Hill Reaction

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THE HILL REACTION: EFFECT OF HERBICIDES (CLEAROUT 41 PLUS AND DIURON 50 WP) IN CORN (ZEA MAYS) LEAVES

JENNY A. ADTOON
ELIEZER B. GALOPE II
SANDY FAYE D. SALMASAN
JYAN DESSE M. SOLANO
MARIANNE KRISTELLE E. YARRA

INTRODUCTION

Photosynthesis sustains almost all of the living world directly or indirectly. It is the process of converting light energy to chemical energy of sugars and other organic compounds. The process depends on the interaction between two sets of reactions, namely the light-dependent and light-independent reactions. The light-dependent reactions convert low energy electrons from splitting of water to a high energy state in the form of NADPH. This process is also coupled with ATP synthesis. The photosynthetic electron transport occurs in thylakoid membranes inside chloroplasts. It was first demonstrated by Robert Hill that photosynthesis could be “uncoupled” if the thylakoid membranes and the stroma components of chloroplast are separated via cell fractionation. The Hill reaction is also referred to as photolysis of water with the subsequent evolution of oxygen. Isolated thylakoids do not contain much of their complement of electron acceptors hence a lot of artificial electron acceptors are used to study the Hill reaction in vitro (the final electron acceptor in vivo is NADP). Many of these artificial electron acceptors accept electrons at different sites along the transport pathway. This study used DPIP dye as an artificial electron acceptor in determining the effects of two herbicides on photosynthetic electron transport by isolated thylakoids of Zea mays leaves. DPIP in analysed by color changes it exhibits – it is blue in its oxidized form and turns colorless when reduced. The shikimate pathway is a biosynthetic sequence in plants to produce the aromatic amino acids phenylalanine, tyrosine, and tryptophan. One of the enzymes of the pathway, EPSP (5-enolpyruvylshikimate-3-phosphate) synthase, is often targeted by herbicides inhibiting its function, consequently shutting down the entire pathway. With this, plants can no longer produce amino acids important for their diet. As a result, plants die after some time. Since only plants undergo shikimate pathway, animals are not affected by the action of the active ingredient of the herbicide. Zea mays, or commonly called mais in the Philippines, is an erect, fast-growing, short-lived annual plant. It has a single main culm and has one or occasionally two branches in the leaf axils in the upper part. The leaves are alternate with broad, sword-shaped leaf blades, parallel veins with a prominent mid-rib, and small ligules. Z. mays is extensively cultivated in the Philippines, and is a staple food is some parts of the country. Its uses include edibility of pollen, seed, and stem; and are a good source of carbohydrates, vitamins A, B, C, potassium, and zinc. The embryo is rich in oil and used widely for cooking, and manufacturing of soaps. Other products made from corn are starch, glue, alcohol, and silk. ClearOut® 41 Plus is a water soluble herbicide solution used for non-selective weed control in cropland systems and non-cropland areas. Its active ingredient is glyphosate as isopropylamine salt. Glyophosate verges on a perfect herbicide more than any other herbicide currently on the market, and was named as the best herbicide in the Philippines due to its affordable price. The herbicidal activity of glyophosate focuses on the inactivation EPSP (5-enolpyruvylshikimate-3-phosphate) synthase enzyme of the shikimate pathway. This pathway does not occur in animals, hence it is harmless and safe to nontarget organisms. Symptoms observed after using ClearOut® 41 Plus include gradual wilting and yellowing (chlorosis) of the plant, at most 4 days after application to annual weeds and at most 7 days on perennial weeds. DCMU or Diuron 50 WP (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is an herbicide used to block photosynthetic...
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