Chapter 10 – transport in multicellular plants

Leaf Tissue Anatomy

Transpiration The internal structure of leaf consists of a variety of cells that carry out different important functions. The cells that are found in the mesophyll layers are not tightly packed, which in turn allow the space around them to be filled with air. In addition, the walls of these cells are wet, and some of the water found there will evaporate into the air spaces. This process therefore allows the air that is inside the leaf to be saturated with water vapour. There are small pores (stomata) that are found on the surface of the leaf. These pores allow the air within the internal spaces of the leaf to be in direct contact with the air outside of the leaf. In addition, these cells must open to allow carbon dioxide from the atmosphere to diffuse into the leaf to be used during photosynthesis. Transpiration occurs when there is a water potential gradient between the air inside of the leaf and the air outside.

Transpiration The rate of transpiration is affected by many different factors. If there is an increase in the water potential gradient between the air spaces in the leaf and the air outside, then the rate of transpiration increases. The rate of transpiration can also be increase if there is an increase in wind speed or a rise in temperature. The opening and closing of the stomata is another factor that affects the rate of transpiration. When the rate of photosynthesis is high, the demand for CO2 is high. This means that the stomata must be open and indirectly increases the rate of transpiration. However, if the water potential gradient is steep, like in dry conditions, a plant will partially close its stomata to prevent the leaves from drying out. This will also reduce the rate of photosynthesis. Transpiration is also important in the cooling of the leaves. As the water evaporates from the cell walls, it absorbs heat energy from these cells, thus reducing their temperature.

Transpiration

From Xylem to Leaf Since water evaporates from the mesophyll cell walls, there needs to be more water drawn into them to replace it. This water will be drawn from the xylem vessels in the leaf. There is a constant movement of water out of these vessels either into the mesophyll cells or along their cells walls. Some of the water is used during photosynthesis, but most of it will eventually evaporate and then diffuse out of the leaf. Hydrostatic pressure is reduced by the removal of water from the top of the xylem vessel. This causes the pressure at the top of the xylem vessel to be much lower than at the bottom, allowing water to move up the xylem vessel because of the pressure gradient. The xylem vessels have strong, lignified walls that will prevent their collapse from the pressure created by the water movement that is under tension.

From Xylem to Leaf Mass flow is what allows the movement of water through the xylem vessels. Water’s cohesive property allows all of the water molecules to move together, while its adhesive property allows the water molecules to stick to the lignin walls of the xylem vessels. Both together allow water to continuously move up the xylem vessels. However, if an air bubble forms within the column, then the water column breaks, an air lock is created, and the water will no longer move upwards. This is due to the fact that the difference in pressure between water at the top and bottom cannot be transmitted through the vessel. This can be prevented due to the small diameter of the xylem vessels. In addition, the pits found along the walls allow water to move out into neighboring vessels, therefore bypassing the air lock. (Air bubbles CANNOT pass through the pits)

Root Pressure A plant can also increase the pressure difference between the top and the bottom of the xylem vessel by rising the water pressure at the base of the vessels. They do so by the active secretion of solutes into the water in the xylem vessels within the root. The cells that surround the xylem vessels use energy to pump solutes across their membranes and into the xylem via active transport. The presence of these solutes now lowers the water potential of the solution in the xylem, drawing in water from the surrounding root cells. By having this influx of water, the water pressure at the base of the xylem vessels increases.

Potometer The rate of transpiration can be difficult to measure since there are many factors that affect it. However, the rate at which a plant stem takes up water is easy to measure. Since a high proportion of water that is taken up by the stem is lost in transpiration, and since the rate of transpiration directly affects the rate of water uptake, then the rate of water uptake by the stem is a goof approximation of the rate of transpiration. A potometer is the apparatus used to measure this rate. A potometer must be completely water and air-tight to prevent air bubbles from breaking the water column.

Xerophytes Xerophytes are certain plants that live in areas that have a short water supply. These plants often have adaptations that will reduce the rate of transpiration. Marram grass (Ammophila arenaria) – The leaves will roll up to expose a tough, waterproof cuticle to the air outside of the leaf. The enclosed, humid space in the middle of the roll will have open stomata. Also, hairs will help trap a layer of moist air close to the leaf surface. Cactus (Opuntia) – Has flattened stems that store water. It has spines as leaves to reduce the surface area from which transpiration can take place.