Plant Transpiration: Process, Importance, and Factors
Plant Transpiration
Plant transpiration is the process of water evaporation from plants. Water is the most abundant substance in plant tissues. However, the aerial parts of plants have poor water economy: only a small portion (1-2%) of the total water entering the roots is retained for photosynthesis. The remaining 98-99% is lost as vapor through transpiration.
Importance of Transpiration
Transpiration is not just a risk to plant life; it’s crucial for several functions:
- Drives water and nutrient transport from roots to leaves.
- Supplies water for photosynthesis.
- Transports minerals from roots for biosynthesis in leaves.
- Cools leaf surfaces.
Factors Affecting Transpiration Rate
Transpiration rate is influenced by environmental and plant-specific factors.
Environmental Factors
Light
Light stimulates stomatal opening, increasing transpiration.
Temperature
Plants transpire more rapidly at higher temperatures because water evaporates faster. At 30°C, a leaf may transpire up to three times faster than at 20°C.
Humidity
The rate of diffusion increases as the difference in substance concentration between two regions increases. When the surrounding air is dry, water diffusion from the leaf increases rapidly.
Wind
Still air around a leaf increases humidity, reducing transpiration. Wind removes moist air and replaces it with dry air, increasing transpiration. When water uptake by roots cannot keep up with transpiration, turgor loss occurs, and stomata close, reducing transpiration and photosynthesis. If turgor loss spreads, the plant wilts.
Soil Water Availability
Plants cannot transpire rapidly if lost water isn’t replaced by soil water.
Plant Factors
Plant factors such as leaf size, shape, and stomatal density also affect transpiration.
Types of Transpiration
There are three main types of transpiration:
1. Stomatal Transpiration
This is the primary form of water loss (up to 90% of total transpiration). It occurs through stomata, connecting intercellular spaces with the external environment. Even when closed, stomata allow water vapor to escape. Plants balance water loss and CO2 uptake by regulating stomatal opening and closing. This mechanism is influenced by environmental factors (light, temperature, humidity, water supply) and internal factors (CO2 concentration, water status, ion content, and phytohormones).
2. Cuticular Transpiration
This involves direct water vapor diffusion through the cuticle, a layer of cutin covering leaf surfaces. It accounts for 1-10% of total transpiration. Cuticular transpiration varies depending on plant type, age, and environmental conditions. Young leaves and those with thinner cuticles have higher cuticular transpiration.
3. Lenticular Transpiration
This occurs through lenticels, small openings in the corky tissue of stems and twigs. It represents a negligible portion (0.1%) of total transpiration and can cause some drying in trees that lose leaves in early winter.
Evapotranspiration
Evapotranspiration is the total water loss from a soil surface with vegetation. It combines transpiration (water loss through plants) and evaporation (water loss from the soil).
EVAPOTRANSPIRATION = TRANSPIRATION + EVAPORATION
Transpiration is highest with dense planting and lowest with sparse planting. Evaporation is minimal with dense planting and maximal with sparse planting.
Exudation
Exudation is the loss of water in liquid form due to mechanical factors like pruning or cutting (e.g., latex-producing plants like Hevea brasiliensis).
Guttation
Guttation is the appearance of water droplets along leaf edges in plants growing in warm, moist environments. It occurs mainly in the morning due to hydrostatic pressure in xylem conduits. The excreted fluid is not pure water but contains salts, sugars, etc. These substances can be deposited on the leaf surface as water evaporates and may be reabsorbed later.
Importance of Light and Phytohormones
Light is crucial for photosynthesis and influences plant orientation. It’s captured in leaf blades and converted into chemical energy. Light also affects stomatal movement, contributing to water ascent in plants.
Phytohormones act at low concentrations to help plants survive stress, such as pathogen attacks and drought.