Re: What is the effect of zinc on plants, and why does it act as it does?

   According to the reference(1), concentration of total Zn in root of wheat 
seedling's cytoplasm can be 0.4mM (i.e. about 25mg/L). However, based on a 
widely used recipe of plant growing medium -- Murashige and Skoog salt(2), 
Zn concentration at 0.03 mM (1.95mg/L of Zn2+) will be enough for many kinds 
of plants. I am not sure what plant your are testing and what the 
environment you are applying on the plant, both would influence your 
experimental result. But generally speaking, plant in 0.1mg/l solution would 
probably show one or several Zn deficiency symptoms, like: young leaves 
growth reduction; leaf margin distorted or puckered; light sensitive and 
interveinel choloroses in leaves; retardation of stem growth(3). However I 
did not find reference about the effects of excessive Zn for plant, probably 
because in nature, the problem for plants is deficiency of Zn rather than 
excess of it. I would like to know your experimental results, especially the 
one about 100mg/L Zn.

   About the function of Zn, you are right, it is mainly about enzyme. Zn, 
which is absorbed as divalent form, is important for more than 80 enzyme's 
function.  Although I can not tell you the names of all these enzymes, most 
of the critical function of Zn in cells is related to its ability to form 
tetrahedral coordination bonds in different cell development stages. For 
example, some amino acids can form tetrahedral coordination bonds with Zn 
rather than with Fe, thus prevents Fe induced production of highly toxic 
hydroxyl radicals. Also, over-oxidation will result as toxic resource to 
cells, especially for electron-transport reactions which are critical part 
for photosynthesis and respiration; Zn might be involved in the expression 
of some enzymes which can decrease oxidative level, thus protect cell from 
oxidative damages. In addition, animal studies showed that Zn is involved in 
restriction of programmed cell death. All the above demonstrated that Zn is 
important for plant growth and development.


Reference:
(1) Cakmak I.Tansley Review No.111- Possible roles of zinc in protecting 
cells from damage by reactive oxygen species. New Pgytologist. 146(2):185-
205, 2000 May
(2) Murashige, T and Skoog, F.(1962) Physiol. Plant.15,485
(3)Frank B Salisbury and Cleon W.Ross. Plant Phyiology. (1992) Wadsworth 
Publish Co.ISBN 0-534-15162-0 Chapter 6.