You should ask yourself, a basic question - why do we root shoots?

Obvious answer  - to get plantlets that can be transferred to pots or into soil.

Do you know osmotic strength of basal MS medium (with and without sucrose)?  It would be very high . I am sure you must be aware that higher the solutes in a solution more would be its osmotic strength (and accordingly both water and osmotic potential would be highly negative).

Accordingly, you should also know the osmotic strength of soil or soil solution. It generally would have much less osmotic strength (unless it is a saline soil).

Therefore, it is inappropriate to transfer plantlets whose roots are accustomed (i.e. adapted) to high osmotic strength environment (i.e. full strength MS medium) to a low osmotic strength environment (i.e. soil). If you do so, your plantlets will collapse/die. Therefore, we try to lower osmotic strength by reducing strength of MS medium to half. We often use one fourth strength.

In addition, growth depends largely on the capacity of cells to take in water. If root(s) has(have) to grow, root initial or root cells must take in water effectively and grow into the medium (irrespective of whether it is semisolid or liquid). If osmotic strength of the medium is high, then root initial or root cells may not be able to take in enough water (therefor there may be limited/restricted growth).

Fully agree with Pardha-Saradhi. But another important factor is to induce some level of nutrient stress. Plants form roots in order to acquire water and minerals. Water or nutrient deficits act as powerful stimulants for rhizogenesis. So 1/4 MS should induce more/faster rooting than 1/2 MS or full MS. 

The answer is simple, they root almost as well in half strength MS as in 1xMS as long as you keep 20 g sucrose per litre. We did the comparison in 1987...and we decided to use half strength MS for everything (not just rooting) because it is cheaper and works well, yes just because it is cheaper, that's the reason. :)

With respect to putting plants from in vitro culture to soil, the main issue is to get them adapted from 100% humidity in a jar to less than that in the greenhouse. So when you wash away the agar freom the roots and plant them in soil, cut away most of the leaves, keep only the youngest, spray the leaves with water, and put them in a plastic bag for 3 days, then cut a corner and after another 2 days take the bag off. Give the roots time to explore the soil before you expose the leaves to normal atmosphere with 60-70% humidity.

The results of comparison between 1x and 1/2x MS for rooting can be different and depend on the species tested. For example, the research from tea (see attached paper), they did show different results, in terms of both the root numbers and the root lengths. The 1/2x strength is better than 1x MS, with statistically significant.

I certainly agree that humidity is the most important factor for acclimatization and survival of in vitro raised plantlets in soil. I suppose most of us are able to overcome this without much problem. I also do not deny the basic fact that different plant species behave differently (and that is the basic reason why tissue culture papers are still being published in relatively good journals). However, I have following comments to make:

if we use full strength MS medium for rooting, besides osmotic adjustment there are two more major problems that one might experience (i) traces of sucrose along with other nutrients, remaining in association, often attract microbes (in particular fungi & bacteria) which often have immense negative impact (I am putting forth practical problems - of course we can theoretically overcome this issue if washed properly and soil mixture is 100% sterile). I do not disagree the cost factor, but researchers bother more about success rather than cost (unless & until one is working for a commercial organization and if that is the case then we have several other ways to take care of cost - for instance replace sucrose with normal sugar we use); (ii) sucrose might also hamper with the development of autotrophic nature.

Yes, we use sugar from the supermarket...and we don't buy MS medium but make it ourself to have higher reproducibility, with the following recipe:

MS stock solutions (dilute as appropriate, add 500 mg MES and 20g sucrose per litre and adjust pH with 5M KOH to pH 5.6-5.8), either with or without the cheapest technical agar (5 grams in 800 mL). To make half strength, just consider the stocks 20x, 200x and 2000x, and just 250 mg MES per litre, but keep sucrose at 2%.

Making it yourself is cheaper and better, and yes, cost factor is also important for basic science lab, even in the UK :)

Macro elements (10X)            1 litre               4 litres

            NH4NO3                      16.5 g              66 g

            KNO3                          19 g                 76 g

            CaCl2, 2H2O               4.4 g                17.6 g

            MgSO4, 7H2O             3.7 g                14.8 g

            KH2 PO4                     1.7 g                6.8 g

Oligo elements  (1000X)

                                                1 litre               0.5 litre           

            ZnSO4, 4H2O              8.6 g                4.3 g

            H3BO3                         6.2 g                3.1 g

            MnSO4, 4H2O             22.3 g              11.15 g

            CuSO4, 5H2O              0.025 g            0.0125 g

            KI                                0.83 g              0.415

            CoCl2, 6H2O               0.025 g            0.0125

            Na2MoO4, 2H2O         0.25 g              0.125

Fe EDTA 1 l (100X)

                                                1 litre               0.5 litre

            FeSO4, 7H2O              2.785 g            1.3925

            Na2 EDTA                  3.36 g              1.68

I find  more logic in P. Pardha-Saradhi's answer. Thanks Dr. Saradhi.Though roots can be induced in full strength MS subsequent acclimatization is not successful plantlets rooted on half MS survive better. 

There is no set/fix rule for rooting and Full strength MS or ½ MS or ¼ MS which is best? It is changes/vary from plant growth stage to stage and species to species. In some cases, Full strength MS showing very good results in rooting compare to reduced strength. Mostly researchers prefer ½ MS because of it is provided low osmotic potential to easy adaptation of plants during acclimatization and economic point of view too. In ½ or ¼ or even 1/8 strength is also create partial nutrient stress, so plants get produce more roots, easy acclimatized and promote plants to become autotropic.

Before transferring plants from in vitro to ex vitro for acclimatization, if 2-3 leaves were keep in plant, and provide 100% humidity for initial 2 weeks and gradually reduced the humidity up to 70-75% . The percent survival of plants was increased up to 90-100%. Even application of Hogland Solution during acclimatization, shown very good result on growth.

Trehalose sugar is also used (In vitro and Ex vitro) to make plants tolerant to abiotic stress. Its plays an important role for normal plant growth & development. The intermediate compound Trehalose-6-Phosphate (T6P) is now confirmed to act as a sensor of available sugar, and induce signalling pathway. It directly influences the type of response to the changing environmental condition. Under in vitro conditions, trehalose has been shown to protect cell & organells from denaturation at high concentration. External addition of trehalose can also clearly be beneficial especially, when plants are facing salt stress. Exogenous addition of low level of trehalose (1-10 µM) to rice plants preserves their root integrity & protects root cells from severe salt stress induced aberrant cell division. Upon rehydration, trehalose is completely or partially hydrolyzed, metabolism resumes and growth is rehydrated. In most plants, trehalose levels are far too low in order to have a function as an osmoprotactent.

Well thats a lot good discussion, but some have mentioned about using 1/4 or 1.8 strength MS medium for rooting but again mentioned 1/2 MS as very much economical for rooting, didnt get this ambiguity. What if the cultures are kept for rooting in MS basal, that may induce maximum stress for rooting.

 @ Gaurab,

Recently I answered another similar question on RG. I found some more documents about this topic. So, post them here.

1. I attach a paper for you. In Discussion section, they listed at least 8 research results which showed the beneficial effect of a reduced culture-medium strength on the in vitro initiation of roots from different plant species (see yellow highlights).

2. In the paper, they also pointed out that "The effect of the medium strength could be possibly associated with particular components of the culture medium". They gave an example: "Castiglione et al. observed a radical reduction in the number and length of roots induced from white poplar (Populus alba) cultures when zinc concentration in the culture medium was increased." 

Yeah, its just because the osmotic potential (less in half), or just water potential (more in half)..

Would a half strenth MS be better for shoot proliferation?

Dear Gaurab Gogoi

Yes, half-strength MS medium/ reduced nutrients promote rhizogenesis. Although basal medium optimization is a key factor in the success of tissue culture, the reduced minerals, particularly macro salts in nutrient medium stimulate root induction and elongation, allowing the plant to stabilize. The concept behind is just mimicking the natural process of plant development to adapt environmental constraints via nutrient availability.

During nutrient stress, plants exhibit a reduced development of the aerial portion and enhanced local branching and development of the root system to maximize uptake of mineral nutrients. Physiologically, the concentrations of nutrients often impact root development by triggering local and systemic signaling pathways. It is significant to note that it should not affect growth and development quantitatively, like oxidation, necrosis, leaf senescence, and abscission.