In article ,
P van Rijckevorsel wrote:
Yes, I did not think this through. All things being equal the total amount
of assimilates will be the same for every day = light+dark-period
(photosynthesis going on until the cut-off point is reached by water
shortage), but respiration (per light+dark-period) will increase 28-fold and
the plant will die.

Hmm. Assuming somebody is providing water and other reasonable care to
these plants, they will respire for 29+ days of the month, and
photosynthesize around the clock for the other 14.5+. They probably
won't grow twice as fast during the round-the-clock light phase than
they would with 12 hours on and 12 hours off, but they will grow at
least somewhat more. Whether they can accumulate enough photosynthate
to survive the dark half of the cycle is problematic, but they will
certainly get etiolated and weakened, and do a poor job of removing CO2
to purify the air for the humans in the habitat, much less provide them
with vegetables. If the plants are being used to purify waste water,
your habitat will end up in deep... umm... unprocessed waste water for
half the month.

I think we may be saying the same thing, but it's hard to tell, since
the word "day" is being used for three very different amounts of time
here. From real life experience, I know that I get lots more than a
day's growth out of 14 terrestrial days of constant light than from one
day, but probably not twice as much as 14 days of 12-hours-on 12-hours-off
lighting under the same conditions, i.e. raising garden transplants
under fluorescent lights in my basement. I check the plants daily and
water and fertilize as needed.

Of course it is entirely academic, as any kind of structure built on the
moon that would keep out a vaccuum would also alter just about all the
circumstances. A meaningful answer is not really possible. Almost certainly
some kind of artificial lighting scheme would be put in effect (perhaps
something involving mirrors and satelites?), almost as a matter of course.

Note that you aren't going to get much light for a good bit of the lunar
day when the sun is near the horizon. Overall, lighting is likely to be
one of the least expensive inputs to this scheme, considering that all
or most of the materials are going to have to be imported from Earth,
and a catastrophic failure could result in lethal CO2 poisoning for
everybody in the habitat. (10% CO2 is lethal to most mammals.)

A Science Fiction book from the fifties will likely go into the matter more
deeply. I remember one of them speculating on the effect of 'virgin' moon
soil on plant growth.

Most science fiction uses handwaving instead of even
back-of-the-envelope calculations for these things. The writer refers
to hydroponics for air quality, fresh veggies and even ignores the
waste water aspects (who wants to eat food grown on sewage in our
culture?), plus a nice park or such for the characters to interact in.
These guys are writing stories, not engineering manuals, and the "hardy
unsung engineers create a paradise fit for women and children by
cleverly building gadgets" story is kind of passe', and was seldom very
practical in the first place. But it sold magazines to teenage boys.

IIRC, since lunar and Martian regosols have never been exposed to
weathering and leaching, they are full of salts to a toxic level for
plants What's more, they've never been exposed to oxygen, so are
generally intensely reducing and will gobble up all that expensive
oxygen you need to breathe and be pretty caustic to handle or try to
grow in.

For a well-written novel that considers the problems of jump-starting
an economy and ecology on Mars, read Kim Stanley Robinson's Red Mars.
It describes the chemical properties of Martian regosols, and how the
characters deal with them.

Interestingly, the main reason the Biosphere II habitat failed was that
the designers didn't take into account that concrete absorbs CO2 from
the air for years after it first sets, converting calcium oxide to
calcium carbonate. The concrete structure constantly drained CO2 from
the closed system, depriving the plants so that food production was
much less than expected. There's a lot to be learned about the details
of maintaining closed systems for any but rather short time periods,
and especially when resupply is extremely difficult and expensive.

It's a lot of fun to speculate about, however.