Question

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

Answer 1

Micronutrients are the nutrients required by the plants in a very minute quantity. The adequate levels for these nutrients are very narrow, a minute decrease in the optimum amount can cause deficiencies, as well as a very small increase in the availability of micronutrients, can lead to the toxicity.

Complete step by step answer: The presence of brown spots surrounded by the development of chlorotic veins is the symptoms caused by the toxic concentration of manganese. The toxicity of manganese causes a decreased uptake of both Mg and Fe, inhibition of Calcium relocation as well as inhibition of binding of manganese with various enzymes. This causes decreased chlorophyll production, and in turn, causes chlorotic veins and brown spots. In adequate amounts, Mn catalyzes the water-splitting reaction which is an important step in the process of photosynthesis.
Considering the other options:
i. Mo: It is an important nutrient required for the nitrogen fixation process in plants. The toxicity of this nutrient can lead to yellow or browning of the plants. Also, it can cause depression in tillering leading to reduced production of side shoots.
ii. Mg: This acts as a carrier for phosphate molecules and is very vital for the metabolism of phosphates. It also helps in capturing the sunlight for photosynthesis. Toxic levels of magnesium inhibit the uptake of calcium as a result of which inhibition of the development of buds, as well as the underdevelopment of roots, occur.
Zn: It is the mineral used in chlorophyll production. It is also important in temperature tolerance of plants. Its toxicity can lead to smaller leaf size, growth retardation, reduced root growth and leaf necrosis.
Thus, the correct answer is option B.

Note: The elements or the nutrients required in relatively larger concentrations are known as macronutrients. This includes carbon, hydrogen, oxygen, sulfur, phosphorus, potassium and calcium. Macronutrients are also known as primary nutrients.