Effects of light deprivation on young and old stems of six varieties of Dahlia
Abstract
The existence of light-dependent anthocyanin production pathways has been known since
at least the early 1900s1), but has not yet been characterized in dahlias. Younger and older
stem nodes of six dahlia varieties – Alpen Diamond, CG Elegance, Fire Mountain, Pooh,
Smooth Operator, Susan Moray – were covered with foil to prevent light from reaching
the stem cells. The covered and uncovered portions were compared to determine the
effects of light deprivation on dahlia stems. Fire Mountain and Smooth Operator showed
no significant difference between the coloring of covered versus uncovered stems, and all
of the four remaining varieties showed greater coloration differences at younger nodes
than at older nodes.
Introduction
Light-dependent anthocyanin pigmentation has been studied in many plants including
maize2) and various different fruits3) but has yet to be characterized in dahlias. Four
dahlia varieties – Alpen Diamond, CG Elegance, Pooh, Susan Moray – were previously
observed to show light-dependent pigmentation (unpublished data). These observations
required duplication and documentation. Although Fire Mountain and Smooth Operator
(both dark-stemmed varieties) were not believed to be light-dependent it was expected
they would show at least some level of light-dependence in their stem coloration.
Materials and Methods
On each of the six plants (one plant per variety), both a younger and an older internode
received foil. The younger internodes in each case were the first below the newest shoots
of growth. The oldest internodes were the last before the stem became woody (and
therefore color difficult to determine). Foil was cut to be 1/2-internode in height (for
whichever node it was made) and a few internode circumferences in width. All 12
internodes had their bottom half covered, with the foil pushed as close to the bottom node
as possible. (See the middle column of figures below for foil placement on each variety).
Foil was left for 8 days and during that time removed only to make periodic observations.
Plants were observed at 5 points: 0 hrs, 24 hrs, 96 hrs, 144 hrs, and 192 hrs (when exp
was ended). Pictures (see below) were taken only before foiling (column 1), with foil on
to show placement (column 2) and after the foil was removed at the end of the
experiment (column 3).
Results
(See images below). All varieties except Fire Mountain and Smooth Operator displayed a
distinct lightening of color beneath the foiled areas of their stems. Absolutely no
difference between foiled and unfoiled areas was observable on Smooth Operator. Fire
Mountain displayed a barely visible lightening under the foil of both internodes. In the
cases of Alpen Diamond and Pooh the contrast between foiled and non-foiled areas of
stem was greater for the younger internodes than the older. This was not the case for
Susan Moray or CG Elegance which both showed approximately equal contrast at their
younger and older internodes.
Discussion
The results indicate that the Alpen Diamond, CG Elegance, Pooh and Susan Moray
dahlia varieties have a light-dependent anthocyanin pathway. In order to better support
this hypothesis additional controls should have been performed and are a logical next step
to these experiments. In particular, a clear covering should also have been applied to the
plants to ensure that lack of light, and not another result of covering, is responsible for the
pigmentation change. Additionally, varieties with light colored stems (those with no
anthocyanin pigmentation) should have been included to see that the foil wasn't actually
affecting a different pathway. In both of these follow up experiments there should be no
pigmentation change between covered and uncovered stems.
The anthocyanin pathway of Smooth Operator does not seem at all light dependent from
these experiments. However, it is possible that it is only light-dependent for a very short
period of time which was missed by taking the first developed internode instead of the
newest shoots of growth.
Fire Mountain provided inconclusive results. It is possible that it too has a very short
period during which anthocyanin production is light dependent, or possibly it takes a
much longer period of darkness before and pigmentation change is observable. Another
explanation attributes the slight lightening of color to cell expansion. As the cells expand,
their pigment is less concentrated and therefore appears lighter. In this case the
anthocyanin production has not changed, the cells have just changed size.
Images
The images below are the initial, with foil, and after pictures for the above experiment.
Each row represents a single internode on a single dahlia variety. The pictures for the
young and old internodes of a given variety are stacked together. The labels below each
picture row tell the dahlia variety and node (young or old).
1)
, 3) Pearce G. W. and Streeter L.R. 1931. A Report on the Effect of Light on Pigment
Formation in Apples. J. Biological Chemistry.
2) Procissi et al. 1997. Light-Dependent Spatial and Temporal Expression of Pigment
Regulatory Genes in Developing Maize Seeds. The Plant Cell. v9 pp.1547-1557