On 11/11/05 20:57, in article ,
"Dave Poole" wrote:

Sacha & Kay wrote:

Sacha:
I'm sure Nick or David will be along to tell us - the autumn colour is
something to do with sugars, IIRC but I find it interesting that the ash
trees have lost their leaves, for example and the oaks and beeches are
holding on to them.

As with anything to do with plants, the development of autumn colour
and leaf fall is complex, but here goes....

In plant leaf cells, chlorophyll molecules absorb blue and red light
so effectively that only green light is reflected (which is why leaves
are green) Also, molecules of carotin and anthocyanins are present
in varying quantities in the leaf and reflect yellow and red/purple
respectively. In most plants, chlorophyll completely masks other
pigments during active growth.

Chlorophyll is unstable and has to be replaced regularly throughout
the life of the leaf. In autumn, shorter days and cooler temperatures
trigger the formation of a water-proof, cork-like substance (suberin)
in a layer between the cells at the base of the leaf in the abscission
zone. This layer progressively reduces the flow of nutrients into the
leaf and chlorophyll production ceases. Later when the layer is
complete and the leaf falls, the suberin protects exposed cells
preventing invasion by harmful bacteria and fungi.

With the reduction in nutrients, the chlorophyll breaks down and this
enables the previously masked carotene and/or anthocyanin pigments to
show through. In some plants, the anthocyanin levels are very high as
the result of a secondary action of simple sugars in the presence of
sunlight (which is why fruits ripen red, orange or purple on the sunny
side). Plants with increased levels of anthocyanins in their leaves
turn red or reddish purple when the chlorophyll has broken down. Not
all tree carry anthocyanins though - the birches, beech and ash only
have carotins.

Rather like an artist mixing primary colours on his palette, the
presence of amounts of carotin, anthocyanins present affect the tone
and vibrancy of the autumnal colours. Dry, sunny weather causes
elevated sugar levels resulting in more anthocyanins in some trees and
richer autumn colours . I've left a lot out, but that just about
covers the basic sequence.

Kay:
there's a big difference in the times when trees shed their leaves -
whitebeam shed a long time ago, mulberry (which is always very late to
come into leaf) is still in full leaf - though yellow up here. The
sycamores opposite hold on to their leaves much longer than the horse
chestnuts.

They do here as well, - Aesculus are the first to colour up will start
even in August if the weather is very dry. Conversely the 'Turkey
Oak' - Quercus cerris holds its leaves until early December and
suddenly turns dark brown within a week, shedding everything a week
later.

Sacha:
Yes but we can remember 5th November parties when all leaves had gone.
These days, they seem to hang on longer and longer! Our young mulberry tree
is in the same phase,

When I first came down here over 20 years ago, I noticed that autumn
was perhaps a week later than in the Midlands, where most leaves were
gone by mid-late October. Nowadays autumn seems to be where winter
first started and the trees are more likely to be stripped by severe
gales rather than conventional leaf fall. In 2003, the horse
chestnuts remained in leaf until November and many tree were carrying
their leaves without significant autumn colouring until late November.
This year seems to be more 'conventional' and most of the leaves that
haven't been stripped in recent gales, will have fallen by the end of
the month.

I can't bear to snip this! Wonderful, David, as always. Even in Jersey,
I remember we had the odd Guy Fawkes night that was bitterly cold and one or
two when we stood in hailstones in our field to watch the fireworks with our
cold and chattering guests! Mulled wine was very popular afterwards with
the parents and I always felt a bit sorry for the children who got apple
juice or something of that sort! We're going to have another go at a
fireworks party tomorrow night in the back field but there is no doubt that
it will not be a really cold evening. I feel the cold very much and can
honestly say that I haven't felt cold once this autumn.
--
Sacha
www.hillhousenursery.co.uk
South Devon
(remove the weeds to email me)