The phytochrome, the light-sensing mechanisms in leaves, rec-
ognizes shorter day lengths. The shorter days and lower
temperatures arrest chlorophyll production. Chlorophyll
breaks down faster than it is replaced, allowing the yellow
and orange pigments to be unmasked.
The molecules reflecting red wavelengths, anthocya-
nins, are water-soluble pigments that occur in the cell sap
creating the red, pink, and purple hues. During these days when
photosynthesis and chlorophyll production are decreasing,
an abundance of sugars accumulates in the leaf. The cool
nights promote a separation layer of cells in the petiole,
where the leaf attaches to the twig, that prevents sugar from
flowing out of the leaf, and also arrests the flow of nutrients
into the leaf. The formation of anthocyanin requires bright
light, a diminishing water supply and the accumulation of
sugars trapped in the leaf.
Temperature,light and water supply are the primary factors that influence
the synthesis of carbohydrates (sugars). Cool, but not freezing,
temperatures favor anthocyanin production. Bright light favors red colors, so red
color often develops on exposed leaves. Water supply also
affects anthocyanin production, with mild drought favoring
bright reds. Rainy days occurring near peak coloration will
decrease color intensity. Late summer droughts can delay
the onset of fall color by a few weeks.
Individual trees of the same species growing together
often show differences in leaf color because of variations in
the amount of sugars in the leaves and genetic predisposi-
tion. Some reach their peak color earlier than others. Varia-
tions among species in the rate of color change reflect differ-
ences in chlorophyll breakdown, production of anthocya-
nin and exposure of carotene and xanthophyll.
[Parphrased from
HERE]
Beautiful plants by the way!
Gr8 job!!
