Heat and Dessication Tolerance of Mungbean Rhizobia

Abstract

Effective symbiosis between legumes and rhizobia increases the yields of leguminous plants. However, various factors affect the viability of rhizobia. Two of the most serious environmental threats to rhizobia are soil desiccation and high soil temperature. High temperature inhibits nitrogen development, plant growth and nodulation capacity while desiccation reduces nitrogen fixation, growth and yields. This paper aims to determine the effect of short-term heat exposure and brief periods of desiccation on the growth of mungbean rhizobia, specifically to screen heat- and desiccation- tolerant Rhizobium as well as to test the nodulation capacity of tolerant strains through inoculation of the rhizobia to the plants using Growth Pouch Method. Two sets of rhizobial strains from BIOTECH collection were used. Strains BVr9, BVr43, BVu7, BVu39, CB756, M6 and S16 were used for the screening for heat tolerance and were exposed to 40°C for 1, 2 and 4 hours. On the screening for desiccation tolerance, strains BIOTECH 1444, BVr9, BVr33, BVr53, CB1015, M6, M7, TAL209 and TAL420 were used and were subjected to desiccation in soil for 1 and 3 days. The populations of rhizobia were determined after the application of heat and desiccation. And then nodulating ability was evaluated by inoculation to 5-day old mungbean (Vigna radiata L.) plants in the growth room. Nodulation was observed after 4 weeks. Rhizobial strain BVr9, a slow grower, and BVu39, a fast grower, were the most heat tolerant strains since they retained their ability to grow and infect its host plant. Strain S16 was able to grow under heat stress but lost its ability to infect the host legume. Strains CB756 tolerated heat stress but with decreased nodulating ability while BVu7 underwent a period of adaptation. Strains M6 and BVr43 maintained their population and nodulating capacity even under heat stress. Strain TAL209 was the most tolerant against desiccation because it retained its ability to grow and infect after 3 days of water stress unlike M6 and M7 that tolerated only 1 day of desiccation. Strains BVr33 and BVrS3 grew but lost their infectiveness. Strains CB1015, TAL420, BVr9 and BIOTECH 1444 failed to survive.