Zinc (Zn) is a micro nutrient as well as an immobile element. Your plant doesn't need much of this nutrient but it does play an important role in your plants size and maturity. The procuction of leaves, stalks, and branches is also needing Zinc(Zn) as well. It also plays an important role in many enzymes in addition to the growth hormone Auxin. It also is there for working with the chlorophyll and helps your plant handle long droughts helping your plants absorb moisture.
Deficiencies can be seen by the bleach spots between the veins which appear in the beginning on the older leaves and then goes on to the immmature leaves. Zinc(Zn) not being a mobile element will show its deficiency on the newer growth.
The PH levels of your plants to keep this element from being locked out is in soil at more than 4.7 and less than 9.5. In hydro and soiless it is 4.25 or less and 8.5 and above to keep this element locked out.
This can be treated by adding organic fertilizer containing Zinc(Zn) or by burying galvanized nails in the soil.
In San Luis Obispo at the Healthy Harvest store they recommend getting one of their fine micro-nutrient products which have small amounts of all the micros your plant could need including Sulfur(S). In particular they like the CNS17 from Botanicare. It works out real nice that as long as your plants aren't flowering it works as an excellent foliar feed so the results will show up much quicker. You can also use this as an additive to your water which works good too.