In the last post we looked at the structure of the xylem vessels, so now we’re going to take a look at their function. Water transport in plants happens up the xylem vessels in a transpiration stream from roots to leaves due to the properties of water and transpiration at the leaves. Let’s break the process down from bottom to top.
Water uptake in the roots
Movement of water in a plant only happens in one direction – that is from the roots upwards. Water enters root hair cells from the soil by osmosis. Then, it has to travel through the root cortex (a thick layer of cells), the endodermis (a single layer of cells), and the pericycle (a final thin layer of cells) before it can enter the xylem vessels. There are two main pathways the water can take:
- The symplast pathway. Water travels through the cytoplasm of cells in the root cortex and endodermis by osmosis (because it has to cross the cell-surface membrane to get into the cytoplasm). The water passes from one cell to the next through plasmodesmata which connect the cytoplasm. Eventually, the water enters the xylem vessels by osmosis.
- The apoplast pathway. Water diffuses through the absorbent cell walls, which are the non-living part of plant cells. It can also diffuse across any small spaces between cells. However, when the water reaches the endodermis, there is a waxy impermeable strip called the Casparian strip in the cell walls. Now the water is forced to cross the cell-surface membrane by osmosis and join the symplast pathway for the rest of the journey to the xylem vessels. The apoplast pathway provides much less resistance to the movement of water so is the main pathway used up until the endodermis.
Movement of water up the xylem vessels
Now the water has entered the xylem vessels and is going to travel upwards. Water has some important properties which we looked at in this article. Water molecules are cohesive (stick to each other) because they form hydrogen bonds with one another. This helps to form an unbroken column of water all the way up the hollow xylem vessels. As water evaporates from the leaves (see next section), the water potential in mesophyll cells lowers and water is drawn out of the xylem by osmosis. This creates tension to pull up the column of water in the xylem, and draw more water into the xylem at the roots by osmosis. This process is called the cohesion-tension theory. Another property of water helping it to travel up the xylem is adhesion – it is attracted to the walls of the xylem vessels, helping it to continue rising upwards towards its destination.
Transpiration at the leaves
Transpiration can happen from any area of the plants surface but mainly happens at the leaves due to the presence of stomata. Stomata are little pores in the leaf surface that open to allow gas exchange. However, this also lets water vapour escape.
Water enters the leaf tissue cells from the xylem vessels using the apoplast pathway (through the cell walls). The water molecules evaporate from the cell walls into air spaces in the spongy mesophyll (see diagram in this article). Then then diffuse out through open stomata down a concentration gradient into the air surrounding the leaf.
Rate of transpiration
There are four main factors that will affect the rate of transpiration:
- Light intensity: if light intensity is high, the stomata are open to let in CO2 for photosynthesis. So more water vapour can diffuse out, and the rate of transpiration is higher. In the dark, the stomata close because the plant cannot photosynthesise.
- Humidity: if the air surrounding the leaves is humid it means there is already a lot of water vapour in the air. This means that the concentration gradient between the inside and outside of the leaf is low, and diffusion of water vapour through the stomata will be slower. So transpiration rate is lower.
- Wind speed: this works in a similar way to humidity. If it’s very windy, water molecules around the outside of the stomata are being constantly blown away, keeping the concentration gradient high. So a higher wind speed means a faster rate.
- Temperature: at a higher temperature, the water molecules have more kinetic energy and evaporate from cells more quickly (dangerously close to chemistry there). This means there are more water molecules inside the stomata and they will diffuse out more quickly along the high concentration gradient.
Bear in mind that when talking about a concentration gradient of water you can also call it a water potential gradient.
Some plants that live in dry conditions (e.g. marram grass and cacti) are adapted to keep the rate of transpiration low and conserve water. For example, marram grass leaves are curled and have stomata sunk into pits surrounded by small ‘hairs’ to trap water vapour nearby and reduce the concentration gradient.
Summary
Water transport in plants is an information-heavy topic, but it all fits into the big picture of some of the things you’ve already learnt about water and plants.
- Water enters the xylem vessels at the roots through the apoplast or symplast pathway.
- Water travels upwards in a transpiration stream due to cohesion, tension and adhesion.
- Water exits the xylem vessels at the leaves through the apoplast pathway and evaporates from cell walls.
- The rate of transpiration is affected by environmental conditions.