Symbiosis of rhizobium and VA mycorrhiza in subabul

Abstract

A survey was conducted at 17 locations in four districts of Kerala for natural nodulation and VA mycorrhizal infection in subabul. The survey revealed that natural nodulation and VA mycorrhizal infection were poor compared to inoculated plants. When all the 17 isolates of rhizobia and four cultures of VA mycorrhizal fungus were tested for effectiveness, the rhizobial isolate R8 and V AM fungus M2 were emerged as most efficient rhizobial and mycorrhizal cultures respectively. An in vitro study conducted revealed that in an acid PH of 6, the rhizobial isolate R8 survived better than other cultures. At pH 8, growth of another isolate R5 was found maximum. However, in an in vivo study, there was not much significance for the soil pH ranging from 6 to 7.1 in influencing various biometric parameters of subabul. In both the pH of 6 and 7.1, the performance of rhizobial isolate R5 and mycorrhizal culture M2 was best. Serological studies revealed that the exotic strains R18 R20 and and isolate from Mimosa indica had serological similarities with the best selected local isolate R8. Fine structure studies of nodules clearly showed the morphological differences between the uninfected nodular tissues and the infected central nodular tissue. The rhizobial infection transformed the normal cells into irregularly shaped cells within which numerous rhizobial cells were visible. In another observation, it was found that subabul plants starts to form nodules only from 15 days of sowing. There- after, the nodule number increased steadily attaining the peak at 70 days of growth and then remained more or less steady. Among various methods of inoculation of the microsymbionts tested, inoculation of both the microsymbionts at the time of sowing in polybag was found good for the better establishment of the plants in the field. In a field study, it was found that inoculation of the local isolate of Rhizobium R8 and mycorrhizal fungus M2 had great influence in increasing all the growth parameters. Standard mycorrhizal culture and local isolate performed equally well. Maximum forage yield was obtained when plants were inoculated with the selected local rhizobial isolate R8 and mycorrhizal fungus M2. Maximum mycorrhizal infection was also seen in the same treatment. Dual inoculation also had significant influence in increasing the leaf protein content. An important observation was that both rhizobial and mycorrhizal inoculation reduced the mimosine content of leaves. However, fertilizer nitrogen increased mimosine content. In short, dual inoculation by Rhizobium and VA mycorrhiza was found necessary for better establishment, growth and low mimosine content of subabul.