9.1 Xylem Transport

• The phloem is responsible for transporting sugars and other organic compounds from the leaves to the rest of the plant.
• Transpiration is the loss of water vapour from the stems and leaves of plants
• Light energy converts water in the leaves to vapour, which evaporates from the leaf via stomata
• New water is absorbed from the soil by the roots, creating a difference in pressure between the leaves (low) and roots (high)
• Water will flow, via the xylem, along the pressure gradient to replace the water lost from leaves (transpiration stream)
• Stomata are pores on the underside of the leaf which facilitate gas exchange (needed for photosynthesis)
• Photosynthetic gas exchange requires stomata to be open
• Transpiration is affected by the level of photosynthesis
• Transpiration is an inevitable consequence of gas exchange in the leaf
• Water is lost from the leaves of the plant when it is converted into vapour (evaporation) and diffuses from the stomata

  • Some of the light energy absorbed by leaves is converted into heat, which evaporates water within the spongy mesophyll
  • This vapour diffuses out of the leaf via stomata, creating a negative pressure gradient within the leaf
  • This negative pressure creates a tension force in leaf cell walls which draws water from the xylem (transpiration pull)
  • The water is pulled from the xylem under tension due to the adhesive attraction between water and the leaf cell walls
• The amount of water lost from leaves (transpiration rate) is regulated by the opening and closing of stomata
• Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
• Dehydrated mesophyll cells release the plant hormone abscisic acid (ABA) when a plant begins to wilt from water stress
• Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
• A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
• Transpiration rates will be higher when stomatal pores are open than when they are closed
• Stomatal pores are responsible for gas exchange in the leaf
• Levels of photosynthesis will affect transpiration
• Factors affecting transpiration rates include humidity, temperature, light intensity, and wind
• The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream

  • Water rises through xylem vessels due to two key properties of water – cohesion and adhesion
• Cohesion:

  • Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
  • Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
  • This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
• Adhesion:

  • Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
  • The xylem wall is also polar and hence can form intermolecular associations with water molecules
  • As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
• Structure of the Xylem

  The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

  • It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water
  • Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
  • The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
  • Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
• Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)

  • Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
  • In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer
• All xylem vessels are reinforced by lignin, which may be deposited in different ways:

  • In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
  • In spiral vessels, the lignin is present in the form of a helix or coil 
• Plants take up water and mineral ions from the soil via their roots and thus need a maximal surface area to optimise this uptake

  • Some plants have a fibrous, highly branching root system which increases the surface area available for absorption
  • Other plants have a main tap root with lateral branches, which can penetrate the soil to access deeper reservoirs of water
• The epidermis of roots may have cellular extensions called root hairs, which further increases the surface area for absorption

  • Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
  • The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions (Casparian strip)
  • Water and minerals are pumped across this barrier by specialised cells, allowing the rate of uptake to be controlled
• Fertile soil contains negatively charged clay particles that mineral ions (cations) may attach to
• Minerals that need to be taken up from the soil include:

  • Mg2+ (for chlorophyll)
  • Nitrates (for amino acids)
  • Na+, K+, and PO43–
• Mineral ions may passively diffuse into the roots, but are more commonly actively uploaded by indirect active transport
• Root cells contain proton pumps that actively expel H+ ions into the surrounding soil
• H+ ions displace positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
• Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton
• Water uptake in roots:

  • Water follows mineral ions into the root via osmosis, moving towards the region with a higher solute concentration
  • The rate of water uptake is regulated by specialised water channels (aquaporins) on the root cell membrane
• Once inside the root, water moves towards the xylem either via:

  • Symplastic pathway: water moves continuously through the cytoplasm of cells connected via plasmodesmata
  • Apoplastic pathway: water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis
• Desert plants (xerophytes) and plants that grow in high salinity (halophytes) possess various adaptations for water conservation

  • Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments
  • Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis
• Xerophytes are plants that can tolerate dry conditions (such as deserts) due to the presence of adaptations
• Adaptations of xerophytes:

  • Reduced leaves: reduces total number and size of leaves to reduce surface area available for water loss
  • Rolled leaves: rolling up leaves reduces exposure of stomata to air, reducing evaporative water loss
  • Thick, waxy cuticle: leaves covered by a thickened cuticle prevents water loss from the leaf surface
  • Stomata in pits: having stomata in pits, surrounded by hairs, traps water vapor and reduces transpiration
  • Low growth: low growing plants are less exposed to wind and more likely to be shaded, reducing water loss
• • CAM physiology: plants with CAM physiology open their stomata at night, reducing water loss via evaporation
• Halophytes are plants that can tolerate salty conditions (such as marshlands) due to the presence of a number of adaptations: