Deficiency of this macronutrient causes older leaves to turn dark green or reddish purple
1. calcium
2. phosphorous
3. nitrogen
4. magnesium

Detailed Explanation:
Correct Answer:
2. Phosphorus

Phosphorus Deficiency in Plants

Phosphorus is a crucial macronutrient for plants, playing a key role in energy transfer, photosynthesis, and the synthesis of nucleic acids and membranes. When plants experience phosphorus deficiency, one of the most noticeable symptoms is the dark green or reddish-purple coloration of the older leaves. This coloration is due to the accumulation of anthocyanin pigments, which are produced in response to the stress caused by insufficient phosphorus.

Phosphorus is essential for several key plant functions, including:

• Energy Transfer: It is a part of ATP (adenosine triphosphate), the energy carrier in cells.

• Root Development: Phosphorus promotes strong root systems, aiding in nutrient and water uptake.

• Flowering and Fruit Production: Phosphorus is vital for reproductive processes in plants.

Symptoms of Phosphorus Deficiency:

• Dark Green Leaves: The lack of phosphorus can lead to a buildup of chlorophyll, giving the leaves a darker shade of green.

• Reddish-Purple Color: Older leaves, in particular, will show a reddish or purplish hue due to the increased anthocyanin production.

• Stunted Growth: Phosphorus-deficient plants may also exhibit stunted growth and reduced yields, as the energy required for cell division is insufficient.

Why Other Options Are Incorrect:

• Calcium (Option 1):
  Calcium deficiency generally causes young leaves to develop distorted growth patterns, but it does not cause the reddish-purple coloration of older leaves.

• Nitrogen (Option 3):
  Nitrogen deficiency often results in yellowing of the leaves, particularly in the older leaves, rather than the reddish or dark green color associated with phosphorus deficiency.

• Magnesium (Option 4):
  Magnesium deficiency leads to interveinal chlorosis (yellowing between veins) on older leaves, not the dark green or reddish purple coloration observed in phosphorus deficiency.

Conclusion:

Phosphorus is an essential macronutrient that significantly influences plant growth and development. A deficiency of phosphorus is most often identified by the characteristic dark green or reddish-purple color of older leaves. Ensuring adequate phosphorus levels is key for healthy plant development, particularly for strong root systems and optimal energy production.