If the goal is to increase vegetative growth some baseline in vitro growth promoting assays would be useful.  Key bacterial properties that promote growth directly is through the production of hormones such as gibberellic acid, cytokinins, particularly auxin (IAA) (Salkowski test for IAA).  The production of enzymes such as 1-aminocyclopropane- 1-carboxylate deaminase (ACCd) to reduce the level of ethylene in the root of developing plants (Penrose and Glick 2003 for ACCd assay). 

Select isolates that can do both, however, place more emphasis on higher ACCd activities than higher IAA production, because too much IAA can inhibit root growth by increasing ethylene production. Another useful property is the formation of biofilms (extracellular matrix). The formation of these structures can provide an almost infinite range of macromolecules beneficial for plant growth and development (simple crystal violet staining). In addition, it helps bacteria to adhere to the root surfaces; enhancing the efficiency of the growth promoting property.

Other properties such as P solubilizing (TCP plate assay), N fixation (nitrogen free media) can be used as screening traits as well.

Of course, these are general guild lines, screening methods must be based on your objectives and there is no grantee that in vitro results will be representative in field trials.

Anyway, good luck, hope this was useful.

Actually the goal is not 'increasing vegetative growth' it's determining "whether bacterial isolates can increase vegetative growth".

By the way thank you for your suggestions. We are considering your biochemical assays though we had Salkowski test in mind for IAA.

Eventual test of these micribial isolates is the field test where soil is inoculated with particular microbial isolate and the inoculated isolate if efficient , performs well while interacting with millions of other microbes which does not happen through in- vitro evaluation..

Please trapping rice at small pot containing autoclaved soil under in vivo condition.

Dear Anoop Kumar Srivastava, 

Thank you for your response. I completely agree with you about interacting of isolates with millions of other microbes in soil condition but following that approach without reducing the isolates number will be onerous and time consuming that's why we were looking for an alternate way to minimizing the number first. After having a considerable number of isolates we will go for field condition as much as it's needed.

Thanks Again.

The best course would be to develop as much diversity  in terms of number of isolates , must  be followed , preferably native isolates and specific crop rhizosphere , if you are looking for a specific crop , fro example rice in your crop. Then , in-vitro studies , you need to carry our depending upon nitrogen fixers, silicate solubilisers, phosphate solubilisers  etc , followed by short term nursery evaluation to shortlist what are the bunch of microbes to be tested in field ,  germination in your case,  study their complimentarity between microbes then real field evaluation , so that you come out finally with a microbial  consortium for rice...