Hi all,
couldn't you make a "indication list" for duckweed e.r. leaves getting yellow - add KNO3 and MgSO4 (I'm not sure if that would be right, but you get the idea).
There is work actually on Duckweed
(Lemna minor) (because of its use in
<"ecotoxicology">), have a look at <"
Biological test methods.....">, but I don't know of any specifically for
Limnobium laevigatum etc.
If we make the assumption that the plant is receiving sufficient PAR, it would be possible to produce deficiency symptoms in culture. The problem is that it is quite time consuming, and you need analytical grade salts (to avoid impurities) to make up the solutions, and you would need some more unusual salts to create solutions lacking chlorine (Cl), sulphur (S) etc. You also need a minimum of six replicates of each culture, for statistical purposes, an initial single cloned plant (to remove genetic effects) and standard growing conditions.
The other problem is that there are 14 separate elements essential for plant growth, but in very differing amounts. If you ignore the three gases - carbon(C), hydrogen(H) and oxygen(O) they are:
Macro
N, K,
P
"Secondary macro",
Mg, Ca, S
Micro,
B, Cl, Co, Cu,
Fe, Mn, Mo, Zn
Because of this, I used a mixture of likelihood, and
<"whether an element is mobile within the plant">, to diagnose deficiencies. I should start with the disclaimer that all the tanks I have are low in all nutrients (deduced from the low conductivity ~ 100 microS) and have soft water. I initially stopped using
Lemna as my "Duckweed" because it never looked happy in the tanks, even when other plants were still in active growth.
Vallisneria is another plant that grows well in our tap water, (hard but low in nutrients), but doesn't survive in the tanks.
By using a floating plant, with access to aerial CO2 levels, that took both light (PAR) and CO2 out of the equation.
This leaves the three macro-elements as the most likely sources of deficiency (because plants need most of them), and plants actually need a lot more N and K than they do P. Nitrogen, P and K are all mobile within the plant, meaning deficiency symptoms will effect older leaves.
We can differentiate between acute P and N & K deficiencies by symptom. Phosphorus deficiency causes older leaves to show purple overtones, and new leaves to be very small. Nitrogen and potassium deficiencies cause new leaves to be small and older leaves to
yellow.
I've never had obvious acute phosphorus deficiency, although this may relate to specifically to
Limnobium, and if you look at the photos in <
"Low maintenance..."> that may be P deficiency around the edges of the older leaves on the larger plant.
Yellow older leaves, and a lack of vigour I get a lot, and usually these can be rapidly greened by the addition of KNO3. As this supplies both nitrogen and potassium I don't know which is the limiting element, and both are mobile within the plant. If your plants show darker leaf veins (interveinal chlorisis)
, that would suggest potassium. Otherwise nitrogen is most likely.
The other possibility for interveinal chlorosis on older leaves is magnesium (Mg), plants need much less of this than they do N or K, but high levels of Ca (calcium), and to some degree K, effect uptake.
If you have hard water magnesium is a much more likely deficiency.
The final element that I've seen deficiency symptoms of is iron (Fe), this is non-mobile so effects newer leaves. I have one tank where the
Limnobium never looks very happy, was always being eaten by the snails and it didn't perk up after the addition of KNO3 or MgSO4.7H2O. I eventually resorted to occasionally replacing the ailing Frogbit with healthier plants from the adjoining tank. These would then start the same slow decline. Looked at in the cold light of day iron deficiency was always a likely cause, but I was resistant to this as an idea. The only real difference from the other tanks is that this tank has a very thin layer of silica sand as a substrate and it is always mulm free. As soon as I added some FeEDTA, the plants greened.
Again
iron deficiencies are much more likely in hard water.
cheers Darrel
