As plants rest quietly in the earth, their leaves waving in the breeze, structures inside their cells are hard at work turning sunlight into sugar using a biochemical process called photosynthesis. Photosynthesis is essential to the survival not only of plants but of all life on earth, as it converts sunlight into usable chemical energy. Several factors affect the rate of photosynthesis, one of which is temperature, producing sometimes unexpected effects.
Increasing Rates of Photosynthesis
To a point, increases in temperature generate higher rates of photosynthesis. However, plants do have an optimum temperature for photosynthesis, and beyond that point, rates drop off--first slowly, then sharply--as temperatures rise. Professor Dénis van Rensburg, writing for the Orchid Society of Northern Transvaal, shows the optimum temperature situated at about 77 degrees. The graph is shaped like an inverted U, showing that the most dramatic differences in rate of photosynthesis occur at extremely high and low temperatures. Changes in mild temperatures, in contrast, produce relatively little effect.
Interaction with Respiration
Unlike photosynthesis, respiration--which uses up the energy created by photosynthesis--increases rapidly as the temperature increases. Professor van Rensburg's graph shows that, at about 95 degrees, respiration consumes all of the energy produced by photosynthesis. At higher temperatures, respiration uses more energy than the plant can produce, which can starve the plant if allowed to continue for too long.
Because plants cannot photosynthesize and, therefore, cannot produce energy in the dark, nighttime temperatures also play a role in the efficiency of photosynthesis. According to Erv Evans, a horticulturist with North Carolina State University, the ideal nighttime temperature is 10 to 15 degrees lower than the daytime temperature, slowing respiration and allowing plants to apply the energy generated through photosynthesis to new growth and flowering.
As Professor van Resenburg's data indicates, photosynthesis decreases as temperatures lower, dropping drastically as temperatures approach the freezing point. Because of this, growth drops off too at low temperatures because the plant is not photosynthesizing enough energy to use for new growth.
Cause of Temperature-Related Differences
Photosynthesis is an enzyme-dependent reaction, which explains why temperature exerts such strong effects on photosynthesis. Enzymes do not perform well at low temperatures, but they also have an optimum temperature--the optimum temperature of photosynthesis illustrated by Professor van Resenburg--after which performance declines and, eventually, stops altogether. The effects of temperature on photosynthesis are caused by the behavior of the enzymes catalyzing the reactions.