Photosynthesis is the process by which plants and some bacteria manufacture glucose. The process can be summarized using the word equation: Carbon dioxide + water (Using sunlight) = glucose + oxygen. The process occurs within special structures called chloroplasts located in the cells of leaves. Optimum photosynthetic rates lead to the removal of greater amounts of carbon dioxide from the local atmosphere, producing greater amounts of glucose. Since glucose levels within plants are difficult to measure, scientists usually use carbon dioxide assimilation as a means to measure photosynthetic rates. Maximum photosynthetic rates vary between plant species but crops such as maize can achieve carbon dioxide assimilation rates as high as 100 milligrams per decimeter per hour (0.075 ounce per cubic foot per hour). In order to achieve optimum growth of some plants, farmers need to keep them in a greenhouse that regulates conditions such as humidity and temperature. There are three temperature regimes over which the rate of photosynthesis changes.
Enzymes are protein molecules used by living organisms to carry out biochemical reactions. The proteins are folded into a very particular shape, and this allows them to bind efficiently to the molecules of interest. At low temperatures, between 0 and 10 degrees Celsius (32 and 50 degrees Fahrenheit), the enzymes that carry out photosynthesis do not work efficiently, and this decreases the photosynthetic rate. This leads to a decrease in glucose production and will result in stunted growth. The installation of a greenhouse heater and thermostat can prevent this from occuring.
At medium temperatures, between 10 and 20 degrees Celsius (50 and 68 degrees Fahrenheit), the photosynthetic enzymes are working at their optimum level, so the rate of photosynthesis is high. Depending on the particular plant in question, the greenhouse thermostat should be set to a temperature within this range. At these optimum temperatures the limiting factor becomes the diffusion of carbon dioxide into the leaves.
At temperatures above 20 degrees Celsius (68 degrees Fahrenheit) the rate of photosynthesis decreases as the enzymes do not work as efficiently. This is also despite the increase of carbon dioxide diffusion into leaves. At a temperature above 40 degrees Celsius (104 degrees Fahrenheit), the enzymes that carry out photosynthesis lose their shape and functionality, and photosynthetic rate declines rapidly. The graph of photosynthetic rate versus temperature therefore has a curved appearance with the peak rate occurring close to room temperature. Thus, a greenhouse or garden that provides optimum light and water will still produce less vigorously if it gets too hot.