14.4 Plant Responses to Environmental Stimuli

Plant Growth Responses

Tropisms: Plant Growth Responses to Directional Stimuli

Tropisms are growth responses where a plant grows toward or away from a directional stimulus in its environment. A positive tropism means growth toward the stimulus, while a negative tropism means growth away from it. These responses allow plants to position themselves for better access to resources like light and water.

The three main tropisms you need to know are phototropism, gravitropism, and thigmotropism.

Phototropism: Growth Response to Light

Phototropism is a plant's growth response to light. Shoots exhibit positive phototropism, bending toward a light source to maximize the surface area available for photosynthesis. Roots often display negative phototropism, growing away from light and deeper into the soil.

The mechanism behind phototropism depends on auxins, a class of plant hormone. Here's how it works:

1. Light hits one side of a shoot tip.
2. Auxins migrate to the shaded side of the shoot.
3. The higher auxin concentration on the shaded side causes those cells to elongate more than cells on the lit side.
4. Unequal elongation bends the shoot toward the light.

Young sunflowers tracking the sun across the sky are a classic example of phototropism in action.

Gravitropism and Thigmotropism: Responses to Gravity and Touch

Gravitropism is a plant's growth response to gravity. Roots show positive gravitropism, growing downward with gravity to anchor the plant and reach water. Shoots show negative gravitropism, growing upward against gravity. This is why a corn stalk grows straight up even if the seed was planted sideways.

In roots, gravity causes dense starch-containing organelles called statoliths to settle to the lower side of root cap cells, which triggers auxin redistribution and directs root growth downward.

Thigmotropism is a growth response to touch or physical contact. When a tendril on a pea plant contacts a trellis, the cells on the contact side grow more slowly than cells on the opposite side, causing the tendril to coil around the support. This is commonly seen in vines and climbing plants that need structural support to reach light.

Plant Rhythms and Cycles

Photoperiodism: Plant Responses to Day Length

Photoperiodism refers to a plant's physiological responses to the relative lengths of light and dark periods in a 24-hour cycle. Plants use photoperiodism to coordinate functions like flowering and dormancy with the appropriate season.

There are three categories:

• Short-day plants flower when the uninterrupted dark period exceeds a critical length. Poinsettias are a common example, flowering naturally as nights grow longer in fall and winter.
• Long-day plants flower when the dark period is shorter than a critical length (meaning day length exceeds a critical duration). Spinach and lettuce are long-day plants that flower in summer.
• Day-neutral plants flower regardless of day length, often based on age or size instead. Tomatoes and corn fall into this category.

A key detail for exams: despite the names, what the plant actually measures is the length of the dark period, not the light period. If you interrupt a long night with a brief flash of light, a short-day plant won't flower because the continuous dark period was broken.

Phytochromes: Light Receptors Controlling Plant Responses

Phytochromes are light-absorbing pigments that act as photoreceptors in plants, controlling processes like seed germination, flowering, and shade avoidance. They exist in two interconvertible forms:

• Pr (inactive form): absorbs red light (~660 nm). When it absorbs red light, it converts to Pfr.
• Pfr (active form): absorbs far-red light (~730 nm). When it absorbs far-red light, it converts back to Pr. Pfr also slowly reverts to Pr in darkness.

Since sunlight is rich in red light, Pfr accumulates during the day. During the night, Pfr gradually converts back to Pr. The ratio of Pfr to Pr effectively tells the plant how long the dark period has been, which is how phytochromes connect to photoperiodism.

For example, seed germination in many species requires Pfr. Seeds buried under a canopy of leaves receive mostly far-red light (leaves absorb red light for photosynthesis), so Pfr levels stay low and germination is suppressed until the seed reaches adequate light.

Circadian Rhythms: Internal Biological Clocks

Circadian rhythms are internal biological cycles with a period of approximately 24 hours. They regulate processes like leaf movements, stomatal opening, growth rates, and flowering.

These rhythms are synchronized to natural day-night cycles, but they persist even under constant conditions (continuous light or continuous dark), which shows they're truly internal clocks rather than just direct responses to light. Flowers opening in the morning and prayer plant leaves folding at night are visible examples.

At the molecular level, circadian rhythms are controlled by genes that regulate the production of specific proteins on a roughly 24-hour feedback loop. Environmental cues like light act as signals that reset the clock each day, keeping it aligned with actual day length.

Plant Hormones and Regulation

Types and Functions of Major Plant Hormones

Plant hormones (also called phytohormones) are chemical signals produced in small quantities in one part of the plant that regulate growth, development, and responses to the environment in target cells and tissues. Five major classes are covered in this unit:

These hormones rarely act alone. Their effects depend on their concentrations and on the ratios between different hormones present in a tissue.

Auxins, Cytokinins, and Gibberellins: Growth-Promoting Hormones

These three hormone classes generally promote growth, though they do so in different ways and sometimes balance each other.

Auxins are produced primarily in shoot tips and developing leaves. They stimulate cell elongation in stems (which drives phototropism, as described above) and promote root initiation in cuttings. At high concentrations, auxins actually inhibit root elongation, which is one reason the auxin redistribution during gravitropism causes roots to bend downward rather than elongate on the high-auxin side.

Cytokinins are produced mainly in root tips and travel upward through the xylem. They promote cell division, stimulate leaf expansion, and delay leaf senescence (aging). The ratio of auxin to cytokinin in a tissue determines whether it develops into roots (high auxin:cytokinin) or shoots (low auxin:cytokinin). This principle is used in plant tissue culture.

Gibberellins stimulate stem elongation by promoting both cell division and cell elongation. They also break seed dormancy and promote fruit development. A practical application: spraying gibberellin on grape clusters increases the space between individual grapes and their overall size, which is standard practice in table grape production.

Abscisic Acid and Ethylene: Stress Response and Ripening Hormones

These two hormones are more associated with inhibition, defense, and developmental transitions than with promoting growth.

Abscisic acid (ABA) acts as a stress signal. During drought, ABA levels rise and trigger guard cells to close stomata, reducing water loss through transpiration. ABA also promotes and maintains seed dormancy under unfavorable conditions. Seeds with high ABA levels won't germinate even if water and temperature are adequate. As ABA levels drop (often after a period of cold or after being washed away by rain), dormancy breaks and germination can proceed.

Ethylene is unique among plant hormones because it's a gas ($C_2H_4$). Its most familiar role is promoting fruit ripening in climacteric fruits like bananas, tomatoes, and apples. This is why placing an unripe banana next to ripe fruit speeds up ripening: the ripe fruit releases ethylene gas that triggers ripening in the unripe fruit.

Ethylene also promotes abscission, the process by which leaves, flowers, and fruit detach from the plant. In addition, ethylene is involved in defense responses to pathogen attack and mechanical stress. Flooding, wounding, and infection all trigger increased ethylene production.