Question

What is the cause of excurrent habit in pinus?
A. Presence of gibberellin
B.Presence of apical dominance
C. High concentration of cytokinin
D. High concentration of ABA

Answer 1

Excurrent Trees are trees that have one central leader better than multi-trunked forms. Excurrent - In arboriculture, a tree that has a straight trunk from the ground level to the very tip. Many trees and arborescent shrubs (tree-like shrubs) are selected for this very excurrent growth structure.

Complete answer: Auxin exerts profound effects on plants. For example, some plants tend to branch out very little when they grow. Growth in these plants occurs mostly from the apical meristem rather than from axillary buds which do not develop as long as the terminal bud is present. Such plants are said to exhibit apical dominance due to the inhibition of axillary bud growth by the apical meristem.
In plants with strong apical dominance, auxin produced in the apical meristem inhibits axillary buds near the apical meristem from developing into actively growing shoots. Thus, this gives rise to an excurrent habit as seen in many evergreen trees like Pinus. The Pinus tree is the pyramid in shape as if we see in simple words the elder branches are larger than younger branches, so it appears like a cone and this pattern is known as an excurrent habit. This is due to apical dominance in the plant, auxin produced in the apical meristem inhibits axillary buds near the apical meristem from developing into actively growing shoots. These trees are pollinated by wind.
So, option C presence of apical dominance is correct

Note: Pinus trees are some of the best-known plants around the world. They possess huge economic importance because they are used for timber trade and are easy to identify because of their characteristic cone-shaped growth and their needle-like leaves.