Introduction to Water and Nutrients in Plants and Properties of Water in Plants

HORT 301 – Plant PhysiologySeptember 21, 2007

paul.m.hasegawa.1@purdue.edu

Plants are autotrophic (self-nourishing) - access essential chemical resources from the environment and synthesize all other necessary molecules

Light or chemical energy is used for acquisition (fixation), assimilation and biosynthesis

Essential chemical resources of plants are necessary for growth, development and survival

Carbon – carbon (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

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

Series of lectures will focus on:

Water and mineral nutrients – properties, acquisition, assimilation and movement (transport) through the plant

Sugar/photosynthetic assimilate movement (translocation) through the phloem, and assimilate allocation and partitioning

Water and Plants - Lectures

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

Water movement into and through plant cells – bulk flow, diffusion and osmosis; water potential

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

Water movement through the plant – water transport in the xylem

Stomatal function in transpiration – control of water loss to the atmosphere, water movement to the shoot and carbon fixation (~500 g H2O transpired/g organic matter fixed)

Water relations and horticulture (Mike Mickelbart)

How or why is water important to plants?

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

Water (H2O) is the most limiting plant resource – essential for plant survival, growth and development, exchanged for CO2

3.1 Corn yield as a function of water availability

Plant volume (fresh weight, size) and cell expansion – water constitutes about 80-95% of cellular volume and “drives” cell expansion (volume increase)

Maintenance of temperature – heat buffer (absorbs heat energy) and other cooling properties of water, continuous movement through the plant because of transpiration (evaporation to the atmosphere) facilitates temperature maintenance (97% of water absorbed by roots is lost by transpiration)

Essential for some biochemical chemical reactions – e.g. carbon fixation

Solvent for ions and organic molecules – most elemental nutrients are absorbed by roots from the soil solution

Molecular structure of water – H2O, two hydrogen atoms (H) bound covalently to oxygen (O), electrons are shared between H+ and O2-

Oxygen has a stronger attraction for the electrons in the covalent bond (more electronegative than hydrogen) creating a negative charge (oxygen) and positive charge (hydrogen) and creating polarity

3.3 Diagram of the water molecule

Separation of the negative and positive charged regions because of the covalent bond angles makes it a polar molecule (molecule has positively and negatively charged regions, although without a net charge)

Hydrogen bonding (weak electrostatic interaction) – Localized negative and positive charges in the water molecule results in formation of hydrogen bonds (HO) that facilitates water molecule aggregation

Water is a solvent for biochemical molecules – Hydrogen bonding between water and ions or polar molecules reduces intramolecular electrostatic interaction and 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 – due to high specific heat and latent heat of vaporization properties

Specific heat - energy required to raise the temperature of a substance

Specific heat of water relative to other substances; water (1.00 cal/g/deg) > alcohol (0.58) > air (0.25) > copper (0.09) @ standard pressure, thermal energy is dissipated to raise the temperature, plant transpires 97% of water taken up by roots

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

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

Water can be compressed forming a positive pressure

Tensile strength – maximum (pulling) force (per unit area) that a water column can withstand without separating causing air spaces, due to cohesion

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

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

Tensile strength, cohesion and adhesion cause capillary movement (capillarity) of water up a small diameter tube (or xylem vessel) from a basal source, and surface tension is the primary driving force for water transport up the xylem

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