SOIL PLANT WATER RELATIONSHIP

• KINDS OF TRANSPORT PROCESS:

  ·       Diffusion

  ·       Osmosis

  ·       Active Transport
• WATER POTENTIAL – free energy of water.
• Water Potential Can be increased by:

  • Heat
  • Pressure
  • Altitude
• *Water moves from regions where water potential is relatively (+) to regions where water potential is relatively more negative (-).
• 1. Water moves whenever there is difference in water potential
  2. If water potential are equal, the two regions are in equilibrium and there is no movement of water.
  3. Water potential must always be considered in pairs.
• HOW DO GROWTH OCCURS IN ACTIVELY GROWING PARTS OF THE PLANT?

  • “Pressure Flow Hypothesis” Long  Distance Transport (Phloem)
  • Source
  • Sink
• LONG DISTANCE TRANSPORT

  PROPERTIES OF WATER:

  1. Cohesion
  2. Adhesion
• “COHESION-TENSION HYPOTHESIS”

  • Trans-stomatal transpiration
  • Transcuticular transpiration
• Water in the plant environment


  • Transports minerals through the soil to the roots where they are absorbed by the plant
  • Principal medium for the chemical and biochemical processes that support plant metabolism
  • Provides physical support for plants under pressure within plant cells
  • Acts as a solvent for dissolved sugars and minerals transported throughout the plant
  • Evaporation within intercellular spaces provides the cooling mechanism that allows plants to maintain the favorable temperatures necessary for metabolic processes
• THE SOIL SYSTEM IS COMPOSED OF 3 MAJOR COMPONENTS:

  1. solid particles (minerals and organic matter)
  2. water with various dissolved chemicals
  3. and air
• WATER IS ABSOLUTELY ESSENTIAL FOR ALL LIVING ORGANISMS

  Evidence:

  • Most organisms are comprised of at least 70% or more water. Some plants, like a head of lettuce, are made up of nearly 95% water;
  • When organisms go dormant, they loose most of their water. For example, seeds and buds are typically less than 10% water, as are desiccated rotifers, nematodes and yeast cells.
  • Earth is the water planet (that's why astronomers get so excited about finding water in space).
  • Water is the limiting resource for crop productivity in most agricultural systems.
• PROPERTIES OF WATER:

  • Water is a liquid at physiological temperatures.
  • Water has a high heat of vaporization.
  • Water has a high specific heat (heat capacity).
  • Water has a high heat of fusion.
  • Water has a high surface tension.
  • The density of water decreases on crystallization.
  • Water is a universal solvent .
  • Water is transparent to light.

   
• Functions of water


  • Major component of cells
  • Solvent for the uptake and transport of materials
  • Good medium for biochemical reactions
  • Reactant in many biochemical reactions (i.e., photosynthesis)
  • Provides structural support via turgor pressure (i.e., leaves)
  • Medium for the transfer of plant gametes (sperms swim to eggs in water, some aquatic plants shed pollen underwater)
  • Offspring (propagule) dispersal (think "coconut")
  • Plant movements are the result of water moving into and out of those parts (i.e., diurnal movements, stomatal opening, flower opening)
  • Cell elongation and growth
  • Thermal buffer
  • Directed the evolution of all organisms
• Morphological features of organisms are a consequence of water availability
• SOIL STRUCTURE - The structural pattern, the extent of aggregation, and the amount and nature of the pore space.

  -       The size, shape, and arrangement of the soil particles and the associated voids (pores) determine the ability of a soil to retain water.
• TRANSPIRATION is the evaporation of water from the aerial parts of plants, especially but also stems, flowers, and roots. 
• Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. 
• Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
• Mass flow is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere.
• PHOTOSYNTHESIS/TRANSPIRATION PARADOX

  CO2 + H2O   --->   (CH2O) n + O2

  *This equation suggests that:

  • Gases, such as carbon dioxide and oxygen, are important in the overall energy metabolism of plants; 
  • Plants must exchange gases with the environment; and
  • In order to obtain carbon dioxide plants will necessarily loose water (transpire).  In other words, transpiration is a necessary evil of photosynthesis.
• THEORETICAL CONSIDERATIONS

  • A large surface area is required for efficient gas exchange.
  • A large surface area for exchanging gases offers a large surface area for desiccation. 
  • Placing the absorptive surfaces inside the organism to reduce desiccation presents a problem - getting the gases to the absorptive surface.
  • Paradox and Compromise.
• transpiration ratio =  mol water transpired / mol CO2 fixed

  If carbon dioxide uptake (or fixation) and water loss are equal, this ratio should be one. In reality, experiments show that this ratio is closer to 200!  Thus, for every 200 kg of water transpired, 1 kg of dry matter is fixed by a plant.
• THE AMOUNT OF WATER LOST BY A PLANT DEPENDS ON THE FF:

  • size
  • surrounding light intensity
  • temperature
  • Humidity
  • wind speed
• Soil- Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.