The yield of a plant is determined by photosynthesis, a physio-chemical process influenced by several internal and external factors. Internal factors, also known as plant factors, are determined by the plant’s genetic make-up and its growth. Factors such as the number, size, age and orientation of the leaves, mesophyll cells, chloroplasts, internal CO2 concentration and the amount of chlorophyll are the internal factors that affect photosynthesis. External factors, on the other hand, include the availability of sunlight, light intensity, incident light, temperature, carbon dioxide concentration and water. Initially, scientists assumed that only an increase in CO2 absorption by the leaves during photosynthesis increased the rate of assimilation, leading to increased plant growth. This opinion underwent a change when experiments conducted by Frederick Blackman showed that photosynthesis is affected by other external factors as well. In fact, Blackman formulated the Law of Limiting Factors which states that “If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. It is the factor which directly affects the process if its quantity is changed.” This fact is utilised by large-scale farmers who grow crops such as tomatoes in a greenhouse where the carbon dioxide-rich atmosphere helps increase the crop yield. Apart from light and CO2 concentration, the rate of photosynthesis is also affected by temperature. In fact, temperature has a major influence on light and dark reactions. Temperature also affects C3 and C4 plants. When C4 plants, whose optimum temperature is 35 degrees Celsius, are subjected to higher temperatures, they register a greater increase in the rate of photosynthesis than their C3 counterparts, whose optimum temperature is 25 degrees Celsius.
Similarly, the rate of photosynthesis of tropical plants remains unaffected by higher temperatures whereas an increase in temperature causes a decrease in the photosynthetic rate of temperate plants as they have a lower optimum temperature than tropical plants. Water is another factor that affects the photosynthetic rate of a plant. Water stress caused by lack of sufficient water forces the stomata to close, water stress causes the leaves to wilt, which reduces their surface area and affects photosynthesis. It is thus, important that water and all the other factors are available at optimum levels as this will ensure a healthy rate of photosynthesis which, in turn, will increase plant yield.