LU2 Water Relations In Plants STU1043 PDF

STU1043 INTRODUCTION TO PLANT PHYSIOLOGY LU2: Water Relations in Plants 1 What will you learn from this LU?  Water movement in plants  Transpiration  Importance of water in plant (Video reflection)  Water stress in plant...

STU1043 INTRODUCTION TO PLANT PHYSIOLOGY LU2: Water Relations in Plants 1 What will you learn from this LU?  Water movement in plants  Transpiration  Importance of water in plant (Video reflection)  Water stress in plant 2 Learning Objective  By the end of this learning unit (LU), students will be able to relate: ◦ An understanding of the transport mineral solutes and water in plants. 3 Water in plants  Major constituent, 80–90 % of fresh weight of herbaceous plants and 50 % of woody plants  Solvent – transport of gas, minerals, and solutes in plants  Reactant – photosynthesis, hydrolysis of starch to sugar 4  Importance of Water in Metabolism cont.  Water gives turgidity to the growing cells and thus maintains their form and structure.  Water is a material that provides mechanical support and rigidity to non- lignified plant cells. 5  Importance of Water in Metabolism cont.  The elongation phase of cell growth depends on absorption of water.  Water is an important metabolic end product of respiration. 6  Transpiration  Translocation Loss of water from plants in the form of vapour. 7  Water is absorbed from the soil through root hair cells.  Water moves by osmosis from root cell to root cell until it reaches the xylem.  It is transported through the xylem vessels up the stem to the leaves.  It evaporates from the leaves (transpiration). 8 Water movement in plants  Water moves from outside into the cell through a semi-permeable membrane.  Difference in osmotic pressure drives movement of water into cells.  Water flows from high to low water potential (water flows from wet places to dry places).  When separated by a semi-permeable membrane, water moves from high osmotic pressure to low osmotic pressure. 9 10 11 12 Transport in Plants: movement up the xylem 1) Root Pressure The movement of water upwards through the plant body is mainly due to root pressure. The movement of water up the stem as a result of root pressure is due to osmotic mechanisms that are created as a result of the active absorption of salt by the roots. 13  Transport in Plants: movement up the xylem cont. 2) Cohesion-Tension Theory Cohesion and tension phenomena is presumed to occur in an unbroken column of water which exists between the soil water and the water of the leaf tissue, enabling the water to move into the root and up the plant from the soil. 14 15  Transport in Plants: movement up the xylem cont.  To sum up: ◦ Root pressure is caused by the influx of water into the roots ◦ Water adheres to the inside of the xylem ◦ Water is cohesive, and forms a column which is pulled up the xylem as a result of transpiration. 16 Passive & Active Transport  Passive ◦ Movement along with water ◦ Occurs through the apoplast (non-living) compartment – cell walls & xylem vessels  Active transport ◦ Movement against gradient of mineral concentration ◦ Occurs in symplast ◦ Uses energy and proton pump 17 18 Casparian Strip An impermeable waxy layer between the cells of the endodermis that stops water and solutes from entering the xylem, except by passing through the cytoplasm of adjacent cells 19 How the casparian strip filters solutes 1. The plasmalemma is selectively permeable so only permitted molecules can get inside 2. Solute molecules cannot pass through casparian strip 3. Solute molecules must detour through the symplast to pass through casparian strip 20 21 Water loss  Cells lose water when solute concentration of membrane is high (eg) salty water.  Cells also lose water via evaporation.  Cells shrink, membranes fold and organelles become distorted (out of shape). 22 Water stress in plant  Water deficit or water stress refers to situations where cells and tissues are less than fully turgid  Causes of water stress: 1. When water loss in transpiration exceeds water absorption. 2. Occurs daily on sunny days. 3. Becomes severe at high temperature and dry soils. 23 Effects of Water Deficits Water stress in plant cont.  General effects – Reduction in: ◦ Plant size ◦ Leaf area ◦ Crop yield  Leaf expansion and vegetative growth are severely inhibited by moderate water stress 24  Water stress in plant cont. Reproductive growth  Particularly sensitive to water stress  Wheat – during initiation of flowering  Soybean – causes flower abortion, seed number and seed size reduction. 25  Water stress in plant cont.  Plant growth depends on: ◦ Cell division ◦ Cell enlargement ◦ Cell differentiation  For cells to expand, they need minimum turgor  Leaf enlargement is reduced or stopped before photosynthesis is much reduced. 26  Water stress in plant cont. Effects of water stress on morphology  Increases leaf thickness (specific leaf weight)  Specific leaf weight = leaf weight / leaf area  Increases amount of cutin and wax on leaves  Increases production of hairs on leaf  Shoot growth more reduced than root growth thus, increase root to shoot ratio 27  Water stress in plant cont. Effects of water stress on photosynthesis  Photosynthesis is reduced by water stress due to: ◦ Closure of stomata ◦ Decreased efficiency in fixation of CO2 ◦ Reduced leaf surface 28  Water stress in plant cont. Effects on Dark Respiration  Rate of respiration decrease with increasing water stress, but the reduction is less than photosynthesis 29  Water stress in plant cont. Other Effects of Water Stress  Decrease in starch content and increase in sugar content.  Hydrolysis of proteins and accumulation of amino acids especially proline.  Increase in abscisic acid (ABA) in leaves (close the stomata). 30  Water stress in plant cont. Beneficial Effects of Water Stress  Sometimes improve quality of plant products ◦ Eg. Aromatic properties of tobacco ◦ Oil content of mint and leaves ◦ Percentage oil in soybean  Quality of pears, peaches and prunes.  Higher seed production in legumes. 31