Leaf abscission in deciduous trees
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Leaf Abscission Mechanisms in Deciduous Trees
Chlorophyll Breakdown and Abscission Zone Formation
Chlorophyll breakdown is a key process during leaf senescence in deciduous trees. The removal of magnesium from chlorophyll by Mg-dechelatase is the first and rate-limiting step in this pathway. This breakdown leads to reduced photosynthetic capacity and triggers the formation of the abscission zone, ultimately resulting in leaf drop. Ethylene production is also stimulated during this process, further promoting abscission. These events mirror the natural progression of leaf senescence and abscission observed in deciduous species, highlighting the physiological role of chlorophyll degradation in leaf fall Ito2022Primka2019.
Hormonal Regulation: Ethylene and Auxin
Ethylene is a well-known promoter of leaf senescence and abscission. In some species, the expression of specific genes, such as DAM6 in Prunus mume, increases ethylene biosynthesis and signaling, accelerating leaf senescence and abscission in autumn. This process is closely linked to the onset of bud dormancy, suggesting that ethylene not only triggers leaf fall but also coordinates seasonal transitions in tree growth .
Auxin, another plant hormone, acts as a long-range signal that can delay leaf abscission. In Populus, exogenous auxin application or inhibition of its transport delays the formation of the abscission zone and leaf shedding, even in the absence of ethylene signaling. This indicates that auxin and ethylene can act independently, with auxin primarily regulating the timing of abscission and ethylene facilitating the cell separation required for leaf drop .
Environmental and Ecological Influences
Environmental factors such as moisture stress, photoperiod, and drought play significant roles in determining the timing and duration of leaf abscission. Trees tend to shed leaves earlier when water deficits become apparent, and the length of the abscission period can vary as an adaptive trait, especially in species growing on infertile soils Escudero1987Gutiérrez-Soto2008Borchert2002. Leaf age has a limited effect on the timing of abscission compared to environmental cues, as trees of different leaf ages often shed leaves simultaneously in response to seasonal drought Gutiérrez-Soto2008Borchert2002.
Phenological variation in leaf abscission is also influenced by climate, with individual trees and populations showing different patterns based on local conditions and seasonal changes. This climate-driven plasticity allows trees to adapt their leaf shedding behavior to varying environmental pressures .
Species Differences and Leaf Damage
Different tree species exhibit varying rates of chlorophyll degradation and abscission layer formation. For example, maples may begin abscission earlier and degrade less chlorophyll than oaks or beeches. However, visible color change is not always a reliable indicator of chlorophyll loss or abscission timing across species .
Leaf damage, such as that caused by herbivores, can increase the likelihood of abscission, especially when a significant portion of the leaf area is removed. However, the timing of abscission for damaged leaves often coincides with the normal period of autumn leaf fall, suggesting that environmental and seasonal cues remain dominant .
Effects of Elevated CO2
Contrary to some hypotheses, increased atmospheric CO2 does not consistently advance leaf senescence or abscission in deciduous trees. In many cases, elevated CO2 either has no effect or delays these processes, indicating that other environmental and physiological factors are more influential in controlling leaf fall timing .
Conclusion
Leaf abscission in deciduous trees is a complex process regulated by hormonal signals, particularly ethylene and auxin, and is strongly influenced by environmental factors such as moisture stress and photoperiod. Chlorophyll breakdown and abscission zone formation are central to this process, with species-specific and climate-driven variations shaping the timing and duration of leaf fall. While leaf damage and elevated CO2 can affect abscission, their impact is generally secondary to the primary physiological and environmental controls.
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Poplar leaf abscission through induced chlorophyll breakdown by Mg-dechelatase.
Induced chlorophyll breakdown by Mg-dechelatase in Poplar leaves leads to leaf abscission, suggesting a physiological role of chlorophyll breakdown in leaf abscission in deciduous trees.
Synchrony in fall leaf drop: chlorophyll degradation, color change, and abscission layer formation in three temperate deciduous tree species.
Maple species degrade less chlorophyll in the fall than oak and beech species, and leaf color change is not a good predictor of chlorophyll degradation levels across species.
Modification of Vegetative Phenology in a Tropical Semi-deciduous Forest by Abnormal Drought and Rain1
Abnormal drought in semi-deciduous forests of Guanacaste, Costa Rica, led to a complex interaction between leaf age and water stress, affecting leaf abscission in trees of different functional types.
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