Iron is of crucial importance for all aquarium plants. It is needed for generating the all-important chlorophyll. Without chlorophyll, the plant cannot photosynthesize. The effects of iron deficiency are immediately apparent: the young leaves appear pale and yellowish (iron-induced chlorosis).
Please note: plants can only absorb divalent "white" iron. This is unfortunately not stable in an aquarium, and is quickly oxidised to become ineffective trivalent "brown" iron. Tap water contains virtually no iron. Aquarium plants are therefore reliant upon regular iron fertilization.