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A table of Photosynthetic adaptations to variations inirradiance(light)

> Now I'm reading 4-5 w/g, overdriving bulbs to get the last lumen possible,
> etc. What happened - have modern plants developed light resistance somehow?
> Ah, progress.  Always "more is better". I long for the good old days.
> George Booth in Ft. Collins, CO (gbooth at frii dot com)

Well more light does not equate better growth in some/numerous cases.
Most folks will find that 2 watts a gallon will do the trick for about any
tank set up unless the tank is real deep etc. I like the growth of many
plants better at lower light values. Things tend to be a bit more flexible
concerning dosing (and mental forgetfulness--"did I add the PO4 yesterday or
was it the day before?"_).

I have some very high light tanks and wanted to see how lighting affects
uptake rates. From these high light tanks I can then come back to a lower
lighting tank and optimize nutrient levels better now knowing what goes on
if things are sped up at a higher rate. It was harder to see the pattern at
lower light and the uptake. But equating the higher light as the same except
higher uptake is not a complete argument but I think it's telling enough to
make the assumption here for our purposes.

From that higher light dosing, I've found that at lower lighting(say 2
watt/gal), I can maintain these same high light nutrient levels(CO2, traces,
NPK etc) but at low lighting and get excellent growth across the board.

Uptake is slower, I'll dose 1x a week instead of 2-3 times a week for
But the maintained levels are the same.
NO3 still 5-10ppm
PO4 1ppm
Lots of traces(I do add this one 2x a week still)
CO2 20-30ppm
This yields very very little algae(less than the high light) but very lush

If you consider lower light shade leaves/plants versus sun leaves/plants,
some interesting comparisons appear. (Sun and shade leaves can occur on the
same plant- we commonly refer to sun as a high light plant and the shade as
a low light plant- sometimes a plants starts off a sun plants and slowly
over the course of the season gets shaded and becomes a shade plant etc)

Some terms can be looked up if folks are unfamiliar with these
units/measures etc. It should be noted that this is for general plant
physiology, note specifically aquatic plants. (From Mahall 2002)

Characteristic:       Low light:          High light:

Irradiance at
light compensation*    low(.5-2)            high(50-100)

Irradiance at
light saturation       low(10)             high(1500-2000)

Rates of dark      
respiration              low                 high

Quantum yield           same or high       same or low

Leaf thickness         thin                 thick

Number of stomata       low                 high

Stomatal conductance    low                 high

Thylakoids/granum       many(100-200)      few (5-10)

Orientation of grana    random             horizontal

Photosynthetic e.t.
components per unit     small               high

Chl a/b                 low(2.5)           high (4)

Total Chl 
unit area               low                high

Total Chl          
unit leaf weight        high               low

Chl/photosynthetic      higher             lower

Soluble protein
(Rubisco) per unit
leaf weight or area     low                high(10X)

Rubisco activity
per area, weight or     low                high(4X)

Photosynthate leaked    low                high
into environment

Basically at lower light, plants are more efficient. Photoxidation is less a
problem, the plant has more time to relocate nutrients from old tissue to
new tissue or from root to shoot. Leaf area generally increases, less
nutrients are required to run photosynthesis, less proteins, enzymes etc to
grow. The plant afford to wasteful at higher light, but if things fall below
the light compensation point, the plants will die. PCP is where the quatum
yield is less than respiration. Intake of the "plant's food" is less than
the energy/maintenance requirements of the plant.

It makes sense not to have all this extra photosynthetic machinery hanging
around if there's not the light to run it. It takes a lot more energy to
maintain all that machinery. If the plant has only a small efficient
photosynthetic machine, it won't need to allocate the respiration /
photosynthate to maintenance. Growth is slower but less wasteful.

Considering electrical cost(some of us are sensitive to this issue),
efficient use of lighting, since it's often the biggest source of electrical
usage, is not a bad goal to consider.

Better nicer growth, less algae problems, lazy dosing, less pruning etc may
be something many are looking for. Lower electrical cost certainly are for

Tom Barr