Introduction to Water and Nutrients in Plants &Properties of Water in PlantsHORT 301 – Plant Physiology

August 29, 2008paul.m.hasegawa.1@purdue.edu

Plant Physiology – plant cell, tissue and organ functions

Independent function of subcellular organelles to integration of cellular, tissue and organ function in planta

Function includes mechanisms and processes of physics, chemistry, biochemistry, cell biology and molecular genetics

Water and Nutrients in Plants

Plants are autotrophic (self-nourishing) – CO2, H2O and minerals areaccessed from the environment using light or chemical energy

These essential “nutrients” are used for biosynthesis of small and large (macro) molecules required for plant growth and development

C, H, O and essential minerals are required for plant growth and development

Carbon – C (as CO2) is fixed and assimilated into sugars from which carbohydrate macromolecules (starch, cellulose), nucleic acids (RNA, DNA), proteins, and lipids are produced

Water (H2O) – absorbed into roots from the soil solution and moved throughout the plant, H and O are essential components of organic molecules

Mineral nutrients – essential elements (excluding C, H and O), usually accessed by roots from the soil solution

Water and Plants - Lectures

Properties of water - structure and physico-chemical properties that are fundamental to the function of water in plants

Water movement into cells and cell expansion – bulk flow, diffusion and osmosis; water potential and cell volume increase (fresh weight growth)

Water absorption/uptake into roots and movement through plants – water movement from the soil solution, absorption by roots and loading into the xylem for movement to the shoot

Stomatal function in transpiration – control of water loss to the atmosphere, water movement from root to shoot, transpiration vs CO2 fixation

Water relations and horticulture (Mike Mickelbart) – translation from chemistry and biophysics to horticultural production agriculture

Why is water important to plants?

Solvent for ions and most organic molecules – essential for biochemical reactions

Temperature control – plant cooling

Ion and solute transport – mineral acquisition by roots, movement from shoot to root, and between tissues and organs

Volume maintenance and cell expansion (volume/fresh weight) – 80 to 95% of the plant is water

Properties of Water in PlantsTaiz and Zeiger, Chapter 3 (p. 37-41)

Water (H2O) is the most limiting plant resource for crop production

3.1 Corn yield as a function of water availability

Molecular structure of water – H2O, two hydrogen atoms (H) form covalent bonds to oxygen (O), electrons shared between H+ and O2- ions

3.3 Diagram of the water molecule

Water is a polar molecule - Separation of the negative and positive charged regions due to angles of covalent bonds causes polarity, molecule has positively and negatively charged regions although without a net charge

Oxygen has stronger attraction for shared electrons hydrogen (more electronegative than hydrogen), creates a local partial negative (oxygen) and positive charges (hydrogens)

Water molecules are linked by hydrogen bonds (weak electrostatic interactions) – Localized negative and positive charges of each molecule results in formation of hydrogen bonds (HO) between molecules (panel A)

Cohesive properties of water is due to intermolecular interaction

Water is a solvent for most plant biochemicals – necessary for majority of biochemical reactions

Most plant biochemicals are hydrophilic (affinity for water) vs hydrophobic (little/no affinity for water)

Hydrogen bonding between H2O, and ions or polar molecules reduces intramolecular electrostatic interactions, which increases solubility

Attraction of water molecules to charged groups in marcromolecules produces a hydration shell that enhances solubility

Temperature buffering and cooling properties of water –water is an effective coolant because of high specific heat and

latent heat of vaporization properties due to intermolecular interactions between H2O molecules

Specific heat – thermal energy required to raise the temperature of a substance; water (1.00 cal/g/deg) > alcohol (0.58) > air (0.25) > copper (0.09)

Heat is “entrapped” in H2O to reduce plant ambient temperature

Plants transpire 97% of water taken up by roots which dissipates heat from the plant to the atmosphere

Latent heat of vaporization - energy required to change the state of a molecule from the liquid to the gas phase

Water (539 cal/g) > alcohol (204), thermal energy is used for vaporization, evaporation at the leaf surface facilitates cooling

Cohesive, tensile strength, adhesive and surface tension properties of water – facilitate water movement in cells, root to shoot

Cohesion – intermolecular attraction of water molecules due to hydrogen bonding (panel A)

3.6 A sealed syringe can be used to create positive and negative pressures

Cavitation – process of air spaces forming in regions where a H2O column separates, disrupts water movement from roots to shoots

Tensile strength – maximum (pulling) force (per unit area) that a water column (formed because of cohesion) can withstand without separating

Water can be compressed forming a positive pressure

Adhesion – attraction of water molecules to a solid phase, e.g. glass tube or cell walls (pores) of xylem vessels

Capillarity - movement of water up a small diameter tube from a basal source, dependent on cohesive and adhesive properties of H2O and surface tension is the primary driving force

Responsible for water movement up the xylem from root to shoot

3.5 (A) Shape of a droplet placed on a solid surface; (B) Capillarity

Surface tension – negative pressure (pull) created at the water-air interface (liquid-vapor) since H2O molecules have greater attraction for each other than for air