Transpiration And Water Movement In Crop Plants

Osmosis

Click here to view a video that explains osmosis membranes and transport.

Osmosis is the process through which water moves (diffusion) through a membrane from an area of higher concentration to an area of lower concentration through a partially permeable or selectively permeable membrane. A selectively permeable membrane is a membrane such as a cell membrane that will selectively allow molecules to pass through it. Such a membrane may for example allow water to pass but not proteins. The movement of compounds from a higher to a lower concentration can be compared by moving down a concentration gradient by diffusion.

In order for osmosis to occur, two solutions must be separated by a membrane that will only let the solvent (water in the case of cells) pass through it. Another prerequisite is that the concentration of the water must be higher in one of the separated solutions. This can be achieved by having a sugar solution as one solution and plain water on the other side. In this way, the water molecules will move into the sugar solution down a concentration gradient.

The water will continue to migrate through the membrane until the two solutions are in equilibrium, meaning that the relative concentrations at both sides of the membrane are the same. In the example the membrane is permeable to only the water, thus only will the water move but the sugar will not migrate, remaining at one side of the membrane.

Selectively or partially permeable membranes are found mostly in living organisms: e.g. Cell membranes and the membrane lining of an eggshell. Osmosis is thus clearly a critical process in living beings. The process controls the constitution of cells and therefore of living tissues. It is important that as little water as possible moves in or out of cells because if too much water enters a cell, it could swell and burst and if too much water exits the cell, the protoplast will shrink and the plasmalemma (outer cell membrane) will pull away from the cell wall ( plasmolysis).

Water Transport in Plants

Click here to view a video that explains the movement of water up the stem and across the leaf.

Plant stems have two major functions. The first is to produce and carry the leaves and “hold’ the leaves up to the sunlight and produce and carry buds and flowers. The second is to hold the vascular system through which sugars and water are transported.

Simplified, a cross and longitudinal section of the stem reveals that a stem is made up of an outer protective tissue layer, ground (filling) tissue and the vascular system.

The vascular system consists of two tissue systems; the phloem through which SUGARS are transported from the leaves (source) to where they are stored or used (sink or target) and the xylem transporting water and mineral nutrients and the primary tissues deriving from the growing point (apical meristem) of the stem.

• The epidermis (Protective tissue). The epidermis in young stems or periderm in older stems.
• The ground tissue. (Cotex and pith)
• Live parenchyma cells, and collenchyma
• Transport system:
  • Primary Xylem – consists of strengthening tissue (dead xylem fibres), transporting tissue (dead xylem vessels) and live cells (xylem parenchyma)
  • Primary Phloem system – consists of transporting tissue (live sieve elements). Live phloem parenchyma and sometimes strengthening tissue (dead phloem fibres)

Secondary tissues deriving from the vascular cambium and cork cambium.

Vascular cambium (only found in Dicots – no secondary thickening growth in most Monocots) Cylinder of embryonic (dividing) cells producing secondary xylem towards the inside and secondary phloem towards the outside of the stem or root:

• Secondary xylem (secondary xylem vessels, secondary xylem fibres and secondary xylem parenchyma in xylem rays and axial XP
• Secondary phloem (secondary phloem sieve elements or sieve tubes, companion cells, secondary phloem fibres, secondary phloem parenchyma

Cork cambium (phellogen) producing:

• Dead cork tissue (phellem) towards the outside
• Live parenchyma cells (phelloderm) towards the inside
• (Phellem + phellogen + felloderm collectively known as perderm, a very important protective tissue)
• Periderm + secondary phloem is commonly known as the bark of a tree that easily breaks off from the wood (secondary xylem) at the vascular cambium when debarking trees

The Role of Stomata in Gas Exchange in Plant Tissues

Stomata are found in the epidermis of leaves, young stems and fruit. Stomata allow gas exchange into and out of the leaf. Water vapour moves out of the leaf and carbon dioxide is allowed in for photosynthesis. If the stomata are closed, transpiration is reduced and photosynthesis is limited. The plant thus has to constantly balance these two processes ensuring metabolism is optimised.

Click here to view a video that explains gas exchange in flowering plants.

In most plants, the stomata are primarily or solely found on the underside of a leaf’s surface but may also b present in young stems and fruit. At times of extreme heat (such as at midday) they will close thereby reducing water loss.

Transpiration and Water Flow in Plants

Transpiration is a biological process in which water evaporates from a plant. The exchange of water vapour from leaves through the stomata is the primary driving force of the process. Transpiration is critical to the metabolic processes in the plant and thus the survival of the plant.

Water Flow

All living organisms need water to survive. Plants need water to maintain the internal pressure or turgidity in cells and tissues. Water is also needed to transport dissolved minerals and elements materials from the soil that may be required for the metabolic process to continue.

Water, containing dissolved minerals enters the plants' roots via root hairs, and is transported via the xylem vessels up the stem to the leaves and actively growing and metabolising part of the plant.

In order to maintain a flow through the xylem system, water evaporates from the leaves via the stomata into the surrounding atmosphere. As the water evaporates from the leaf surface, more water enters the roots to replace the evaporated water. This causes a sucking system allowing water to be drawn from the roots to leaves in a continuous stream through the plant. This capillary suction action is known as the transpiration stream or transpiration tension and is similar to the suction effect found in a wick. Much of this water will not enter cells but will pass by the cells and exit the plant directly.

Wilting

Wilting is the loss of rigidity or turgidity of non-woody plant parts. Plant cells and tissues lose turgidity when the turgor pressure in the cells decreases towards zero. The pressure is reduced when the volume of water in the cell decreases below the ideal. Permanent wilting leads to plant death.

Click here to view a video that explains turgor pressure.

Decreased cell water content occurs due to:

• Drought, where soil moisture decreases below that which plants can maintain water uptake
• High salinity, which causes water to diffuse out of plant cells
• Saturated soil conditions, where roots are unable to obtain sufficient oxygen or infections that may clog the vascular systems.

Control of Transpiration

Because water is not always abundant, most plants must control transpiration in order to prevent excess water loss. The surface of plant leaves is covered in a waxy, semi-waterproof coating called the cuticle. This layer ensures a minimal loss of water (evaporation) through the cuticle of epidermal cells. The stomatal pore is flanked by two specialised bean-shaped guard cells. The guard cells’ inner walls are thicker and firmer than their outer walls. When a plant begins to wilt through excess transpiration, the guard cells become flaccid due to the lack of water, straighten and so doing, close the pore.

When thousands of stomata shut on a leaf, the rate of transpiration is drastically reduced. The plant though, continues to absorb water via the roots, replenishes the plant cells to a point where they swell and become turgid. As the guard cells swell, the thin, outer wall stretches and become curved, resulting in the widening of the stomatal pore between them. More water vapour passes through the open stomata and transpiration increases until the plant begins to wilt again. This self-adjusting system regulates the plant's water loss by means of transpiration. The concentration of the plant hormone, abscisic acid, also increases when the leaves of the plant wilt as it causes the guard cells to close the stomata.