BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.3. Thermoregulation

19.3.3. Plant adaptations to high temperatures

In many Regions of the world, high air temperatures are combined with Water scarcity, and plant adaptations observed in these areas are often related, on the one hand, to drought tolerance and, on the other hand, to The Need for evaporative cooling.

Plants cannot escape extreme heat by moving into the shade like animals; therefore, they avoid overheating through structural and physiological adaptations. The aerial PARTS OF THE plant are most exposed to high temperatures, and leaves possess the greatest surface area. Leaves are characterized by a thin Structure and a high surface area-to-volume ratio, which facilitates gas exchange and Light absorption. In addition, this Morphology is best suited to prevent overheating damage. A thin leaf has a relatively low heat capacity and thus generally adopts the ambient Temperature. In plants of hot climates, the associated risk of overheating is mitigated by The formation of a shiny cuticle secreted by the epidermis, which reflects a significant portion of the incident sunlight, thereby preventing heat absorption. The large leaf surface area contains numerous Stomata through which Transpiration takes place. In hot, dry weather, a plant may lose up to 0.5 kg of water per hour per 1 m² through transpiration, which is equivalent to 350 W of thermal energy per 1 m² and accounts for approximately half of the total energy absorbed by the plant. All these mechanisms enable the plant to regulate its temperature to a certain extent.

19.1. Why are plants at risk of overheating at temperatures above 30 °C and high air humidity, whereas the risk is lower under dry conditions?

On hot, sunny days, plants frequently wilt. This happens because water loss through transpiration outpaces its uptake from the soil, causing the turgor pressure of parenchyma Cells to drop. However, wilting can also be observed in greenhouse plants exposed to excessively high temperatures accumulating within the leaves, despite an adequate water supply. It is possible that in this case, a specific protective mechanism operates to reduce leaf heating by drooping, i.e., by decreasing the surface area exposed to direct sunlight. Nevertheless, wilting invariably indicates a shortage of water for Photosynthesis and leads to growth arrest. When the temperature drops, even a severely wilted plant very quickly restores the turgor of its Tissues.

Plants adapted to arid conditions are called xerophytes. They possess A wide variety of structural adaptations that enable them to survive (sec. 20.10). In most cases, these adaptations are designed to reduce water loss, but their characteristic narrow leaves simultaneously minimize solar heating. Nevertheless, the mechanisms of high-temperature tolerance are primarily physiological in nature. One physiological mechanism employed by plants (not only xerophytes) in arid environments to avoid wilting is the synthesis of large amounts of Abscisic acid. Abscisic acid triggers stomatal closure and leaf shedding, which prevents wilting and overall organismal death.