Leaf Area Influence On Transpiration Depends On External Environment

• As leaf area increases, the transpiration rate also increases. However, there are exceptions to this rule.
• Small leaves have higher transpiration rates in hot and dry regions for better thermoregulation and cooling effects.
• Smaller leaves have higher transpiration in cold, dry regions with high radiation to provide better hydraulic movements and nutrient uptake.

Transpiration determines water and nutrient availability, water use efficiency, and thermoregulation. Around 90% of transpiration occurs through leaves. The influence of the leaf area on transpiration is not yet completely understood, but this information can be valuable in comprehending plant water balance, especially in a changing climate. This article covers some trends in the relationship between leaf area and transpiration rate.

Plant Water Relationship and Leaves

Transpiration is the water vapor loss from plants, mainly through leaf stomata, with only 10% lost through cuticles. Besides determining water and nutrient availability and thermo regulation, transpiration is closely associated with photosynthesis through stomatal conductance. As stomata close to reduce transpiration, photosynthetic and carbon fixation rates will slow down. So, a reduction in transpiration is usually associated with less biomass accumulation. Plants must ideally balance gas exchanges to maintain growth and productivity.

At the plant level, water use efficiency is called transpiration efficiency (TE). TE is the ratio of biomass accumulated to the water transpired in a given time.

Internal and external factors influence transpiration.

• Internal factors: Among the internal factors, leaf area, leaf mass, shape, boundary layer, orientation, leaf thickness, wax layer, and number of trichomes on the surface, number, shape, and location of stomata can influence transpiration.
• External factors: Ambient temperature, air humidity, wind, light, and soil water content will influence the transpiration rate in plants.

Plants are modular and regulate the size and number of organs, especially leaves, in response to their environment. Leaf size is the average leaf area and dry mass. It is measured for all the leaves of simple-leaved species and for leaflets individually in compound-leaved species. Leaf area varies over six orders of magnitude among terrestrial plants. Hence, leaf size has physiological and ecological significance.

Leaf Area Effects

Leaf area is the foliar trait that most impacts transpiration rates. Generally, larger leaves transpire more than plants with small leaves. However, plants can reduce the number of leaves to reduce total leaf area, control water loss, and avoid dehydration, such as creosote, true poplars,  black walnut, etc. Curling leaves also reduce the area exposed to the sun and wind and increase resistance to water vapor loss.

Wide variations exist in transpiration rates per unit of leaf area among species. Still, differences in total leaf area measured by leaf area index (LAI) will compensate for the differences in individual transpiration rate per unit of leaf area.

However, the leaf size depends on soil water availability and hydraulic movement. Humid regions have larger leaves, while arid regions have smaller leaves. Small leaves are seen in the arid areas with hot and cold temperatures. Also, as tree height increases, there is less hydraulic movement, more exposure to radiant sunlight, and the size of leaves decreases.

As leaf size decreases, the number of leaves will increase. This tradeoff between leaf size and number is global. Moreover, smaller twigs have more leaf intensity in temperate regions, with many located at short internodes, while larger branches will have fewer and bigger leaves.

Figure 1: “Leaf water loss rate (k) in relation to individual leaf area (A) and individual leaf mass (B) of 10 simple-leaved tree species, analyzed using linear mixed models,” Wang et al. 2019. ( Image credits: https://doi.org/10.3389/fpls.2019.00058)

Transpiration in Arid Regions

The relationships between leaf area and transpiration rates are different in arid regions. A strong negative correlation exists between transpiration and leaf size across tree species in dry areas with intense radiation. This correlation exists in hot and cold regions.

Dry and hot regions

Small leaves in dry and hot regions are well known. Plants in deserts have smaller leaves and higher transpiration for faster cooling.

Dry and cold regions

Increasingly, several research findings show that in dry and cold temperate regions with high radiation, higher transpiration rates are seen in smaller leaves, as shown in Figure 1. The leaf size is genetically controlled in these instances. A Chinese study found that 16 tree species in a region with larger leaves had lower transpiration rates to reduce water loss and increase water use efficiency. Another study showed that grasses across the globe from cool and dry areas had shorter, narrower leaves with high transpiration. The higher transpiration in smaller leaves ensures enough water and nutrient absorption to support carbon fixation and plant growth.

Also, black poplars in France had varying transpiration rates within a region based on leaf size. Highly productive species originating from cool locations have large leaf areas with low transpiration rates and high transpiration efficiency.

Boundary Layer

The higher transpiration rate in small leaves can be due to the boundary layer.

The boundary layer is a thin film of still air surrounding the leaf surface. Water vapor lost from leaves must move through this boundary layer, which restricts the flow, slowing transpiration rates. As the boundary layer increases, the transpiration rate reduces. Plants increase the boundary layer in many ways to minimize transpiration rate through more leaf hairs, sinking stomata, or increasing leaf size.

Hence, leaves with more leaf area and boundary layer have slower transpiration and species with small leaves have less boundary layer and faster transpiration. Smaller leaves lose more water but also cool faster, which is crucial in environments with high light intensity that occur in hot and cold places in the world.

Another way the boundary layer influences leaf temperature is by regulating heat transfer. More boundary layers are associated with more heat transfer through convection, so small leaves with less boundary stay cooler.

Implications of Size-Dependant Transpiration

Leaves account for over 30% of the plant’s hydraulic resistance, so the considerable variation in leaf area becomes significant in dry environments. Though other leaf traits, especially stomata density, are substantial, leaf size can be overwhelmingly important in regulating transpiration. Therefore, the leaf area’s role becomes essential in the changing climate with large fluctuations in temperature and rainfall.

Large leaves are better in moister, cooler, and low-radiance environments. Large-leaved species have higher biomass investment costs to develop an extensive root system to stay cool in hot environments. So, smaller leaves are advantageous in hot and dry environments and areas with cold, dry, and high radiation.

The variation in leaf size and transpiration also has ecological implications. Lower transpiration rates in large leaves can increase plant water use efficiency, but it can lead to less cooling and nutrient uptake under high temperatures.

Leaf Area Importance

Leaf area is a standard plant trait widely used in research, as it is the largest organ of the plant that interacts with the environment. It is also relatively easy to measure than underground root systems and internal physiological parameters. As recent research shows, we still have to learn much about the influence of this common trait on plants and ecology. Modern tools like the CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter allow non-destructive, accurate measurements in real-time so that scientists can further explore the role of this versatile plant trait.

Contact CID Bio-Science Inc. to find out more about our leaf area meters.

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