Long Distance Transport of Water
- Diffusion is a slow process, suitable for short distances. For example, molecule movement across a typical plant cell (about 50 μm) takes approximately 2.5 seconds.
- Long-distance transport systems are necessary to move substances faster over extended distances.
- Movement of substances in bulk (en masse) from one point to another due to pressure differences is called mass (bulk) flow. Examples include the movement of water, minerals, and food.
- In mass flow, substances (in solution or suspension) move together at the same pace, like in a flowing river. In contrast, diffusion involves independent movement of substances based on their concentration gradients.
- Bulk flow is driven by either a positive hydrostatic pressure gradient (e.g., water through a garden hose) or a negative hydrostatic pressure gradient (e.g., suction through a straw).
- Bulk movement of substances over long distances through conducting tissues (xylem and phloem) is called translocation.
Absorption of Water by Plants
- Water and minerals are absorbed by diffusion through millions of root hairs at the root tips, which increase the surface area for absorption.
- Absorbed water moves deeper into root layers via two pathways:
- Apoplast pathway
- Symplast pathway
1. Apoplast Pathway
- It consists of a continuous system of adjacent cell walls, interrupted only by the casparian strips in the endodermis of the roots.
- Water moves exclusively through intercellular spaces and cell walls without crossing the cell membrane.
- Movement through the apoplast depends on a gradient and occurs via mass flow.
- The apoplast offers no barrier to water movement.
- As water evaporates into intercellular spaces or the atmosphere, tension develops in the continuous water stream in the apoplast. Mass flow occurs due to the adhesive and cohesive properties of water.
2. Symplast Pathway
- It involves a system of interconnected protoplasts.
- Water travels through the cytoplasm, with intercellular movement occurring via plasmodesmata (junctions between neighboring cells through which cytoplasmic strands extend).
- Water must enter cells through the cell membrane, making movement slower and dependent on a potential gradient.
- Symplastic movement may be aided by cytoplasmic streaming. For example, in Hydrilla leaves, chloroplast movement due to cytoplasmic streaming is visible.
- Most water flow in roots occurs via the apoplast since cortical cells are loosely packed, allowing movement without resistance. However, the endodermis is impervious to water due to the casparian strip (a band of suberized matrix). Water is directed to non-suberized wall regions, then moves through the symplast, crossing a membrane to reach the xylem.
- Water movement through the root layers is ultimately symplastic in the endodermis, the only pathway for water and solutes to enter the vascular cylinder.
- In young roots, water enters directly into xylem vessels and tracheids, which are non-living conduits and part of the apoplast.
- Some plants have additional structures for absorption. For example, mycorrhiza, a symbiotic association of a fungus with a root system, enhances absorption. Fungal hyphae absorb mineral ions and water from the soil, while roots provide sugars and nitrogen compounds. Some plants, like Pinus, require mycorrhizae for seed germination and establishment.
Water Movement up a Plant
Water moves up a stem against gravity, requiring energy.
Root Pressure
- As ions are actively transported from the soil into the vascular tissues of roots, water follows its potential gradient, increasing pressure inside the xylem. This positive pressure is called root pressure.
- It helps push water to small heights in the stem.
- In early morning with high atmospheric moisture, cut a soft plant stem horizontally near the base. Drops of solution ooze out due to positive root pressure.
- At night and early morning, when evaporation is low, excess water collects as droplets around special openings of veins near the tips of grass blades and leaves of herbaceous plants. This liquid-phase water loss is called guttation.
- Root pressure provides only a modest push and plays no major role in water movement up tall trees. Its primary role is re-establishing continuous water chains in the xylem, which often break under transpiration-induced tensions.
- In most plants, the majority of water transport occurs via transpiration pull.
Experiment to Prove Existence of Root Pressure
Transpiration Pull
- Water flows upward through the xylem at high rates (up to 15 m/hr).
- Water is primarily pulled through the plant due to transpiration pull, the driving force of transpiration. This is explained by the cohesion-tension-transpiration pull model of water transport.