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Gibberellins are used to end seed dormancy and promote flowering, while rooting powder contains synthetic auxins. Free notes and 14 questions answered.
Plant Hormones in Agriculture and Horticulture is part of Plant hormones (biology only) in GCSE AQA Higher Biology.
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GCSE AQA Higher
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Gibberellins are used to end seed dormancy and promote flowering, while rooting powder contains synthetic auxins. Free notes and 14 questions answered.
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Use this lesson to stay inside Plant hormones (biology only) while connecting the detail back to the wider Homeostasis and response topic.
Read the core explanations from Plant Hormones in Agriculture and Horticulture before testing yourself from memory.
Farmers face constant battles: weeds stealing nutrients from crops, seeds refusing to germinate, cuttings dying before they root. Plant hormones solve these problems by hijacking the plant's own control systems.
Think of plant hormones as chemical messengers - like texts between different parts of the plant. Auxins say "grow longer", gibberellins say "wake up and flower". Scientists have learned to send fake messages to control exactly what plants do and when.
Here's the clever trick: synthetic auxins kill weeds by making them grow themselves to death. When you spray a high concentration of synthetic auxin on a weed, it forces uncontrolled cell division and elongation.
The weed grows so fast it can't transport water and nutrients properly. Stems become weak and twisted, leaves curl up, and the plant dies within days. It's like forcing someone to sprint until they collapse.
**Trap**: Many students think auxins always promote growth. Actually, high concentrations kill plants while low concentrations promote growth. It's all about the dose.
When you take a cutting from a plant, it has no roots to absorb water. Most cuttings die before they can grow new roots. Rooting powder contains synthetic auxins that trigger rapid root development.
Worked example: How Rooting Powder Works
A gardener takes a rose cutting and wants it to develop roots quickly.
Answer: Synthetic auxin reduces rooting time from months to weeks, dramatically improving success rates.
Commercial growers use this technique to produce thousands of identical plants from a single parent. Every rose bush in a garden centre probably started as a cutting treated with auxin.
Some seeds refuse to germinate even in perfect conditions - they're dormant. This evolved as protection against germinating at the wrong time of year. Gibberellins override this safety mechanism.
Picture a seed as a locked safe. Inside are all the nutrients needed for growth, but the lock won't open until conditions are perfect. Gibberellins act like a master key, forcing the lock open regardless of conditions.
Brewers use this principle to make beer. Barley seeds are soaked in gibberellin solution to trigger simultaneous germination. This converts starch to sugars, which yeast then ferments into alcohol.
Many plants only flower at specific times of year, triggered by day length or temperature. Gibberellins can force flowering out of season, allowing year-round production of flowers and fruits.
Greenhouse growers spray gibberellins on plants to produce flowers for Valentine's Day, even in winter. The hormone tricks the plant into thinking spring has arrived.
**Trap**: Students often confuse auxins and gibberellins. Remember: auxins control growth direction and rooting, gibberellins control germination and flowering timing.
Tissue culture grows entire plants from tiny pieces of tissue in sterile laboratory conditions. This technique relies heavily on precise hormone concentrations to control what the cells become.
Think of it like programming cells. High auxin concentrations tell cells "become roots". High cytokinin concentrations say "become shoots". The ratio determines what grows.
Worked example: Tissue Culture Process
A laboratory wants to produce 1000 identical orchid plants from a single leaf.
Answer: Tissue culture produces thousands of identical plants in months rather than years, using hormone ratios to control development.
This technique produces disease-free plants and preserves rare species. Many houseplants are now produced this way because it's faster and more reliable than traditional propagation.
Exam questions about plant hormones often test whether you understand the difference between natural plant responses and commercial applications. Here's how to tackle them systematically.
**Common Mistake**: Writing "auxins make plants grow" without specifying concentration. In weed killers, auxins kill plants through excessive growth. Always mention the dose-dependent effect.
**Mark-winning phrases**: "synthetic auxin", "concentration-dependent effect", "commercial advantage", "selective herbicide", "breaks dormancy".
Notice how the sample answer gets marks for: naming the hormone type, explaining the mechanism, describing the effect, and explaining selectivity. This structure works for most plant hormone questions.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Gibberellins are used to end seed dormancy, forcing seeds to germinate, and to promote flowering out of season. Brewers soak barley seeds in gibberellin solution to trigger simultaneous germination, converting starch to sugars that yeast ferments into alcohol.
Seed dormancy is a state where viable seeds delay germination even when environmental conditions appear suitable. It evolved as protection against germinating at the wrong time of year, and it can be broken by triggers including gibberellins.
Rooting powder contains synthetic auxins that trigger rapid root development in plant cuttings. The cut end of the stem is dipped in the powder and the auxin triggers cell division at the cut surface, so new roots appear within 2-4 weeks instead of months.
A selective herbicide is a weed killer that targets specific plant types, usually broad-leaved weeds, while leaving crops like grass unharmed. It often uses synthetic auxins at precise concentrations.
Synthetic auxins kill weeds by making them grow themselves to death: a high concentration forces uncontrolled cell division and elongation, so the weed cannot transport water and nutrients properly and dies within days. As selective herbicides they target broad-leaved weeds while leaving crops like grass unharmed.
In tissue culture, a precise ratio of auxins and cytokinins in the nutrient medium controls what cells become: a high auxin to cytokinin ratio produces roots and a low ratio produces shoots. This produces thousands of genetically identical, disease-free plants in months rather than years.
Auxins control growth direction and rooting, and are used in weed killers and rooting powders. Gibberellins control germination and flowering timing, and are used to break seed dormancy and promote flowering. High auxin concentrations kill plants while low concentrations promote growth.
Auxins do not always promote growth; the effect depends on the dose. Low concentrations promote growth, but high concentrations of synthetic auxin kill plants by forcing uncontrolled growth, which is how auxin weed killers work.
In brewing, barley seeds are soaked in gibberellin solution to trigger simultaneous germination. This converts starch to sugars, which yeast then ferments into alcohol to make beer.
Gibberellins are sprayed on plants to force flowering out of season, allowing year-round production of flowers and fruits. Greenhouse growers use them to produce flowers for Valentine's Day, even in winter.
With rooting powder, new roots appear on a cutting within 2-4 weeks, whereas without auxin the process takes months or fails completely. The synthetic auxin triggers cell division at the cut surface.
In tissue culture, a high auxin to cytokinin ratio tells cells to become roots, while a low ratio tells them to become shoots. The ratio of the two hormones determines what grows from the callus.
Synthetic auxins kill weeds by forcing uncontrolled cell division and elongation at high concentration. The weed grows so fast it cannot transport water and nutrients properly, so stems become weak and twisted, leaves curl up, and the plant dies within days.
Tissue culture produces thousands of genetically identical plants in months rather than years, gives disease-free plants and preserves rare species. Many houseplants are produced this way because it is faster and more reliable than traditional propagation.