a) Is there any range of salinities that stress mangroves to start using their phisiological strategy.
Since R. mangle and A. germinans utilize different mechanisms, i will try to answer by plant.
R. mangle filters salt at the root-water interface and filters over 99% of the salt. i get the impression that it is a more passive, "always on" mechanism because the filtration mechanism is not affected by temperature and poisons as you would expect other metabolic functions to be affected (Scholander 1968)
A. germinans also uses ultrafiltration to some degree, although experimental results vary widely, from 95% to 30% effectiveness (tomlinson 1986) This too, is presumably always in effect. A. germinans also has salt glands for secretion, unlike the red mangrove. At what concentration these kick in, I dont know. It may always be on, it may kick in above a threshold, or perhaps it is a graded response. I am getting a paper soon that hopefully offers some clarity on this topic. The big picture is that both species can readily handle salt concentrations at least 2.5 times higher than those of seawater.
That doesnt answer your question in terms of specific numbers, but I still havent found many.
b) Do mangroves really need salt or not ? Why ?
Mangroves are considered facultative halophytes, ie they do not need salt to grow but tolerate it. There is some evidence that a complete absence of salt is detrimental (Field, 1984). A good way of characterizing it is that freshwater is a physiological requirement, but saltwater is an ecological requirement, because it eliminates certain competitors (tomlinson 1986)
c) How much salt A. germinans can excrete (per day, per month)?
I have no idea, nor have I found out any experiments that focus on this, but I am still looking. I imagine this might be a difficult measurement to obtain.
R. mangle does not have salt secretory structures although salt can build up on the leaf surface in small quantities through normal transpiration.
d) Why these phisiological mechanisms(excluding salt & salt excreter) make the mangroves more successful than other plant in that kind of environment ?
Maybe as simple as because an excess of sodium ions is toxic to most plants, and the ones that have evolved methods of handling salt can survive there. There are other plants that can tolerate higher salinities, but these are often much smaller, and therefore could be shaded out by the mangroves (Tomlinson 1986) Other proven strategies include succulence and sequestering salt in organs (leaves) that are senesced.
e) Do you think the plants ability to use the energy (ATP available)for reproduction & growth is affected if this energy is used mostly for the salt exclusion or salt excretion ? Why ? Explain.
I'm sure it does, at least in some ways. There is relatively no information on the cellular/molecular workings of R. mangle's ultrafiltration mechanism, so this is a tough call. As previously noted, this plant's mechanism seems to not be so much a metabolic as a physical barrier. A. germinans salt glands are another story. These salt glands probably work full time, because salt that builds up as an indirect result of transpiration throughout the day keeps getting secreted even after the leaf is put on distilled water. Secretory cells have more mitochondria, lack chloroplasts, and secrete in the dark (not supporting a link to photosynthesis). Furthermore, presence of ATPase inhibitors decrease secretion. (Dschida et. al., 1992).
How all these factors affect competition between the magroves is a good question. (and one that i wont speculate on right now)
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