Transpiration

Water which is lost by a plant as water vapour. Water moves into the plant via the root hairs, which are mostly located towards the apical (or tip) region. Water moves through the outer cells that surround the xylem within the root and into the xylem ready for further movement within the plant.

The xylem consists of two key features: tracheids, which have perforated sides, and vessels, which have perforated sides as well as perforated ends. This network of interconnected cells provides the means by which water is moved up through a plant.

Water is lost, as water vapour, through the stomata in the leaves to the atmosphere through the process of evaporation. This evaporation creates a negative pressure (essential a suction effect) in the xylem which helps pull water up through the xylem cells by the action of cohesion of water molecules to other water molecules and adhesion of water molecules to cell walls.

Where little, if any, water is evaporated, in particular due to humid, cool conditions which can readily arise overnight, then surface water can be present on grass leaves as dew in the morning, which is where atmospheric moisture has condensed onto the grass leaves.

Root pressure can also force up water through the xylem, especially when little negative pressure exists at the leaf surface during the night-time (i.e. when transpiration rates are low). This produces water droplets, which are exuded through specialised pores called hydathodes, on the tips and edges of grass leaves, including from the leaf veins at the end of cut leaves: This process is called guttation.

A plant which has adequate supplies of water in its cells will be called 'turgid', which means rigid, whilst a plant which lacks water and is wilting is called 'flaccid', which is limp and weak.

Approximately 95% of water loss is through the stomata, whilst 5% is through he cuticle.

Transpiration can be affected by the following features or conditions:
• A deeper rooting depth and more extensive rooting density allows the plant to explore for more availability of water, therefore allowing it to continue to transpire and develop. This also aids in resistance to drought conditions.
• Grass length ¯ a larger leaf area will typically transpire more than a shorter and smaller leaf area.
• Grass species and their ability to adapt to available moisture conditions, e.g. red fescue and rolling of leaf.
• Dense swards can lose less water than bare soil initially can through evaporation.
• Light ¯ increasing light levels increase the amount of water that is transpired.
• Relative humidity, especially within the turf sward can reduce transpiration rates.
• Dry, exposed, windy conditions can increase the rate of transpiration.
• Low temperatures reduce water uptake.
• Waterlogged and anaerobic soil conditions reduce water uptake.