Basic Plant Physiology & Function

Plant physiology is the biological process by which plants are able to grow and flower.  This process involves the combined synergistic operation of three primary functions which produce the food and energy that enable plants to develop.  These characteristics are common to almost all plants.
The physical makeup of plants consists of about 80% water, that water is retained in and around all plant cells and tissue.
Water flow throughout a healthy plants system is accomplished by osmosis or diffusion.  This water movement is crucial to a plants development because it is essential in order to conduct photosynthesis as well as carrying dissolved nutrients and minerals to all areas of the plant.
Leaves are the engines of a plant.  They are naturally well suited to bring together the raw materials; water, carbon dioxide and light to produce energy for the plant in the form of sugars (glucose) by means of the complex process of photosynthesis. The resulting sugars are then transported from the leaves for use elsewhere in the plant via the plants vascular system.  Leaves accomplish this task in three basic stages: first, the leaf uses pigments in the form of chlorophyll cells to capture light energy.  Second, the effect of light on the chlorophyll cells causes a flow of electrons which splits the water into hydrogen ions and oxygen.  And third, the hydrogen ions are used in the reduction of carbon dioxide to produce sugars.
The flow of water, sugars and nutrients through this vascular system is made possible by a process called transpiration.  Transpiration is a form of controlled evaporation of water from the plant.  This evaporation takes place through tiny pores on the leaf surface called stomata and helps create a pressure gradient to draw new water up from the roots into the xylem, or stem, where nutrients (minerals) can be carried up the plant in the transpiration stream until they arrive where they are needed.  At this stage, the dissolved nutrients can diffuse or be transported into the cells where they are required.  Evaporation (transpiration) helps keep the movement of water going up the plant, creating a central transport system known as the transpiration stream.

Principles of growth
Plants convert CO2 and H2O into glucose under the influence of light.  Glucose is the chemical building block responsible for the structure and sturdiness of the plant.  From glucose, the plant then creates cellulose, the material which gives plants their fibrous structure. (Glucose is, in fact, stored light energy).  The chemical process by which carbon dioxide and water are converted into glucose is called photosynthesis (from the Greek ‘photos’ = light, and ‘synthesis’ = to compose).  Chlorophyll is the pigment in leaves which gives them their green color and is indispensable for this process.  Chlorophyll molecules absorb light energy and transfer that energy in a usable form to facilitate the generation of Glucose.    If all the conditions are right, the following chemical reaction occurs:
6CO2 + 12H2O = C6H12O6 (glucose) + 6O2 (oxygen) + 6H2O
A number of facts can be deduced from this formula.  First, to get one part glucose, we need six parts CO2 and 12 parts H2O.  So initially it would seem that less water is necessary.  When we look at the chemical formula, six parts water are also produced next to the 6 parts oxygen, and 1 part glucose.  However, research has shown that in the chemical process, 12 parts water are needed.  The ‘excess’ water is used in the intermediate steps.  The water does not re-appear until the end of the process.
Also, for photosynthesis to occur properly there must always be sufficient carbon dioxide available, a gas which is naturally present in the atmosphere.  If there is a deficiency of CO2, plant growth will be impeded.  In addition, when CO2, H2O and light energy are utilized by plants to conduct photosynthesis, not only glucose, but oxygen is also generated during the process.   The oxygen produced is actually a by-product of plant growth.
And because an atmosphere containing oxygen is a primary condition required for animal life, there exists an obvious symbiotic relationship between plants and animals.
In fact, animal metabolisms produce the converse of what plants do.  Animals consume then convert glucose and oxygen into carbon dioxide and water which gives them energy, allowing the heart and lungs to function.  CO2, a gas which is exhaled by animals, can again be used by plants for photosynthesis.  It can be thought of as a cycle.  The glucose made by plants also serves as their energy source. Plant processes such as the intake of water, production of seeds, flowers and fruits all require energy derived from this internally generated glucose.
The process of photosynthesis actually involves two different chemical reactions. The first is called photolysis. In photolysis, water is broken down into oxygen (O), and hydrogen (H).  Both light and chlorophyll are necessary for photolysis. This chemical reaction is called the light response.  The second chemical reaction is called the dark response.  As the term suggests, no light is necessary for the dark response to function.  With dark response, carbon dioxide is converted into glucose, with the help of the hydrogen produced during the light response.  The distinction between the light and dark reaction is of great interest to the home grower because it gives insight into the manner and time period which plants should be illuminated, and just as importantly, kept in darkness.  Plants grow optimally only when a correct balance is found between the light and dark reactions.

Osmotic processes
Osmosis is the processes by which water and nutrients are absorbed through plant tissue.  The osmotic process is based on the principle that a plant’s exterior will permit some materials to pass through, and deny the entry of un-useful material.  Osmosis is made possible because the cell walls of plants are semi-permeable.
One important function served by osmosis is that it gives sturdiness to plant cells.  When water is absorbed by plant cells, they become saturated and the stalk and leaves stand upright.  If insufficient water is available, the plant cells will slowly lose water through evaporation, cell strength diminishes and the plant wilts. Another way for a plant to lose its sturdiness is for osmosis to work in the reverse direction.  This can occur if the concentration of dissolved materials in the water being fed to the plant becomes too high and actually impairs the plants’ ability to absorb water.  Under this circumstance, plants may actually release water and become less sturdy.  A common example is the addition of too high a dosage of fertilizer to plants.  Over-fertilization frequently causes plants to dry out and burn.
A second important function of osmosis is the ‘hitch-hiking’ of dissolved nutrient salts as an added component of water that, through osmosis, make their way into plant cells.  Nutrients are necessary to allow certain growth processes to take place. The nutrient salts also cause many varieties of plants to develop various properties; flowers, fruit, and fragrances are prime examples.
In general, plants require nourishment from the following materials in a water solution:  nitrogen, phosphorus, and sulfur for the construction of cells; magnesium to manufacture chlorophyll; potassium, calcium, and magnesium for osmotic processes; water for growth, the transport of nutrients, and for sturdiness; iron, boron, copper, manganese, and zinc as building materials.
Most, but not all, of the nutrients needed by plants are sufficiently present in our ordinary tap water.  The law of minimums plays a great role in the feeding of plants.
Material that is present in too small a quantity is a limiting factor on the plant’s health.  So-called ‘deficiency disease’ appears when a plant does not receive one or more nutrients.  For example, a shortage of iron causes rather white leaves, while a shortage of nitrogen causes reduced growth and yellowed leaves.  Usually deficiency disease not only has a direct effect (an unhealthy plant doesn’t grow well), but also impairs a plants resistance to external attack.  If needed materials are lacking, the chance for infection by molds, insects and vermin increases.
In order to raise healthy plants, we need further amplification of the materials which, by nature, appear in our water.  This comprises primarily nitrogen (N), phosphate (P), and potassium (K).  Specifically formulated nutrient solutions containing these materials are readily available to all levels of growers and are referred to as NPK solutions.  Individual nutrients fall into three distinct categories according to their importance.  The most essential are the NPK elements which are referred to as the primary nutrients; the secondary nutrients being, magnesium (Mg), and calcium (Ca).  Finally, there is a group of micro-nutrients, also called trace elements or minerals; they include: Sulfur (S), iron (Fe), manganese (Ma), boron (B), zinc (Zn), and copper (Cu) along with a few others of minimal importance.

Intake and transport of materials
Water, and the nutrients dissolved in it, is absorbed through the root hairs of the plant.  The condition of the soil plays an important role in this absorption.  Hard, tightly compacted dirt allows little space for water to reach the root hairs, whereas lose soil permits ample moisture to reach the root zone, and a rockwool substrate used hydroponically all but guarantees a good water supply.
Root hairs play a very important role in facilitating plant nourishment.  When their function is impaired, the plant receives too little water and food, thus growth is retarded.  Root hairs are very sensitive; they can easily be damaged by exposure to air and light.  Moreover, you can irreparably harm them by careless transplanting, or just by exposure.  The uptake of water and nutrients through the root system requires energy from the plant, so the oxygen and glucose which power this operation are essential to healthy plant function.
Ultimately, temperature is a limiting factor.  Even if you take care to provide sufficient water and nutrients, the growth of the plant will be impeded if the temperature of the soil or other grow media is too low.  This is one of the reasons why most outdoor plants grow very slowly during the winter.  For hydroponic gardens, the ideal water temperature should be maintained between 68 – 71 degrees Fahrenheit.  This temperature range permits the reservoir to hold the highest oxygen content allowing for the root zone to maximize nutrient uptake.
Because a temperate grow media brings about suitable root zone conditions for the transport of water and nutrients, this insures that these materials end up in the leaves.  Two forces are responsible for this: the suction power of the leaves, (they lose moisture by evaporation, causing suction to occur), and so-called root pressure.  Root pressure can be observed when the stem of a plant is cut off.  The resulting wound will secrete a liquid know as plant sap.  The suction force of the leaves depends on the evaporation of water through the leaves.  Pores in the leaf’s surface called stomata are responsible for this evaporation process.  Along with the evaporation of water, they provide principally for the intake of carbon dioxide (CO2) from the air.  And the oxygen which is produced as a byproduct of photosynthesis is then respired by means of leaf stomata.
In a previous paragraph, we have seen that plants lose their sturdiness if they lose too much water.  The stomata are possessed of a mechanism to prevent that from occurring; they can close.  Generally, a stoma will be open if there is light, (thus providing for CO2 intake, and for optimal suction power of the leaves), and close if it’s dark (when no CO2-intake, or evaporation is necessary).  If the air is extremely dry, the stomata can also close during the day.  For stomata to work properly, a clean surrounding is necessary since a stoma can become blocked with dirt particles.

Factors influencing the growth of plants
In summary, keeping in mind that the growing environment itself is the single most important factor influencing plant development, the principal requirements for the optimal growth and flowering of plants that should be of greatest concern to urban gardeners include the following factors:
Air temperature of 74 – 84 degrees Fahrenheit during the day and no cooler than 60 degrees Fahrenheit during the dark cycle; reservoir temperature of 68 – 71 degrees Fahrenheit; pH level between 5.6 & 6.1; abundant CO2 content in the air; adequate ventilation; correct light intensity and wavelength; adequate amounts of water and nutrients; good soil composition/grow media; genetically sound seeds or cuttings/clones; understanding plant physiology.