Leaves213-page10 - (enzymatic degradation activated by...

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Leaves - 10 Leaf Abscission (Loss of leaves) The loss and replacement of leaves in plants is a normal process. A perennial which loses all of its leaves at one time in response to seasonal or climate differences, is termed deciduous. All leaves, however, have a finite life span, and are lost from the plant, to be replaced by newer leaves produced in the shoot meristems. The process of leaf loss is called abscission, and is controlled by hormones. The abscission zone is located at the base of the petiole in a region of undifferentiated, small parenchyma cells. Their walls contain no lignin, and the vascular cells in the abscission zone are also reduced in size. The process of abscission is initiated and proceeds as follows: The parenchyma cells start dividing rapidly. They secrete a layer of suberin in the walls nearest the stem The middle lamella, cell walls and cells of the abscission zone dissolve
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Unformatted text preview: (enzymatic degradation activated by ethylene) • Leaf abscises The hormones (which will be discussed later) involved in leaf abscission are: • Auxin • Ethylene Associated with abscission, and occurring prior to the loss of the leaf, is a process that involves degrading and moving a number of solutes, minerals and other substances from the leaf tissue into the stem and roots. The degradation of chlorophyll and change in leaf color which results, is often referred to as "fall color". As chlorophyll degrades the natural carotenoids become prominent. In addition, some leaves may accumulate anthocyanins in their vacuoles, adding reds or bluish reds to the fall color. Photoperiod is important in triggering the fall change in pigmentation. Summer moisture also affects fall color. There are a number of web sites that discuss fall color....
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