ecological questions

Mon Jan 27 09:09:34 EST 1992

Pam Krannitz asks:
"Why does Arabidopsis require such low light levels (200 umol m-2 s-1) for
optimum growth under controlled conditions, when it must experience much
higher irradiance in the field?"

There are all sorts of possible answers to this (including the
suggestion that 200 umol m-2 s-1 is the *average* light intensity it
sees - photosynthesis happens on cloudy days and during late afternoons
aswell), but my favourites are these:

1. Arabidopsis isn't used to growing in an environment where nutrients
aren't a scarce commodity, so when there's an excess it pumps as much as
it can into major leaf proteins - i.e. RUBISCO and LHCII. The latter
would be unfortunate for the plant in high light, since PSII would
suffer, *inhibiting* photosynthesis.

2. "Controlled" conditions don't really mimic natural conditions. The
lights suddenly cut in in the morning, and the poor plants, which have
been merrily respiring, suddenly get the full blast of the fluorescent
tubes rays, and the photosynthetic apparatus suffers as a result. In the
field, the plants would be gently woken up.

3. Under higher intensity lights, the plants get much warmer than they
would like, they dry out (unless they're being grown hydroponically) and
are stressed as a result. Also, using ordinary fluorescent tubes doesn't
agree with them in general - there's too much PSII light and not enough

Acclimation to light intensity is an interesting phenomenon in itself - and
it's not as simple as it might seem. Resource allocation (which is what we're
really talking about) is affected by drought stress, NO3/NH4 levels, sink/
source demands - indeed any potentially limiting resource.

Robin Walters

Robert Hill Institute,
University of Sheffield,
P.O.Box 594
Sheffield S10 2UH

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