Question

DCMU a herbicide kills plants by
(a) Checking respiration due to inhibitory effect on dehydrogenase
(b) Inhibits photosynthesis, as it inhibits PS-I
(c) Inhibiting photosynthesis as it inhibits PS-II
(d) Inhibits photosynthesis of water since it inhibits PS-I

Answer 1

Hint: The full form of DCMU is Dichlorophenyl dimethyl urea. It shows specific binding toward Plastoquinone (PQ), which is a molecule involved in the light- dependent reaction.

Complete answer:
DCMU kills the weed by inhibition of carbon dioxide fixation, as it is a strong inhibitor of photosystem- II. DCMU acts by blocking electron flow at the guanine acceptors of PS- II (water- plastoquinone oxidoreductase) pigment or pigment system, by attaching itself with the binding site called plastoquinone.
The electron transport chain is an important part of photosynthesis- II, through which the electrons are transferred to generate ATP (energy) . When the electron flow is blocked as a result of the herbicide, no production of ATP occurs and the photosynthetic process is hindered.
- The reaction center for PS- II is P680.
- Pigments of PS- II are found in the opposed portion of grana lamellae.
- PS- II receives electrons due to the photolysis of water and is involved only in the non- cyclic photophosphorylation.
- The full form of ATP is Adenosine Triphosphate.
- The ratio of chlorophyll to carotenoid content is 3-7: 1.
So, the correct answer is, ‘Inhibiting photosynthesis as it inhibits PS- II.’

Note: PS- II performs non- cyclic photophosphorylation in conjunction with PS- I, which means when the same electrons released by two pigment systems PS- I and PS- II are not received back and are used at different places simultaneously producing the ATP with the help of light.