The question raises very basic issues, which seems to not understood properly yet. The scion-stock relations are well establshed, which we find in many research works as well as reviews. However, the underlying physiology is not clear. In the case of some dwarfing rootstocks, root stock restricts the nutrient flow the to scion, probably due to the anatomical differences. However, there are other mechanisms also reported. We can be easily mislead by apparent mechanisms, but deep underlying physiology is yet largely unknown in my opinion. 

The rootstock sets foot in the ground to support and nourish the Scion. So, logically it must be resistant / tolerant  to soil constraints such as lime, fungal diseases, nematodes, soil insects as well as abiotic stresses due to various toxic chemicals in the soil and water logging. Then comes the issues of the interaction with the scion ranging from giving it the latter the architectural growth looked for to accommodate special cropping practices or  to produce a fruits with determined qualities sought by the market.

Dr Kalidasu, you have nailed the central theme of my question . My main intention of raising such a issue was to explore the hypotheses available with researchers across to  get apprised with latest updates. Otherwise , use of rootstock in offering resistance against diseases , especially against soil borne diseases and  various abiotic stresses , rootstock utility is unquestionable. However , it is so interesting that how use of rootstock  aids in elevating the production or various fruit quality parameters. How , under such conditions , the various physiological and biochemical mechanisms operate across rootstock to scion . And what about scion to rootstock , very little is known about this very important aspect of such mutualistic combination.

I totally agree with you sir. There is an urgent need to study the underlying physiological mechanisms instead of complacent with the apparent benefits. As this will lead to numerous avenues to manage the scion-stock relations in a novel way, which we cannot even imagine now.

usually root stocks are used with objective of resistance to soil borne diseases caused by fungi, bacteria and also infestation of nematodes. also there are other required characteristics of the root stock in terms of root development, density, spreading anchoring abilities. also other physiological and anatomical features which are preferred for water and nutrient absorption, 

There are many good reasons why rootstocks evolved into the standard in propagation of perennial tree crops.  It is safe to say that many of the world's tree fruit industries would not exist in the modern world without suitable combinations of scions and rootstocks.  However, it may not be broadly recognized that the reasons for using rootstocks  and their impact  vary considerably among crops.  For example, with citrus, rootstocks are essential because they have large effects on many traits like tree size and yield.  Apple rootstocks affect primarily tree size, pest and disease tolerance and soil adaptation.  The real answer for using rootstocks?  Profit!

Effectiveness of rootstock in raising the productivity through control on canopy size , nutrient absorption , and offering resistance to many of the pests and diseases  in  perennial crops is beyond doubt . Rootstocks also played an important role in cutting the long juvenility of seedling origin plantation .These are simply unquestionable  impacts as far as role of rootstocks is concerned. I flagged off the underlying reasons to define these responses through the sequence of events , physiological or biochemical in nature so that  the rootstock options can be more befittingly utilised under varied conditions. Another very basic issue which I raised was that in a given rootstock-scion combination , what exactly happens from rootstock to scion or vice -versa in response to various biotic and abiotic stresses. What kind of mechanistic pathways are involved. How compatibility between  rootstock and scion is defined physiologically or biochemically. Why am I raising this issue , since no genetic transformation takes place in a given rootstock-scion combination .But still , possibly through  molecular signals rootstocks effects are expressed so distinctively . Can we decode those signals, so mutualistically getting exchanged between rootstock and scion and vice-versa in response to any external stimuli(s)  . I invite Drs Giridhar, Boulif, Awadalla, William and others  to take stock of my response, and respond. 

Most of the responses have covered the subject. What is left now is research into the grey areas of the physiological bases of the stock-scion relationships to further extend the frontiers of knowledge in the subject.

Lets have the responses from other researchers working on variety of fruit crops. We really have very little understanding on the underlying principles of rootstock-scion compatibility across different perennial crops .

Perhaps the participants in this conversation realize that when applied and basic types of research are being considered or debated to address a problem, the question often is resolved by deciding to begin with the applied approach in order to first determine if any effects [in this case, rootstock] actually exist.  Once that research is conducted, it is followed by the basic research or at best, the applied and basic work proceeds simultaneously.  Unfortunately, it follows that at least three things are true: [1] new rootstocks have become available with increasing speed and they need to be evaluated; [2]  when resources are limited the decision is to spend them on applied research; and [3]  little success has been achieved by those studying rootstock metabolism and physiology of any perennial tree crop.

Yes, the proposed work is needed.  Interstock research clearly shows that the rootstock is the component that influences the scion.  Some research suggests that the mediator is water physiology.  An elegant study was conducted by Frank Gardner who showed that a young fruit taken from a tree on sour orange and grafted to a tree on rough lemon developed the characteristics of fruit raised on rough lemon.  For researchers interested in vegetative compatibility, look at the older research published by Randy Moore.  For a novel approach to studying citrus compatibility, look for a publication by K.B. Bevington.  There are many hormone reports that lead no where.  For those who wish to start on the matter of scion-rootstock relationships, note that no one has yet to plant trees on rootstocks that differ in vigor, yield and fruit quality and measure all events that growth as the plants grow and produce fruit.  For example, why and how does a tree on Flying Dragon trifoliate orange, a dwarfing rootstock, grow for 10 years and be only half the the height and canopy size as a tree on rough lemon?  One could imagine many explanation, but none are published.

Interesting William . You have added many dimensions  to such an important and equally  interesting conversation reaching climax of revelations about the rootstock-scion compatibility , but surely in terms of applied research only . Some classical examples  can be cited without any doubt , use of Dog Ridge ( initially selected in Texas ) rootstock , known for tolerance against drought and salinity and rough lemon  rootstock , known for mild tolerance against Phytophthora-induced foot and root rot diseases ( Although does not fit into high planting density because of its vigorousness nature )   in revolutionarising the Indian grape industry and citrus industry , respectively. Use of trifoliate orange in Satsuma mandarin industry of  China and Japan for that matter .  Let me cite another example on use of  sour orange as rootstock  in Brazillian  Pera sweet orange industry . This rootstock as a rootstock works so excellently as a rootstock but moment used in combination  with  sweet orange as a scion of sour orange , the magnitude of damage to the industry due to so called QUICK  DECLINE  SYNDROME is known to researchers worldover. But , still we do not have any plausible explanation , as to what happened functionally to such combination . In a rootstock-scion compatibility ,  greater flow of phenols from rootstock to scion , offers better preparedness to scion to face the consequences of salinity . Such physiological explanations are meagre , which you also admit .

What is the purpose of this conversation?  With my last contribution I tried to provide some career perspective and explain why basic research has not been pursued to understand the  physiology of citrus scion-rootstock relationships.  In the last reply to my comments, classical examples are mentioned, but "classical examples" of what? Those of us, including myself,  who spent a career working in the field of citrus rootstocks, are familiar with a million examples of rootstock effects.  The citrus tristeza effect on sour orange is well known and has even been studied. actually.  The anatomy of the bunion has been characterized and some physiological studies have been published.  However, it is true that given the molecular tools available today, we do not know what happens at that level.  One of the most interesting studies of this nature was conducted by a PhD student in Australia named Angela Snowball.  She employed immunology to look at phase change: juvenile to mature to better understand the advent of flowering.  Many years ago, Israeli researchers looked at the relationship of such compounds like phenols to salinity and other rootstock traits.  Much of that work was published in reports.  The results did not seem to impact our understanding of rootstock influences.

I am not a molecular biologist, but a retired, old school horticulturist.  However, it is quite clear that today's molecular tools are incredible.  The time is right to look at scion-rootstock relationships with those tools, but only with clear objectives and understanding of the past.

The discussion has gone round and come back to my comment that more studies are needed to push the frontiers of knowledge of the physiological bases of the root stock-scion relationships.

 The purpose of such  discussion on the subject  will never be complete  with just few passing references . Those who are working in the field far so long  , they have alot to share  through  this discussion . Let them have this opportunity to share their own thoughts /experiences . And those experiences , by no means will be last word(s) . Let us  accept this fact , while dealing with rootstock effects , by far one of the  the most researched issues . It is difficult to describe rootstock effects  in few lines/paras /pages . On the other hand ,our all the learned  participants must   also acknowledge  various other glaring issues that  remain to be rigorously persued  through latest molecular tools throwing some more meaningful additions over existing  state of knowledge. I think , we are all unanimously agreeing on this issue . Lets see , what our other participants ( Who have not so far participated )  have in stock to share.

We often debate, why should we use rootstock when rootstock properties are not always translated to scion or well rootstocks do not bring any genetic transformation in scion properties ?

Root-stocks are essential to overcome some problems in the soil such as salinity, water logging or diseases where the variety of the scion can not produce or success. for example, some mango varieties are grafted on some others to overcome salinity problem of the soil. in some other cases, root stock promotes better growth and production and better fruit quality of scion

Root stocks are the perfect solution to soil problems such as diseases, salinity..ect. Not only in fruit trees but recently also in vegetable crops specifically in cucurbitceae family. Moreover, it can also improve scion growth compared to the same variety. it is the best solution that saves us money and efforts on the long run for many soil associated problems.

Can you please elaborate more on the anatomical , physiological and genetic basis of rootstock - scion comparability, since its often claimed about better performance of scion with the use of superior quality of rootstock...

Senior researcher have added enough arguments for understanding the rootstock importance and also identify the research gaps. here i would like to add that there is need to focus on the rootstock fine root system because different rootstocks are recommeded for drought and water log conditions that might be differences in fine root system to resist unfavorable conditions.

You very rightly pointed. One of the glaring gaps that even today exist is the lack of on- field studies using hot spots of the problems , viz., salinity , drought , waterlogging, acidity etc.Most of such studies have been undertaken using potted studies and when such studies are replicated into matching fields, they conspicuously fail to deliver...