Question

Assertion: The prominent symptom of manganese toxicity is the appearance of brown spots surrounded by chlorotic veins.
Reason: Excess of manganese may induce deficiencies of iron, magnesium and calcium.

A.Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B.Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C.Assertion is correct but Reason is incorrect
D.Both Assertion and Reason are incorrect

Answer 1

Chlorosis is defined as a condition in which the green pigmentation is lost. Chlorosis and brown spots are usually the result of oxidative stress and upsetting of photosynthetic apparatus. This condition occurs due to deficiency of mineral elements.

Complete answer:
Manganese, molybdenum and zinc, all three are the kinds of essential micronutrients while magnesium is one of the essential macronutrients that is required for proper completion of a plant's life cycle and its physiological activity.
Despite being an essential nutrient, excess manganese can produce toxic effects on plants as opposed to its role in growth and metabolism of plants.

Chlorosis and brown spots are usually the result of oxidative stress and upsetting of photosynthetic apparatus. This condition occurs due to deficiency of mineral elements.
Manganese performs a role in the water splitting step of photosystem II to generate electrons in the electron transport chain and therefore it is needed for oxygen evolution. It performs as an activator as well as cofactor of various enzymes in various biochemical reactions.
It also performs re in ATP generation and other metabolic processes. It is therefore quite necessary for plants, but, in low levels. It is observed that a deficit of manganese is harmful for the organism, but when manganese is present in excess it has evidently produced permanent toxic effects on plant cells.

The amount of manganese phytotoxicity is directly proportional to the surplus amount of manganese accumulated. More 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 essential mineral elements and functioning of enzymes and controls oxidative stress leading to stunted growth and occurrence of necrotic brown spots.

Hence, the correct answer is option (A)

Note: The deficiency of manganese mineral element affects the photosynthetic processes of plants because it is a necessary plant nutrient just like magnesium, molybdenum and zinc but even slight excess of manganese has severe repercussions.