Properties and functions of water in soil and plants

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Information about Properties and functions of water in soil and plants

Published on May 27, 2018

Author: Dasanal


Properties and functions of water in plants and soil: Properties and functions of water in plants and soil PRESENTED BY KIRAN DASANAL UG13AGR1879 CONTENTS: CONTENTS A) Molecular characteristics of water B ) Important properties of water C ) Functions of water in plants D) Functions of water in soils E) Research paper F) Conclusion G) References A) Molecular characteristics of water: A) Molecular characteristics of water Water is a substance of unique electro-physical and electro-chemical properties which are arise from its peculiar asymmetrical atomic configuration. Water molecule has dipole character . Water has a high dielectric constant of 81 . The angle between the imaginary lines joining the two H nuclei with the O nucleus is found 104.5 0 Slide 4: Formation of water molecule and Angle Dipole character of water molecule B) Important properties of water: B) Important properties of water 1 ) Specific heat 2 ) Heat of vaporization 3) Cohesion and surface Tension 4) Adhesion 5) Spontaneous Transport 6) Water potential (i) In plants (ii) In soil (iii) In air 1. Specific heat : 1 . Specific heat Heat required to increase the temperature of 1g of a substance by 1°C is called specific heat. The specific heat of water is 1 calorie/gram °C which is higher than any other common substance helps it to maintain relatively stable internal temperatures in plants and soils despite large fluctuations in atmospheric temperatures. Slide 7: Water resist changing in temperature because of high specific heat. 2. Heat of vaporization : 2. Heat of vaporization Heat of vaporization of water is very high, being 540 to 580 cal/g of water.This leaves a cooling effect on the water evaporating surfaces of soils, plants and other bodies. 3) Cohesion and surface Tension : 3) Cohesion and surface Tension The ability of two water molecules to cohere to each other (by H-bonding) rather tenaciously is called cohesion. Surface tension could be defined as the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water. Surface tension at liquid-air interface, is higher than the surface tension within interior of the liquid. : Surface tension at liquid-air interface , is higher than the surface tension within interior of the liquid. 4) Adhesion : 4) Adhesion The attraction between water molecules and other bonding surfaces known as adhesion. Water molecules hold firmly to solid substances rich in O and N atoms, e.g. soil colloids, cellulose , proteins and glass. A soil colloid particle holds the first molecular layer of water by force of ‘adhesion’ and its subsequent layers by ‘cohesion’ . 5) Spontaneous transport: 5) Spontaneous transport Movement of energy from its higher level to its lower level , or of any substance from its higher energy state towards its lower energy state without the aid of any external energy or force, when there is no hindrance to do so. In which donor loses energy and the recipient gains it. 6 Water potential : 6 Water potential Water potential : Water potential Spontaneous movement of water from one region to another is explained in terms of free energy of water. The free energy is the principle of thermodynamics. It deals with energy change. The tendency of molecules moves depend upon the free energy and that is called chemical potential . And the chemical potential of water is nothing but water potential. Cont…..: Cont….. Water potential is inside plant and in soil is expressed by the Greek letter Ψ (psi). Measured in bars (one bar = 0.987 atm ). The term Water potential was coined by Slayter and Taylor (1960). The Water potential of pure water at atmospheric pressure is zero. Definition of water potential: Definition of water potential Water potential can be defined as the difference between the free energy of water molecules in pure water and the free energy of water in a solution (or cell sap). i) Water potential in plants: i ) Water potential in plants Water potential ( Ψ w ) in plants is deals with the osmotic relations of plant cells. Components of Water potential in plants Matrix potential (matric potential) ( Ψ m ) Osmotic potential (solute potential) ( Ψ s ) Pressure potential ( Ψ p ) Ψ w = Ψ  m + Ψ  s + Ψ  p 1 Matrix potential (matric potential) (Ψm) : Matrix potential (matric potential) ( Ψ m ) Matric is the term used for the surface (such as soil particles, cell wall, protoplasm etc) to which water molecules are adsorbed. It has negative value. In case of plant-water relationship ; it is not considered because it does not play a significant role in movement in water. Ψ w = Ψ  s + Ψ  p 2 Osmotic potential (solute potential) (Ψs): Osmotic potential (solute potential) ( Ψ s ) About three-fourth water present in plants is in the form of solution of variable solutes of different concentrations, called cell sap . The quantum of water potential lowered by the presence of osmotically active solutes in the plant tissues called solute potential. In the normally growing crop plants the solute potential in leaf cell vacuoles are found to vary from -10 to -20 bars. Pressure potential (Ψp): Pressure potential ( Ψ p ) Cont…..: In plants turgid cell possess outer turgor pressure which is equal to inward cell wall pressure. It has positive value. In the normally growing plants the crop leaves are found to possess pressure potential of +3 to +5 bars during daytime and up to +15 bars during night time. ( why ? ) Cont….. ii) Water potential in soil: ii) Water potential in soil There are three main sources causing reduction in water potential in soil below zero bar. These sources are solutes present in the soil colloidal soil matrix And force of gravity Osmotic potential (Ψs) : Osmotic potential ( Ψ s ) Water potential reduced due to the presence of the solutes called osmotic potential. In salt affected soil the osmotic potential may become a very significant component of soil water potential, which may lowered to the extent that the plant roots may fail to absorb water from it, even when it was sufficiently moist. Cont…..: Cont….. Sometimes in extreme cases, water may even move out from plant roots into the soil by process of reverse osmosis . Desiccation of plants may then be very rapid, except halophytes . In all normal agricultural soils the osmotic potential as a component of water potential is ineffective and can, therefore, be ignored. Matric potential (Ψm) : Matric potential ( Ψ m ) The adhesion of water molecules to soil matrix lowers the water potential in proportion to the intensity of the adhesion. In soils, the matric potential values are very high in the wet range (-0.1 to -0.31 bar) and low in the dry range (-0.7 to -15 bars). It determines the movement of water within the soil, as well as from soil into the plant roots. Gravitational potential (Ψm) : Gravitational potential ( Ψ m ) It is importance only in saturated soils where free water loses its energy and moves in response to gravity as drainage water. In unsaturated soils where water is held against gravity by forces of adhesion and cohesion , gravitational potential of water can be easily overlooked. Cont….: Cont…. Reviewing the three components of water potential in soils it is clear that in normal agricultural soils the osmosis and gravitational potentials do not influence the soil water and plant relations. Here matric potential alone determines the availability of moisture to the plant roots. Ψ w = Ψ  m iii) Water potential in Air: iii) Water potential in Air In atmosphere the relative humidity (RH) of its air determines the water potential prevailing at a given time. Water potential at R.H. of around 100%,is near zero bar. but at 50%RH,it drops to -1000 bars and at 10% R.H. to about -3200 bars. Lower the water potential in atmosphere, greater is its strength to induce transpiration in plants and surface evaporation in soils. C) Functions of water in plants : C) Functions of water in plants Water maintains turgidity in plant cells and tissues. only in turgid cells normal biochemical functions can takes place. because water acts as common place in these reaction and turgidity makes room in cells for these activities. Cont…..: Cont….. The very basic physiological function of plants, called photosynthesis , is dependent upon water as source of hydrogen for the purpose. Cont…..: Cont….. Water is a major constituent (75-85%) of cytoplasm which is the basic substance of life. Upon dehydration, cytoplasm as well as nucleoplasm (= protoplasm) are denatured. cytoplasm Cont…..: Cont….. Water movement from root to top of plant and create transpiration pull due to cohesion force of water molecule. Cont…..: Cont….. Water is the solvent in which mineral and organic solutes are translocated to respective sinks in plants. Water maintains temperatures in plants to within their physiological limits. D) Functions of water in soils: D) Functions of water in soils Plants absorb water mainly from soils through their roots. so to say, the presence of adequate amount of water in soils is primary for plant growth. Soil moisture acts as buffer in preventing fast changes in soil temperatures with changing atmospheric temperatures. Cont…..: Cont….. Water acts as solvent of mineral nutrients which is necessary for conducting several chemical reactions and for maintaining nutrients in their ionic forms for their rapid absorption by plant roots. Soil moisture is necessary to enable proper tillage of the soil to obtain desired tilth . E) Research paper: E) Research paper Turner N.C., Wright G.C., Siddique K.H.M. Nayyar et al. (2006) Adaptation of grain legumes (pulses) to water-limited environments, Adv. Agron. 71, 123–231. Economic yield reduction by unavailability of water in some representative pulse crops: Economic yield reduction by unavailability of water in some representative pulse crops Crop Growth stage Yield reduction References Chickpea Reproductive 45–69% Nayyar et al . (2006) Pigeon pea Reproductive 40–55% Nam et al. (2001) Common beans Reproductive 58–87% Martínez et al . (2007) Soybean Reproductive 46–71% Samarah et al. (2006) Cowpea Reproductive 11–60% Ogbonnaya et al. (2003) F) Conclusion : F) Conclusion G) References: G) References Water in relation to soils and plants- O. P. GUPTA Boscoss Biology II PUC textbook –Dr. K. Rajendra , Precilla D’ silva, Anita Fernandes. Google Slide 41: THANK YOU

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