• Question: how do leaves know when to start going brown in autumn, and is it the same time every year regardless of the weather and light conditions?

    Asked by wookiee to Amelia, Clem, Izzy, Sarah on 11 Mar 2014.
    • Photo: Isabel Webb

      Isabel Webb answered on 11 Mar 2014:

      Leaves are green because of a chemical called chlorophyll, which is used for photosynthesis, which is a reaction taking carbon dioxide from the air and turning it into sugars, which they then use for energy (just like we eat sugars). The chlorophyll needs light to power the reaction, and so uses the sunlight around the trees.

      In the winter there isn’t enough light to power the entire plant. As a solution, the plant spends the summer storing lots of food for the winter to help it survive. The tree also then loses it’s leaves, so that it has less of itself to supply energy to. The first step in this is to stop making chlorophyll. When the chlorophyll starts to be lost, the true colour of leaves comes through. The brown colour is caused by other chemicals in the leaves, but this is covered up by the green of chlorophyll.
      Once the plant has saved some of the useful chemicals back from the leaves, the leaves die and fall off.

      Plants decide when to lose their leaves by sensing the temperature and light. Light is sensed by the same system that chlorophyll works in and we understand this well. The plant determines how long the day is and uses that to judge time – this is also used to determine when to flower. Temperature sensing is less understood, but we know that a set of signals is used to send the message. The plant relies on temperature and light to ensure it doesn’t lose its leaves too early or too late.

      To answer the second part of your question, no, this isn’t the same time every year. Different trees sense the light and temperature differently, which is why some trees lose their leaves earlier than others. A few years ago when we had a very warm summer, and you could see some trees completely bare whilst others still had brilliant green leaves.

    • Photo: Clemence Bonnot

      Clemence Bonnot answered on 11 Mar 2014:

      In autumn, most of the plants became dormant. This means that their growth is slowed down. This phenomenon is because with less light it is more difficult for plant to produce energy using the sun light (we call this process photosynthesis). It is also due to the fact that proteins that are making the energy particles (called enzymes) are also slowed down by the cold and became less efficient.
      This is also true for animal enzymes (some animal cope with that by sleeping all winter like reptiles and some mammals (hibernation)).
      When the plants are dormant they do not produce energy but they are still alive and consequently need to have some energy stocked to be able to bloom at the beginning of spring. Making flower is very energy demanding.
      In order to have energy plants suck,recycle and stock most of the compounds present in their leaves. This process is the reason leaves dry and wither. Isabel already explained why they loose their green color which happen at the same time.

      Recycling the leaves compound and stopping to produce chlorophyll happens when the plant perceive the good signals from its environment. These signals are: shorter days than summer + cooler temperature. They are the opposite of the spring signals that make them produce the flowers: longer days than winter and warmer temperature.
      In addition plant need to have felt a certain number of days of summer (long warm day) to be able to feel the autumn signal. And this is the same for spring, plants need to feel cold and short days of winter for a certain number of days to flower. This necessity is a security filter, to be sure that the plant will not loose its leaves too early because of a stormy cold day of summer, or produce the flower too quickly in winter because of a sunny warmer day in winter. If they were doing so, plants could not have enough energy for all winter or lose all its flower because of ice and that could lead to the plant death.
      So the time plant loose their leaves or make flower depend on the environment, if summer is longer plant will tend to loose their leaves later. But because it also depend of the length of the day which is dependent on the position of earth around the sun (which is more or less the same every year at the same moment of a season) the differences between each year isn’t very big.

      That said, some plant like the algae I work on event growing in growth chamber with a control light and temperature still recognize the seasons. Plant as animals are able to recall a rhythms of cyclic changes the usually undergo. It takes numerous generation of plants growing in growth chambers to make them forget when you take a plant from the wild.

    • Photo: Amelia Frizell-Armitage

      Amelia Frizell-Armitage answered on 11 Mar 2014:

      Some fantastic answers here for you from Izzy and Clem. I thought I would just put in a word for evergreen plants.

      The trees and shrubs you are asking about that lose their leaves in winter are called Deciduous. This happens, as mentioned in previous comments, because plants need to conserve their nutrients and water during the winter months and green leaves are expensive to maintain.

      However, some trees have adapted to be Evergreen, which means they stay green and don’t lose all their leaves over winter. Instead of this they constantly lose and replace leaves throughout the year. Evergreen plants usually grow in places with low nutrient levels because it takes a lot of nutrients to grow a leaf. A plant in a low nutrient environment can save more nutrients by growing leaves that last a very long time and keeping these all year round, rather than growing leaves that only last a short period of time and losing these over winter. This means they don’t have to spend lots of nutrients on growing a whole new set of leaves the following spring.

      It’s all about the plant doing a cost/ benefit analysis and working out how it can save the most nutrients.

      Examples of evergreens are confiers (christmas trees) and trees from tropical rainforests.

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