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Time signals

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Dr Dario Stefanelli, plant physiologist from Agriculture Victoria, investigates the behaviours of plants. Physiology is the study of action and reaction that happens in an organism. Like all organisms plants respond to external stimuli, the response being behaviour. Plants cannot move and are anchored to a particular place so they have a lot of different mechanisms to perceive and react to their environment. Plants do perceive time but do not measure it in the matter of hours and such that humans do. When humans lived in caves, however, time was a matter of light on or light off and different intensities of the light would tell them where and when they were in a day. The sequence of the temperature each successive day would tell them where they were in a year, therefore the succession of seasons. Plants perceive time exactly that way through their receptors that receive light.

Plants, then, understand day and night and the seasons of the year. The day and its length are especially important as it is when plants produce carbohydrates through photosynthesis. This can’t be done at night so when they transpire at this time they use stored carbohydrates. Plants keep track of the sequence of light and dark and discriminate between high and low temperatures. They correlate this information into 'growing degrees days' or 'growing degrees hours'. 

These are similar to storing things in a bucket. When that accumulation reaches a certain value, certain action happens. For instance, if fruit plants do not perceive a certain amount of cold in the winter they will not flower. That is why wheat seeds are put in cold storage so they will then germinate when planted. Other plants like wattle perceive smoke to germinate, still others use light or the kind of light. A bout of warm weather in winter might mean that plants flower early which can be a problem if there is subsequent frost that kills them. For fruiting plants that would mean a huge economic loss.

The issue of temperature change during the year is also a problem for fruiting plants. If the temperature does not stay cold for long enough they don’t flower. Or on the opposite once they get the rightA of chilling hours they could flower too early, which means they could not get pollinated as there are not enough insect pollinators around, again creating huge economic losses. Deciduous plants are a good example of how plants perceive light and temperature. 

If the temperature during the day is still high, (ie 30°C) and drops considerably at night, (ie below 10°C) then the plants perceive it as autumn. This in turn starts a process of slowly siphoning out the nutrients of leaves into their roots so the leaves become yellow. As the day and night temperatures drop and the length of daylight drops the plant detects this to be deep autumn or winter which is the signal to drop all its leaves. This is why deciduous plants near streetlights keep their leaves on branches near the light because plants perceive light as well as temperature. Modern greenhouses control the amount of light, the intensity of light, the general temperature, the drop in temperature between day and night, the air humidity and the air flow in order to stimulate their highest production possible. Now with LED lights they can even change the kind of light and can increase particular parts of the spectrum (say red) that helps plants in particular moments to increase their growth and production. 


Some varieties of fruiting plants, like tomatoes, don’t care whether they are grown indoors or out. Others are better fruiting or easier to manage when grown outside. That is why nurseries ask you where and how you want to grow your plants so you buy the right variety. Definitely, though, fruit grown in natural direct sunlight do generally taste better.
Left: 20 hectare glasshouse raised tomatoes Guyra NSW (Mark Sim ABC Rural http://www.abc.net.au/news/2013-12-09/foreign-investment-horticulture-costa/5143844

Right:'Braeburn' pear blossom, Spring 2012, Applethorpe Research Station, QLD (Photo courtesy Dr Heidi Parkes, DAFF QLD) 
Survival of the fittest also occurs in plants, climate playing its part. Plants that are not flowering due to changed or suboptimal climate conditions don’t produce seeds so they will disappear from an area. Conversely their widespread seed dispersal would see them flower in new areas more conducive to their growth and flowering, therefore colonising places that were prohibitive before. The life cycle of plants relies on complex time signals.  

Dr Dario Stefanelli was interviewed for A Question of Balance by Ruby Vincent. Images from Dr Stefanelli or as acknowledged above. Summary text by Victor Barry November 2016.

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