Allelochemicals are a wide variety of chemicals and effects with terpenoids. Sometimes Allelochemicals allow for mutual relationship between organisms. They are present in all plants, roots, leaves etc. Biggest producer for Allelochemicals are plants, these chemicals are mostly non-toxic but can be sometimes more toxic than pollutants. These are in huge amount in plants and protect them from insects, fungi and bacteria. But sometime these can be lethal to crop plants and significantly reduce growth of crop plants. Allelochemicals are released in environment through plants via roots and leaves. This includes phenolic acid groups such as benzoic and cinnamic acid, alkaloids, terpenoids and many more (Rice, 1984). These modify growth and development of plants. Allelochemicals alter variety of physiochemical processes such as photosynthesis in which it is hard to separate the primary from secondary effects. Photosynthesis is the basic essential process for plant growth, in which the plants, algae and trees use light energy to drive the synthesis of organic compounds. The photosynthetic process in plants and algae occurs in small organelles known as chloroplasts that are located inside a plant cell. Photosynthesis reaction is divided into two parts. The "light reaction" part which consisting electrons and protons transfer reaction. And the "dark reaction" part which contains synthesis of carbohydrates using CO2. The light reactions are made up of protein complexes, electron carriers, chlorophyll and lipid molecules. In the light reaction, there are two reaction center such as photosystem I and photosystem II. In this electrons were transferred from H2O to NADP+ and reduced to NADPH. The NADPH combining with ATP gives energy for the dark reactions of photosynthesis, mostly known as the Calvin cycle or the photosynthetic carbon reduction cycle. Calvin cycle occurs in aqueous phase of the chloroplast and involves series of enzymatic reactions (Singh & Thapar, 2003).
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Photosynthesis is greatly influenced by environmental factors such as light, temperature, CO2 concentration, water condition and microbes. Reduction of CO2 adjustment has been widely observed in many plants after treatments with Allelochemicals. Allelochemicals can potentially inhibit the performance of the three main processes of photosynthesis, electron transport and the carbon reduction cycle. Examples of some common Allelochemicals are shown in Figure 1 in appendix section.
Impact of Allelochemicals on stomata
Allelochemicals treatments frequently resulted in a decrease on stomatal conductance together with loss of leaf turgor. In cucumber, allelopathic agents would result in stomatal conductance in hours after treatment (Yu et al., 2003). In the case of stomatal limitation, reduced stomatal conductance is generally accompanied by decreased intracellular CO2 concentration. On the other hand, non-stomatal limitation is characterized by a reduced stomatal conductance and an increased intracellular CO2 concentration (Farquhar & Sharkey, 1982). From the response of CO2 integration rate to intracellular CO2 concentration. Stomata function is influenced by a lot of factors such as water status and potassium concentration. There is no enough evidence that Allelochemicals is involved in the regulation of stomatal aperture. Since root is the first thing that come into contact with Allelochemicals in most cases, decreased water intake and ion uptake are most possible mechanism involved.
Photoinhibition and electron transport
Photoinhibition uses light energy to carry out two main reactions; oxidation of H2O and the reduction of plastoquinone. Many environmental factors reduce the capacities of photosynthetic system to utilize incident light, leading to a photoinhibition process. Photoinhibition of photosynthesis is characterized as a reduction in significant return of photosystem II photochemistry and a decrease in Chlorophyll fluorescence. A few studies showed that Allelochemicals or phytochemicals from higher plants, cyanobacteria and algae exhibited inhibition to ATP generation, uncoupled electron transport and phosphorylating electron flow. One of the studies found that trachyobanoic acid, an isolated form of isotephane heterophylla, acts as hill reaction inhibitor and inhibited uncoupled photosystem II electron from H2O to DCPP and they concluded that a perturbation in the thylakoids at the level of LHC II occurred.
In theory, very little information is available as to the accumulation of the tested Allelochemicals in chloroplast and leaves. For higher plants, Allelochemicals must be absorbed in roots and transported through stems to chloroplasts before it works as inhibitor of photosystem II electron transport. Showing disruption of photosystem II electron transport in vivo doesn't mean that it is the primary mechanism of the allelochemical-mediated inhibition of photosynthesis. However, regardless of this; some allelochemicals in aquatic ecosystems may have direct effects on electron transport as they easily reach and enter the photosynthesis part of the algae.
Metabolism of Carbohydrate and effects of allelochemicals on the process
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Since CO2 adjustment is regulated, carbohydrate metabolism is also thought to be associated with CO2 adjustment. A significant increase in CO2 adjustment rate is observed in plants in response to a switch from ambient to non-photorespiratory conditions. In some cases, CO2 adjustment rate doesn't increase with this switch due to limitation of thylakoid ATPase synthase activity arising from insufficient return of inorganic phosphate. (Pi) to chloroplasts cause by the adjustment of triose phosphatases. Many allelochemicals also have a great impact on phosphate uptake resulting in overall metabolism of carbohydrate to slow. Moreover, growth treated with allelochemicals increased carbohydrate metabolism and carbohydrate content.
Effects on photosynthetic productivity
It is clear that allelochemicals treatments significantly decreased plant biomass together with reduced leaf area and stunt plant growth. Allelochemicals also have key effects on cell division and enlargement causing in poor plant and leaf growth. Therefore, decreases in capacity to capture photosynthetically active radiation are also a main factor in determining the reduction in the photosynthetic productivity of the allelochemical exposed plants.
General overview of the examples of allelochemicals causing these effects on photosynthesis
Allelochemicals and many other natural products and herbicides inhibit photosynthesis. An example of this is Juglone which is produced by black walnut tree. This allelochemical inhibits electron transport and also inhibits photosystem II by binding to the plastoquinone site, causing competitive inhibition. Photosynthesis has similar structure in terms of its process, hence causing effects on photosynthesis. This also inhibits Co-enzyme Q (CoQ) reduction in mitochondria. Another example of this is Atrazine which acts similar to Juglone by binding to plastoquinone site. Structures of these are shown in figure 1 under appendix section.
The major components of photosynthesis that are typically affected by allelochemicals in plants are discussed above. Generally allelochemicals in the rhizosphere usually change the plant-water relation by distributing the membranes of root cell and the water stress-induced changes is one of the candidates for the reduced CO2 activity. Many of these allelochemicals such as phenolic groups of benzoic and cinnamic acid causes the general membrane damage to the plant leaves and roots and decrease photosynthesis activity among plants. This is main reason for decreased crop plants, when using high amounts of herbicides. Production of these allelochemicals is regulated process by environmental conditions such as UV radiation in sunlight, day length, stress and elicitors. Overall, there are many different effects of allelochemicals on photosynthesis and overall photochemistry of plants. This is mainly due to the up-regulation of allelochemical production by environmental condition by plants and roots.