M Deiveekasundaram’s research while affiliated with Annamalai University and other places

The phenomenon of flocculation in Azospirillum brasilense (MTCC-125) was studied under different combinations of carbon and nitrogen sources. Fructose and Potassium nitrate at a pH of 6.4 in the cultural medium favour a higher bio-floc production. The biofloc was studied for root adhesion and its survival efficiency in the rhizoplane and rhizosphere of certain crops such as sorghum and sunflower under dryland condition. It has been demonstrated that the flocculated cultures of Azospirillum were found to have maximum adhesion to the root surface and higher survival rate in the rhizoplane and rhizosphere under different moisture stressed conditions as compared to the log phase cells of Azospirillum.

The effect of plant growth promoting rhizobacteria such as Azotobacter, Bacillus and Pseudomonas was tested separately or in combination in Catharanthus roseus for two consecutive years (2005 and 2006). The combinations of above mentioned PGPR strains significantly increased plant height, root length, root girth and alkaloid content in C. roseus in comparison to the control. In addition, all nutrient contents (N, P, K, Ca and Mg) were also significantly increased as compared to the control. The maximum N, P, K content was obtained from the combination of PGPR treatment. The results of this study suggest that PGPR applied in combination have the potential to increase the plant growth, alkaloid content and nutrient content of C. roseus.

Catharanthus roseus (L.). G. Don. is become one of the important medical plants. This plant contains more than 100 alkaloids distributed in all parts of the plant, but in different proportions. The following aspects of Catharanthus are covered in this paper: diversity of alkaloids, phenolic compounds in Catharanthus roseus, role of different phytohormones in alkaloid improvement, diversity of microorganisms occurrence in C. roseus root exudates of C. roseus, role of PGPB interaction with root exudates, role of PGPB and AM fungi in improvement of alkaloid is discussed. Special attention has been paid in this to the effect of PGPB on plant biomass and alkaloid yield.

A preliminary study was made on four medicinal plants viz., Ocimum sanctum L., Coleus forskholii Briq, Catharanthus roseus (L.) G. Don. and Aloe vera in order to identify and enumerate the rhizosphere, non-rhizosphere and diazotrophic microorganisms in soil. The diazotrophic bacterial population studied includes Azospirillum, Azotobacter and Pseudomonas. The rhizosphere bacterial populations were 23.33 x 10(6)g(-1) in O. sanctum followed by C. roseus (20.46 x 10(6)g(-1)), A. vera (18.44 x 10(6)g(-1)) and C. forskholii (16.64 x 10(6)g(-1)). The fungi populations were 19.44 x 10(4)g(-1) in C. roseus, 18.66 x 10(4)g(-1) in O. sanctum, 16.44 x 10(4)g(-1) in A. vera and 14.22 x 10(4)g(-1) in C. forskholii. The actinomycetes population was 12.22 x 10(5)g(-1) in O. sanctum, 10.44 x 10(5)g(-1) in C. roseus, 8.44 x 10(5)g(-1) in A. vera and 6.22 x 10(5)g(-1) in C. forskholii. The diazotrophic bacterial population of Azospirillum, Azotobacter and Pseudomonas is 8.2 x 10(4)g(-1), 12 x 10(4)g(-1), 6 x 10(4)g(-1) in the rhizosphere soil. In all the four medicinal plants the microbial population is more in the rhizosphere soil, when compared to non-rhizosphere soil. These results are helpful in developing a biofertilizer consortium for these commercially grown medicinal plants.

A field study was conducted to evaluate the effectiveness of arbuscular mycorrhizal fungi (AMF) and phosphorus levels (100, 150 and 200 kg) for increasing biomass yield and ajmalicine content in a medicinal plant (Catharanthus roseus). The plants treated with 150 and 200 kg P2O5/ha along with AMF had the maximum plant height, number of leaves, root biomass, phosphorus content, root colonization, spore count and ajmalicine content 120 days after planting when compared with the control plants. The results suggested that these treatments could be recommended for enhancing biomass and alkaloid content in C. roseus.

An experiment was conducted on Catharanthus roseus to study the effect of seed treatments with native diazotrophs on its seedling growth and antioxidant enzyme activities. The treatments had significant influence on various seedling parameters. There is no significant influence on dry matter production with the diazotrophs, Azospirillum and Azotobacter. However, the vital seedling parameters such as germination percentage and vigour index were improved. Azotobacter treatment influenced maximum of 50% germination, whereas Azospirillum and Azotobacter were on par with C. roseus with respect to their vigour index. There was significant difference in the population of total diazotrophs. Azospirillum and Azotobacter between rhizosphere and non-rhizosphere soils of C. roseus had the same trend and were observed at various locations of the study. The activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) were increased to a significant extent due to the treatment with diazotrophs.