In the Northern Hemisphere this is the time of year when the leaves of deciduous plants turn color and fall off. Why do they change color and why are some leaves so red?There are two different answers to the question. The first one deals with the trigger for leaf senescence. It's the shortening of daylight hours that starts the process and from the time it is triggered by photoperiod the process proceeds in a manner that is not influenced very much by the environment, including whether the weather is cold or hot (Keskitalo et al. 2005). What this means is that the leaves all fall off at about the same time each year. The intensity of leaf color, on the other hand, is affected by the weather. Warm weather tends to produce a less spectacular display of fall colors.
The second answer addresses the reason for leaf color. It has to do with senescence. In the autumn the leaves of deciduous trees fall off the tree to prepare for winter. As the leaves die, the tree attempts to salvage as much nitrogen and carbohydrate as it can. While the photosynthetic apparatus is winding down it is more likely to produce free radicals and oxidative damage [Superoxide Dismutase Is a Really Fast Enzyme]. To prevent excess damage the leaves produce pigment molecules that block some of the light and reduce levels of photosynthesis. Red pigments, such as anthocyanins are especially effective (Feild et al. 2002).
Anthocyanins are only produced in the autumn. They are not found in leaves during the summer and their main role is to block sunlight from the photosynthesis machinery during leaf senescence. Other leaf colors are due to the unmasking of accessory pigments as chlorophyll breaks down. The regular pigments such as carotenoids (orange) [Vitamin A (retinol)] and xanthophylls (yellow) become more prominent because their breakdown is delayed [Why Leaves Change Color].
The intensity of the color is influenced by the composition of the soil. When the soil is deficient in nitrogen the tree needs to recover more nitrogen from the leaves before they fall off. This leads to increased production of anthocyanins in order to prolong the period when the leaf cells can remain metabolically active to export nitrogen and carbohydrates [Why do autumn leaves bother to turn red?].Feild, T.S., Lee, D.W. and Holbrook, N.M. (2002) Why leaves turn red in autumn. The role of anthocyanins in senescing leaves of red-osier dogwood. Plant Physiol. 127:566-574. [PubMed]
Keskitalo, J., Bergquist, G., Gardeström, P. and Jansson, S. (2005) A cellular timetable of autumn senescence. Plant Physiol. 139:1635-48. [PubMed]
WD (Bill) Hamilton had some ideas about this. Carl Zimmer wrote a piece about it a few years ago. According to Schaefer & Wilkinson he "proposed that coevolution between plants and herbivorous insects explains the bright autumnal colouration of leaves. Accordingly, plants invest in bright signals to reduce their herbivore load, whereas insects use these bright signals to identify less-defended hosts more efficiently."
ReplyDeleteThat always seemed to me one of Hamilton's stranger ideas. The sunscreen hypothesis makes good sense, and there is recent evidence supporting it (the correlation with nitrogen-poor soils mentioned by Dr. Moran). Can we agree that, whatever the ultimate explanation, the proximate fact of energy-intensive pigment synthesis strongly suggests "adaptation"?
ReplyDeleteI notice that Nature has a piece about this in one of their recent news articles.
ReplyDeletesven dimilo says,
ReplyDeleteThat always seemed to me one of Hamilton's stranger ideas.
Wow! Hamilton had many strange ideas. This one seems pretty mild to me, even though it's almost certainly wrong.
Can we agree that, whatever the ultimate explanation, the proximate fact of energy-intensive pigment synthesis strongly suggests "adaptation"?
Yes. The synthesis of anthocyanins in those species that make them seems to call out for an adaptive explanation.
On the other hand, the coloration due to unmasking of carotenoidsand xanthophylls doesn't.