Drying of surface soil decreased Lupinus angustifolius root length and manganese uptake in a split-root experiment

In a glasshouse, a split-root experiment was used to determine the ability of lupins (Lupinus angustifolius L.) to take up manganese (Nn) from dry soil either when young or at mid-flowering of the primary branches. Three soil-watering regimes (maintained at field capacity, maintained below wilting point, and alternating from field capacity to well below wilting point) were imposed after taproots had grown through topsoil and into a nutrient solution below. Four sequential harvests (11, 22, 37, and 49 days after sowing) were taken to determine the effect of soil drying on lupin growth, Mn uptake, and soil-extractable Mn. Soil drying early in the lupin plant's life stopped the growth of lateral roots in the soil and slowed the growth of roots grown in subsoil solution and the growth of lupin tops. Soil drying decreased uptake of Mn in the tops to 13% of that under continuous wet soil conditions. Of the 13%, most (11%) was taken up while the soil wets drying. Soil re-wetting enabled the plants to resume uptake of Mn and soil re-drying (just before anthesis) decreased the Mn concentration in the lupin stems to 4.8 mu g/g, whereas stems of lupins grown in the wet and dry soils contained 10.3 and 3.3 mu g/g, respectively. Easily reducible and plant-available soil Mn were not affected by soil wetting and drying treatments. This study confirms that the uptake of Mn by lupin may be severely restricted by drying of surface soil at both the beginning and the end of the lupin plant's life. The decrease in root length rather than the chemical form of Mn restricted Mn uptake.