Senescense
The
process in a tree that produces Autumn colour and causes leaves
to fall is known as Senescense. Senescense is normally initiated
when the nutrients produced by the leaves, slows down in the
reduced light and temperature levels of Autumn. Shorter days,
longer nights and changing light quality all help throw internal
switches within the tree itself.
Senescense
physically begins by the production of a seal across the base
of the leaf. On the leaf side of the seal, the leaf cells weaken
and start to become thinner. On the opposite side of the seal,
the food transport cells (phloem) become more tenuous.
The
cambium cells continue to supply water to the leaves to replace
any moisture lost through evaporation as normal. All the time
the leaf base cells weaken until it is only the water connections
that hold the leaf onto the tree. One by one the leaves lose
their hold and fall to the ground. The wound left on the tree
is the outward face of the constructed wall established to
keep the cold of Winter outside.
Changing
Colours
The
green colour of leaves comes from chlorophyll. Chlorophyll production
in a tree slows with the onset of senescense in Autumn, at the
same time, longer nights and cooler temperatures help to break
the chlorophyll in a leaf apart.
As
the base of the leaf begins to seal, leaf starches (or stored
food) is broken apart and is shipped out of the leaf into the
tree itself. What chlorophyll remains continues to power the
remaining live cells within the leaf itself.
As
the green of chlorophyll breaks up and disappears from sight,
other pigments remain in the leaf; red, orange and yellow Carotenoids
and yellow and tan Xanthophylls.
These
pigments were originally made to shield and protect the chlorophyll's,
now the chlorophyll's have gone, the remaining colours can be
seen.
Behind
all of these brightly coloured pigments lies the deep brown
of the tannins and the light browns of the tree tissues. As
the leaf colour eventually fades, it is these browns of the
leaf skeleton that are left.
Improving
Autumn Colour in Bonsai
When
the chlorophyll component of a leaf breaks down quickly and
early in Autumn, the remaining coloured pigments are brighter
and last longer; Autumn colour is improved. There is no real
precise way of promoting this but there are factors that can
improve colour;
>Cold early Autumn nights speed up the start of Senescense
and increase the strength and length of time that the coloured
pigments can be seen.
>Leaves containing lower levels of chlorophyll as a result
of midsummer leaf-cutting/ defoliation, or trees that have been
fed low doses of nitrogen through the year will take less time
for the chlorophyll to exit the leaf and will also display better
Autumn colours.
