All the leaves are brown



So the falling, gold, red and brown leaves tell me that Autumn (fall, l’Automne) is here. The colours are amazing, but why do the leaves change from green to these colours?

Green is the colour of fuel

Leaves are green because of chlorophyll, a pigment molecule found within the leaves (and in bacteria and some algae). This green pigment absorbs sunlight and utilises its energy to transform water obtained from the roots, and carbon dioxide in the air, into glucose (which the plants use as a fuel source) and oxygen.

These reactions take place in cells called chloroplasts in two stages. In stage one, chlorophyll is energised by sunlight to make ATP (a chemical energy transporter). At the same time, water molecules are ‘split’ to make electrons available to another molecule which along with ATP, in stage two of photosynthesis, turns carbon dioxide from the air into the components required to make glucose.

So why the transition to brown?

A sign of the times

The colour change signals a decrease in moisture (water) and sunlight, meaning that chlorophyll becomes degraded and leaves become less green. This is of course associated with seasonal transitions.

Interestingly, longer ‘green seasons’ has been found to be a marker of increased carbon dioxide availability, warmer temperatures and altered precipitation. It has also been discussed that these changes in Autumn length can affect competiveness between plant species, promoting greater growth of parasitic or destructive plant species.

Plants in space?

Because plants absorb carbon dioxide from the air and produce water, using plants as a life support in space exploration is an attractive idea. However, much needs to be known about the effect of zero gravity, altered atmospheric gas concentrations and cosmic radiations on plant growth and function.

It is already known that there are subtle micro -structural differences between terrestrial and space grown plants. For instance, some proteins were found to be less abundant in space grown plants meaning basic plant functions could be reduced. Conversely, there were also groups of proteins that were more abundant resulting in increased activity for certain plant functions. For example, proteins involved in flavour, were found to be over abundant in space grown plants!

Another consideration is light cycling. Terrestrial plants have evolved in a 12-hour light/dark cycle, and during dark cycles, plants release carbon dioxide that can affect the breathable gases concentrations in enclosed spaces such as spacecraft.

Finally, the effect of gravity needs to be considered. Water is drawn up from the roots into the main stem of the plants to the leaves. The effect of zero gravity on this function needs to be understood and researched further, as well as the affect on the cell membranes.

Final thought

Before you stop to smell the roses, maybe you should consider the browning of the leaves as well, and how one chemical is in part responsible for the air that we breathe, and possibly allowing humans to survive space exploration!




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