PS inhibition by herbicides

Complied By

MEDIDA SUNIL KUMAR

Acharya BN. G. Ranga Agricultural University

Department of Agronomy

Agricultural College, Bapatla

Introduction to Photosynthesis

Physico-chemical process by which

photosynthetic organisms use light energy to drive

the synthesis of organic compounds

Photosynthesis inhibitors These shut down the photosynthetic process in susceptible plants

by binding to specific sites within the plant's chloroplasts

Inhibition of photosynthesis could result in a slow starvation of the

plant

Plant experiences a more rapid death that is believed to be due to

the production of secondary toxic substances

Families within the mode of action Triazines (amitrole , atrazine, cyanazine, simazine & trietazine)

Uracils (Isocil , Bromacil, Terbacil etc., )

Phenyl Urea's (Diuron, Isoproturon, Fluometuron etc., )

Benzothiadiazoles (Bentazon etc.,)

Nitriles (Bromoxynil, Ioxynil etc.,)

Pyridazines (Pyridafol, Credazine etc.,)

Mobile

Non – mobilePost emergence herbicides

Curtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

PS Inhibition Mechanism

Inhibition of Carotenoid biosynthesis

Inhibition of Protoporphyrinogen channeling

Inhibition of Photosystem II electron transfer

Uncoupling of Photosystem I electron transfer

Curtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

Inhibitors of Carotenoid Biosynthesis

Carotenoids

Widely distributed in nature and are synthesized by all photosynthetic

and non- photosynthetic organisms.

Wide group of lipophilic pigments that provide a series of colors,

including yellow, orange and red.

Mostly found intracellularly at the chloroplast and chromoplast

membranes in plants

Participating in the light-harvesting process and as photoprotectors of

the photosynthetic apparatus

Precursors of abscisic acidCurtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

Play three essential protective roles

Ability to quench triplet chlorophyll molecules back to the ground

state

Quench singlet oxygen molecules (which are destructive) back to

the normal triplet state (oxygen is unusual in that its triplet state is

more stable than its singlet state).

Quenching the Photosystem reaction centers when overexcited in

very bright light. For this last role, zeaxanthin, a specific

Carotenoid, is produced from violaxanthin that is normally present

in the chloroplast. Ex: Norflurazone  inhibit the desaturase enzymes and block the

biosynthesis of carotenoidsCurtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

Carotenoids play an important role in dissipating the oxidative energy of singlet O2 (1O2).

In normal photosynthetic electron transport, a low level of photosystem-II reaction center chlorophylls in the first excited singlet state transform into the excited triplet state (3Chl). This energized 3Chl can interact with ground state molecular oxygen (O2)to form 1O2.

In healthy plants, the energy of 1O2 is safely quenched by carotenoids and other protective molecules.

Carotenoids are largely absent in herbicide-treated plants, allowing 1O2 and 3Chl to abstract a hydrogen from an unsaturated lipid (e.g. membrane fatty acid, chlorophyll) producing a lipid radical.

The lipid radical interacts with O2 yielding a peroxidized lipid and another lipid radical.

Thus, a self-sustaining chain reaction of lipid peroxidation is initiated which functionally destroys chlorophyll and membrane lipids. Proteins also are destroyed by 1O2.

Destruction of integral membrane components leads to leaky membranes and rapid tissue desiccation

Carotenoid biosynthesis pathway

Symptoms

Initial whitening or bleaching of new flush

Later on whitening or bleaching of older

Finally chlorosis & necrosis as a result of photo-oxidation 

Protoporphyrinogen oxidase (Protox), an enzyme of chlorophyll

and heme biosynthesis catalyzing the oxidation of

protoporphyrinogen IX (PPGIX) to protoporphyrin IX (PPIX).

Protox inhibition leads to accumulation of PPIX, the first light-

absorbing chlorophyll precursor.

PPGIX accumulation apparently is transitory as it overflows its

normal environment in the thylakoid membrane and oxidizes to

PPIX. PPIX formed outside its native environment probably is

separated from Mg chelatase and other pathway enzymes that

normally prevent accumulation of PPIX. http://wssa.net/wp-content/uploads/WSSA-Mechanism-of-Action.pdf

Inhibition of protoporphyrinogen channeling

Light absorption by PPIX apparently produces triplet state PPIX

which interacts with ground state oxygen to form singlet oxygen.

Both triplet PPIX and singlet oxygen can abstract hydrogen from

unsaturated lipids, producing a lipid radical and initiating a chain

reaction of lipid peroxidation.

Lipids and proteins are attacked and oxidized, resulting in loss of

chlorophyll and carotenoids and in leaky membranes which allows

cells and cell organelles to dry and disintegrate rapidly

Ex: Acifluorfen, sulfentrazone etc.,

http://wssa.net/wp-content/uploads/WSSA-Mechanism-of-Action.pdf

Inhibition of protoporphyrinogen channeling

Diphenylether (PPO) Herbicide injury on Soybeans Acifluorfen injury on soybean

Electrons transport in Photosystems

Diversion of Electrons in Photosystem I

Herbicides interacts with ferredoxin, competing with NADP+ as an

electron acceptor.

When the herbicide is reduced by an electron, it rapidly transfers

the electron to oxygen, forming highly reactive superoxide.

Ex: paraquat, diquat etc,.

Curtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

• Bipyridyliums are examples of herbicides that accept electrons from photosystem I and are reduced to form an herbicide radical.

• This radical then reduces molecular oxygen to form superoxide radicals. Superoxide radicals then react with themselves in the presence of superoxide dismutase to form hydrogen peroxides.

• Hydrogen peroxides and superoxides react to generate hydroxyl radicals. Superoxides and, to a lesser extent, hydrogen peroxides may oxidize SH (sulfhydryl) groups on various organic compounds within the cell.

• Hydroxyl radical, however, is extremely reactive and readily destroys unsaturated lipids, including membrane fatty acids and chlorophyll.

• Hydroxyl radicals produce lipid radicals which react with oxygen to form lipid hydroperoxides plus another lipid radical to initiate a self-perpetuating chain reaction of lipid oxidation.

• Such lipid hydroperoxides destroy the integrity of cell membranes allowing cytoplasm to leak into intercellular spaces which leads to rapid.

• leaf wilting and desiccation.• These compounds can be reduced/oxidized repeatedly

http://wssa.net/wp-content/uploads/WSSA-Mechanism-of-Action.pdf

Inhibitors of Photosystem II• Electron transfer chain is the reduction of Plastoquinones (PQ) by the D1 protein in the

thylakoid membrane.

• These group herbicides act as inhibitors of binding (D1 protein) and block the binding of

PQ and stops CO2 fixation and production of ATP and NADPH2.

• Inability to re-oxidize QA promotes the formation of triplet state chlorophyll which

interacts with ground state oxygen to form singlet oxygen.

• Both triplet chlorophyll and singlet oxygen can abstract hydrogen from unsaturated lipids,

producing a lipid radical and initiating a chain reaction of lipid peroxidation.

• Lipids and proteins are attacked and oxidized, resulting in loss of chlorophyll and

carotenoids and in leaky membranes which allow cells and cell organelles to dry and

disintegrate rapidly

• Phenylcarbamates, pyridazinones, triazines, triazinones, uracils, amides, ureas,

benzothiadiazinones, nitriles, & phenylpyridazines.

• Some compounds in this group may also inhibit carotenoid biosynthesis (fluometuron) or

synthesis of anthocyanin, RNA, and proteins (propanil), as well as effects on the

plasmalemma (propanil) Curtsy: Plant and Soil Sciences eLibrary, University of Nebraska 

Plastoquinones

Diuron inhibits electron transport in PS-I

Summary• Herbicides that inhibit the normal production of protoporphyrin IX, a photosensitizing

molecule, cause severe photodynamic damage.

• Herbicides that inhibit biosynthesis of carotenoids deprive plant cells of the

photoprotection given by these molecules, permitting damage from chlorophyll mediated

photosensitization.

• Inhibitors of electron transfer from Photosystem II block  photophosphorylation and starve

the cell of the energy normally produced by photosynthesis.

• And finally, some herbicides act by diverting high-energy electrons from Photosystem I to

generate damaging superoxide and other free radicals.

• Although each of these four classes of herbicides has a distinct mode of action, each

interferes with the plant's ability to safely handle the high energy present in sunlight.

Curtsy: Plant and Soil Sciences eLibrary, University of Nebraska