Plant Growth Regulators

NAA (Naphthalene Acetic Acid) is a synthetic plant hormone in the auxinfamily. It is used as an ingredient in many commercial plants rooting horticultural products. 

• NAA and 2,4-D (2.4-dichloro phenoxy acetic acid) are both synthetic auxins. 
• IAA and IBA (Indole Butyric Acid) are isolated from plants. 
• NAA does not occur naturally and is toxic to plants at high concentrations. 

Therefore, B) NAA is the correct answer.

Let’s analyze each statement:
A. All GAs are acidic: Gibberellins (GAs) are a class of acidic plant hormones. This statement is true.
B. Auxins are antagonists to GAs: Auxins and gibberellins often work synergistically to promote growth, so they are not antagonists. This statement is false.
C. Zeatin was isolated from coconut milk: Zeatin is a type of cytokinin that was first isolated from immature corn kernels (Zea mays), not coconut milk. However, coconut milk does contain other cytokinins. This statement is false.
D. Ethylene induces flowering in Mango: Ethylene is a gaseous plant hormone that promotes flowering in some plants, including mango. This statement is true.
E. Abscisic acid induces parthenocarpy: Abscisic acid (ABA) generally inhibits growth and is not associated with parthenocarpy (fruit development without fertilization). Auxins and gibberellins are involved in parthenocarpy. This statement is false.

F. Skoog’s experiments on tobacco callus cultures showed that callus proliferation required auxin along with extracts of vascular tissues, yeast extract, or coconut milk. These extracts contain cytokinins, which, together with auxin, are essential for cell division and growth. Coconut milk is a particularly rich source of cytokinins (specifically kinetin). Abscisic acid (ABA) is a plant hormone involved in growth inhibition, dormancy, and stress responses, and it does not promote cell division and proliferation in callus cultures.