Abstract
Microbial inoculants are becoming more available as sustainable alternatives to fertilizers and other agrichemicals in broad-acre cropping. However, with the exception of legume inoculants little is understood about effective delivery and survival of the inoculum. Legume inoculants are applied to both seed and soil but seed inoculation is the most economical technique. Large quantities of pasture seed in Australia are inoculated by commercial seed coating companies, but the long-term survival of seed-applied inoculum is variable and monitoring of viability requires specialist microbiology skills and facilities. The aim of our research was to define optimum storage conditions for survival of rhizobia on legume seed and evaluate water activity as a means of monitoring shelf-life. The relationship between survival and water activity varied according to seed species, inoculum preparation, coating ingredients, initial water activity and time suggesting that storage conditions would need to be defined for each different combination. Although drying seeds after coating significantly reduced viable numbers of rhizobia, survival of rhizobia on dried commercially coated lucerne seed after 11 weeks was less variable than seeds that had not been dried. The highest numbers were maintained when seeds remained dry with water activities of between 0.47 and 0.38. The quality of inoculated seed could be improved by reducing the death rate of inoculum during preparation and providing optimum storage conditions for long-term survival.
Highlights
Interest in application of microorganisms to agricultural crops and pastures as a means of supplying a sustainable source of nutrients, increasing nutrient-use efficiency or controlling disease has increased in recent years
When polyvinyl alcohol GL05 was used to apply peat cultures of Rhizobium leguminosarum bv. trifolii TA1 to seeds of subterranean clover (Trifolium subteraneum), survival of TA1 was not affected by water activity of the seeds after 24 h and 1 week storage at different relative humidities (Figure 2A)
Relationships between survival of rhizobia on legume seed and water activity of coated seeds were established after storage of inoculated seeds at different relative humidities and constant temperature
Summary
Interest in application of microorganisms to agricultural crops and pastures as a means of supplying a sustainable source of nutrients, increasing nutrient-use efficiency or controlling disease has increased in recent years. The aim of legume inoculation is to provide high numbers of viable effective rhizobia to the rhizosphere to allow rapid colonization, nodulation and nitrogen fixation by the selected inoculant strain in order to maximize legume yield potential [2,3]. The need to inoculate is dependent on the number of effective rhizobia present in soil where legumes are to be sown. Rhizobial numbers in soil may be adequate if the host legume has been sown previously but can decline under both permanent and transient abiotic edaphic stress [5]. Where stress-related suppression of rhizobial numbers has not occurred, intraspecific competition can be a problem; where less effective populations are present in soil or have emerged through horizontal transfer of nodulation genes [6,7]
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