Allelochemicals and photosynthesis

Abstract

Allelopathy in natural and agricultural ecosystems is receiving increasing attention because allelochemicals significantly reduce the growth of other plants and the yields of crop plants (Rice 1984; Korner and Nicklish 2002; Leu et al. 2002; Inderjit and Duke 2003). Allelochemicals are found to be released to environment in appreciable quantities via root exudates, leaf leachates, roots and other degrading plant residues, which include a wide range of phenolic acids such as benzoic and cinnamic acids, alkaloids, terpenoids and others (Rice 1984). These compounds are known to modify growth, development of plants, including germination and early seedling growth. Allelochemicals appear to alter a variety of physiological processes and it is difficult to separate the primary from secondary effects. There are increasing evidences that allelochemicals have significant effects on cell division, cell differentiation, ion and water uptake, water status, phytohormone metabolism, respiration, photosynthesis, enzyme function, signal transduction as well as gene expression (Singh and Thapar 2003; Inderjit and Duke 2003; Belz and Hurle 2004, Science 2003). It is quite possible that allelochemicals may produce more than one effect on the cellular processes responsible for reduced plant growth. However, the details of the biochemical mechanism through which a particular compound exerts a toxic effect on the growth of plants are not well known. Photosynthesis is the basic physico-chemical process for plant growth, by which plants, algae and photosynthetic bacteria use light energy to drive the synthesis of organic compounds. The overall equation for photosynthesis is deceptively simple, in fact, a complex set of physical and chemical reactions must occur in a coordinated manner for the synthesis of carbohydrates (Figure1). The photosynthetic process in plants and algae occurs in small organelles known as chloroplasts that are located inside cells. The photosynthetic reactions are traditionally divided into two stages-the "light reactions,"which consist of electron and proton transfer reactions and the "dark reactions," which consist of the biosynthesis of carbohydrates from CO2. The light reactions occur in a complex membrane system (the photosynthetic membrane) that is made up of protein complexes, electron carriers, antenna pigment (chlorophyll) and lipid molecules. In the light reactions, two different reaction centers, known as photosystem II and photosystem I, work concurrently but in series. As a result, electrons were transferred from a water molecule to NADP+, producing the reduced form, NADPH. The NADPH together with ATP formed by the light reactions provides the energy for the dark reactions of photosynthesis, known as the Calvin cycle or the photosynthetic carbon reduction cycle. Calvin cycle occurs in the aqueous phase of the chloroplast and involves a series of enzymatic reactions. It is firstly catalyzed by the protein Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase), which attaches CO2absorbed through stomata to a five-carbon compound and at last produce two molecules of a three-carbon compound. Subsequent biochemical reactions involve several enzymes such as Fapase and at last triose phosphates is produced (Raghavendra 1998). Photosynthesis is greatly influenced by environmental factors such light, temperature, CO2concentration, water condition and microbes. Recent studies showed that allelochemicals also significantly influenced photosynthesis. A reduction in CO2assimilation has been widely observed in many plants after treatment with allelochemicals. It is evident that allelochemicals can potentially impair the performance of the three main processes of photosynthesis, the stomatal control of CO2supply, the thylakoid electron transport (light reaction), and the carbon reduction cycle (dark reaction). The detailed mechanism for the reduced assimilation induced by allelochemicals in most studies, however, remains largely unclear.