Bud dormancy is a temporary suspension of visible growth of any plant structure containing a meristem, which is a group of cells that can divide and differentiate into various tissues and organs. Bud dormancy is an essential strategy for perennial plants, especially deciduous fruit trees, to survive harsh conditions and synchronize their growth and development with seasonal changes. Understanding the mechanism of bud dormancy is crucial for improving the cultivation and breeding of deciduous fruit trees. Artificial regulation of bud dormancy can help cope with climate change and avoid frost damage or poor flowering. Creating new cultivars with different chilling requirement can expand the geographical range and adaptability of fruit trees. Moreover, studying bud dormancy can provide insights into other aspects of plant biology, such as flowering time, stress tolerance, and perennialism.
Types of Bud Dormancy
Bud dormancy can be classified into three types: paradormancy, endodormancy, and ecodormancy.
Paradormancy in Bud
Paradormancy in bud is a phenomenon that occurs when a plant's buds remain dormant even after the environmental conditions are favorable for growth. This can happen due to a range of factors, such as hormonal imbalance, genetic variation, stress, or disease. Paradormancy in bud can affect the quality and quantity of the plant's yield, as well as its resistance to pests and diseases.
Causes of Paradormancy in Bud
One of the main causes of paradormancy in bud is the lack of a sufficient amount of gibberellins, which are plant hormones that stimulate cell division and elongation. The apical meristem, which is the growing tip of the stem or root, produces gibberellins. When the apical meristem is removed or damaged, the production of gibberellins is reduced, and the buds below it remains dormant. This is called apical dominance, and it is a way for the plant to allocate its resources to the main stem.
Another cause of paradormancy in bud is the presence of abscisic acid (ABA), which is another plant hormone that inhibits growth and induces dormancy. The buds produce ABA themselves, and it acts as a signal to prevent premature sprouting. Environmental factors, such as temperature, light, water, and nutrients influence the ABA levels. When these factors are unfavorable for growth, ABA level increases, and the buds enter dormancy. When these factors become favorable again, ABA level decreases, and the buds break dormancy. However, sometimes ABA levels remain high even when the environmental conditions are optimal for growth. This can happen due to genetic variation among different cultivars or individuals of the same species. Some plants have a higher sensitivity or responsiveness to ABA than others, and they require a stronger stimulus to break dormancy. This can also happen due to stress or disease, which can impair the metabolism or transport of ABA within the plant.
Consequences of Paradormancy in Bud
Paradormancy in bud can have negative impacts on the plant's performance and productivity. For example, paradormancy in bud can delay flowering and fruiting, which can reduce the quality and quantity of the harvest. Paradormancy in bud can also affect the timing and synchrony of flowering and pollination, which can reduce the chances of successful fertilization and seed formation. Paradormancy in bud can also affect the distribution and density of branches and leaves, which can alter the plant's architecture and morphology.
Paradormancy in bud can also have positive impacts on the plant's survival and adaptation. For instance, paradormancy in bud can protect the buds from frost damage or drought stress, which can occur during winter or summer. Paradormancy in bud can also allow the plant to escape herbivory or predation, which can occur during spring or autumn. Paradormancy in bud can also enable the plant to adjust its phenology or life cycle to match the changing climate or environment.
Examples of Paradormancy in Bud
Some examples of plants that show paradormancy in bud are:
- Apple: Apple trees have buds that are sensitive to ABA and require a period of chilling to break dormancy. However, few apple cultivars have buds that remain dormant even after sufficient chilling has occurred. This can result in delayed or uneven flowering and fruiting.
- Grape: Grape vines have buds that are influenced by gibberellins and apical dominance. Pruning can stimulate bud break by removing the apical meristem and increasing gibberellin production. However, some grape cultivars have buds that resist pruning and remain dormant even after gibberellin application.
- Potato: Potato plants have tubers that are modified stems with buds that can sprout into new plants. However, some potato cultivars have tubers that produce elevated levels of ABA and remain dormant even under favorable conditions. This can prevent premature sprouting and extend storage life.
Solutions to overcome Paradormancy in Bud
There are methods used to overcome paradormancy in bud and induce growth. These methods are:
- Pruning: Pruning is the removal of unwanted or excess parts of the plant, such as branches, leaves, flowers, or fruits. Pruning can stimulate growth by relieving apical dominance and increasing gibberellin production. Pruning can also improve light penetration and air circulation within the canopy, which can enhance photosynthesis and respiration.
- Grafting: Grafting is the joining of two different parts of plants, such as stems or roots. Grafting can induce growth by transferring gibberellins from one part to another. Grafting can also introduce new genes or traits into the plant, such as disease resistance or drought tolerance.
- Hormone application: Hormone application is the spraying or soaking of synthetic or natural hormones onto the plant or its parts. Hormone application can induce growth by mimicking or enhancing gibberellin activity or inhibiting ABA activity. Hormone application can also modify other aspects of plant development, such as flowering or fruiting.
- Chilling: Chilling is the exposure of low temperatures to the plant or its parts for a certain period. Chilling can induce growth by reducing ABA levels or increasing gibberellin levels. Chilling can also break seed dormancy or vernalization requirement.
- Scarification: Scarification is the mechanical or chemical treatment of seeds to weaken their coat or covering. Scarification can induce germination by allowing water and oxygen to enter the seed. Scarification can also break physical or chemical barriers that prevent germination.
Ecodormancy in Bud
Ecodormancy occurs when the bud has the potential to grow but unfavorable environmental factors, such as low temperature or short-day length restrain its growth. Ecodormancy is also known as spontaneous dormancy, induced dormancy, or natural dormancy, which means that the growth of buds is inhibited by unfavorable external conditions, such as low temperatures or a short photoperiod. However, unlike endodormancy, buds in ecodormancy have acquired the potential to resume growth and can break dormancy when conditions become favorable again. Ecodormancy usually occurs after endodormancy is released and before budburst and flowering in spring.
The transition from endodormancy to ecodormancy is a crucial step for the flowering and fruiting process of deciduous fruit trees. The timing and duration of this transition can determine the quality and quantity of blooming and fruit production. A better understanding of the molecular and physiological mechanisms of this transition can help fruit growers to optimize their management practices and cope with climate change. Less is known about the molecular regulation of ecodormancy in bud. Some studies have suggested that ecodormancy is controlled by a balance between growth-promoting and growth-inhibiting signals. For example, FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1 (TFL1) are two antagonistic genes that regulate flowering time in response to environmental cues. FT promotes flowering by activating floral meristem identity genes, while TFL1 inhibits flowering by maintaining vegetative meristem identity. The expressions of FT and TFL1 may be modulated by temperature, photoperiod, vernalization, and phytohormones during ecodormancy. Another example is DORMANCY-ASSOCIATED LATE EMBRYOGENESIS ABUNDANT (DALEA) genes, which are stress-responsive genes that encode proteins with protective functions against dehydration, salinity, cold, heat, and oxidative stress. DALEA genes may be involved in both endodormancy and ecodormancy regulation by modulating water status, membrane stability, protein folding, and antioxidant capacity in buds. In conclusion, ecodormancy in bud is a complex phenomenon that involves multiple factors and interactions at different levels. Ecodormancy plays an important role in ensuring the proper timing and quality of flowering and fruiting in deciduous fruit trees.
Endodormancy in Bud
Endodormancy is an internal state of the bud that prevents it from growing even under favorable conditions. Endodormancy is also known as rest or deep dormancy, which means that the growth of buds is inhibited by internal factors within the buds themselves. Endodormancy is a state of arrested growth that occurs in the buds of deciduous fruit trees in autumn and winter. It is a physiological process that helps the plants survive the cold and unfavorable conditions and prevents them from sprouting prematurely during warm spells. Endodormancy is induced by internal factors within the plant, such as phytohormones and gene expression, and can only be released by a certain period of exposure to low temperatures, called chilling requirement. Chilling requirement (CR) is the amount of chilling needed to overcome endodormancy and varies among different species and cultivars of fruit trees. The chilling requirement varies among different species and cultivars of fruit trees, and it is essential for flowering and fruiting in the following spring. Endodormancy is the most important and complex type of bud dormancy for deciduous fruit trees, as it determines the timing and quality of bud break, flowering, and fruit production.
The molecular regulation of endodormancy involves various phytohormones, epigenetic modifications, and gene expression changes. Among them, two groups of genes play central roles in endodormancy maintenance and release: SHORT VEGETATIVE PHASE (SVP) and Dormancy-associated MADS-box (DAM) genes. SVP and DAM genes belong to the MADS-box family of transcription factors that regulate various aspects of plant development, such as flowering time and floral organ identity. SVP and DAM genes are highly expressed in dormant buds and repress the expression of genes involved in growth resumption, such as FLOWERING LOCUS T (FT) and APETALA1 (AP1). The expression of SVP and DAM genes is downregulated by chilling exposure, which relieves the repression of growth-promoting genes and allows bud break.
The CR of fruit trees is influenced by both genetic and environmental factors. Quantitative trait loci (QTL) analysis is a powerful tool to identify the genetic factors that control CR. Several QTLs for CR have been detected in different fruit tree species, such as pear, peach, apple, cherry, and apricot. Some candidate genes underlying these QTLs have been identified, such as DAM genes in peach and pear, FT genes in apple and cherry, and C-repeat binding factor (CBF) genes in apricot.
Regulation of Endodormancy
The regulation of endodormancy is a complex and dynamic process that involves multiple factors and pathways. Some of the key players are:
- Abscisic acid (ABA): ABA is a plant hormone that accumulates in the buds during endodormancy induction and maintenance. It inhibits bud growth by suppressing cell division and elongation, and by inducing dormancy-related genes. ABA also enhances the cold hardiness of the buds by increasing the accumulation of sugars and proteins that act as cryoprotectants.
-SHORT VEGETATIVE PHASE (SVP) and Dormancy-associated MADS-box (DAM) genes: SVP and DAM are transcription factors that belong to the MADS-box gene family. They are expressed in the buds during endodormancy and repress the expression of genes involved in floral development, such as APETALA1 (AP1) and LEAFY (LFY). SVP and DAM also interact with other genes that regulate endodormancy, such as FLOWERING LOCUS T (FT) and TERMINAL FLOWER1 (TFL1).
-Epigenetic regulation: Epigenetic regulation refers to the modification of DNA or histones that affects gene expression without changing the DNA sequence. Epigenetic regulation plays an important role in endodormancy by altering the accessibility of DNA to transcription factors and enzymes. For example, DNA methylation and histone acetylation are involved in the regulation of SVP, DAM, FT, TFL1, AP1, and LFY genes during endodormancy.
Importance of Endodormancy
Endodormancy is a key adaptive trait that enables deciduous fruit trees to survive harsh winter conditions and synchronize their growth with seasonal changes. By preventing bud burst during mild spells in autumn and winter, endodormancy avoids the risk of frost damage to the buds and flowers. By requiring a certain amount of chilling to break dormancy, endodormancy ensures that bud burst occurs at an optimal time for pollination and fruit set in spring.
Endodormancy also has practical implications for fruit growers and breeders. By selecting cultivars with different chilling requirements, growers can choose varieties that are suitable for their local climate and avoid problems such as delayed or uneven bud break, poor flowering, or reduced yield. Breeders can use quantitative trait loci (QTL) analysis to identify genes that control chilling requirement and use them for marker-assisted selection or genetic engineering to create new cultivars with desired chilling requirements.
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