Major Types of Plant Stimuli

Phototropism

Phototropism is the growth response of plants to light. This phenomenon is not limited to plants; it can also occur in fungi. In plants, phototropism involves the movement of cells in response to light, facilitated by a hormone called auxin. Auxin accumulates on the side of the plant that is farther from the light, causing these cells to elongate more than those on the light-exposed side. This differential growth results in the plant bending toward the light source, known as positive phototropism. Conversely, growth away from the light is termed negative phototropism. While most plant shoots demonstrate positive phototropism, roots generally show negative phototropism, though geotropism can also influence root growth. Some vine shoots, for instance, exhibit negative phototropism, allowing them to grow towards dark, solid structures for climbing. The shoot is the part of the plant responsible for light direction responses. A common misconception that cutting off the shoot prevents a plant from growing toward light has been disproven.

In plants like Arabidopsis thaliana, blue light triggers phototropism through blue light receptors called phototropins. Other plant light receptors include phytochromes, which detect red light, and cryptochromes, which sense blue light. Different plant organs may respond differently to various light wavelengths: stem tips generally show positive responses to blue light, while root tips respond negatively. Both stem and root tips exhibit positive responses to red light.

Auxins play a crucial role in phototropism. These hormones cause the expulsion of protons through proton pumps, which lowers the pH on the shaded side of the plant. This acidification activates expansins, enzymes that weaken the cell walls by breaking bonds, and disrupts hydrogen bonds in cellulose. As a result, the cell walls become less rigid, leading to cell swelling and the resulting mechanical pressure that drives the plant’s phototropic response.

Short-Day Plants

Short-day plants flower when the length of the night exceeds a critical duration. They do not flower under long days or if exposed to brief artificial light during the night; they require an uninterrupted dark period to initiate flowering. Natural nighttime light, such as moonlight or lightning, is insufficient to interrupt this process.

Typically, short-day plants flower as days shorten and nights lengthen, particularly after June 21 in the Northern Hemisphere (summer to fall). The necessary dark period for flowering varies among species and varieties. Flowering in these plants is influenced by photoperiod, which triggers the transition from producing leaves and lateral buds to floral buds. Some species need to undergo a juvenile stage before flowering can be induced. Examples of short-day plants include:

Chrysanthemum

Coffee (Coffea)

Euphorbia

Strawberry (Fragaria)

Tobacco

Lemma

Day-Neutral Plants

Day-neutral plants, such as cucumbers, roses, and tomatoes, do not rely on photoperiod to initiate flowering. Instead, they flower based on other factors like reaching a certain developmental stage or responding to environmental cues, such as vernalization.

Long-Day Plants

Long-day plants require fewer hours of darkness to induce flowering and typically bloom in late spring or early summer as days lengthen. In the Northern Hemisphere, the longest day occurs around June 21, with days becoming shorter after this date until December 21. This pattern is reversed in the Southern Hemisphere. In some regions, the concept of winter and summer may refer to rainy and dry seasons rather than temperature extremes.

Examples of obligate long-day plants include:

Dianthus

Henbane (Hyoscyamus)

Oats (Avena)

Ryegrass (Lolium)

Clover (Trifolium)

Carpathian Bellflower (Campanula carpatica)

Gravitropism

Gravitropism, also known as geotropism, is the directional growth or turning movement of plants and fungi in response to gravity. Charles Darwin was one of the first to scientifically document this phenomenon, noting that roots exhibit positive gravitropism by growing in the direction of gravitational pull, while stems exhibit negative gravitropism by growing away from it. This behavior can be easily observed in a potted plant: when the plant is tilted onto its side, the stem will curve upward, demonstrating negative gravitropism, while the roots will grow downward, showing positive gravitropism. Although herbaceous stems can bend slightly, most of the observed curvature is due to changes in growth direction rather than actual bending.

In roots, gravitropism is regulated by the plant hormone auxin through a process known as polar auxin transport. An increased concentration of auxin on the lower side of the root inhibits cell expansion, causing the root to bend downward in response to gravity. This differential growth in the elongation zone of the root leads to the characteristic root curvature.

The term 'tropism' refers to plant movements triggered by external stimuli. In this context, geotropism describes how roots grow downward into the soil in response to gravity. Plants typically exhibit anisotropic growth, where roots grow downward and shoots grow upward. This growth pattern persists even when the plant is rotated or flipped, as gravity consistently influences root and shoot orientation.

A similar mechanism operates in plant stems, although the response to auxin differs. In shoots, a higher concentration of auxin promotes cell expansion, contrary to the effect seen in roots. This differential sensitivity to auxin explains why stems and roots respond oppositely to gravity. In roots, auxin accumulates on the lower side, inhibiting cell expansion and causing downward curvature. In stems, auxin accumulation on the lower side promotes cell expansion, leading to upward curvature.

Thigmotropism

Thigmotropism refers to the growth or movement of an organism in response to touch or contact stimuli. The term is derived from the Greek word for touch. This phenomenon is commonly observed in plants that grow around surfaces such as walls, pots, or trellises. For example, climbing plants like vines develop tendrils that wrap around supporting structures. When these tendrils come into contact with an object, cells that are touched produce auxin, which is then transported to the untouched cells. This auxin redistribution causes the untouched cells to elongate more rapidly, resulting in the tendrils bending around the support.

Additionally, some seedlings exhibit a 'triple response' when exposed to ethylene, a plant hormone. This response causes the stem to thicken, grow more slowly, and curve, thereby initiating horizontal growth. A notable example of plant movement is seen in Mimosa pudica, known for its rapid response to touch. When its leaves are touched, they close and droop. However, this movement is classified as a nastic movement rather than a tropism. While tropisms are directed responses to external stimuli, nastic movements occur independently of the stimulus direction.

Hydrotropism

Hydrotropism is a plant growth response where the direction of growth is influenced by a water concentration gradient. A common example is plant roots growing in humid air, bending toward areas with higher humidity levels. The process begins when the root cap detects water and sends signals to the elongating part of the root, guiding its growth.

Observing hydrotropism in underground roots is challenging because roots are not easily visible, and gravitropism (growth response to gravity) often plays a more dominant role than hydrotropism. Unlike other environmental stimuli such as gravity, light, or touch, water gradients in soil are not stable, as water moves readily through the soil and moisture levels fluctuate constantly.

As a result, most hydrotropism research has been conducted in controlled laboratory settings, with roots grown in humid air rather than soil. The ecological significance of hydrotropism in soil-grown plants remains unclear due to limited research in this area. However, the discovery of mutant plants lacking hydrotropic responses may offer new insights into its natural role. Hydrotropism could also be important in space environments, where it might help roots orient in the absence of gravity.

Common misunderstandings

The increased root growth in moist soil zones compared to dry ones is typically not due to hydrotropism. Hydrotropism involves a root bending from a dry to a wet area, but roots simply require water to grow. Roots in moist soil will naturally grow and branch more than those in dry soil because they have the necessary water for growth.

Contrary to popular belief, roots cannot detect water inside intact pipes through hydrotropism and do not break pipes to access the water. Additionally, roots are unable to sense water several feet away and grow towards it. Hydrotropism, if present, likely operates only over very short distances, possibly just a few millimeters.

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