Hi all,
What will you see on the floating plants that will indicate low light?
Leaf death occurring more quickly than leaf growth, pale colour and a dwindling of the leaf rosette, this is known as <
"Etiolation">.
At that point you've discounted CO2 (plant has access to atmospheric CO2 at 400ppm) and nutrients (via EI), so that only leaves light.
If the light intensity (really <"
PAR">) doesn't reach <"
light compensation point">, increasing the length of the photoperiod won't help, but increasing the PAR will.
If light intensity exceeds LCP, and nutrients, including carbon, are available the plant will show a pretty rapid greening, followed by growth, response.
The situation in a tropical water course would be slightly different from the situation in more temperate latitudes. In the tropics light intensity is very high, and a floating plant would need to be able to deal with very high light levels. Again this is why Amazon Frogbit is useful for the "Duckweed Index", it can grow quite happily over a large range of light intensities without showing light damage (although leaf colour may change and the red marking become more pronounced).
Here is Amazon Frogbit enjoying itself as an "invasive alien" in S. California (from <
http://plants.ifas.ufl.edu/node/740>).
Why plants scorch
Plants have evolved in differing environments, and a very dark green plant like
Anubias spp. or Bolbitis heudelotii, will have evolved in a shady, low light environment. The plant is very dark green because it has a lot of chlorophyll to harvest as many of the scarce passing photons as it can. Producing chlorophyll has a "cost" to the plant, and natural selection via "survival of the fittest" will winnow out those plants carrying genes that produce sub-optimal amounts of chlorophyll (either too much or too little) for the conditions the plant evolved in.
If you place a very dark green plant into intense light for any extended period, Clive's <"
photon torpedoes"> will damage them.
Why tropical plants are different
Tropical plants forest plants differ from those from more temperate latitudes in the way recieve the PAR they require. Rather than having continual diffuse light they will get in frequent ephemeral patches of very high intensity light, and areas that don't get any sun-light won't have any vegetation (not enough PAR).
An end result of this is that a plant like an
Anubias can adapt to higher light, when it has grown emersed with access to aerial CO2.
This is one of mine that has become emersed, the scorch marks are thermal damage from the leaves being in contact with the light unit (when it had spiral CFL lamps rather than LEDs).
cheers Darrel