Understanding Plant Dwarfness and Growth Control

Plants exhibit various growth patterns, and sometimes, they might be dwarf or short in height. This dwarfness can be due to genetic factors or environmental conditions. Plant growth and development are regulated by special chemical substances called plant growth regulators or phytohormones. These hormones influence various processes, including cell division, cell elongation, differentiation, and organ formation.

The question asks which substance can control or overcome dwarfness in plants by treating them. Let's look at the functions of the options provided:

Functions of Plant Growth Regulators

• Cytokinin: Primarily promotes cell division, delays senescence (aging), helps in the growth of lateral buds, and promotes chloroplast development. While it promotes growth, it is not the primary hormone known for dramatically increasing stem elongation and overcoming dwarfness.
• Gibberellic acid: This is a well-known plant growth regulator that promotes stem elongation, seed germination, and flowering in some plants. Gibberellins cause cells to elongate and divide in the stem. Treating dwarf varieties of plants with gibberellins can significantly increase their height, effectively controlling or reversing the dwarf characteristic.
• Ethylene: This is a gaseous hormone primarily associated with fruit ripening, senescence, abscission (shedding of leaves, flowers, and fruits), and also affects root growth and development. It generally inhibits stem elongation in many plants, especially in response to stress. Therefore, it would not be used to control dwarfness.
• Auxin: Auxins are involved in cell elongation (especially in stems and coleoptiles), apical dominance, root formation, and preventing abscission. While auxins promote elongation, gibberellins are generally more effective and specifically known for promoting stem elongation to overcome genetic dwarfism in many plant species.

Based on the functions described, Gibberellic acid is the phytohormone most commonly used to promote stem elongation and overcome dwarfness in plants, particularly genetic dwarfism.

Therefore, treating plants with Gibberellic acid can control dwarfness by stimulating stem elongation.

Revision Table: Plant Hormones and Effects

Additional Information on Controlling Plant Dwarfness

Controlling dwarfness with Gibberellic acid (\(\text{GA}_3\)) is a classic example of how plant hormones are used in agriculture and horticulture. It is particularly effective for dwarfism caused by mutations affecting gibberellin synthesis or response. For example, many dwarf varieties of maize, peas, and rice grow to normal height when treated with Gibberellic acid.

However, not all types of dwarfism respond to Gibberellic acid treatment. Dwarfness might be caused by environmental factors like lack of nutrients, water stress, or unfavorable temperature, or by genetic mutations affecting other pathways not related to gibberellins. In such cases, addressing the underlying cause (like providing nutrients or water) or using different genetic varieties would be necessary.

The application of Gibberellic acid is often done by spraying a solution onto the leaves and stems of the dwarf plants. The concentration used depends on the plant species and the desired effect.

In summary, Gibberellic acid is widely recognized and used for its ability to promote significant stem elongation and overcome dwarfism in responsive plants.