Absorption of water by plants
- Water is absorbed along with mineral solutes, by the root hairs, purely by diffusion.
- Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.
- Once water is absorbed by the root hairs, it can move deeper into root layers by two distinct pathways:
- apoplast pathway
- symplast pathway
- The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at the casparian strips of the endodermis in the roots.
- The apoplastic movement of water occurs through the intercellular spaces and the walls of the cells.
- The apoplast does not provide any barrier to water movement and water movement is through mass flow.
- As water evaporates into the intercellular spaces or the atmosphere, tension develop in the continuous stream of water in the apoplast, hence mass flow of water occurs due to the adhesive and cohesive
properties of water.
- The symplastic system is the system of interconnected protoplasts.
- Neighbouring cells are connected through cytoplasmic strands that extend through the structure called plasmodesmata.
- During symplastic movement, the water travels through the cytoplasm and intercellular movement is through the plasmodesmata.
- The inner boundary of the cortex, the endodermis is impervious to water because of a band of suberised matrix called the casparian strip.
- Water molecules are unable to penetrate the layer, so they are directed to wall region and the water then moves through the symplast and again crosses a membrane to reach the cells of the xylem.
- Once inside the xylem, water is again free to move between cells as well as through them.
- Some plants have additional structures associated with them that help in water and mineral absorption.
- A mycorrhiza is a symbiotic association of a fungus with a root system.
- The fungus provides minerals and water to the roots, in turn the roots provide sugars and N-containing compounds to the mycorrhizae.
- Some plants have an obligate association with the mycorrhizae. For example- Pinus seeds cannot germinate and establish without the presence of mycorrhizae.
Fig. apoplast and symplast pathways