Falling Leaves and Hormones

Why do leaves and fruits fall naturally from the plant?  Before we blame it on gravity, let us look at the role of some hormones.

"Everyone must take time to sit and watch the leaves turn." 

~Elizabeth Lawrence

Abscission is the physiological process plants go through to separate entire organs, such as leaves, flowers and fruits.  This is the natural process that allows plants to discard organs to conserve their resources.  Plants will develop leaves and flowers as much as they are capable to support.  Sometimes environmental conditions become unfavorable that maintaining all the organs such as leaves and fruits will put the whole plant in jeopardy.  It is a survival mechanism for a plant under stress. 

During the process of abscission, a weakened layer known as abscision zone develops at the base of the petiole.  This is a layer of specialized cells that develops into a scar tissue.  Here's a science fiction idea for you: Imagine that you have an unwanted conditon on your arm.  You are in so much pain and becoming very weak.  Without doing anything, you notice that a scar tissue begins to develop around your armpit separating your arm from your body.  The separation would start from the outer layer of your skin and going deeper until every last cell that is holding your arm in place has turned into clean scar.  That is called abscision; fortunately it happens only in plants.

There are three plant hormones that are known to be responsible for abscission: 
1.  Ethylene is the senescence and ripening hormone.
2.  Auxin is a growth hormone. 
3.  Abscisic Acid (ABA) is the stress hormone.
These hormones have other important functions in the plant but for the sake of explaining abscision we will focus on the above descriptions.

The interaction of these three hormones results in the falling of leaves in the fall.  Abscisic acid plays a role in the way plants respond to diverse range of environmental conditions.  Changes in temperature and photoperiod are interpreted by the plant as stress.  Under stress, the plant increases its abscisic acid levels while reducing the auxin levels.   Reduced auxin levels slow cell elongation process which means that growth is minimized or stopped.  The plant then shifts priorities from growth to survival until conditions become favorable for growth.    In the meantime, a part of the plant will interpret the cessation of growth as senescence- ethylene levels will increase to facilitate all senescence-related processes.  As the leaves stop growing, the nutrients in the senescing leaves will be mobilized to the other parts of the plant (branches, roots, etc.) depleting the leaves from nitrogen.  The yellowing of the leaves is a reflection of this process.  When low temperatures combine with bright sunny days in the beginning of fall, the leaves will tend to have a red tint in them.  This is because the sunny days will still induce the leaves to photosynthesize but since the development of the abscision zone or the scar tissue is already in process, sugars are trapped in the leaves.   The beautiful colors of the leaves after all are a symptom of a stressed plant.  The falling leaves are a sign of a plant that is purging itself in order to survive a stressful period.