1. Introduction to Water and Nutrients in Plants & Properties of Water in Plants HORT 301 – Plant Physiology August 29, 2008 paul.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) – CO 2, H 2 O and minerals are accessed 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

2. C, H, O and essential minerals are required for plant growth and development Carbon – C (as CO 2 ) is fixed and assimilated into sugars from which carbohydrate macromolecules (starch, cellulose), nucleic acids (RNA, DNA), proteins, and lipids are produced Water (H 2 O) – 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

3. 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 CO 2 fixation Water relations and horticulture (Mike Mickelbart) – translation from chemistry and biophysics to horticultural production agriculture

4. 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 Plants Taiz and Zeiger, Chapter 3 (p. 37-41)

5. Water (H 2 O) is the most limiting plant resource for crop production

6. Molecular structure of water – H 2 O, two hydrogen atoms (H) form covalent bonds to oxygen (O), electrons shared between H + and O 2- ions 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)

7. Water molecules are linked by hydrogen bonds (weak electrostatic interactions) – Localized negative and positive charges of each molecule results in formation of hydrogen bonds (H  O) between molecules (panel A) Cohesive properties of water is due to intermolecular interaction

8. 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 H 2 O, 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

9. 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 H 2 O 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 H 2 O 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

10. 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)

11. Cavitation – process of air spaces forming in regions where a H 2 O 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

12. 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 H 2 O and surface tension is the primary driving force Responsible for water movement up the xylem from root to shoot Surface tension – negative pressure (pull) created at the water-air interface (liquid-vapor) since H 2 O molecules have greater attraction for each other than for air