Question

Appearance of brown spots surrounded by chlorotic veins is a permanent toxicity symptom of
A. Mn
B. Mo
C. Mg
D. Zn

Answer 1

Hint: Chlorosis is a condition in which the green pigmentation is lost. Chlorosis and brown spots occur due to oxidative stress and upsetting of photosynthetic apparatus.

Complete answer:
The correct answer is A.
Manganese, molybdenum and zinc, all three are the types of essential micronutrients while magnesium is one of the essential macronutrients needed for proper completion of a plant’s life cycle and its physiological activity. Despite being an essential nutrient, excess manganese produces toxic effects on plants as opposed to its role in growth and metabolism of plants.
Manganese participates in the water splitting step of photosystem II to generate electrons in the electron transport chain and therefore is required for oxygen evolution. It acts as an activator as well as cofactor of various enzymes in various biochemical reactions.
It also participates in ATP generation and other metabolic processes. It is therefore quite essential for plants, but, in low levels. It is known that a deficit of manganese is harmful for the organism, however, an excess of manganese has evidently produced permanent toxic effects on plant cells.
The extent of manganese phytotoxicity is directly proportional to the surplus amount of manganese accumulated. Excess of manganese injures the photosynthetic apparatus which causes both interveinal and marginal chlorosis (yellowing or lightening of green leaves).
It interferes with the absorption and utilisation of necessary mineral elements and functioning of enzymes and produces oxidative stress leading to stunted growth and occurrence of necrotic brown spots.

Note: Deficiency of manganese affects the photosynthetic processes of plants since it is an essential plant nutrient just like magnesium, molybdenum and zinc but even slight excess of manganese has severe repercussions.