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