Date: Wed, 5 Aug 1998 19:27:58 -0500 From: "Benjamin F. Lasseter" <benjamin@twister.tamu.edu> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg2597$foo@default> Subject: photosynthesis thoughts
> The more prominent, light cycle (daytime), is what necessitates you
>shower your precious plants with light. The dark cycle (nighttime of the
>equivalent) is one of the reasons why there is a limit to the photo
>period which your lights are set (however, if you use our sun as a
>light....) I could go into more depth, but there really is no reason. I
>am only writing this because I have been growing concerned with the
>growers whom I have seen set timers for 18-20 hour photo periods, or
>abandon the timer all together and just leave the lights on 24 hours a
>day.
>
>...
>
>I'm not saying that no night-like darkness is bad for your plants, but I
>believe it to be so.
Trevor,
This is an interesting point, and you are correct that there indeed
two different reactions that occur in photosynthesis. I am not certain,
however, that your recommendations are correct.
The "dark" reaction, as it is usually called, is more specifically
"light-independent". It is light-independent because it is energetically
favorable. However, it occurs equally as much during the daytime as it
does at night (see the exception below). The "light" reaction, or more
accurately the "light-dependent" reaction, is an energetically unfavorable
process, which is why it requires light to occur. The light-dependent
reaction feeds into the light-independent reaction. In general, the rate
of the light-dependent reaction is much greater than that of the
light-independent reaction, so you can observe the light-independent
reaction occurring long after the light-dependent reaction has stopped.
There are two exceptions that I am aware of. Under conditions of
high carbon dioxide levels and low oxygen levels, the light-independent
reaction is hindered at a single step. Some plants have an alternate
pathway which they can use, but most cease photosynthesis when CO2 levels
are too high. The other exception is "photorespiration". This is a
catabolic process requiring, to the best of my knowledge, both the light
and dark reactions. But I have to go look it up again. Memory is failing
me. Can anyone out there remind me?
In spite of my basic disagreement with you, I do agree that the
mechanics of plant physiology cannot be thought about too much.
Benjamin F. Lasseter
Texas A&M University
P.S. By the way, is there anything one can do to save one's porch plants
from drought conditions such as the ones currently prevailing in Texas?
(over 100 F every single day, almost no rain since the beginning of June,
no relief in sight) I have lost quite a number of my best plants to this,
in spite of regular watering. I moved my VFTs indoors, but that has
drastically cut down the amount of light they can get. Only my moss roses
seem to thrive on the heat. Can anyone offer advice?
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