Abstract
Moisture stress and method of inoculation greatly affected the number and distribution of infected root hairs and nodules of young seedlings of Trifolium subterraneum. A reduction of soil moisture from 5·5 to 3·5% (−0·36 to −3·6 × 105 Pa) significantly decreased the number of infection threads and completely inhibited nodulation, although the number of rhizobia in the rhizosphere was unaffected. At low soil moisture levels the root hairs were abnormally short and swollen. Infection and nodulation were little affected between 5·5 and 9·5% moisture (−0·36 to −0·089 × 105 Pa). Distribution of infected root hairs depended on the initial placement of the inoculum; with the inoculum mixed evenly through the soil, infection threads occurred at discrete foci along the root. With seedlings inoculated at planting, infection threads were restricted to the top 1–2 cm of root, even at the highest soil moisture tested. Watering increased the number of infections in plants grown at 3·5% moisture; nodules were formed at a rate equivalent to non-stressed plants. Watering also enabled movement of the seedling-borne inocula; new infections were formed along the root surface bearing mature root hairs.
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