THE presence of plant growth regulators in rhizosphere soil is of much ecological importance and the use of synthetic plant growth regulators for the betterment of agricultural industry has increased substantially during the recent years.

Plant growth regulators may be considered as a new generation of agro-chemicals after fertilizers, pesticides and herbicides. These are organic compounds other than the nutrients, which affect the physiological processes of growth and development in plants when applied in extremely low concentrations. There are five major groups of compounds:these are auxins, gibberellins, cytokinins, ethylene and abscisic acid which are synthesized endogenously in higher plants and are also termed as plant hormones. It is now well accepted that normal plant growth and development throughout ontogeny is controlled by these compounds produced by the plant itself.

Some significant research endeavours have shown that microbial production of auxins can be increased by several-folds upon addition of its suitable precursor to the culture media or soil. The precursor may provide a continuous source of biologically active substances like auxins due to the activities of rhizosphere microbiota which could affect the plant growth because of the intimate contact between the rhizosphere microbiota and the plant roots. This is better than a one time application of biologically active substance. Tryptophan is considered an efficient physiological precursor of auxins in higher plants as well as for microbial biosynthesis of auxins.

Inoculation with specific micro-organisms could affect plant growth through production of plant growth regulators in the rhizosphere, particularly when these metabolites are released as a result of precursor-inoculum interactions. Diazotrophs such as azotobacter and azospirillum have been and are being used frequently as inoculants in many regions of the world since the first decade of last century. These micro-organisms have been reported to enhance plant growth and yield, even without the addition of precursors of various plant growth regulators. It is very likely that the use of these inoculants, along with suitable precursors of plant growth regulators, can further improve plant growth and development.

Keeping in view the above background, a series of lab, pot and field trials were conducted to utilize the tryptophan dependent microbially derived auxins for the betterment of maize crop with the major objectives of isolation and identification of the diazotrophs, determination of the in-vitro biosynthesis of auxins by these diazotrophs, determination of the tryptophan-dependent auxin biosynthesis in soil, improving the growth and yield of maize through tryptophan-derived auxins produced by diazotrophs and evaluation of the efficacy of this approach under fertility and salinity stressed conditions.

By using specific media, a number of azotobacter and azospirillum strains were isolated from the maize rhizosphere and were screened for their potential to produce auxins. The tryptophan-dependent auxin biosynthesis in soil was monitored. Growth-promoting activities of the diazotrophs were investigated by conducting trials under gnotobiotic conditions. Strains and tryptophan level showing promising results were selected and used in subsequent experimentation to test the effectiveness of the precursor inoculum interaction for improving the growth and yield of maize. The salient features of the results recorded are as under:

1. Ten strains of each bacterium, azotobacter and azospirillum were isolated from the maize rhizosphere and their auxin production was measured by the colorimetry. The efficient auxin producers strains of azotobacter and azosirillum were selected for further experimentation.

2.Inoculation of seeds with the most efficient auxin producing Azotobacter and Azospirillum cultures resulted in the highest seed germination percentages in plate experiments conducted in the laboratory.

3. Three maize varieties were screened for their responsiveness to inoculation and tryptophan either applied alone or in combination by conducting the plate experiments.

4. The same experiments were repeated either with azotobacter or azospirillum strain using the more responsive variety Golden. The most promising results in terms of root elongation and weight of maize were obtained with the combined application of azotobacter/azospirillum.

5. Further, plate experiments were conducted to study the response of maize to azotobacter or azospirillum inoculation in the presence or absence of tryptophan under low and high nutritional and salinity levels. In most of the cases, tryptophan in combination with Azotobacter/ Azospirillum gave significantly positive effects on root elongation and weight under normal and stressed conditions.

6. Leonard jar experiments were conducted to study the effect of afzotobacter/azospirillum and/or tryptophan on the growth of maize shoots under low and high nutritional and salinity levels. Although response to the treatments varied with a change in nutritional level or salinity level. However in most of these trials, the response was more obvious to the combined application of inocula plus tryptophan compared to uninoculated/untreated control and inoculation alone.

7. Under normal nutritional conditions, combined application of the azotobacter with tryptophan significantly improved the maize growth, by increasing its fresh and dry shoot and root weights. Under the low fertility conditions, plant height, fresh shoot weight, fresh and dry root weights were increased significantly in response to azotobacter+tryptophan. However under the nutrient-rich environment, this treatment had positive effect on growth parameters but significant response was only observed in case of dry root weight.

8. The effectiveness of this approach was also tested in pot experiment. The selected azotobacter strain and various levels of tryptophan were applied to maize grown under the normal, salinity-stressed and the fertility-stressed conditions. The results revealed that the azotobacter and tryptophan when applied alone or in combination had significant effects on the maize growth under normal and salinity-stressed soil conditions.

9. The results of field trials revealed that the combined application of azotobacter and tryptophan had more pronounced effects on maize growth and yield, however, application of tryptophan alone gave significantly higher results than that of the control. It may imply that soil indigenous microflora was also active in synthesising the auxins from the tryptophan. Conclusions and future recommendations: These were the first comprehensive studies ever conducted to improve the growth of maize crop through substrate-dependent microbially derived auxins under the normal, salinity and the fertility-stressed conditions. The results obtained are extremely encouraging with regard to the physiological response of maize to auxin precursor tryptophan and/or to the azotobacter/azospirillum inoculation.

Since the plant growth regulators/hormones are biologically active at extremely low concentrations and sensitivity of crops to plant growth regulators/hormones may vary substantially, so there is a need to screen crops/varieties against various levels of hormones/precursors. The success of this approach may provide a breakthrough for improving the production of various crops under the fertility and the salinity-stressed conditions. In brief, the hypothesized approach has great potential of improving the growth and yield of test crops, but intensive future work is needed to develop this technology for its commercial use by the farmers.