Long Distance Transport of Water Absorption

Introduction

Plants require water for growth, performing photosynthesis, and for their entire survival. Despite this significance, they retain less than 5 percent of the water absorbed for cell expansion and growth. The rest of the water gets delivered to the atmosphere, a process known as transpiration. 

To generate sugars, plants need to absorb carbon dioxide from the atmosphere through small pores in their leaves, known as stomata. Nevertheless, the opening of stomata makes the plant lose most of the water collected through the long distance transport of water absorption. The correlation between transpiration and photosynthesis form a significant balance in the existence of plants.

Mass Flow

The movement of dissolved nutrients into a plant during absorption, which occurs due to the differences in pressures, and water potentials between the two points, is called mass flow or bulk flow. This mass flow results from transpiration (the process of pulling water through roots and withdrawing it into the air through leaves). This makes the mass flow of water equivalent to the water transpired eventually. During this process of long distance transport of water absorption, substances (both in solution and suspension) are driven at the same pace. 

Absorption of Water by Plants

All the water consumed by plants is absorbed through the roots. Through a process known as osmosis, these roots absorb the water present in the soil. Minerals and ions are absorbed by active transportation compared to the concentration gradient. Root hair cells adapt themselves to grasp water and mineral ions by elongating the surface area to elevate the absorption rate. 

Soil solution should be higher in water potential against the root hair cell. Only this will allow water entry into the root hair cell. Then the absorbed water passes through the cortical cells, endodermis, and pericycle, to enter the xylem vessel. 

Water moves deeply into the root layers through two different tracks, namely Apoplast and Symplast.

• Apoplast Pathway 

The apoplast is the set of adjoining cell walls excluding the Casparian strips of the endodermis in the roots. This apoplastic movement takes place through the intercellular spaces and the cell walls. In fact, this movement prevents crossing through the cell membrane since it is a gradient-driven procedure. This apoplast does not act as a barrier to water movement, and water movement occurs through mass flow. In this movement, the mass flow of water results from the adhesive and cohesive properties of water. 

•  Symplast Pathway

The system of interconnected protoplasts is called the symplastic system. The water moves through the cells, their cytoplasm, and through the plasmodesmata during this system. This movement is influenced by the gradient. Water enters the cells via the cell membrane, making this movement relatively slower. As the cortical cells are wobbly attached, most of the long distance transport of water absorption in the roots happens through the apoplast since they do not resist the water flow. Eventually, the water movement through the root layers is symplastic in the endodermis. Unlike apoplasts, xylem vessels and tracheids are abiotic water transporting channels.

The Endodermis

The long distance transport of water absorption, starting through root layers, is symplastic in the endodermis. The endodermis does not allow water to pass through it due to the presence of a band of suberised matrix called the Casparian strip. Water molecules can not make their way to the layer, so they are mended through the membranes’ wall regions. This is because the wall regions are not suberised. The water moves through the symplast and crosses the membrane again to reach the xylem cells. 

Mycorrhiza 

Mycorrhiza is an interactive associate of a fungus with a root system. The fungal strings form a network around the young root, or they choose to penetrate the root cells. The hyphae own a huge surface area that draws minerals, ions, and water from the soil, and that too from a greater rate of soil than roots. The source of minerals and water for the roots is the fungus. As a return, the roots deliver sugars and Nitrogen enriched compounds to the mycorrhizae. 

Root Pressure

Since numerous ions are delivered into the vascular tissues of the roots from the soil, water follows, leading to an increase in the pressure in the internal xylem structure. The positive pressure generated is what we call root pressure. 

Conclusion 

• Long distance transport of water absorption takes place through a mass flow system. When pressure differences help move dissolved nutrients and other substances in plants, it is known as mass or bulk flow
• Through diffusion, plants absorb minerals and water using their root hairs
• There are two pathways for water to advance to the root layers:

  • Apoplast pathway

  • Symplast pathway