# Investigating Phototropism & Geotropism ## Phototropism in plant shoots - Plant shoots are **positively phototropic** - they grow towards light - This maximises light absorption for photosynthesis - In the late 19th and early 20th centuries, various experiments investigated the mechanism of phototropism - Many experiments used **coleoptiles** - protective sheaths surrounding young grass shoots

Phototropism is a growth response towards or away from a directional light source

## Historical phototropism experiments ### Darwin's Experiment (1880) - Removing the tip of a coleoptile **stopped the phototropic response** to unidirectional light - To check this wasn't just due to wounding, Darwin covered coleoptile tips with opaque caps - Covering the tip also stopped the phototropic response - **Conclusion**: The tip of the coleoptile is responsible for detecting light ### Boysen-Jensen's Experiment (1913) - Replaced the cut tip back onto the coleoptile with a **gelatin block** between them - The phototropic response was **restored** - This showed the stimulus for growth was a **chemical (hormone)** that could travel through gelatin - Boysen-Jensen then inserted a mica barrier (impermeable to chemicals) halfway through the coleoptile below the tip: - On the **lit side**: phototropic response occurred - On the **shaded side**: phototropic response did NOT occur - **Conclusion**: The chemical stimulus is produced at the tip, travels down the shaded side, and causes growth on that side (rather than inhibiting growth on the lit side) ### Paál's Experiment (1919) - Cut off the coleoptile tip and replaced it **off-centre** in the dark - The side with the tip grew more than the other side, causing the coleoptile to **curve** - **Conclusion**: In light, phototropism is caused by a hormone diffusing through plant tissue and stimulating tissue growth ### Went's Experiment (1926) - Placed cut coleoptile tips on gelatin blocks, allowing hormones to diffuse into the block - Placed the hormone-containing block **off-centre** on a coleoptile in the dark - The side with the block grew more, causing curvature - The **greater the hormone concentration**, the more the coleoptile curved - **Conclusion**: Confirmed the hormone mechanism and demonstrated concentration-dependent effects

These experiments collectively proved phototropism works via a chemical messenger (later identified as auxin/IAA)

## Controlling growth by elongation - **Indole-3-acetic acid (IAA)**, an auxin, is a specific growth factor in plants - IAA is synthesised in the growing tips of roots and shoots (the meristems where cells divide) - IAA coordinates phototropisms by controlling growth through elongation - IAA molecules: - Are synthesised in the meristem - Pass down the stem to stimulate elongation growth - Activate **expansin** proteins in cell walls - Expansins loosen bonds between cellulose microfibrils - This makes cell walls more flexible - Cells can then elongate ## The phototropic mechanism in shoots - Phototropism affects shoots and the top of stems - **IAA concentration** determines the rate of cell elongation in the region of elongation - Uneven IAA distribution on either side of a shoot causes uneven growth - When shoots grow towards light = **positive phototropism** - Described as "positive" because growth occurs towards the stimulus ### How it works: - In shoots, **higher IAA concentrations** result in a **greater rate** of cell elongation - Experiments show IAA moves from the illuminated side to the shaded side of a shoot - Higher IAA concentration on the shaded side causes faster cell elongation there - This causes the shoot to bend towards the light

In shoots: more IAA = more growth. In roots: more IAA = less growth (opposite effects!)

## Geotropism in plant shoots and roots - Gravity affects both plant shoots and roots, but in different ways ### Negative geotropism (shoots) - When shoots grow **away from gravity** = negative geotropism - Gravity modifies IAA distribution so it accumulates on the **lower side** of the shoot - As with phototropism, IAA increases growth rate in shoots - This causes the shoot to grow upwards ### Positive geotropism (roots) - When roots grow **towards gravity** = positive geotropism - In roots, **higher IAA concentrations** result in a **lower rate** of cell elongation - IAA accumulates on the lower side of the root and **inhibits** cell elongation - The lower side grows at a slower rate than the upper side - This causes the root to bend downwards

Geotropism (gravitropism) is a growth response to the directional stimulus of gravity

## Opposite effects of IAA in shoots vs roots The relationship between IAA concentration and growth rate is fundamentally different in shoots compared to roots. This explains their opposite tropisms:

- In **shoots**: increasing IAA concentration increases growth rate (positive relationship) - In **roots**: increasing IAA concentration decreases growth rate (negative/inhibitory relationship) - This opposite response to the same hormone explains why shoots and roots can respond differently to the same stimulus ## Practical: Investigating the effect of IAA on root growth Experiments can investigate how IAA affects root growth in seedlings. ### Apparatus - Seedlings (same age and species) - Cutting tile - Scalpel - Light source - Lightproof container - Blocks of agar (all same volume) - Marker/pen - Test tubes - Water ### Method 1. Use scalpel to cut a 1cm section from the root tip of each seedling 2. Mark the root tips at 2mm intervals 3. Divide root tips into three groups and place in test tubes of water - Water keeps the plant tissue alive **Group A - Treatment 1:** - Remove the ends of root tips using the scalpel - Transfer root cuttings (with end removed) to an agar block - Apply uniform light source **Group B - Treatment 2:** - Transfer intact root tips to an agar block - Place lightproof container over seedlings to prevent light entering **Group C - Treatment 3:** - Transfer intact root tips to an agar block - Apply directional light source to one side of root tips 4. Leave all roots in their treatment conditions for 3 hours 5. Use the 2mm marker lines to determine if growth has occurred 6. Note if growth has been even on both sides ### Results and analysis **Group A (tips removed):** - Roots grow evenly on both sides - IAA is synthesised in root tips, so removing them means no IAA is produced - There is no inhibition of cell elongation **Group B (no light):** - Roots grow slightly less than Group A but evenly on both sides - Equal IAA concentration on both sides of root tip - Inhibition of cell elongation is equal on both sides - Roots don't grow as long as Group A due to presence of IAA **Group C (directional light):** - Cells on the **illuminated side** grow longer than those on the shaded side - Greater IAA concentration on the shaded side - This results in greater inhibition of cell elongation on shaded side - Illuminated side grows at faster rate - Roots bend **away from light** - negative phototropism

Remember: IAA has opposite effects in roots vs shoots - inhibits root growth but promotes shoot growth

### Limitations - Although same species used, **plants still have genetic differences** - Certain genotypes may be more prone to bending - May have slightly different sensitivities to IAA - The 2mm marks may get **smudged or move** - If root is mishandled, marks can be altered - This affects results accuracy - **Evenness of growth hard to determine** with naked eye - Only general comments can be made about whether growth is even on both sides - Quantitative measurement is difficult ## Key principles summary | Factor | Shoots | Roots | |--------|--------|-------| | Effect of high IAA | Increases growth rate | Decreases growth rate | | Phototropism | Positive (towards light) | Negative (away from light) | | Geotropism | Negative (away from gravity) | Positive (towards gravity) | | Where IAA accumulates in light | Shaded side | Shaded side | | Where IAA accumulates with gravity | Lower side | Lower side | | Result | Bends towards light/away from gravity | Bends away from light/towards gravity |