Transport in Plants Test - 3

The transmembrane (or vacuolar) pathway is the route followed by water that sequentially enters a cell on one side, exits the cell on the other side, enters the next in the series and so on. In this pathway, water crosses at least two membranes for each cell in its path (the plasma membrane on entering and on exiting).

The process of exudation of liquid drops from the edges of leaves is called guttation. Usually it occurs through stomata-like pores called hydathodes. It is this process by which fully turgid plants remove extra water. The cause of guttation is mainly root pressure (when the root pressure is high and rate of transpiration is low guttation takes place). Hydathodes are stomata-like pores generally present at the tips or margins of leaves of those plants that grow in moist shady places (e.g., Tropaeolum).

Water and minerals are absorbed by root hairs. Water moves radially through the root before it begins its ascent towards the leaf. It moves through different tissue of the root in apoplastic and symplastic manner. Water enters the root through the root hair, travel through the cortex in an apoplastic manner., reaches the endoderms, the presence of casperian strips prevent apoplastic movement through endodermis. From here it travels simplistically through pericycle, protoxylem (exarch condition of root) to metaxylem and then ascends upwards.

If a difference in water potential exists between the two regions, spontaneous movement of water will take place, provided the movement can occur between them. The direction of flow will be energetically down hill, that is from the region of higher water potential (root hair) to the region of lower water potential (xylem) via cortex from less negative water potential (e.g., −1 kPa) to more negative water potential (e.g., −2kPa).

In symplast pathway, water passes from cell to cell through their protoplasm. It does not enter cell vacuoles. The cytoplasm of the adjacent cells are connected through plasmodesmata. Symplastic movement is aided by cytoplasmlc streaming of individual cells.

Apoplast pathway consists of non-living parts of plant body i.e., cell walls and intercellular spaces. In apoplast pathway, water passes from root hair to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The pathway provides the least resistance to movement of water. However, it is interrupted by the presence of impermeable ligno-suberin Casparian strips in the walls of endodermal cells.

Concentration of minerals in the soil is lower than in root. They dissolve in water and move around soil in the solution. Root hair cells are adapted to absorb the water out of soil by osmosis. Root hair cells have carrier molecules on their surface that pick up the minerals and move them into the cell against the concentration gradient.

Roots are poorly developed in hydrophytes and the water absorption takes place through general surface of the plant body.

If we cut a small soft-stemmed plant on a day , When there is plenty of atmospheric moisture early in the morning we will soon see drops of solution ooze out of the cut stem, this comes out due to the positive root pressure. If we fix a rubber tube to the cut stem as a sleeve we can actually collect and measure the rate of exudation and also determine the composition of the exudates Root pressure is a positive pressure that develops in the xylem sap of the root of some plants. It is a manifestation of active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce transpiration. It is more during rainy season in the tropical countries and during spring in temperate habitats.

The loss or excretion of water in the form of liquid droplets from the leaves and other parts of uninjured or intact plant is called guttation. It was first studied by Bergerstein in 1887. All plants do not show guttation. It is restricted to about 345 genera of herbaceous and some woody plants. Common examples are garden nasturtium, oat and other cereals, balsam, tomato, cucurbits. Guttation takes place through special structures called hydathodes. They are usually found on the margins and tips of the leaves.