Water Management in Horticultural Crops

1. Plant-water potential

• Energy status of water in plant cells is determined by three major factors viz., turgor pressure (p) imbibitional pressure (m) and solute or osmotic pressure (s).
  
• Pressures arising both from gravitational forces and intercellular pressure can be included in the turgor pressure term. Total potential of water in plants can be expressed as indicated below:


Where,
ψ=total water potential
Ψp= turgor potential (equivalent to pressure potential in soils)
Ψm= imbibitional potential (equivalent to matric potential in soils) and
Ψs=solute or osmotic potential.

1.1. Relative Water Content

• Relative water content (RWC) is the ratio of actual water content to water content at saturation (fully turgid) and is generally expressed as percentage.
  
• Actual water content is obtained by subtracting dry weight (DW) of the sample from the fresh weight (FW).
  
• Water content at saturation is the difference between saturation weight or turgid weight (TW) and dry weight.
Major areas of water-plant relationships in irrigation water management are:

• Water absorption and
• Water loss or transpiration.
1.2. Water absorption by plants

• Plants absorb water from soil through roots, rain and water sprays through foliage.
• Young roots offer largely the water absorbing surface in actively growing annual plants, while they offer relatively a small fraction of the total absorbing in old perennial plants and trees.
• A young growing root tip consists of a root cap, a zone of maximum meristematic activity, region of rapid cell elongation and a region of quick cell differentiation and maturation.
• A rapid absorption of water occurs through younger part of the root immediately basal to the meristematic region. It is usually the area where root hairs grow extensively; Root hairs are thin walled protuberances of the epidermal cells. They present relatively large absorbing surface. The xylem elements develop to conduct water up the plant system. Suberization of cell walls reduces the permeability to water. But a considerable volume of water is absorbed, though slowly, through suberized roots in older plants. The role of such roots in water absorption is very important as they comprise the largest portion of a root system in older plants and trees and offer relatively large water absorbing surface.
1.3. Water absorption processes

• Water absorption by plants occurs by two processes namely active absorption and passive absorption.
  
• In active absorption plants play an active part.
  
• In passive absorption water is absorbed mechanically through roots without plants playing an active role and plants present simply the absorbing surfaces.
1.4. Factors affecting water absorption

• Water absorption by plants is influenced by atmospheric, soil and plant factors.
1.5. Water conduction

• Water is conducted from the root surface to leaf surface through the plant body.
  
• The difference of ψ air and ψ root surface results in the ascent of water.
  
• The transpiration from leaf surface sets up imbibitional forces in the mesophyll cells that are transmitted through the hydrodynamic systems in the plant to the root surface.
  
• Water moves in liquid form from the soil to leaf cells through root cells and the conductive system of xylem. It moves in vapour form from leaf cells to the air through intercellular spaces in the leaf and stomatal openings.
• The xylem functions in water conductivity. The water conduction is based on the cohesion theory.