Plant Growth and Development · Lesson 8 of 10
Gibberellins and Cytokinins: Elongation and New Growth
“Contrast gibberellin-controlled stem elongation with cytokinin-supported cell division, shoot formation and delayed ageing.”
• Identify typical gibberellins and explain bolting, seed and stem responses. • Explain gibberellin applications in fruits, sugarcane, malting and conifers without assuming universal outcomes. • Describe cytokinin sources, natural zeatin and the historical kinetin distinction. • Explain how cytokinin interacts with auxin in shoot development and delays leaf senescence.
Gibberellins: a family linked with longitudinal extension
A large family of acidic diterpenoid plant growth regulators that influence cell elongation, developmental transitions and several reproductive or germination-related processes.
More than one hundred gibberellin-related compounds have been identified, traditionally designated GA₁, GA₂, GA₃ and so on. Gibberellic acid, GA₃, is a historically important and widely studied example. Many gibberellins can promote growth in suitable tissues by increasing cell expansion or altering gene expression and enzymatic activities. Their effects differ among species and stages, so a gibberellin treatment does not make every plant axis elongate indefinitely.
How gibberellins create an elongated plant axis
A stem has nodes where leaves or buds arise and internodes between those nodes. If internodes lengthen, the whole axis becomes taller even if the number of nodes changes little. Gibberellin signalling can enhance internode elongation and produce visibly longer stems. The classic bakanae rice seedlings illustrate excessive elongation associated with fungal gibberellin production. This is a growth-pattern change rather than a simple increase in leaf area.
Rapid elongation of internodes just before the development of a flowering stem, particularly conspicuous in plants with a basal rosette habit.
In rosette plants such as beet and cabbage, leaves initially arise on a short axis with compact internodes. A developmental transition permits rapid extension of the flower-bearing stem: bolting. Gibberellins can promote this elongation under suitable conditions. A plant that bolts is not necessarily producing more rosette leaves; it is changing its axis architecture to support reproductive development.
Gibberellins in fruit and crop development
The source lists several agricultural and horticultural applications: gibberellin treatments can elongate grape stalks, influence apple shape and delay aspects of senescence, permitting fruits to remain on plants for longer under some conditions. In sugarcane, growth of stem internodes is especially important because the stalk stores sucrose. Appropriate gibberellin applications may extend internodes and increase yield. The source mentions increases of as much as 20 tonnes per acre in particular reported settings, but the value must not be assumed as a standard result for every field.
Spraying juvenile conifers with suitable gibberellin treatments can hasten the transition to reproductive maturity and earlier seed production. In grain germination, gibberellins promote mobilisation of stored reserves, including activation of hydrolytic enzymes in some cereal seeds. This supports the use of GA₃-related treatments to speed malting in brewing. Malting depends on enzymatic modification of stored grain nutrients; gibberellin action is not the same as simply adding alcohol or sugar from outside.
Cytokinins promote division and help shape organ development
Plant growth regulators, many of them adenine-derived, that promote cytokinesis and influence the development and maintenance of shoots, leaves and other tissues.
Kinetin was an early experimentally identified cytokinin-active compound derived from laboratory DNA preparations. It is not considered a normal endogenous plant hormone in the original discovery context. Zeatin is an example of a naturally occurring cytokinin, found in maize kernels and coconut milk. Natural cytokinins are synthesised in regions associated with active cell division, including root apices, developing shoot buds and young fruits. They can move through the plant and act together with other developmental signals.
From division to new shoots and chloroplasts
Cytokinin activity promotes cellular division and can support development of new leaves, chloroplasts, lateral shoot growth and adventitious shoot formation. Growth is not produced solely by dividing cells; newly formed daughter cells must also expand and differentiate. Cytokinins therefore participate in patterning and tissue development as well as in the more direct promotion of cytokinesis. A root or shoot meristem integrates multiple signalling pathways, so the same applied cytokinin can produce different outcomes depending on where and when it acts.
Cytokinins can overcome apical dominance
An active shoot tip often inhibits lateral bud growth through a hormone-regulated network. Cytokinins can promote the activation and elongation of some axillary buds, opposing aspects of apical dominance. This does not mean that all buds grow independently of auxin; it means the balance of signals influences bud activation. In tissue culture, the relative presence of auxin and cytokinin is also used to influence whether cultured cells maintain proliferative callus or develop shoot or root structures, though a precise ratio-to-outcome rule varies by plant and culture system.
Leaf senescence and nutrient mobilisation
A regulated stage of ageing in which a leaf or another organ undergoes characteristic physiological decline, often involving breakdown and redistribution of materials.
A senescing leaf breaks down chloroplast components, loses chlorophyll and mobilises nutrients that may be reused by the plant. Cytokinins can help delay these changes and promote nutrient mobilisation into actively maintained tissues. Delayed senescence is not immortality: it shifts the timing or strength of ageing responses under suitable conditions. The same leaf ultimately remains subject to developmental and environmental limitations.
Why gibberellins and cytokinins are not synonyms
| Comparison | Gibberellins | Cytokinins |
|---|---|---|
| Representative | GA₃ | Zeatin |
| Familiar major response | Internode elongation or bolting | Cell division, shoot development |
| Developmental example | Stem transition from rosette to tall flowering axis | Lateral bud activation and new leaves |
| Applied context | Grape stalks, malting, sugarcane, conifer seed production | Cultured tissue growth and shoot formation |
| Ageing-related role | Can delay senescence in certain organs | Can delay leaf senescence and support nutrient redistribution |
Both groups can have growth-promoting roles, but the cellular and tissue-level outcomes differ. Gibberellins often emphasise axis elongation or developmental transitions; cytokinins strongly influence cell proliferation and shoot patterning. Their joint effects with auxin, ethylene and ABA can alter what one observes in real tissues. The source presents these as broad physiological tendencies, not as exclusive reactions controlled by one substance at a time.
A hormone application may change size, shape or timing under suitable conditions, but it cannot guarantee a numerical yield increase. Species, concentration, time, nutrition and other growth conditions matter.
Quick Quiz
Quiz
Which regulator is strongly associated with bolting in rosette plants?
What does GA₃ designate?
Why are gibberellin treatments useful in malting?
What is a natural cytokinin?
Which substance was historically isolated from heated DNA preparations?
Cytokinins can promote:
How can cytokinin influence apical dominance?
Which cytokinin-related effect helps keep leaves functional longer?
What is the key structural difference between rosette and bolting stages?
Why should the quoted large sugarcane yield increase not be treated as universal?
Practice Problems
- Define bolting and describe how gibberellins influence it.
- Give four gibberellin applications and explain the biological action in each.
- Explain the difference between kinetin and zeatin.
- Explain why cytokinins can delay leaf senescence.
- Compare the respective roles of auxin, gibberellin and cytokinin in branching and elongation.
Bolting is rapid internode elongation just before the flowering stage, especially apparent in rosette plants such as beet or cabbage. Gibberellins can promote the elongation that produces the taller flowering axis.
Key Takeaways
• Gibberellins (including GA₃) promote internode elongation and bolting in suitable plants. • Gibberellins have applications in grain malting, fruit development, sugarcane and conifers. • Kinetin is a historically isolated cytokinin-active compound; zeatin occurs naturally. • Cytokinins support cell division, new leaves, lateral buds and chloroplast development. • Both groups can delay aspects of ageing, but their effects depend on tissues and other signals.