Brown heart disease, also known as "boron deficiency disorder," is a condition that primarily affects plants, particularly those with a high demand for boron. It is characterized by the browning and necrosis of the heart leaves of plants, which can lead to stunted growth and reduced crop yields.

Boron is an essential micronutrient for plants, playing a crucial role in various physiological and biochemical processes. It is involved in cell wall formation, carbohydrate metabolism, hormone regulation, and the uptake and utilization of other nutrients.

Here is a detailed explanation of why boron deficiency leads to brown heart disease:

1. Role of boron in cell wall formation:
- Boron is necessary for the synthesis and stability of pectic substances, which are major components of the cell wall.
- Without sufficient boron, plants cannot form a proper cell wall structure, leading to weakened cell walls and tissue damage.
- The heart leaves, being young and rapidly growing, are particularly vulnerable to boron deficiency.

2. Impact on carbohydrate metabolism:
- Boron plays a significant role in carbohydrate metabolism, particularly in the transportation of sugars within the plant.
- It helps regulate the movement of sugars from source tissues (where they are produced) to sink tissues (where they are utilized).
- A deficiency of boron disrupts this process, resulting in the accumulation of sugars in the heart leaves and subsequent tissue damage.

3. Hormone regulation:
- Boron is involved in the regulation of plant hormones, such as auxins and cytokinins.
- These hormones play crucial roles in plant growth and development, including cell division, elongation, and differentiation.
- Boron deficiency disrupts hormone balance, leading to abnormal growth patterns and the development of brown, necrotic heart leaves.

4. Interaction with other nutrients:
- Boron deficiency can also affect the uptake and utilization of other essential nutrients, such as calcium and magnesium.
- It impairs the mobility of these nutrients within the plant, further exacerbating tissue damage and browning of the heart leaves.

In conclusion, brown heart disease is primarily caused by a deficiency of boron in plants. Boron is essential for cell wall formation, carbohydrate metabolism, hormone regulation, and nutrient uptake. Its deficiency disrupts these processes, resulting in tissue damage and the characteristic browning of the heart leaves.