External factors affecting photosynthesis:

Concentration of carbon dioxide Carbon dioxide is the major limiting factor for photosynthesis. The Increase in the concentration of Co2 in the atmosphere initially increases the photosynthetic rate but higher concentrations of Co2 can be inhibitory to photosynthesis.

The CO2 is present in a very low concentration in the atmosphere. (between 0.03 and 0.04 per cent). Increase in concentration up to 0.05 percent can cause an increase in CO2 fixation rates, beyond this the levels can become damaging over longer periods. The C3 and C4 plants do not respond to the higher concentration of CO2 at low light intensity. At high light intensities, both C3 and C4 plants show increase in the rates of photosynthesis. The C4 plants show saturation at about 360 µlL­­­­­­-1 while C3 show saturation at levels that are beyond 450 µlL­­­­­­-1. Thus, current availability of CO2 levels is limiting to the C3 plants.

The fact that C3 plants respond to higher CO2 concentration by showing increased rates of photosynthesis leading to higher productivity has been used for some greenhouse crops such as tomatoes and bell pepper. They are allowed to grow in carbon dioxide enriched atmosphere that leads to higher yields.

Temperature

Both light and dark reactions are affected by temperature, although the light reaction is affected to a lesser extent as compared to dark reaction.
The dark reactions involve a lot of enzymatic reactions and is temperature controlled. The C4 plants show enhanced photosynthesis at higher temperatures while C3 plants have a much lower temperature optimum. Different plants show different temperature optimums for photosynthesis depending on there habitat. Plants living in tropical habitats have a higher temperature optimum than the plants adapted to temperate climates. Water deficit Water does not affect the photosynthesis process directly. Water stress causes stomatal closure thereby reducing the CO2 availability. Moreover, the surface area of leaves and the metabolic activity of the leaves is reduced as the leaves wilt in water deficit conditions. Soil water Water also helps to maintain the temperature required for photosynthesis, it helps in the uptake of minerals and ions from the soil and helps in stomatal opening and closing as well. Only 1 percent of the soil water is used by the plants in processes such as photolysis of water and to carry out biochemical reactions.

Air pollution

Some of the pollutants in the atmosphere are dust particles, smoke, fly ash, hydrogen fluoride, carbon monoxide, sulfur dioxide and nitric oxide. These pollutants affect the photosynthesis in following ways: Reduced Light Penetration: Particulate matter and pollutants in the air can scatter and absorb sunlight, leading to decreased light intensity reaching plant surfaces.

Stomatal Closure: Pollutants like sulfur dioxide and nitrogen dioxide can lead to the closure of stomata—tiny openings in leaves responsible for gas exchange. Stomatal closure limits the intake of carbon dioxide (CO2), which is a vital component for photosynthesis. Alteration of Chlorophyll Content: Some pollutants can disrupt chlorophyll synthesis and degrade existing chlorophyll molecules, thus affecting the rate of photosynthesis. Disruption of the electron transport chain: The smoke and the dust particles have heavy metals that can inhibit the electron transport chain. Internal factors affecting photosynthesis

Leaf Orientation:

The orientation of leaves relative to the sun influences the amount of light they receive. Plants with leaves that can adjust their orientation (through movements like phototropism) optimize light exposure while minimizing heat and radiation stress.

Leaf Age:

In comparison to mature leaves, developing leaves possess a lower concentration of chlorophyll, resulting in a reduced photosynthetic rate during their early growth stages. As leaves progress in maturity, the chlorophyll content within them gradually rises, leading to an increase in photosynthesis. Conversely, aged leaves exhibit a minimal photosynthetic activity due to the degradation of chlorophyll, which subsequently lowers the overall rate of photosynthesis.

Leaf anatomy:

Leaf structure plays a pivotal role in determining the photosynthetic capacity, achieved through alterations in mesophyll thickness and the expansion of surface area to accommodate chloroplasts, which are crucial for facilitating gas exchange.

Mesophyll cells

Chloroplast Distribution: Cells with higher chloroplast density, such as palisade mesophyll cells, maximize light absorption. Surface Area for Light: Large surface area of mesophyll cells, especially those with specialized shapes like palisade and spongy mesophyll cells, allows for increased exposure to sunlight

Cell Arrangement: The arrangement of mesophyll cells, particularly in palisade and spongy layers, optimizes light penetration and distribution within the leaf. Internal CO2 concentration Higher internal concentrations of CO2 ensure an adequate supply of this molecule, promoting the efficiency of the Calvin cycle. Amount of chlorophyll Chlorophyll helps in light absorption and converts light energy into chemical energy during the process of photophosphorylation.