TRANSPIRATION Only small amount of water absorbed by the plant is utilized. The excess water is lost as water vapours from the aerial parts (transpiration). Demonstration of transpiration Transpiration differs from evaporation; Transpiration is a vital physiological process for which living tissues are essential. Evaporation is a purely physical process.

Types of Transpiration Stomatal Transpiration It occurs through stomata in dicots, monocots, and aquatic plants), also known as Foliar transpiration. Cuticular Transpiration (Peristomatal transpiration); It occurs through the cuticle or epidermal cells of the leaves and other exposed parts of the plant.  This transpiration is about 10% of the total transpiration. Lenticular Transpiration; It occurs through woody stems through lenticels. Bark Transpiration: It occurs through corky covering of the stems.

Types of Stomata The stomata are minute pores which occur in the epidermis of the plants. Each stoma remains surrounded by two kidneys or bean shaped epidermal cells the guard cells. The stomata may occur on any part of a plant except the roots. The epidermal cells bordering the guard cells are called accessory cells or subsidiary cells. Usually in the leaves of dicotyledons the stomata remain scattered whereas in the leaves of monocotyldons they are arranged in parallel rows.

1. Ranunculaceous or Anomocytic The stoma remains surrounded by a limited number of subsidiary cells which are quite alike the remaining epidermal cells. The accessory or subsidiary cells are five in number.

2. Cruciferous or Anisocytic The stoma remains surrounded by three accessory or subsidiary cells of which one is distinctly smaller than the other two. 3. Rubiaceous or Paracytic The stoma remains surrounded by two subsidiary or accessory cells which are parallel to the long axis of the pore and guard cells. 4. Caryophyllaceous or Diacytic The stoma remains surrounded by a pair of subsidiary or accessory cells and whose common wall is at right angles to the guard cells.

Mechanism of Stomatal Transpiration Takes place in the leaf during the day in 3 steps: (i) Osmotic diffusion of water from xylem to the intercellular spaces through mesophyll cells. (ii) Opening and closing of stomata (stomatal movement) (iii) Simple diffusion of water vapours from intercellular spaces to outer atmosphere through open stomata.

(i) Osmotic diffusion of water from xylem to the mesophyll cells, then to intercellular spaces (air spaces) above stomata. - Changing of D.P.D & O.P. of mesophyll cells: decreasing upon drawing of water from xylem, and increasing upon releasing of water vapours into the intercellular spaces.

(ii) Opening and closing of stomata (stomatal movement) Structure of stomata (Subsidiary cells; Guard cells) Guard cells differ from other epidermal cells; contain chloroplasts and peculiar thickenings; - peculiar thickenings on their adjacent surfaces (in closed stomata) - peculiar thickenings on surfaces adjacent to stomatal pore (in open stomata) - Radial arrangement of cellulose microfibrils in the walls of guard cells; allow lengthwise elongation and prevent crosswise elongation of turgid guard cells.

- Stomata open due to accumulation of osmotically active substances in the guard cells; turgid guard cells. - Stomata close due to depletion of osmotically active substances in the guard cells; flaccid guard cells. - Movement of water takes place from region of less negative water potential to region of more negative water potential.

- There may be several different factors or mechanisms which create osmotic potential in the guard cells and control stomatal movements: --- (a) hydrolysis of starch into sugars --- (b) synthesis of sugars or organic acids. --- (c) active pumping of K+ ions accompanied by Cl- or organic acids counter ions.

(a) Starch-Sugars Interconversion Theory -- the effect of pH on starch phosphorylase enzyme during day and night . ---(starch phosphorylase reversibly catalyses the conversion of starch + inorganic phosphate into glucose-1-phosphate) --- During day, high pH, starch (insoluble) →glucose-1-phosphate (soluble) →lower osmotic potential (i.e. lower water potential) →turgid guard cells →the stomata open. --- During dark, low pH, glucose-1-phosphate→starch →higher osmotic potential (i.e. higher water potential) →flaccid guard cells →the stomata close.

Another mechanism (of metabolic reactions) is suggested to open and close stomata: - (i) glucose-1-phosphate should be further converted into glucose and inorganic phosphate for the opening of stomata, and - (ii) metabolic energy in the form of ATP would be required for the closing of stomata; ATP probably comes through respiration.

(b) Synthesis of sugars or organic acids in guard cells - during daylight, photosynthesis in guard cells →soluble sugar (glucose) → decreasing water potential → opening of stomata. During daylight, photosynthesis, CO2 concentration decreases in guard cells →pH increases →formation of organic acids (chiefly malic acid) →production of protons (H+) which operate in an ATP-driven-proton-K+ exchange pump →protons move into the adjacent epidermal cells and K+ ions move into the guard cells →decreasing water potential of guard cells →opening of stomata.

(c) ATP-Driven Proton (H+)-K+ Exchange Pump Mechanism in guard cells. This mechanism is much more widely accepted. - accumulation of K+ ions in guard cells during daylight (due to pumping out of H+ from the guard cells into the adjacent epidermal (or subsidiary) cells in exchange with pumping out of K+ ions from the adjacent epidermal cells into the guard cells. - H+-K+ exchange →entry of Cl- anions into guard cells - accumulation of K+, Cl-, and organic acid counter ions →sufficient enough to significant decrease of water potential of guard cells →turgid guard cells →open stomata.

(iii) Simple diffusion of water vapours from intercellular spaces to outer atmosphere through open stomata.

Significance of Transpiration 1- Movement of Water (upward movement, i.e. ascent of sap; arguments) 2- Absorption and Translocation of Mineral Salts: -- Absorption of water and absorption of mineral salts are entirely independent processes. -- Translocation or distribution of salts may be facilitated by transpiration through translocation of water in the xylem elements.

Significance of Transpiration (Cont.) 3- Regulation of Temperature: -- prevent plants from excessive heating. -- arguments (transpiration cannot account for the total dissipation (loss) of heat energy from the leaves and other parts of the plant: --- plants kept under intense sunlight with their stomata plugged by vaseline do not show much increase in their temperature. --- Xerophytes having structural modifications and adaptations to prevent excessive heating can withstand higher temperatures without appreciable damage to protoplast.

Factors Affecting Stomatal Movements 1- Light --- the amount of light required to achieve maximal stomatal opening varies with the species. --- CAM plants --- duration --- different wave lengths of light --- photosynthesis 2- Carbon dioxide concentration --- Reduced CO2 conc. favours opening of stomata; Increased CO2 promotes stomatal closing. --- intercellular CO2; atmospheric CO2. 3- Temperature 4- Water deficit and abscisic acid (ABA) --- water-stressed plants completely close their stomata; hydropassive control. --- accumulation of ABA causes closing of stomata; hydroactive control; external application of ABA.