Grevillea plants, native to Australia, are generally well-adapted to warm, sunny conditions. However, when planted in exceptionally hot climates, they can exhibit leaf burn and scorch. From the plant's physiological perspective, this is not merely a cosmetic issue but a sign of significant stress affecting its core functions. The plant is struggling to maintain a balance between acquiring essential resources and protecting its internal structures from damage.
At the heart of leaf scorch is a water crisis. Grevillea leaves have a large surface area relative to their volume. In intense heat, the rate of transpiration (water loss through leaf pores, or stomata) can exceed the rate at which the roots can absorb water from the soil. This creates a negative water potential within the plant. To conserve water, the plant will close its stomata. While this reduces water loss, it also halts the intake of carbon dioxide, which is essential for photosynthesis. Consequently, the plant's energy production slows down, leading to reduced growth and overall vigor.
When the leaf cells lose too much water, they plasmolyze—the living protoplast within the cell shrinks away from the cell wall. This process is often irreversible and kills the cells, resulting in the characteristic brown, crispy, "burned" appearance, typically starting at the leaf margins and tips which are furthest from the vascular supply. Furthermore, high temperatures and intense sunlight generate reactive oxygen species (ROS) within the leaf tissues. These are destructive molecules that can break down chlorophyll (the green pigment) and damage cell membranes and proteins. The plant's natural antioxidant systems can become overwhelmed, leading to the visible yellowing and browning as the leaf tissues die.
The problem often originates below ground. Grevilleas possess proteoid roots, which are dense clusters of fine rootlets designed to efficiently scavenge nutrients in poor soils. However, these roots are also sensitive to environmental extremes. In hot climates, soil temperatures can rise dramatically, especially in containers or poorly mulched beds. High soil temperatures can directly damage these fine roots, impairing their ability to absorb water and nutrients precisely when the plant needs them most. A compromised root system cannot support the water demands of the foliage, accelerating the cycle of water deficit and leaf scorch.
While Grevilleas are sun-loving, the combination of high temperature and extreme light intensity can cause photodamage (photoinhibition). The photosynthetic machinery in the leaves can only process a finite amount of light energy. When light absorption exceeds the plant's capacity to use it for photosynthesis, the excess energy can cause the formation of the destructive ROS mentioned earlier. This is akin to a solar panel overheating in direct, scorching sun. The plant cannot effectively dissipate this energy, leading to the breakdown of chlorophyll and the appearance of bleached or pale, scorched patches on the leaves, particularly those facing the afternoon sun.
Water is the medium through which nutrients move from the soil into the roots and throughout the plant (the xylem stream). During periods of water deficit, this flow is disrupted. Essential elements like potassium, which helps regulate stomatal opening and closing, may become less mobile within the plant. An imbalance in potassium can further hinder the plant's ability to manage its water loss efficiently. Additionally, the damaged root system in hot soil is less effective at absorbing nutrients, potentially leading to deficiencies that can mimic or exacerbate the symptoms of scorch, creating a compound stress situation for the Grevillea.
