MadSci Network: Botany
Query:

Re: coleus leaf, a red leaf and red leaves of plum, different to each other in

Date: Sun Jun 1 21:14:09 2008
Posted By: Alex Brands, Post-doc/Fellow, Biological ciences, Lehigh University
Area of science: Botany
ID: 1210082814.Bt
Message:

The red color you see in leaves is due to the presence of various pigments called anthocyanins, and to a lesser extent, carotenoids (carotenoids are usually yellow or orange). Why do plants make these other pigments?

Plants capture some of the energy of sunlight through the process of photosynthesis. Central to this process is the green pigment chlorophyll, which absorbs light mainly in the blue and red wavelengths. The absorbed light provides the energy for photosynthesis. To an extent, increased light intensity results in increased photosynthesis. At some point, the photosynthetic machinery is working at its maximum potential, and any increase in light intensity beyond that does not increase the rate of photosynthesis. In fact, any increase beyond that can decrease the rate of photosynthesis (photoinhibition) and creates a physical stress on the plant. To address this, the plant makes the other pigments mentioned above. They serve to absorb excess light (especially the wavelengths not absorbed by chlorophyll), thus preventing that light from damaging the cell. Carotenoids can also play a role in photosynthesis by capturing light energy and passing it on to chlorophyll.

In plants whose leaves appear entirely red or purple, such as those of some plum trees, chlorophyll is present in the leaf and photosynthesis is occurring just as it does in green plants. However, there is so much purple pigment in the leaf that it appears purple, so the green chlorophyll is not obvious to the casual observer. Because the anthocyanins are reflecting some of the red light, this may reduce the amount of red light reaching the chlorophyll in the leaves, but enough gets through to power photosynthesis.

Another example of this can be found in the red leaves of poinsettias. As described in this abstract: http://www.springerlink.com/content/jm12261r1p355522/

The red leaves of poinsettias show similar levels of photosynthesis as the green leaves when exposed to normal levels of light. However, the red leaves can withstand higher intensities of light before experiencing photoinhibition. This is because the red anthocyanins are absorbing some of the excess light.

In a plant with variegated leaves, such as coleus, chlorophyll may not be present throughout the leaf. Some varieties of coleus have white variegation. In these areas, there is no chlorophyll, and thus no photosynthesis occurring. So if there are areas of coleus leaf with no pigment, it is possible that in some coleus varieties, the red areas have only anthocyanins, and no chlorophyll. In that case, no photosynthesis would be taking place in that part of the leaf.

I was not able to find much research specifically on coleus, but did find the following abstract: http://www.plantphysiol.org/cgi/content/abstract/59/1/114

The results of that paper indicate that the rate of photosynthesis does vary quite a bit among varieties of coleus that have different colored leaves. Keep in mind though that all of them are performing photosynthesis through the action of chlorophyll absorbing red and blue light.

Alex Brands

References:
Pomar F and Ros Barceló A (2007) Are red leaves photosynthetically active? Biologia Plantarum 51(4):799-800

Rouhani I and Khosh-Khui M (1977) Variations in Photosynthetic Rates of Fourteen Coleus Cultivars Plant Physiology 59:114-115


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