Question

\(2,4-\mathrm{D}\) does not show herbicidal action on (a) Chicory led ones (b) Monocotyledonous (c) Mature dicotyledonous (d) Mature monocotyledonous

Short answer

Herbicidal action of 2,4-D is not shown on: Monocotyledonous (Option b), Mature dicotyledonous (Option c), Mature monocotyledonous (Option d), and Chicory led ones (Option a).

Step-by-step solution

Identify the characteristics of 2,4-D

2,4-D is a selective herbicide, which essentially affects broadleaf plants or dicotyledonous plants and does not harm monocotyledonous plants or grass-like plants. Furthermore, it doesn't affect the mature plants. Chicory is also a dicotyledonous plant, but remember 2,4-D does not affect mature plants

Match the given options with the characteristics of 2,4-D

As known, 2,4-D does not affect monocotyledonous plants. Thus, Option b and d already meet the criteria. It also doesn’t have an effect on mature plants, thus, Option c, which is a mature dicotyledonous, is also suitable.

Identify the exceptions

Although Chicory is a dicotyledonous plant, it's necessary to highlight that the type of 2,4-D doesn't show any herbicidal action on it. Although it's not given if Chicory let ones are mature or not, it is known that it's an exception to the rule.

Key concepts

Selective Herbicides

Herbicides are used to manage weed populations in agricultural and landscaped areas. Among them, selective herbicides are well-regarded for their ability to target specific types of plants without harming others. This specificity is critical when you want to eliminate weeds without damaging the crops you are trying to grow.

Selective herbicides, like 2,4-D, are formulated based on the biological differences between plant types. They exploit these differences to suppress unwanted vegetation while leaving the desired plants unaffected. Herbicide selectivity is a fundamental characteristic that allows farmers and gardeners to control weeds in a crop that would otherwise compete for resources such as light, water, and nutrients. This selectivity must be well-understood to avoid accidental damage to non-target plants.

Dicotyledonous Plants

Plants are classified into two major groups based on the number of cotyledons, or seed leaves, they have: monocots and dicots. Dicotyledonous plants, commonly referred to as dicots, have two cotyledons. These plants typically have broad leaves with net-like veins and their flower parts are usually in multiples of four or five.

Dicots include a vast array of species including most broadleaf weeds, which are often targeted by selective herbicides like 2,4-D. This herbicide is particularly effective against these types of plants because it mimics the action of natural plant hormones, causing uncontrolled growth that eventually kills the weed. However, the age of the plant can affect the efficacy of the herbicide; mature dicots are often less susceptible.

Monocotyledonous Plants

In contrast to dicots, monocotyledonous plants, or monocots, are identified by their single cotyledon. Monocots include grasses, grains, and lawn turfs which are usually not affected by selective herbicides designed for broadleaf weeds. Characteristics of monocots include parallel veins in the leaves, and flower parts that are typically in multiples of three.

The cellular structure and growth patterns of monocots differ substantially from those of dicots, making them naturally resistant to certain types of herbicides, such as 2,4-D. Therefore, these herbicides can be applied to fields or lawns with monocot plants without damaging them, allowing for effective weed control in areas with mixed vegetation.

Herbicide Selectivity

The concept of herbicide selectivity is pivotal in agricultural sciences and directly relates to the ability of a herbicide to kill certain plants while sparing others. It's the basis for the application of herbicides in crop production and ecosystem management. Factors contributing to selectivity include the herbicide's mode of action, application rate, timing, and formulation.

2,4-D is an excellent example of a selective herbicide, demonstrating selectivity based on the plant's stage of growth and structural differences between monocotyledonous and dicotyledonous plants. Its effectiveness is maximized by applying it when weeds are actively growing and at a point in their lifecycle when they're most vulnerable. Mature plants, having developed resistance mechanisms or physical barriers, tend to be less affected.