I can't believe I allowed myself to fall for Ric's tricks.
Below my post RIC......obviously not by better judgement I said--Nickel, Selenium,etc. as a secondary answer instead of iodine.
The answer is NI--Nickel.
The 5 cent answer!!!
Funny to say the least because when you are looking for an answer it is right before you!
Nickel is a key component of selected enzymes involved in N metabolism and biological N fixation. Plants suffering from Ni deficiency show necrosis initiating from the tip of the leaf.