Basic Plant Phys questions

Jonathan B. Marder marder at agri.huji.ac.il
Tue Dec 1 04:04:23 EST 1998


"Janice M. Glime" wrote in message
<199811251619.LAA06947 at login.ice.mtu.edu>...

>My first impulse was to say no, but then I remembered an article I read
a
>long time ago.  Someone used a strobe light for a short period (2
minutes,
>I think) at the beginning and end of the day (using a long-day
>photoperiod) and caused plants in the greenhouse to have as much
>photosynthesis as if they had had the intense light all day.

Sorry to intervene like this again Janice, but as a photosynthesis
specialist this strikes me as completely wrong.  We can verify this if
you cite the relevant article. Perhaps 2 min illuminations might affect
daylength, but surely not the daily photosynthetic production.
Photosynthesis is an energy-dependant process, and is reduced with a
shorter daily illumination period. Also, you cannot necessarily replace
lost hours with higher intensity because photosynthesis can become
light-saturated or even photoinhibited

[snip]
>The next question is whether lightning has the right spectrum.
>Anyone know the right physics to answer that?

Lightning is basically white light, so has the right spectrum to drive
photosynthesis. However, brief isolated flashed of light are not really
effective because of problems of biochemical activation etc.

>> 2)  Can plants (are there any that can) use moonlight for
photosynthesis?
>> I am guessing it is possible as moonlight is reflected white light
from
>> sun.  Does anyone know of any examples?
[snip
>  So, my next question is whether there is a threshhold for light level
to
>activate a chlorophyll molecule, or if it is quantitative, resulting in
>few molecules being activated in the low light of the moon and more
energy
>lost to respiration than gained in photosynthesis.  And is
>photorespiration totally inactive in moonlight?  What is the
determining
>factor for it, light quality or quantity?


There is no threshold for chlorophyll activation, but there are
thresholds for the biochemical processes.First problem is with
photosystem II which reduces plastoquinone to plastoquinol (2 electrons,
2 photoacts) and oxides water to O2 (4 photoacts per O2). The separate
photoacts have to occur within a few seconds or the previous step is
lost to back reactions. This has been checked with light-flash
experiments. Under moonlight, photons would arrive at each reaction
centre at too slow a rate to overcome this problem.

Regarding photorespiration, this can only occur under conditions of
active photosynthetic generation of ATP and NADPH - so moonlight is much
too low intensity.

Help this clears up some things,


Jonathan B. Marder   <MARDER at agri.huji.ac.il>
Department of Agricultural Botany, The Hebrew University of Jerusalem
Faculty of Agriculture, P.O.Box 12, Rehovot 76100, ISRAEL
Phone: +972 8 9481918   Fax:   +972 8 9467763
Web page:    http://www.agri.huji.ac.il/~marder





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