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plant growth in different light spectra

Wolfgang Ruehle ruehle at mzdmza.zdv.uni-mainz.de
Tue Mar 17 11:31:15 EST 1998

Husain Qutbuddin wrote:
> I am doing an experiment in which I am exposing plants to different =
> spectra of light for a length of time (a few days) and then measuring =
> their length increase in that period of time. Although growth is not a =
> direct effect of photosynthesis it would be expected that the plants =
> will grow more when they are exposed to the violet, blue and red =
> spectra, because the plant pigments absorb these ranges most. But I am =
> also comparing the increase of length of the plants when exposed to =
> these ranges to its increase when their exposed to normal light. Can =
> anyone predict if the plants will grow faster when exposed to only =
> normal light compared to its growth in violet, blue or red? Assuming of =
> course that the growth will be a consequence of the plant =
> photosynthesizing faster.
> Any specific book references which give answer to this question will =
> also be appreciated.
> Thankyou.

Often the influence of light quality is overestimated especially when
green light is compared to red or blue. This may be due to the depiction
of chlorophyll-spectra in the logarithmic scale of absorbance units. In
addition to this mathematical aspect the optical path through a leaf is
lengthened by reflections at the cell/intercellular interface. This
effect is more pronounced at wavelength with low absorption (Ruehle and
Wild, Planta 146:551-557 (1979). Therefore a bean leaf absorbs about
90-95% of the incident red and blue light but in the green region the
absorption is still about 60% (Balegh and Biddulph, Plant Physiol.
46:1-5 (1970). According to this paper the quantum yield in the green is
only 25% lower than that of the red or blue light. Compared to 'normal'
light I would not expect big differences in photosynthesis between
red,blue and even green light if all experiments are carefully done with
equal quantum fluxes as pointed out by Keith E Henderson's posting. A
more drastic drop in the quantum yield is due to a selective excitation
of photosystem I with far-red light. This however requires very pure
monochromatic light.

In addition to effects of light quality on photosynthesis the
photomorphogenetic effects will influence the lenght of a plant more
drastic than photosynthesis, especially when only low intensities are
applied. So if you find differences in your experiment consider those
rather than photosyntesis.

W. Ruehle

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