In plant nutrition, how is nitrogen taken up by higher plants from their environment? Consider the following statements and choose the correct combination: I. In the form of inorganic nitrates or nitrites II. In the form of organic compounds III. Directly taken up as nitrogen gas from the atmosphere.

Introduction / Context:
Nitrogen is an essential nutrient for plants because it is a key component of amino acids, proteins, nucleic acids, and chlorophyll. However, the way plants obtain nitrogen from their environment is not straightforward. This question checks your understanding of the nitrogen cycle and the specific chemical forms in which higher plants actually absorb nitrogen, by asking you to evaluate three statements about nitrogen uptake and select the correct combination.

Given Data / Assumptions:

• Statement I: Plants take up nitrogen in the form of inorganic nitrates or nitrites.
• Statement II: Plants can receive nitrogen in the form of organic compounds.
• Statement III: Plants directly take up nitrogen gas from the atmosphere.
• We are discussing higher plants, not nitrogen fixing bacteria themselves.

Concept / Approach:
In the nitrogen cycle, atmospheric nitrogen gas is converted into plant usable forms by biological fixation (microorganisms) or industrial and natural processes. Higher plants generally absorb nitrogen mainly as nitrate ions (NO3 minus) and ammonium ions (NH4 plus) from the soil solution. Some plants can also take up simple organic nitrogen compounds in certain conditions. However, higher plants do not directly absorb nitrogen gas (N2) from the air; that role is played by specialised microorganisms such as Rhizobium, Azotobacter, and blue green algae. The approach is to examine each statement and see if it matches these established facts.

Step-by-Step Solution:
Step 1: Evaluate Statement I. Inorganic nitrates (NO3 minus) and nitrites (NO2 minus), often converted to ammonium, are standard forms of nitrogen absorbed by plant roots. This statement is correct. Step 2: Evaluate Statement II. Plants can obtain nitrogen in organic forms when organic matter in the soil is mineralised and when dissolved amino acids or other organic nitrogen compounds are absorbed by roots. This can occur in natural ecosystems and in some cultivated conditions, so Statement II is essentially correct. Step 3: Evaluate Statement III. Most higher plants do not have the biochemical machinery to fix atmospheric nitrogen gas directly. Nitrogen fixation is carried out by symbiotic and free living bacteria, not by plant root cells themselves. Thus higher plants do not directly take N2 gas from the atmosphere for nutrition, making Statement III incorrect. Step 4: Combine the evaluations. Statements I and II are true, while Statement III is false. Step 5: The option that matches this pattern is "Only I and II".

Verification / Alternative check:
Standard biology textbooks state that plants absorb nitrogen chiefly as nitrate and ammonium from the soil solution. They also explain that nitrogen fixing bacteria convert atmospheric nitrogen into ammonia which is then transformed into nitrates and other compounds. The emphasis is that plants depend on microbial activity for converting nitrogen gas, rather than doing it themselves. Where organic nitrogen uptake is discussed, it is treated as an additional pathway, not a direct use of N2. This consistent description across sources confirms that only I and II describe forms that plants can take up, while direct uptake of atmospheric nitrogen by higher plants is not correct.

Why Other Options Are Wrong:

• Only I and III: Includes direct uptake of atmospheric nitrogen, which higher plants cannot perform.
• Only II and III: Again, includes the incorrect idea that plants directly use nitrogen gas from the air.
• All I, II and III: Treats all statements as correct, but Statement III is not valid for higher plants.

Common Pitfalls:
A frequent misconception is that because legumes and some other plants are associated with nitrogen fixation, the plant itself directly absorbs nitrogen gas from the atmosphere. In reality, special bacteria in root nodules fix nitrogen, and the plant receives fixed nitrogen in assimilable forms. Another pitfall is ignoring the role of soil solution and thinking that plants can pull nitrogen gas from the air around leaves in the same way they take in carbon dioxide. Remember that only specialised microorganisms can break the strong triple bond in nitrogen gas, and plants rely on them to obtain nitrogen in usable forms.

Final Answer:
Nitrogen is taken up by higher plants in the form of inorganic nitrates or nitrites and in the form of organic compounds, that is, Only I and II.