The correct option is IAA and 2,4-D.
Explanation
Auxins are a class of plant hormones (phytohormones) that play a crucial role in the regulation of plant growth, particularly in cell elongation, apical dominance, and root initiation. Based on their origin, auxins are classified into two categories: natural auxins and synthetic auxins.
Detailed Classification
- Natural Auxins: These are produced endogenously (internally) by the plant, primarily in the growing apices of stems and roots.
- Indole-3-Acetic Acid (IAA): This is the principal and most ubiquitous natural auxin found in plants.
- Indole Butyric Acid (IBA): Also isolated from plants and classified as a natural auxin.
- Synthetic Auxins: These are chemical compounds synthesized in laboratories that mimic the physiological activity of natural auxins. They are widely used in agriculture and horticulture.
- 2,4-D (2,4-Dichlorophenoxyacetic acid): A synthetic auxin commonly used as a herbicide (weedicide) to kill broad-leaved weeds.
- NAA (Naphthalene Acetic Acid): Another common synthetic auxin used to prevent fruit drop and induce flowering.
Option Analysis
- IAA and 2,4-D: IAA is a natural auxin, and 2,4-D is a synthetic auxin. This correctly matches the order requested.
- NAA and 2,4-D: NAA and 2,4-D are both synthetic auxins.
- IBA and NAA: While IBA is natural and NAA is synthetic, IAA is the primary representative of natural auxins, and IAA and 2,4-D presents the most standard contrast between the principal natural auxin and a widely used synthetic one.
- IAA and IBA: IAA and IBA are both natural auxins.
Key Takeaway
IAA and IBA are natural auxins isolated from plants. NAA and 2,4-D are synthetic auxins produced chemically. 2,4-D is extensively used as a weedicide.