From our perspective as cacti, sunburn is a severe form of physical damage, not unlike a severe injury for other organisms. Our skin, the epidermis, and the underlying chlorophyll-rich tissues are designed to handle intense light, but there is a limit. When the ultraviolet radiation and heat exceed our tolerance—often because we were moved too abruptly from shade to full sun or during an extreme heatwave—the cells literally cook and die. This damage destroys the delicate chloroplasts responsible for photosynthesis and ruptures cell walls, leading to the characteristic pale yellow, white, or even brown, corky patches you see. It is a traumatic wound that compromises our primary function: making food from sunlight.
The first step in our recovery is to isolate the wounded area. We cannot repair dead cells; they are lost to us. Our biological response is to create a protective barrier between the healthy, living tissue and the damaged, dead tissue to prevent infection from pathogens like bacteria or fungi. We do this by forming a corky layer of scar tissue, often called a callus, directly underneath the sunburned area. This process, known as suberization, seals the wound and prevents further moisture loss and the entry of diseases. The discolored, scarred patch will become permanent and will never revert to its original green color, as those cells are gone forever.
Our long-term health depends on our ability to manage the aftermath of the injury. If the sunburn is localized and only affects a small section of a stem, the recovery strategy is one of compensation. The healthy green parts of our body will increase their photosynthetic activity to produce enough energy to sustain the entire plant and, crucially, to support new growth. Over time, as we continue to grow, the sunburned scar may become less prominent relative to our overall size, eventually being overshadowed by new, healthy stems or pads. However, if the sunburn is extensive and encircles a significant portion of the plant, it can be catastrophic. This girdling damage can fatally disrupt the internal transport of water and nutrients, leading to a slow decline and eventual death, as we cannot generate enough energy or move resources to survive.
While we possess innate healing mechanisms, the actions of our caretaker are crucial for a successful outcome. The most important intervention is to immediately provide protection from further intense sunlight, typically by moving us to a location with bright but indirect light or by providing shade cloth. This allows us to dedicate our energy to healing instead of defending against further radiation. It is also vital to adjust watering practices; our damaged tissue cannot utilize water as efficiently, and our reduced photosynthetic capacity means we are more susceptible to rot if the soil remains too wet. The caretaker must resist the urge to cut away the sunburned parts, as this creates fresh, open wounds that are far more vulnerable to infection than our naturally formed callus.
