Factors that Affect Photosynthesis
A number of factors have been found to influence the rate of photosynthesis in plants. These include both external factors such as the intensity and quality of light, the availability of carbon dioxide and water, and the temperature, and internal factors such as the amount of chlorophyll in the leaf tissue, the structure and age of leaves and the degree of opening of stomata.
No photosynthesis occurs in the dark. However, a plant continues to use oxygen and give off carbon dioxide in the process of cellular respiration which occurs in light as well as darkness. When light intensity increases photosynthesis begins and then, along with increase in the intensity of light, photosynthesis gradually increases in rate. The rate of photosynthesis goes on increasing until the overall gas exchange between the plant and the environment is reversed, that is, more CO2 is utilised and more O2 is given out. The light intensity at which photosynthetic CO2 uptake just balances respiratory CO2 release is called compensation point. Light intensities higher than that at compensation point, favour increased photosynthesis and storage of food by plants for their growth.
When light intensities are increased beyond the compensation point, the rate of photosynthesis continues to increase more or less proportionately. However, a stage comes, ultimately, when the plant becomes light saturated when light becomes no longer a limiting factor in determining the rate of photosynthesis.
If the carbon dioxide concentration available is increased at the light saturation level the rate of photosynthesis is again increased. This will go on until at the new light intensity carbon dioxide concentration again becomes a limiting factor. In land plants the bulk of CO2 enters the plants through stomata. Therefore, when the stomata are closed the availability of CO2 is also decreased resulting in a decrease in the rate of photosynthesis. In field conditions, when all other factors are not rate limiting, the availability of CO2 becomes a limiting factor, influencing the rate of photosynthesis.
In plants temperature affects photosynthesis and cellular respiration quite differentially. The rate of photosynthesis decreases with increase in temperature, whereas cellular respiration increases with an increase in temperature up to a certain point. It has been found that lower the temperature, lower the compensation point and that the maximum photosynthesis is achieved at lower temperatures. At higher temperature, increased cellular respiration necessitates increased utilisation of the photosynthetic products, resulting in less storage of materials in plants.
In dry weather and in drought conditions water is found to be the factor limiting the rate of photosynthesis. The non-availability of water also influences photosynthesis indirectly as it affects the availability of carbon dioxide. In hot weather, the stomata are closed to prevent excessive water loss through transpiration. This adversely affects photosynthesis, because the availability of CO2 is also decreased in such situations. Apart from these, the structure and age of leaves are also factors that affect the photosynthetic efficiency of the plant. Young mature leaves show maximum photosynthesis when compared to immature or old leaves.
Crassulacean Acid Metabolism
Crassulacean acid metabolism (CAM) refers to a mechanism of photosynthesis that is different than already discussed C3 and C4 pathways. This occurs only in succulents and other plants that normally grow in dry conditions. In CAM plants, CO2 taken up in the night is fixed in the same way as it happens in C4 plants to form malic acid, which is stored in the vacuole. The malic acid thus formed during the night, is used during the day as a source of CO2 or photosynthesis to proceed via the C3 pathway. Thus, CAM is a kind of adaptation that allows certain plants (for example pineapple) to carry out photosynthesis without much loss of water, which is inevitable in plants with C3 and C4 mechanisms.
Translocation of Photosynthesis
Photosynthesis or photoassimilates, i.e. the energy-rich carbon compounds formed during the process of photosynthesis, are transported out of the leaf to non-photosynthetic organs and tissues, such as roots, stem tissues and developing seeds and grains. This long distance transport of photosynthates occurs through phloem and is known as translocation. The translocation of photosynthates to storage organs plays a significant role in determining crop yield. Sucrose is the principal form of carbohydrates that is translocated from leaf to the non-photosynthetic plant organs. It is a non-reducing sugar, and hence, chemically stable. Because of this property, sucrose does not react with other substances during translocation through phloem.
The photosynthates provide energy to the non-photosynthetic tissues through respiration. In storage organs, they are stored in the form of starch or as other carbohydrates.
Significance of Photosynthesis
Photosynthesis is vital for life on planet earth. It is the only process that links the physical and the biological world. It helps in conversion of the solar energy into organic matter which makes bulk of the dry matter of any organism. The plant biomass or dry matter, derived through photosynthesis supports humans and all other heterotrophic organisms living in the biosphere. Presence of oxygen in the atmosphere is also an outcome of photosynthesis. This oxygen helps living organisms in two ways:
- In efficient utilization of the energy-rich molecules (carbohydrates formed during photosynthesis) through respiration, and
- In making ozone (O3) in the outer layer of atmosphere, which helps in stopping the highly destructive ultraviolet (UV) rays from reaching the earth. Without oxygen, life of all aerobic organisms including humans is not possible. Agricultural productivity is also totally dependent on photosynthesis. Scientists are currently engaged in genetically manipulating this process for further increasing productivity of agricultural crops.
Another mode of obtaining energy in some different species of bacteria is chemosynthesis, which is different from photosynthesis. The process of carbohydrate synthesis, in which the organisms use chemical reactions to obtain energy from inorganic compounds, is called chemosynthesis. For example, bacteria of genus Nitrosomonas oxidize ammonia to nitrite. The energy released during oxidation is used by the bacteria in the same way as plants use energy from sunlight during photosynthesis, for converting carbon dioxide to carbohydrates. Such bacteria are called chemosynthetic autotrophs.