Questions and Answers
What effect do cytokinins have on apical dominance?
What is the role of ethylene in fruit ripening?
Where are natural cytokinins primarily synthesized in plants?
What is kinetin a modified form of?
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What type of growth does ethylene promote in plants?
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Which substance was first discovered as a cytokinin?
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What effect do cytokinins have on leaf senescence?
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Which of these processes is promoted by ethylene?
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What is the primary natural source of zeatin?
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Which of the following is a synthetic activity of cytokinins?
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Study Notes
Growth Measurement
- Growth at the cellular level results from an increase in protoplasm, difficult to measure directly.
- Measurable parameters for growth include fresh weight, dry weight, length, area, volume, and cell number.
- A single maize root apical meristem can produce over 17,500 new cells per hour.
- Watermelon cells can increase in size up to 350,000 times, highlighting the difference in growth types.
- Pollen tube growth is measured by length, while dorsiventral leaves show growth through surface area increases.
Growth Rates
- Growth rate is defined as increased growth per unit time and can be calculated mathematically.
- Growth can exhibit arithmetic or geometric patterns.
- Arithmetic growth occurs when one daughter cell continues to divide post-mitosis, resulting in linear growth (example: constant root elongation).
- The mathematical expression for linear growth is represented as: Lt = L0 + rt, where Lt is length at time t, L0 is initial length, and r is growth rate.
Plant Growth Regulators (PGRs)
- 'Bakanae' disease in rice is caused by the fungus Gibberella fujikuroi, which produces gibberellic acid.
- Auxins, first isolated from human urine, include indole-3-acetic acid (IAA) and synthetic variants like NAA and 2,4-D.
- Auxins stimulate rooting in stem cuttings and play a significant role in flowering, fruit retention, and apical dominance.
- Gibberellins are another type of promotory PGR.
Auxins
- Auxins are produced in growing apices and are vital for plant growth.
- They prevent early fruit and leaf drop, while promoting the abscission of mature leaves.
- Apical dominance results from auxin production, inhibiting lateral bud growth unless the shoot tip is removed.
- Auxins also induce parthenocarpy, prevalent in tomatoes, and are effective as herbicides (e.g., 2,4-D).
Cytokinins
- Cytokinins, discovered as kinetin, promote cytokinesis and are found in regions of rapid cell division.
- Zeatin, isolated from corn-kernels, represents a naturally occurring cytokinin.
- These hormones stimulate new leaf production, chloroplast formation, and adventitious shoot growth.
- Cytokinins counteract apical dominance and assist in delaying leaf senescence through nutrient mobilization.
Ethylene
- Ethylene is a gaseous PGR synthesized in ripening tissues and senescing leaves.
- It initiates horizontal growth in seedlings, participates in fruit ripening, and accelerates respiration rates (respiratory climactic).
- Ethylene promotes organ senescence and abscission, and aids in breaking dormancy of seeds and buds, enhancing germination processes.
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Description
Explore the methods and parameters used to measure plant growth at the cellular level. This quiz covers various measurable aspects including weight, length, and cell count, highlighting unique growth examples like maize roots and watermelon cells. Test your understanding of how these parameters can reflect growth dynamics in plants.
