MadSci Network: Botany

Re: Why do plants turn brown when they die, and why are they green in life?

Date: Sun Apr 4 21:29:57 1999
Posted By: Astrid Dijkgraaf, Staff, Tech support forest ecology, Department of Conservation
Area of science: Botany
ID: 907275239.Bt


Hi Katy

Why are plants green?

Well you are spot on about it being chlorophyll inside the chloroplasts that gives a plant the green colour. There are other pigments in the leaves too, such as xanthophylls (yellows) and carotenoids (yellows, oranges and reds). These pigments are also used in photosynthesis but occur in lesser quantities than the green chlorophyll. The combinations of the different pigments make different shades of green.

Now the reason that plants look green is that they are trying to obtain energy from the sun using a particular part of the light spectrum, mainly the red and infra red wavelengths. If you remember from your physics classes the colour you see is the colour that is reflected from the object, the other colours are absorbed. So in the case of green plants, the green wavelength is reflected and all the other colours, especially reds and blues, are absorbed to drive the energy cycle in the plants.

Chlorophyll does best in the red (around 670 nm) and blue (around 500 nm) areas of the spectrum. That's why many plants have the additional pigments (xanthophylls and carotenoids) called accessory pigments that feed light energy to chlorophyll "a" from light. Chlorophyll is almost useless in the green part of the spectrum, and doesn't absorb that colour. That is why most plants are green.

There are different colours of plants too.

However, that does depend a bit on where the plant lives. Many seaweeds are red or brown or blue-green in colour because the red wavelengths don't penetrate water very well. In shallow water the seaweeds are still green, but as you get further and further underwater they become much more brown or red in colour. That is because these plants use a different part of the light spectrum to drive the photosynthetic cycle. Can you use the colour wheel on this page to work out which colours seaweeds absorb?

Another common exception to the green colour are plants such as begonias. They often have red underneath the leaves and green upper surfaces. This is because begonias live in quite dark parts of the forest (the red coloured begonias get leaf burn in full sunlight). The upper green surface catches the little bit of direct light that penetrates the dense leaf canopy of the forest. The lower red leaf surface collects the red and brown colours that are reflected from the ground surface; just to get that little bit more energy..Here are some other plants with various light requirements.

Help we are breaking up

The chloroplasts are little machines that capture energy from the sun, but they do wear out. Plants have a choice, they can either keep making more machines (chloroplasts filled with chlorophyll) to replace the broken down ones, or they can close down the factory (the leaf) and begin afresh somewhere else. Evergreen plants, such as pines and tropical plants, keep replacing the broken down chloroplasts until the actual structure of the leaf starts to break down, then that particular leaf is closed down after most of the nutrients are reabsorbed. Deciduous plants, like maples, have a slightly different philosophy on life. They only make short term factories (leaves) and start reabsorbing nutrients in autumn.

The reason that leaves go various shades of yellow, orange or red is because the elements that make up the chlorophyll are reabsorbed most - they are needed most in another leaf or another time to make more chlorophyll. The other pigments, such as the xanthophylls (yellows) and carotenoids (yellows, oranges and reds), are partially reabsorbed after most of the chlorophyll is gone. That is why you get those brilliant autumn colours for a while, and then everything goes a brown colour. I'm not entirely sure why brown is the dead colour for plants, but on the other hand whenever I try to mix paint colours to get a different shade I often end up with a muddy brown. So maybe it is just a case of the left over colour combine to make a brown.

The dead arise?

Your last question "are plants really dead when they die?", well yes. When plants die, they are really dead. However, there are a lot of plants that have various states of dormancy. They might look as if they are dying, but actually, they are only closing the factories (leaves), packaging and storing all the nutrients and getting ready for another season next year. The maples and other deciduous trees close up shop for the winter because at such low temperatures the leaf structure is easily damaged, the chlorophyll reaction doesn't work as well either, and there is very little light available to drive the photosynthetic reaction in any case. So, the deciduous trees "sleep" over the winter, no point in wasting effort in a difficult season.

Bulbs, such as tulip and onions, die back once they have achieved their purpose - to become pollinated and spread seed around. Once they've achieved that, they don't bother to stick around to be eaten. Much safer to pull your head in under ground. Saves on nutrients too, because those dang herbivores won't get them if you reabsorb them into the bulb. So, again, they aren't dead, just dormant.

I hope that I've managed to answer you questions Katy.


Cheers from Astrid

To check how much you really know about photosynthesis have a go at this quiz.

And if you have trouble with the quiz you might find some answers on this page.

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