Q.A gardener finds some broad-leaved dicot weeds growing in his lawns. What can be done to get rid of the weeds efficiently?

The Maestro of Plant Growth: Understanding Phytohormones

Plants, much like animals, have their own intricate chemical messengers called phytohormones (or plant hormones). These tiny molecules, produced in minute quantities, orchestrate nearly every aspect of a plant's life cycle – from germination and growth to flowering, fruiting, and senescence. What makes them fascinating is their versatility: a single hormone can have vastly different effects depending on its concentration, the specific plant species, the developmental stage, and its interaction with other hormones. Our task is to find one such versatile hormone that performs two specific roles: promoting flowering in pineapple and inducing parthenocarpy in tomato.

Let's break down the question and explore the candidates.

1. Deconstructing the Question's Demands

   We are looking for a single plant hormone that fulfills two distinct criteria:

   • Promotes flowering in pineapple: Pineapple (a bromeliad) has unique flowering requirements.
   • Induces parthenocarpy in tomato: Parthenocarpy is the development of fruit without prior fertilization, resulting in seedless fruits.

   This requires us to identify a hormone with a dual role, which immediately narrows down our options.

2. Evaluating Auxin: The Growth Regulator Extraordinaire

   Auxins are a class of plant hormones primarily known for their role in cell elongation, apical dominance, and root formation. However, their influence extends much further, making them a strong candidate for our question.

   • Promoting Flowering in Pineapple:

     While ethylene is famously known for inducing flowering in pineapple (and is often used commercially for this purpose), auxins also play a crucial role. Synthetic auxins, such as NAA (naphthaleneacetic acid), are widely used in pineapple cultivation to synchronize flowering. At specific concentrations, auxins can trigger the flowering process in pineapples. This effect is often mediated by the auxin's ability to stimulate the plant's own production of ethylene, which then acts as the direct flowering signal. So, auxin acts as an upstream signal or a direct inducer depending on the context and concentration.

     Watch out

     It's a common misconception that only ethylene promotes flowering in pineapple. While ethylene is a potent and direct inducer, auxins are also effectively used, often by stimulating endogenous ethylene production, to induce or synchronize flowering in pineapple.

   • Inducing Parthenocarpy in Tomato:

     This is one of auxin's most well-established roles.

     Parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules, resulting in seedless fruits.

     In many plants, including tomatoes, the developing seeds produce auxins, which are essential for fruit growth. If fertilization doesn't occur, there are no developing seeds to produce these auxins, and the fruit typically aborts. However, by applying exogenous auxins (either naturally occurring or synthetic), the ovary can be stimulated to develop into a fruit even without fertilization, leading to seedless tomatoes. This is a common horticultural practice. …

The key distinction here is between movement and growth. In biology, when we talk about an organism's response to light, the specific term depends on whether the organism can move from place to place (motile) or is fixed in place (non-motile).

For motile organisms like bacteria, algae, or animals, light-directed movement is called phototaxis — think of a moth flying toward a flame or a single-celled alga swimming toward brighter water. For non-motile organisms, especially plants, the response to light involves differential growth rather than locomotion. A plant cannot pack up and move to a sunnier spot; instead, it grows toward the light by elongating cells on the shaded side more than those on the lit side. That growth-based orientation is phototropism.

Let's walk through each option to see why only one fits.

1. Phototaxis — This is the directional movement of a motile organism (or a freely moving cell) toward or away from light. The organism physically changes location. Since the question specifies a non-motile organism or part, this cannot apply. A rooted plant does not exhibit taxis.

2. Photokinesis — This refers to a change in the speed of movement (not direction) in response to light intensity. It is a non-directional response — the organism moves faster or slower but not toward or away from the light source. Again, this requires motility, so it is irrelevant for a non-motile organism. …