involves the heat generated by certain light sources, which is primarily a function of Infrared (IR) radiation. All radiation above 700 nm is considered infrared, and while it is not used for photosynthesis, it is readily absorbed by the water content within plant leaves. This absorption significantly raises the leaf temperature, leading to thermal damage. High leaf temperatures cause proteins to denature and cell structures to break down, resulting in rapid desiccation and scorched, crisp tissue.
This is common in indoor growing environments where artificial light sources emit high levels of IR. The resulting heat stress causes the stomata to close to conserve water, which halts photosynthesis and can quickly lead to localized tissue death.
Recognizing and Treating Light Damage
Acute light damage presents with specific visual symptoms that distinguish it from nutrient deficiencies or disease. Characteristic signs include bleached, white, or pale yellow patches (chlorosis) and crisp, brown, or necrotic spots, often appearing on the leaves closest to the light source. Leaves may also curl inward or wilt as the plant attempts to reduce its surface area exposed to the intense radiation. This damage often appears suddenly on the most exposed foliage, unlike a disease that spreads progressively.
The first step in treatment is to immediately move the plant to a dimmer location or increase the distance from an artificial light source. For outdoor plants, light damage can be prevented by “hardening off,” which involves gradually increasing the duration of sun exposure over several weeks. Severely damaged leaves will not recover and should be pruned away to allow the plant to focus its energy on new, healthy growth.