BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. TRANSPORT IN PLANTS

13.3. Transpiration and Water Movement Through Leaves

13.3.6. Environmental Factors Affecting Transpiration

Plants possess A number of adaptations that enable them to reduce Water loss in arid environments. Such features are termed xeromorphic and are discussed in more detail in Chapter 20. Species adapted to low humidity and regular droughts are called xerophytes (or xeromorphs). Plants adapted to conditions of adequate moisture are known as mesophytes, although they may also exhibit certain xeromorphic traits.

Temperature

Temperature has the most profound effect on Transpiration. The higher the temperature, the faster water evaporates from mesophyll Cells and the more saturated the air inside the leaf becomes with water vapor. Simultaneously, a rise in temperature leads to a decrease in the relative humidity of the air surrounding the plant. Both of these phenomena steepen the water molecule gradient between the intercellular air spaces and the surrounding atmosphere. The steeper this gradient, in turn, the higher The rate of diffusion. Alternatively put, The water potential inside the leaf increases, while outside it decreases.

Solar radiation increases leaf temperature. The paler the leaves, the more solar radiation they reflect and the less rapidly they heat up. A pale leaf coloration is typically associated with a dense covering of epidermal hairs or a waxy coating—characteristics that are considered xeromorphic.

Humidity and Water Vapor Pressure

Low humidity around the leaf promotes transpiration because the water vapor diffusion gradient (the water potential gradient) between the intercellular spaces and the surrounding atmosphere becomes steeper. Conversely, as the water vapor concentration in the air increases, i.e., as humidity rises, the diffusion gradient flattens. Atmospheric water potential also decreases with increasing altitude due to falling atmospheric pressure. Consequently, high-altitude plants frequently exhibit xeromorphic traits that serve to reduce transpiration rates.

Examples of such adaptations include sunken Stomata—stomata located at the bottom of epidermal grooves or folds where localized high humidity builds up, thereby slowing down transpiratory water loss. In some species, such as Ammophila (marram grass), the entire leaf blade rolls into a tube so that the stomata are enclosed within, creating a humid internal atmosphere (Fig. 13.11). Similarly, if a leaf is covered with a dense layer of epidermal hairs or scales, escaping water vapor is trapped among them, which also reduces the rate of transpiration.

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Fig. 13.11. Transverse section of a xeromorphic leaf of marram grass (Ammophila); the arrangement of various Tissues is clearly visible. The leaf is shown in a rolled state. In response to higher air humidity, it partially unrolls.

Air Movement

In still air, a boundary layer of moisture-saturated air forms around the leaf, making the diffusion gradient between the intercellular spaces and the external atmosphere relatively shallow. The slightest air movement strips away this boundary layer; therefore, wind increases the rate of transpiration, with the most pronounced effect occurring in light breezes. In strong winds, however, the stomata close and transpiration ceases.

As noted previously, leaf pubescence or surface scales create a micro-zone of stagnant air and thus moderately slow down transpiration.

Light

Light intensity affects transpiration because stomata are typically open in the light and closed in the dark. Consequently, plants lose relatively little water at night (mainly through the cuticle or lenticels). In the morning, the stomata open and the rate of transpiration increases.