Simple growth criteria like fresh , dry weights and relative water content will give a basic idea

Growth pouch assay can be one of the best way for screening of PGPB at plant level

I suggest both in vitro assays using petri dishes etc. and in vivo studies using different kinds of soils. Further a range of soils and at least three treatment levels are recommended.

This article may be of assistance:

ARE LABORATORY BIOASSAYS FOR ALLELOPATHY SUITABLE FOR PREDICTION OF FIELD RESPONSES?

INDERJIT1 and LESLIE A. WESTON2,*

Plant species selection depend on individual, if some body is looking for agriculture crops then he/she can select a important crop of the region where they want to apply the PGPR. Growth condition depends on plant one select. The microbial part is try to make microbe free environment so that the plant growth promotion could be related to the PGPR isolate only. Sterilization of seeds or plant-lets, soil or growth medium, and pots.

In my experience it is unwise to generalize from any particular condition when identifying such growth promotion. Therefore, begin with the exact condition that you hope to see the effect. Use the same genotype of plant for all experiments. Use soil with the nutrient level, texture, organic-matter composition and microbial flora that your application calls for.

Once you have show the plant growth promotion to be repeatable, then you can begin to vary conditions to see to what extent it can be generalized.

It is tempting to begin with aseptic conditions, but doing so makes it less likely that the process affected by the putative PGRP is limiting growth. In that case you would fail to identify bacteria that have the effect you seek. You would also be likely to get false positives, identifying bacteria that only establish if there is no competition.

si se tratan de bacterias promotoras del crecimiento es mejor los ensayos con fiohormonas y luego cuantificaciones en ensayos con sustratos estériles.