Photosynthesis is influenced by both Environmental and Genetic (internal) elements.
Environmental elements include light, carbon dioxide, temperature, soil, water, and nutrients.
Internal or Genetic elements affecting the leaf include protoplasmic factors, chlorophyll content, leaf shape, and end-product accumulation. Several critical elements are highlighted below:
1. Concept of Cardinal Values: Numerous environmental elements affect metabolic processes. The magnitude of each element affects the rate of a metabolic process. Sachs (1860) identified three crucial values for each factor, referred to as the cardinal values or magnitude points. These are the minimal, optimal, and maximal values. The cardinal minimum value is the factor magnitude below which the metabolic process cannot proceed.
The optimal value is the one at which the metabolic process is most rapid. The maximum is the magnitude of a factor at which the process comes to a halt. A metabolic process’ rate declines at magnitudes below and above the optimum until the minimum and maximum values are attained.
2. Principle of Limiting Factors: Liebig (1843) proposed the law of minimal speed, which asserts that the rate of a process is constrained by the speed (rapidity) of the slowest element. However, Blackman (1905) added the “concept of limiting factors” to it. It claims that when several factors influence the speed with which a metabolic process occurs, the rate of the process is restricted by the pace (rapidity) of the slowest factor. This is often known as “Blackman’s Law of Limiting Factors.”
The slowest factor, or limiting factor, is the one whose magnitude increases exactly proportional to the rate of the metabolic process (here photosynthesis).
The validity of Blackman’s law of limiting factors has been questioned. For example:
(i). It has been demonstrated that the rate of a process cannot be increased indefinitely by increasing the availability of all known factors.
(ii) Toxic compounds and inhibitors are not covered by the Blackman principle.
(iii) Some researchers have demonstrated that two or more factors can simultaneously control the rate of a reaction.
3. External Factors: Carbon dioxide, light, temperature, water, oxygen, minerals, contaminants, and inhibitors are examples of external elements that can influence the rate of photosynthesis.
1. Effect of Carbon dioxide: One of the raw materials, carbon dioxide, has a considerable effect on the rate of photosynthesis. Carbon dioxide is generally found in the atmosphere at concentrations of 0.03 to 0.04 percent by volume. It has been demonstrated experimentally that increasing the carbon dioxide level of the air by approximately 1% results in a similar increase in photosynthesis when the intensity of light is also increased.
2. Effect of Light: Solar radiation is the ultimate source of light for photosynthesis in green plants, which travels in the form of electromagnetic waves. Around 2% of the total solar energy reaching the planet is used for photosynthesis, whereas around 10% is used for other metabolic activities. The intensity, quality (wavelength), and duration of light are all variable.
The effects of light on photosynthesis can be classified into three categories:
(i). Intensity of Light: The total amount of light that a plant perceives is determined by its overall shape (i.e., height and leaf size, etc.) and leaf arrangement. Around 80% of the light that strikes a leaf is absorbed, 10% reflected, and 10% transmitted. The lux meter is used to measure light brightness. The effect of light intensity varies by plant, for example, it is more in heliophytes (plants that thrive in the sun) and less in sciophytes (plants that prefer shade) (shade-loving plants). The rate of photosynthesis improves with increasing light intensity for a full-plant, except when the light intensity is extremely high, at which point the phenomenon of Solarization occurs (i.e., Photo-oxidation of different cellular components including chlorophyll). Additionally, it affects the opening and closure of stomata, which affects gas exchange. The light saturation point is the value of light saturation at which an additional increase does not increase CO2 absorption.
(ii) Quality of Light: Photosynthetic pigments absorb visible light between 380 and 760 m. Chlorophyll, for example, absorbs blue and red light. In blue and red light, plants typically exhibit a high rate of photosynthesis. The highest rate of photosynthesis has been reported in red light, followed by blue light and yellow light (monochromatic light). The green light has a negligible effect. Photosynthesis occurs at its peak in white light, or sunlight (polychromatic light). On the other hand, red algae photosynthesis is maximal in the green light, while brown algae photosynthesis is maximal in blue light.
(iii) Duration of Light: Photosynthesis is favored by a longer duration of the light period. In general, plants benefit from 10 to 12 hours of light per day. Plants may actively photosynthesize in the presence of continuous light without being harmed. Photosynthesis proceeds at a constant rate regardless of the duration of light.
3. Effect of Temperature: The rate of photosynthesis increases significantly as temperature increases, assuming that other factors such as CO2 and light do not act as a limiting factor. In the dark stage, the temperature affects the rate of enzyme-controlled processes. When the light intensity is low, the reaction is constrained by the limited supply of reduced coenzymes, and any increase in temperature has a negligible influence on the total rate of photosynthesis.
At high light intensities, the rate of photosynthesis is governed by the enzyme-controlled dark stage, and Q10 = 2. When the temperature exceeds approximately 30°C, the rate of photosynthesis abruptly decreases due to the inactivation of enzymes by heat.
4. Effect of Water: While the amount of water required for photosynthesis is less than 1% of the total water absorbed by the plant, any variation in the amount of water absorbed by the plant has a considerable effect on its rate of photosynthesis. Under normal conditions, water does not appear to be a limiting factor, as chloroplasts often contain a sufficient amount of water.
Numerous experimental findings demonstrate that in the field, the plant can tolerate a wide range of soil wetness without affecting photosynthesis and that photosynthesis is only slowed when wilting occurs. Some of the effects of drought may be secondary when stomata close in response to water deprivation. Drought has a more particular influence on photosynthesis due to protoplasm dehydration.
5. Effect of Oxygen: Excess O2 may act as an inhibitory factor in the process. Increased O2 supply accelerates respiration while lowering the rate of photosynthesis using common intermediary molecules. The oxygen concentration in the atmosphere is approximately 21% by volume and rarely varies. O2 is not a photosynthetic limiting factor.
The Warburg effect occurs when the oxygen concentration in the air falls. [Reported in 1920 in Chlorella algae by German scientist Warburg]. This is because increasing O2 levels inhibit RuBP-carboxylase competitively, i.e., O2 competes with CO2 for active sites in the RuBP-carboxylase enzyme. This issue is explained by the phenomenon of photorespiration. If the amount of oxygen in the atmosphere drops, photosynthesis in the C3 cycle will increase while the C4 cycle will remain the same.
6. Effect of Minerals: Light processes (photolysis of water/oxygen evolution) require Mn++ and CI– for optimal operation.Mg++, Cu++, and Fe++ ions are required for chlorophyll production.
7. Pollutants and their Inhibitors: In smoke, nitrogen oxides and hydrocarbons combine to generate peroxyacetyl nitrate (PAN) and ozone. It is well established that PAN inhibits the Hill reaction. Diquat and paraquat (often referred to as Viologens) inhibit electron transport between Q and PQ in PS II.
Monouron (Chlorophenyl dimethyl urea), Diuron (Chlorophenyl dimethyl urea), and DCMU (Chlorophenyl dimethyl urea) are other photosynthetic inhibitors (Dichlorophenyl dimethyl urea), Bromocil, and Atrazine, which all work in the same way as violates. Potassium cyanide appears to have no inhibitory effect on photosynthesis at low light intensity.
4. Internal Factors: The critical internal components that affect the rate of photosynthesis are as follows:
1. Protoplasmic factors: In protoplasm, there is an unknown component that regulates the rate of photosynthesis. This variable affects dark reactions. Many plants show a drop in photosynthetic rate when exposed to temperatures above 30°C or strong light intensities, implying that this unknown component is an enzyme.
2. Chlorophyll content: Chlorophyll is a critical component of photosynthesis on the internal level. The amount of CO2 fixed in an hour by one gram of chlorophyll is referred to as the Photosynthetic or Assimilation number. It is typically stable within a plant species but occasionally changes. The absorption rate of a species’ variegated variety was found to be greater than that of its green leaves variety.
3. Accumulation of end products: Food accumulation in the chloroplasts slows photosynthesis.
4. Structure of leaves: The amount of CO2 that enters the chloroplasts is determined by leaf structural characteristics such as the size, position, and behavior of the stomata, as well as the number of intercellular gaps. Other characteristics like the thickness of the cuticle and epidermis, the presence of epidermal hairs, and the amount of mesophyll tissue all have an effect on the amount and quality of light reaching the chloroplast.
5. CO2 Compensation Point: It is the value or position in light intensity and ambient CO2 concentration at which the rate of photosynthesis is just equal to the rate of respiration in photosynthetic organs, resulting in no net gaseous exchange. The light adjustment point for shade plants is 2.5-100 ft. candles and 100-400 ft. candles for sun plants. The CO2 compensation point in C4 plants is quite low (0-5 ppm), whereas it is quite high in C3 plants (25-100 ppm). Because there is a net loss of organic matter due to non-green organ respiration and dark respiration, a plant cannot exist at the compensation point for long.