[Ian_Thomas]

Ian_Thomas  asked the Naked Scientists:

Hello Chris

I hear your show on Radio 702 in South Africa and enjoy it immensely.

I have a question for you. In the Kruger National Park there is a bush Phyllanthus reticulates or Potato bush. It is called this because in the evening it smells like cooking potatoes. I presume that it smells more in the evening because the air is cooler and denser and somehow this causes the scent to be stronger: hopefully, you can explain why. My main question is that if you go up close to the bush the scent is weaker and often absent. It is quite strong if you are a short distance, about two, or three meters away from the bush. I have tried this a few times and think that I am correct in this statement. It seems illogical, but hopefully you have an explanation.

Kind regards

Ian Thomas

What do you think?

[Don_1]

I am not familiar with this plant, but would suggest that it perhaps releases a strong scent during hours of twilight and darkness because it is dependant on a nocturnal animal/insect to pollinate it. Thus releasing a strong scent during daylight would be a waste of energy.

[dentstudent]

The potato appears to be pollinated by bees and through self-pollination. Do bees collect at night? The other thing to consider is that just because we can smell it, doesn't mean that that is when it will be pollinated. There may be other smells released at other times that we are less sensetive to but which attracts the bees. Are bees attracted by "smell"?

[Don_1]

Some speices of Phyllanthus are pollinated by moths, but I wouldn't like to say this particular one is.

[RD]

My main question is that if you go up close to the bush the scent is weaker and often absent.
It is quite strong if you are a short distance, about two, or three meters away from the bush.
I have tried this a few times and think that I am correct in this statement.
It seems illogical, but hopefully you have an explanation.

When you are close to the plant its smell is constantly detectable, so you become acclimatised to the smell: your brain ignores this constant stimulus. When you move a certain distance from the plant its odour is intermittently detectable and the brain pays attention to this changing stimulus...

This is why we 'get used to' or 'acclimatise' to smells (particularly useful in farm yards and hospitals !) and eventually stop noticing them. The purpose of this dynamic response is that it prevents sensory overload, and enables you to continue to perceive new information.

http://www.thenakedscientists.com/forum/index.php?topic=67.msg104#msg104

[Ian_Thomas]

Hello

I understand that we may 'get used to' or 'acclimatise' to smells to which we are continually exposed, hospitals etc. However, with plants we are not continually exposed to them, especially if they are growing outdoors. In my experience when we go closer to plants, the smell gets stronger not weaker and that is why I find this Potato bush unusual.
 

[chris]

Hi Ian

This is a really interesting observation. What we know about smells and odours is that these are volatile chemicals that diffuse through the air, enter the nose and then dock with chemical receptors on the olfactory epithelium. These receptors signal, via nerves, to the brain that the odourant is present. The overall "smell" that we experience are the combined signals from all of the different odourant receptors. The equivalent situation with light is to think of the colour you "see" as the sum of the different wavelengths of light reaching your retina.

But what scientists have begun to discover about the olfactory system is that the same odourant molecule can bind to more than one type of chemical receptor, and in some cases binding to the other receptor can shut off the response from the first. Usually the odourant molecule has a very high affinity for its main receptor, but at much higher concentrations is can bind, albeit more weakly, to other secondary receptors.

In the case of your potato bush, perhaps when you get close enough to be experiencing very high odourant concentrations the molecule is activating other chemical receptors which are shutting off the main one, hence the apparent diminution in the smell strength.

Another possibility is that the plant is making a second odourant compound but which is present at much lower concentrations. This second agent might be a competitive antagonist (blocker) of the action of the first. For instance it might be able to, with high affinity, lock on to the chemical receptor that enabled you to smell the plant in the first place, but without activating this receptor. Therefore, close to the plant the smell appears to diminish as the second compound begins to block your ability to smell it.

A third possibility is that a combination of these effects is happening. A second molecule, at low concentration, might be binding to another receptor in the nose which in turn shuts off the signals from the first.

Plants do resort to this kind of chemical subversion to attract specific types of animals or pollinators. By producing a class of attractants at high concentration, but then a class of repellents that work at close range to the plant but to which only some kinds of animals are sensitive, plants can restrict access to only a desired species of pollinator.

In terms of how odours can change throughout the day, this was beautifully demonstrated by Irene Terry last year when she found that cycads alter their flower temperatures during the day to alter the production of scents and thus manipulate the behaviour of pollinating bugs:

http://www.thenakedscientists.com/HTML/content/interviews/interview/802/

Chris

 

[wannabe]

Some speices of Phyllanthus are pollinated by moths, but I wouldn't like to say this particular one is.

Phyllanthus seems to be one species vs the potato bush under discussion being refered to as:Solanum ellipticum is known as Potato Bush and under the more ambiguous name of "bush tomato". This from wikipedia

[wannabe]

Some observation/comment to Chris' excellent tutorial on chemo receptor function in sensory systems: combining chemical to produce taste seemingly produces a unique and separate quality more readily than what I observe in smelling. This being personal observation only and not having tested it in "double blind studies" can only be seen as anecdotal. Here's the anecdote: when in the pasture and fresh cow manure is about where I am it's distinct (and lovely) smell is unique and recognisable. When Bessie urinates right next to this I smell the urine distinctly and it does not seem to combine into the lovely scent of a '57 Montrachet.
And to add to the possible reasons why the odor strengthens with distance, distance equates time and were it not possible that the odor evolves, say through  oxidation, as time evolves?

[Ian_Thomas]

Hi, Thank you everyone for all the interest, knowledge and insights. Regarding the name of the bush, Potato bush is the common name, but I would think that there are many different species of plants that have the common name of 'Potato bush'. I therefore think that it is better to work with the scientific name of Phyllanthus reticulatus. Hopefully this would avoid any confusion with other plants.