Alfalfa,Medicago sativa L., in highly weathered, acid soils
SummaryRooting into acid subsoils would be a desirable trait for alfalfa which should result in better water extraction and yield. In this study, the rooting depth into acid subsoils and top yield of alfalfa plants selected for acid tolerance were investigated in a repacked profile with a limed, fertile topsoil, but unamended, acid subsoil. The effects of subsoil modification by CaCO3 and CaSO4·2H2O addition on alfalfa rooting, top growth, and water extraction were also studied.Plants from acid selections rooted deeper into acid subsoil when compared to control plants (selected under limed conditions). However, the reverse response was found in the CaSO4·2H2O treated subsoil. There were no differences among selections for total top yield for any subsoil treatment. Water extraction from the lower subsoil and top growth yield (data pooled by selections) were improved mainly in the subsoil treatment containing the highest addition of CaCO3.
Effect of Xanthobacter, isolated and characterized from rice roots, on growth of wetland rice
With an autotrophic, N-free medium, Xanthobacter populations were isolated from the roots of wetland rice grown under field conditions. Xanthobacter populations ranged from 3.2×104 to 5.1×105 colony-forming units (cfu) g-1 of root and averaged 47-fold higher on the root or rhizoplane than in the neighbouring nonrhizosphere. Characterization studies indicated dissimilarities in carbon utilization and motility among the isolated Xanthobacter strains and other recognized Xanthobacter species. Under gnotobiotic conditions, the population of one isolate, Xanthobacter sp. JW-KR1, increased from 105 to 107 cfu plant-1 1 d after inoculation when a rice plant was present, but declined to numbers below the limit of detection (<104 cfu assembly-1) after 3 d in the absence of a plant. Scanning electron microscopy revealed Xanthobacter as pleomorphic forms on the rhizoplane. To assess the effect of Xanthobacter on plant growth, rice plants were grown under greenhouse conditions in plant assemblies containing sand and half-strength Hoagland's nutrient solution with and without nitrogen. Plants were either inoculated with 105 cfu Xanthobacter g-1 of sand or left uninoculated. After 40 d, plants without nitrogen showed no significant differences in top or root dry weight, plant height, root length, or number of tillers or leaves, whether the plants were inoculated or uninoculated. However, when nitrogen was added, inoculated plants had a significantly larger top dry weight (15%) and number of leaves (19%) than uninoculated plants. Under conditions of added and no added nitrogen, acetylene reduction assays showed Xanthobacter sp. JW-KR1 produced <0.1 (below detection limit) and 7 nmol C2H4 plant-1 h-1, respectively. Under the conditions studied, the results suggest that both Xanthobacter and wetland rice derive some benefits from their association.
Nodulation ofMedicago sativa in solution culture
SummaryAcidity reduced nodule numbers at pH below 5.5, and virtually prevented nodulation at pH 4.5. In this range (pH 5.5 to 4.5) it did not affect root growth or the number of root hairs.In lightly inoculated solution cultures, acidity inhibited the extensive multiplication of Rhizobium which was necessary to establish a rhizosphere population sufficiently large to induce nodulation.In heavily inoculated solutions, containing 108 to 109 Rhizobium cells per litre, Rhizobium rapidly accumulated in the rhizosphere by a pH-independent process which seemed to be largely mechanical and to involve little multiplication. Thus large rhizosphere populations accumulated within a few hours even at pH 4.4. Nevertheless, acidity still prevented root hairs from curling and becoming infected. Raising the pH from 4.4 to 5.4 at this stage allowed curling and subsequent steps to proceed without delay.Shortly after curling was completed, the pH could be lowered to 4.4 without hindering the development of infection and the normal completion of nodulation. Hence the prevention of nodulation by acidity, in heavily inoculated solution cultures, can be attributed to prevention of a step which approximately coincides with the curling of root hairs. This step occupied less than 12 hours of the 4 to 7 days required for visible nodules to appear.