How it Works
The plant is represented as a series of connected segments forming a stem (above ground) and root (below ground). Each segment has a target angle influenced by auxin concentration on its left and right sides.
Light causes phototropin activation on the illuminated side of the stem, triggering lateral auxin transport to the shaded side. Higher auxin on the shaded side promotes faster cell elongation there, bending the stem toward the light. Gravity redistributes auxin downward in roots; since high auxin inhibits root cell elongation, the upper side (less auxin) grows faster, bending the root downward.
Root curvature: κ = -α_grav · (A_low - A_high) / (A_low + A_high)
Auxin ratio r = (light_intensity · sin(θ_light)) + gravity_signal
Segment bend rate: dθ/dt = κ · sensitivity
Frequently Asked Questions
What is phototropism?
Phototropism is the directional growth response of a plant toward (positive) or away from (negative) a light source. Stems typically show positive phototropism, bending toward light to maximize photosynthesis.
What is gravitropism?
Gravitropism is the growth response to gravity. Roots exhibit positive gravitropism (grow downward) while shoots show negative gravitropism (grow upward), ensuring proper plant orientation regardless of seed position.
What is auxin and how does it work?
Auxin (indole-3-acetic acid, IAA) is a plant hormone that promotes cell elongation in stems at low concentrations and inhibits root elongation at high concentrations. Its redistribution causes differential growth that bends the organ.
What is the Cholodny-Went theory?
The Cholodny-Went theory states that tropisms result from the lateral redistribution of auxin to the shaded or lower side of a plant organ. The unequal auxin distribution causes unequal growth rates on the two sides, producing curvature.
How quickly do plants respond to light?
Initial auxin redistribution can begin within minutes of unilateral light exposure. Visible bending typically occurs over 30 minutes to several hours, depending on species, light intensity, and growth rate.
What photoreceptor detects light for phototropism?
Phototropins (phot1 and phot2) are the primary blue-light receptors responsible for phototropic responses. They are activated by UV-A/blue light (350–500 nm) and trigger downstream signaling cascades that relocate auxin.
How do plants sense gravity?
Specialized cells called statocytes contain starch-filled plastids (statoliths) that sediment under gravity. This sedimentation is detected by the cell, triggering asymmetric auxin transport via PIN proteins to produce gravitropic curvature.
Can phototropism and gravitropism interact?
Yes, the two responses can compete or cooperate. A horizontal plant placed in light from above will show both a gravitropic upward bend and a phototropic bend toward the light source, with the resulting angle reflecting both signals.
What is the role of PIN proteins in tropisms?
PIN (PIN-FORMED) proteins are auxin efflux transporters that control the direction of auxin flow. Their asymmetric localization on cell membranes determines which side of an organ receives more auxin, driving differential growth.
Are tropisms relevant to agriculture?
Yes, understanding tropisms helps optimize crop growth. Controlled light environments in vertical farming exploit phototropism, and gravitropism research informs how plants might grow in microgravity for space agriculture.