You would do best to avoid issues of turgor by growing within a
very humid atmosphere, something easy to obtain when dealing with a
single plant. In my opinion, you need to decide what it is that you
are measuring: the extention of the plant, its dry mass or its
wet mass. You are interested in the effect of light: ask yourself
what model you are using and what measurement would best validate
the model.
The easiest way to measure wet mass is to weigh the plant with
tissue-dried roots, which is not very hard to achieve in
hydroponic culture. More elaborate systems use changes in magnetic
inductance in large coils and some field systems use infra-red
absorbtion from a standard candle source in a temporary enclosure.
Dry mass can only be measured destructively: you start with many
matched plants and sacrifice these in sets over time so as to
get adequate staistics. The actual hight of a plant could use the
system that you propose, you could time-lapse photograph it or
you could use a ruler! Nastic movemnets (wobblings and flexings
during diurnal changes and growth) will throw all but the most
judgemental systems out of the window for precise measurements.
The real question, perhaps, is what you are trying to measure.
If you think that all other things being equal, then light
causes growth in ways not related to direct photosynthesis, then
it would be sensible to concentrate on the parts of the anatomy
of the plant which grow: the meristems and the cells downstream
of them which are still plastic. Focused observations will
tell you something that whole-plant views may not.
_________________________________________________
Oliver Sparrow
ohgs at chatham.demon.co.uk
