Nightlife explained

Pumpkins grow at night because their nocturnal they love the
nightime........

 
Seriously:

It is true that on most all occasions where someone was able to take
measurements frequently, the fruit puts on most of its weight at night. 
There is a physiological reason for this and Brock just about put his
finger on it.  There are two factors that allow the individual cells inside
a fruit to expand.  One is turgor pressure and the other is the elasticity
of the cell membrane.  There is a plant growth regulator (hormone) that
when present in the right concentration makes the cell wall fluid and
flexible thus able to expand.  Then turgor pressue is what actually pushes
the cell wall outward and thus it grows.   Turgor pressure changes with
water relations in the plant....just as brock said.   If you  pick a head
of leaf lettuce during the middle of a hot August day it is fairly limp as
compared to a head that you cut early in the morning right at dawn.  I'm
sure all of us have seen the rigidity of most plants in the early morning
hours.   Brock is right that the reason for this is that the plants water
uptake is able to get ahead of the evapotranspiration that is going on
during the day.  Its all a function of how much water the plant is able to
take up verses how much is being evaporated.  

At night the plant is not actively photosynthesizing so the stomates are
mostly closed, thus reducing evapotranspiration.  Also the relative
humidity is lower so the potential difference is lower.  You can think of a
growing cell like a balloon.  The turgor pressure is the air you blow in it
to blow it up and the hormone is present to make the rubber flexible enough
to expand.....and so they grow mostly at night.  

Or maybe they just like the night life....

Rick
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