The correct answer is (2) Plastochron. This term specifically denotes the time interval between initiation of successive leaf primordia at the shoot apical meristem.​

Option Analysis

(1) Apical Dominance (Incorrect)

Apical dominance refers to hormonal inhibition (mainly auxin from shoot apex) of lateral bud growth, promoting main axis elongation. It regulates branching, not leaf primordia timing.​

(2) Plastochron (Correct)

Plastochron measures the developmental time lapse—from initiation of one leaf primordium (at shoot apex) to the next. Expressed as plastochron index (PI), it quantifies shoot growth rate; shorter plastochron yields more leaves.

(3) Phyllotaxy (Incorrect)

Phyllotaxy describes the spatial arrangement/pattern of leaves on stem (e.g., alternate, opposite, whorled; spiral angles like 137.5° Fibonacci). It governs position, not temporal interval.​

(4) Parastichy (Incorrect)

Parastichy denotes visible spiral lines (parastichies) connecting leaves in phyllotactic patterns, observed in mature shoots. It’s a phyllotaxy descriptor, unrelated to primordia formation timing.​

Time interval between successive leaf primordia, termed plastochron, governs shoot architecture in plants. At the apical meristem, it marks rhythmic primordia initiation, influencing leaf number and plant form.

Plastochron Mechanics

During one plastochron, the shoot apex transitions: rounded mound → widening → leaf buttress formation → new primordium. Plastochron index (PI) = N + (D/log R), where N=leaf number, D=widest leaf width, R=older leaf width; tracks developmental age.​

Exam Confusions Cleared

• Apical dominance: Auxin-mediated axillary suppression.​

• Phyllotaxy: Leaf positioning (spatial).​

• Parastichy: Spiral patterns in leaf arrangement.​

Research Insights

Rice mutants (pla1/pla2) shorten plastochron via altered meristem division, producing more nodes—key for crop architecture breeding.