Wednesday, September 22, 2010

Why Rhododendrons Leaves Are Turning Yellow Now

A reader has posted the question of why Rhododendron leaves are turning yellow now in the Newland area.  Actually, they are turning yellow all through the southern Appalachians.  This is a normal occurrence.  Rhodos (as I will call them) can retain their leaves for up to 7 years if in shady conditions.  But each fall, Rhodos shed their oldest leaves.  This is a natural process and no cause for concern.  Rhododendron maximum (the most common forest Rhodo) is now dropping the oldest set of leaves during this time of year, and you can see yellow/golden leaves all through the forest understory.  The reason the leaves turn yellow is that just prior to leaf fall, the chlorophyll degrades and exposes pigments in the leaf known as xanthophylls, which are yellow. 

You can age the leaves on a Rhodo by counting the leaf whorls.  The uppermost whorl of leaves formed this past spring while the next whorl formed in the spring of 2009.  You can keep counting back along the stem to determine how many years of leaves your Rhodo is holding on to.  Rhodos that grow in high light often retain fewer age classes of leaves (maybe only the last two or three year's worth) than those growing in deep shade. This is because leaves in full sun perform higher rates of photosynthesis and probably age faster, thereby hastening their time of demise. In addition, for a leaf to make a positive contribution to the plant, it must recoup all the costs of its construction and maintenance (losses of carbon due to respiration) and then carry on photosynthesis for a while to produce enough sugars to justify it's existence.  And in the shade, rates of photosynthesis are low, so it takes a longer time to accumulate those sugars than a leaf in high light.  Also, high light can cause the accumulation of toxic compounds that stimulate senescence (ageing). These compounds are known as reactive oxygen compounds, and in the leaf can cause damage to the photosynthetic machinery. After several summers and winters in high light (and winters are the worst for causing damage from high light) the leaves degrade and fall off.


By the time a leaf is several years old, it is no longer as active as a new leaf (just like people to some extent!!) and it contributes less in the way of sugar production for the plant.  For example, older leaves have lower rates of photosynthesis (the process by which plants convert sunlight into sugar).  At some point, the cost/benefit ratio may become negative - that is, the plant loses more carbon through respiration than it takes in via photosynthesis, and the leaf becomes a liability for the plant.  This may be one reason the plant jettisons these oldest leaves.  However, the exact physiological trigger is not completely known at this time. 

So, no need to worry!  You are simply seeing the natural process of leaf senescence.
Thanks for writing!! 

Note that my latest fall color predictions are now posted here, on VISITNC.COM and my own fall color page.

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