There are three major physiological functions that occur in plants that foster plant growth and development.  They are photosynthesis, respiration, and transpiration.

Photosynthesis

Photosynthesis is the process whereby plants, algae, and certain bacteria harness light energy from the sun and convert it to chemical energy in the form of sugars. These sugars or carbohydrates become food used to fuel plant growth.

How Does Photosynthesis Work?

During the process, light is absorbed by pigments (primarily chlorophyll) located in the chloroplasts of a plant’s cells and used as energy to convert carbon dioxide and water from the air and roots into sugars used by the plant as food and oxygen which gets released into the atmosphere.  If any of the key ingredients (light, water, carbon dioxide, chlorophyll) required for photosynthesis are missing, the process will cease.  Prolonged periods where one or more of the required ingredients are missing will result in the death of the plant.

Plant cells typically contain several pigments including chlorophyll a, chlorophyll b, carotenoids, and anthocyanins.  Pigments produce color based on the wavelengths of light it reflects.  For example, chlorophyll is the pigment in the membranes of chloroplasts that makes plants appear green, because it reflects green light - however, it absorbs other light waves, primarily red and blue light waves. 

Chlorophyll a is the primary participant in the light reactions of photosynthesis, but other pigments participate indirectly like carotenoids.  Carotenoids, for example, which are red, orange, or yellow, cannot participate directly in photosynthesis.  Instead, they pass the sunlight energy they absorb to chlorophyll, and are therefore called accessory pigments.  These additional pigments allow the plant to capture more sunlight. 

Without sunlight, photosynthesis cannot occur.  How much light is needed for photosynthesis to occur varies from species to species, but typically as the amount of sunlight increases, photosynthetic activity increases.  As a result, more food is produced for the plant to use for growth.

Some plants like tomatoes require maximum sun for optimal fruit production.  There are only a few varieties of tomatoes which will produce fruit in a greenhouse setting between late fall and early spring because of the minimal amount of sunlight available for photosynthesis.  Other shade loving plants like azaleas and dogwoods can produce enough food to grow at an optimal rate with less sunlight.

Water is necessary for photosynthesis to occur for several reasons.  Water creates turgor pressure, so that the tissues in roots, stems, and leaves remain firm.  During photosynthesis, water is split into hydrogen and oxygen.  The oxygen is releases into the air while the hydrogen is used to create carbohydrates and other compounds for storing energy.  Water is also needed to transport dissolved minerals from the roots to other parts of the plant that need them for growth and photosynthesis.

Carbon dioxide is required for photosynthesis and is absorbed from the atmosphere through open stomata.  Carbon dioxide is abundant in the earth’s atmosphere, so it is not typically a limiting factor in the production of food except possibly in airtight greenhouses.  However, this situation can be remedied with proper ventilation to allow sufficient outside air to enter or with carbon dioxide generators.  In small greenhouses, dry ice can also be used to produce carbon dioxide.

Temperature can affect the rate at which photosynthesis occurs.  The optimal temperature range for maximizing the rate of photosynthesis is between 65° and 85° F.