What is the cause of excurrent habit in Pinus

Options

A. Presence of gibberellin
B. Presence of apical dominance
C. High concentration of cytokinin
D. High concentration of ABA

Explanation

### Step-by-Step Solution:

1. **Understanding Excurrent Habit**:
- The excurrent habit refers to a growth pattern where the tree has a tall, conical, or pyramidal shape. In this structure, the main stem is dominant over the lateral branches, leading to a pointed top.

2. **Identifying Characteristics of Pinus**:
- In Pinus (pine trees), the older branches are larger compared to the younger branches. This characteristic contributes to the overall pyramidal shape of the tree.

3. **Role of Apical Dominance**:
- Apical dominance is a phenomenon where the growth of the main shoot is prioritized over the growth of lateral buds. This is due to the presence of auxins (plant hormones) produced at the shoot tip.

4. **Influence of Auxins**:
- Auxins promote the elongation of the main shoot while inhibiting the growth of lateral buds. This results in the vertical growth of the tree, contributing to its excurrent habit.

5. **Conclusion**:
- The cause of the excurrent habit in Pinus is primarily due to apical dominance, which is facilitated by the presence of auxins at the shoot tip. This leads to a taller and more pointed structure as the lateral branches do not grow as much as the main stem.