Very good question , you have raised , must appreciate it . Fro Zn to be really effective to deliver its response , it has to enter through  availability , absorption into roots , translocation , and accredition at the required metabolic sites of the plant. All these processes are inter-dependent , but genetically controlled through involvement of sequence of biotechemical/physiological events known as nutriomics. Zinc uptake is mediated by two overlapping processes , one called as high velocity ; low affinity system , and another class called  as low velocity ; high affinity system  ( operates under low Zn conditions ).

Zn absorption is facilitated by the presence of Zn transporters into the root symplasm .These Zn transporters are mostly proteinous in nature , which participate in long distance transport of Zn through xylem . While , phloem mobility of Zn is low , since Zn is not mobilised from older leaves under low Zn concentration . Hence  , Zn -influx has been hypothesized as free Zn2+ ions as well as Zn complexes with non-protein amino acid  . called phytosiderophores or phytometallophores. The potential mechanisms incuding root Zn uptake and translocation , Zn sequestration in leaves and biochemical utilisation of Zn , are often claimed to be either root specific trait or shoot specific trait under genetic control at the sub-cellular level.  Under such transport , Zn is much better mediated through complexation as part of carbonic anhydrase ( Metalloenzyme) and dismutase-controlled ( Cu/Zn-SOD ) under Zn-surplus conditions .  That is even today , the Zn like most of the micronutrients absorption linked Zn-efficiency  is considered  as trait term of the plant where root configuration has a strong impetus on the utilisation of Zn at the plant organ/tissue level. Zinc efficiency of a plant is also considered as shoot mediated trait ( In addition to root mediated trait )  through subcellular Zn compartmentation for cytoplasm Zn and biochemical utilisation of cellular Zn incorporating Zn as a cofactor of Zn requiring enzymes  under low Zn , is usually operational  .

Some studies show 70% or more of Zinc is transmitted into the plant via mycorrhizal fungi. 

Since these are microscopic and only a small group of researchers concentrate on them they are much neglected.

The mechanism that Phosphate interferes with Zinc is likely related to the ability of plants to sense Phosphorus level and not encourage infection when the level in adequate.

When the mycorrhizae surface area in soil is compared to the naked root the court of interaction it represents an enormous increase of hundreds to thousands of times the noninfected naked root.

Phosphorus and Zinc are both needed to optimize yield and quality. The ability to increase the role of Phosphorus in the plant for legumes can be related to nodulation. When legumes are well nodulated it stimulates the extension of mycorrhizal fungi and the exudation of substances from the root plus mycorrhizae into the soil.

Plants that are well nodulated and well colonized with mycorrhizae can withstand drought and environmental issues better. The increased exudations in the presence of Rhizobia and mycorrhizae also stimulate the biological stimulating rhizosphere bacteria. Together they can create an environment of sustainable health.

The studies I was directing at Rodale Institute prove the concept that based on biological input for the field legume components high comparative intensive field crop yields are available without synthetic fertilizer amendment. Sometimes withdrawing the inputs is part of the system solution.

Dr Singh , your question baffles many of the researchers across globe to really crack the concrete mechanisms leading to improved Zn -use efficiency vis-a-vis mechanisms involve din Zn-uptake . Even today , higher root uptake , efficient utilization of  root absorbed Zn and enhanced translocation of absorbed Zn are considered some of the important traits of Zn-efficiency .  I am providing you a link to get more information ion the subject . link.springer.com/article/10.1007%2FBF00197584. Regards

Sir, I want to know what parent material and minerals are the best source of zinc in the soil.

Pual you have already answered about it. What is the screening mechanism of Zn efficient cutlivers/varieties in annual crops.

Dr Singh , basalt is the richest parent material with regard to Zn concentration . please go through the responses provided by Paul in my questions . He has very ably responded on this issue , while discussing about the Fe-Zn interaction . 

There was question about how to screen for Zinc efficiency. I believe this would need a two step process. The first would be a in vitro screening using growing the varieties lets say a seed plant in a complete nutrient solution but having zero, trace, and optimum concentration of Zinc and running a sand experiment where irrigation solution was dripped on the test plants. Once the varieties of in vitro efficiency were found they should be re tested under field controls with appropriate controls. The varieties which would work in these situations could be followed under deficiency environments and crossed with a susceptible donor variety to follow the nature of the genetic action of the adaption mechanism. Is it recessive, dominant single or multiple gene. With the latest molecular tools the exact gene(s) could be identified and situated on the chromosomes of interest and context in the plant genetic material. You would need quite a dose of patience and dedication and interest to do this work but it certainly is a worthy goal.

Paul, can it be possible by determining Zn content of the different varieties of same crop in Zn deficient soils and then compare as an efficient Zn absorber variety.

Dear Doctor Singh,

The ability to utilize Zinc is best determined in a soil known to be Zinc deficient. Genotype variety trial will be run and ideally you can have similar blocks where an ameleriorated dose is added. I would suggest a adjacent but separate block kept as a control.

 It would be good to identify a variety which does well in both the deficient and supplemented experimental condition.

Besides the yield performance it would good to have plant analysis of genotypes which can be effective in the low Zinc situation. By applying copper and Phosphorus a low low Zinc condition can be created by nutrient interaction.

If you could advance a soil analysis I would give my insights on what I think is constraining and some opportunities related to the management and experimentation.

I hope your efforts to further the interest of the food system flourish.

Be Happy and Do Not Worry

Paul

Nice answer. Bio-fortification of Zn is possible in many crops. But this type of products are not coming in the market due to unawareness. 

Dear Doctor Singh

It has been a blessing to be able to interact with you. You are a man of passion and wisdom. Thank you for who you are and what you are doing.

Have a good day. Paul

Thanks for nice comment.