Correct Option
The correct option isPlants cannot directly assimilate atmospheric nitrogen.
Explanation
Nitrogen is a critical macronutrient required for the synthesis of amino acids, proteins, nucleic acids (DNA and RNA), and chlorophyll. Although the Earth's atmosphere is composed of approximately 78% nitrogen gas ($N_2$), this molecular form is chemically inert due to the strong triple bond holding the two nitrogen atoms together.
Option Analysis
- Nitrogen is the only essential macronutrient for plants. is Incorrect: Nitrogen is not the only essential macronutrient. Plants also require Phosphorus (P) and Potassium (K) as primary macronutrients, alongside secondary macronutrients such as Calcium, Magnesium, and Sulfur.
- Plants cannot directly assimilate atmospheric nitrogen. is Correct: Plants lack the enzyme nitrogenase required to break the stable triple bond of atmospheric nitrogen ($N_2$). Therefore, they cannot assimilate nitrogen directly from the air. They can only absorb nitrogen in "fixed" mineral forms, such as nitrate ($NO_3^-$) or ammonium ($NH_4^+$), dissolved in soil water. This necessitates the addition of nitrogenous fertilizers or the use of nitrogen-fixing bacteria in agriculture.
- Nitrogen facilitates the hydraulic conductivity of roots. is Incorrect: While nutrient status affects overall plant health, nitrogen does not primarily function to facilitate the hydraulic conductivity of roots. Its main role is in metabolism and structural growth.
- Nitrogen acts as a primary pesticide against soil pathogens. is Incorrect: Nitrogen acts as a nutrient, not a pesticide. It promotes vegetative growth. Conversely, excessive nitrogen application can sometimes make plants more succulent and susceptible to pest attacks and diseases.
Key Takeaway: The necessity for nitrogen supplementation arises because plants are physiologically incapable of utilizing atmospheric nitrogen ($N_2$) and must absorb it in fixed forms like nitrate or ammonium from the soil.