In general, longer the plant is in contact with nitrate, there would be

Correct option is A

The correct answer is (A) Constant accumulation of nitrate reserve

Explanation:
• Plants absorb Nitrogen primarily in the form of Nitrate ($NO_3^-$). Once inside the plant, nitrates must be reduced to ammonia for the synthesis of amino acids and proteins.
• If the rate of nitrate uptake from the soil exceeds the rate at which the plant can reduce and utilize it, the excess is stored in the vacuoles of cells.
• Under conditions where a plant has continuous and prolonged access to high levels of soil nitrate, it will continue to accumulate these reserves, primarily in the stems and leaves.
• This can lead to dangerously high nitrate concentrations in forage crops, which can be toxic to livestock (Nitrate Poisoning) if consumed in large quantities.

Information Booster:
• Nitrate accumulation is often higher during periods of drought, low light intensity, or deficiency of other nutrients (like Molybdenum) which are needed for nitrate reduction.
• Forage crops like oats, sorghum, and maize are particularly known for accumulating nitrates under high-N fertilization regimes.

Additional Knowledge:
• Nitrogen fixation (Option B): This refers to the conversion of atmospheric $N_2$ into ammonia by bacteria; high soil nitrate actually *inhibits* biological nitrogen fixation in legumes.
• Denitrification (Option D): This is a soil process (conversion of $NO_3^-$ to $N_2$ gas by bacteria under anaerobic conditions), not a plant internal process related to reserves.