Which macronutrient is crucial for chlorophyll production and its deficiency leads to yellowing of leaves?

The macronutrient vital for chlorophyll production is nitrogen, which plays a fundamental role in the synthesis of chlorophyll molecules necessary for photosynthesis. Chlorophyll is crucial for absorbing sunlight and facilitating the plant's conversion of light energy into chemical energy. When a plant experiences nitrogen deficiency, it cannot produce sufficient chlorophyll, causing the leaves to turn yellow, a condition known as chlorosis. This yellowing starts in older leaves and can spread to younger foliage as nitrogen is remobilized within the plant.

In contrast, potassium is primarily involved in regulating various physiological processes, including water uptake and enzyme activation, but it does not play a direct role in chlorophyll synthesis. Phosphorus is essential for energy transfer within the plant, particularly in ATP (adenosine triphosphate) formation, but it does not specifically influence chlorophyll production directly. Calcium, while important for cell wall structure and signaling, does not contribute to chlorophyll synthesis either. Therefore, nitrogen is the correct answer as its deficiency directly leads to yellowing of leaves due to insufficient chlorophyll production.