Leaf Senescence and Abscission

Leaf Drop Habit
    deciduous:  replace all leaves yearly; all leaves are the same age.
    evergreen:  rate of leaf turnover is slower, from 2 years to >30 years (bristlecone pine); leaves are different ages

Generally find the evergreens in places with longer winters (higher altitudes, northern latitudes) or year-round growing conditions (tropics, subtropics).  In temperate regions, most (but not all) conifers and some angiosperms have the evergreen habit.

Conifers have a leaf morphology and anatomy that are adapted to decrease desiccation and snow holding (and therefore decrease limb damage from weight of snow).
    thin, aerodynamic shape
    thick cuticle 

    sunken (recessed) stomata

    lose all leaves at once (fall), grow new leaves in the spring

    broad, flat leaves =>  excellent solar collectors for photosynthesis, but the shape increases desiccation and snow holding.  Therefore, drop the leaves before winter.
    thin cuticle; no waxy endodermis
Programmed Cell Death (PCD)
there is a series of steps gone through, in order, as cells die 
leaf abscission is an example of PCD   
other examples of PCD:  formation of aerenchyma, development of tracheids and vessel elements

PCD during leaf abscission:

    ↓proteins and nucleic acids
    ↓organelles:  chloroplasts first, nucleus last
The tree scavenges materials, esp. N and organic C, from the leaves before leaf drop.  The materials are stored in the parenchyma cells of the wood and bark over the winter.  The vascular tissue (transport route) and nucleus (instructions) must be maintained until the salvage operation is over.  The abscission zone forms as the mobilization of leaf material ends.

A layer of cork cells forms at the base of the petiole.
Cells to the leaf-side of the cork layer are degraded.
Leaf falls off of the tree.
The cork layer is visible as the leaf scar.

Pigments responsible fot the colors seen in leaves

    yellows and oranges 
    fat-soluble pigments found in chloroplasts

        accessory pigments for additional light collection for photosynthesis
        photoprotection of chlorophyll and photosynthetic apparatus
    unmasked as chlorophyll is destroyed and not replaced

    reds and purples      
    water-soluble pigments found in vacuoles

    production increases with dry, sunny days and cool (not freezing) nights (need sugars to synthesize the anthocyanins)

Various environmental factors affect the intensity of the colors and duration of the fall color show:  temperature, moisture.

Trigger to initiate PCD ==>  increasing night length due to seasonal changes in photoperiod
The ratio of two forms of phytochrome essentially tells a plant how long it has been in the dark.

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12 September 2013