NEET Biology Notes Transport in Plants Transpiration
The loss of water in the form of water vapours from living tissues of aerial parts of plants, is called transpiration. About 95% water absorbed by roots of plant, is lost by transpiration an^l only 5% is used by plant.
Transpiration is of three types, i.e. stomatal transpiration (80-90%), cuticular transpiration (3-9%) and lenticular transpiration (0.1-1%).
Transpiration mainly occurs through stomata (i.e. minute pores generally present on leaf epidermis). Each stomata is bordered by two specialised epidermal cells called guard cells, which are generally kidney-shaped or bean-shaped. Guard cells, are surrounded by other specialised epidermal cells called subsidiary cells or accessory cells.
Potometer measures the transpiration rate of plant (e.g. Ganong’s potometer, Farmer’s potometer) and comparative rate of transpiration of two leaf surfaces is measured by cobalt chloride paper method.
Transpiration do not takes place in submerged aquatic plants.
Opening and Closing of Stomata
Opening and closing of stomata (i.e. stomatal movements) are governed by change in osmotic pressure or turgidity of guard cells. When guard cells are turgid, stomata open and when flaccid, stomata close.
Transpiration is affected by several external factors, i. e. temperature, light, humidity, wind speed. Plant factors that influence transpiration are number and distribution of stomata, number of stomata open, water status of the plant, canopy, structure, etc.
The transpiration driven ascent of xylem sap depends on
- Cohesion Mutual attraction between molecules.
- Adhesion Attraction of water molecules to polar surfaces.
- Surface tension Water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
These three forces contributes to the high tensile strength of water.
The process of photosynthesis also needs water, which is supplied by the system of xylem vessels from the root to the leaf vein. As water evaporates through the stomata, since the thin film of water over the cells is continuous results in pulling of water, molecule by molecule into the leaf from the xylem.
Because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air. This creates a pull.
Theories Explaining Opening and Closing of Stomata
Important theories to explain the opening and closing of stomata are strach-sugar interconversion theory, Steward theory and active K+ transport theory.
Starch vs Sugar Interconversion Theory
This theory was proposed by Lloyd (1990). In guard cells the amount of starch is high during night and low in day. According to Sayere (1926), the stomatal aperture opens at high pH (low H+ concentration) and closes at low pH (high H+ concentration).
In light, pH of guard cell is high and the amount of starch decreases and that of sugar increases. The sugar increases the osmotic pressure of guard cell, which leads to endosmosis and the stomatal aperture opens.
When pH is low the guard cells become flaccid.
The summary of opening of stomata is :
According to Steward (1964) the osmotic pressure of guard cells will not be appreciably affected unless glucose 1-phosphate is further converted to glucose and inorganic phosphate.
Active K+ Transport Mechanism
This mechanism was given by Levitt (1974). Change in turgor pressure of guard cells is actually a result of reversible absorption and loss of potassium ions (K+) in the guard cells.
Accumulation of K+ occurs in the guard cells during the day in response to light and is sufficient to account for increase in turgidity. In darkness, ions move out of the guard cells into surroundirfg epidermal cells. The uptake of K+ is balanced by the uptake of Cfr or by H+ released from organic acids like malic acid or by the negative charges of organic acids when they loss H+. Severe drought stress or intense solar radiations induce the formation of abscisic acid, which induces the closure of stomata. It blocks the active excretion of H+ from guard cells.
The summary of the process is
Malic acid further dissociates to form H+ and malate anion. The uptake of K+ ions is balanced by one of the following :
- uptake of Cl
- transport of H+ ions from organic acids such as malic acid.
- by negative charges of organic acids when they lose H+ ions.
Thus, all these factors leads to the opening of stomata. The stomata closure is due to, excretion of K+ from guard cells surrounding epidermal and subsidiary cells. Thus, stomatal closure is considered to be brought about by a passive or highly catalysed excretion of K+ and Cl” from the guard cells to the epidermal tissue in general and the subsidiary cells in particular. It is believed that subsidiary cells have an active reabsorption mechanism of K+
Role of ions in opening of stomata
These are inhibitors of transpiration. These may be metabolic inhibitors like PMA (Phenyl Mercuric Acetate) ABA (Abscisic Acid) and aspirin or film forming like silicon emulsions, waxes, etc.
At night or early morning, when evaporation is low, excess water collects in the form of droplets around special openings of veins, near the tip of grass blades and leaves of many herbaceous plaPts. Such water loss in its liquid phase is called guttation. The process of exudation of liquid drops from the edges of leaves is called guttation and it usually occurs through stomata like pores called hydathodes.
- Uptake and Transport of Mineral Nutrients
Most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP. Some ions also move into the epidermal cells passively. Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasm of the epidermal cells. Transport protein of endodermal cells are control points, where plant adjusts the quantity and types of solutes that reach the xylem. The root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
- Translocation of Mineral Ions
The chief sinks for the mineral elements are the growing regions of the plant, such as the apical and lateral meristems, young leaves, developing flowers, fruits and seeds and the storage organs.
Unloading of mineral ions occurs at fine vein endings, through diffusion and active uptake by these cells. Mineral ions are frequently remobilised, particularly from older, senescing parts. Some structural elements like calcium are not remobilised. Xylem transports only inorganic nutrients, while phloem transports only organic materials.
- Phloem Transport of Food
Sucrose is the main food transported by the vascular tissue, phloem from a source (the leaf) to sink (which stored food). The source sink relationship is variable due to the season and needs. The direction of movement in the phloem can be upwards or downwards, i.e. bidirectional. In xylem, the movement of water is always unidirectional, i.e. upwards. Phloem transports mainly water and sucrose as sap, but also translocates other sugars, hormones and amino acids.
- Mass Flow Hypothesis of Translocation
The mechanism used for translocation of sugars from source to sink is called pressure flow or mass flow hypothesis. The sucrose after synthesis moves into the companion cells and then into the living phloem sieve tube cells by active transport. As the osmotic pressure builds up, the phloem sap moves to the areas of lower pressure. Active transport is necessary to move the sucrose out of the phloem sap and into the cells, which will use the sugar-converting it into energy, starch or cellulose.
As sugars are removed, the osmotic pressure decreases and water moves out of the phloem. Phloem tissue is composed of sieve tube cells, which form long columns with holes in their end walls called sieve plates.
As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins and the sap moves through the phloem.
During this, at the sink, incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem and water passes out, returning to the xylem.
A simple experiment called girdling, was used to identify the tissues through which food is transported. This experiment proved that phloem is responsible for the translocation of food- When a plant is girdled (phloem removed), roots will die first.