Question

The only gaseous PGR is (a) IBA (b) Kinetin (c) Terpenes (d) Ethylene

Short answer

The only gaseous PGR among the given options is Ethylene.

Step-by-step solution

Understand PGRs

Plant growth regulators (PGRs) are chemicals that shape the growth and development of plants. There are many PGRs and they can be solid or liquid in nature. However, there is only one that is gaseous.

Evaluate each option

Evaluate each option by understanding the nature of each PGR on the list. \n (a) IBA (Indole-3-Butyric Acid) is a plant hormone in the auxin family and is primarily used in horticulture for vegetative propagation of plants from stem and leaf cuttings. It is usually a solid. \n (b) Kinetin is a type of cytokinin, a class of plant growth substances that promote cell division. It is usually found in the form of a synthetic crystalline solid. \n (c) Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, which are often strong-smelling. They can be both solid and liquid but not gaseous. \n (d) Ethylene is a hydrocarbon which is produced in all higher plants and is a gaseous PGR under natural conditions.

Identifying the gaseous PGR

Based on the above evaluations, it is clear that Ethylene is the only gaseous PGR among the given options. So, option (d) is the correct choice.

Key concepts

Ethylene in Plants

Ethylene plays a critical role in the life of plants, even though it might seem unusual to think of a gas as a plant growth regulator. It's unique among plant hormones because it exists as a gas under natural conditions. This substance is key to various aspects of a plant's life cycle, ranging from seed germination to fruit ripening and even the shedding of leaves.

For instance, when fruit begins to ripen, it often releases more ethylene, which in turn accelerates the ripening process in nearby fruit, a phenomenon known as 'the ripening hormone' effect. Ethylene also plays a role in response to environmental stress, such as drought or flooding, and can act as a signal that prompts the plant to adapt to these conditions. To fully understand its impact, one should consider ethylene's role not only in facilitating growth processes but also in enabling plants to cope with their ever-changing environments.

PGRs in Plant Development

Plant growth regulators, or PGRs, are a fundamental component of plant biology, crucially guiding the growth and development of plants. They encompass a wide range of substances, including auxins, cytokinins, gibberellins, abscisic acid, and ethylene, each with distinct roles in plant physiology.

PGRs are involved in nearly every aspect of plant development such as cell division, elongation, aging, and even plant responses to environmental stimuli. For example, auxins are renowned for their ability to stimulate stem elongation and root initiation, whereas cytokinins are pivotal in promoting cell division, and thus, plant growth.

Integral Role in Agriculture

In agriculture, understanding and manipulating these PGRs can lead to enhanced crop yields, better quality fruit, and more efficient agricultural practices. It is the knowledge of how these regulators can be used, or at times inhibited, that holds great potential for tailoring plant growth to meet human needs.

Plant Hormones

Plant hormones, also known as phytohormones, are naturally occurring organic substances that influence physiological processes at low concentrations. They are the chemical messengers that plants use to communicate and coordinate their growth and development. Unlike animal hormones, plant hormones are not produced in specialized glands and often have their effects at the site of production or are transported to other regions of the plant to exert their influence.

• Auxins: Promote stem elongation, root growth, and differentiation.
• Cytokinins: Stimulate cell division and growth, delay aging of leaves.
• Gibberellins: Stimulate stem elongation, seed germination, and fruit development.
• Abscisic acid: Inhibits growth, helps close stomata, and plays a role in stress responses.
• Ethylene: Influences fruit ripening and flower wilting, and is associated with response to stress and senescence.

Each hormone has a specialized role but also works in concert with others, creating a complex web of interactions that determines a plant’s growth and development.