Root growth and nitrate uptake by wheat (Triticum aestivumL.) following wetting of dry surface soil

Abstract

The nitrate uptake capacity of surface roots of spring wheat (Triticum aestivum L. cv. Kulin) was investigated following wetting of dry surface soil. Plants were grown to stem elongation stage with adequate watering at depth while the surface soil was allowed to dry. Eight weeks after sowing, water or a l5 N-nitrate solution was added to the surface soil to simulate rainfall. Root growth and nitrate uptake were measured up to 4 d after wetting on plants with unconfined nodal root growth and on plants with the majority of nodal roots confined within small vials. Prior to wetting, plants from both nodal treatments had seminal roots with collapsed cortices along the upper 10 cm and many short, viable lateral roots. Nodal roots, where present, were only a few cm long and unbranched. Only plants with unconfined nodal roots were able to take up nitrate within the 24 h before any new root growth. By 2 d after wetting there was significant growth of the seminal lateral roots, and rapid growth and branching of nodal roots. From 2d after wetting, plants with confined nodal roots also took up nitrate, presumably due to the growth of the seminal lateral roots. Hence it appears as though the nodal roots in the unconfined treatment could immediately take up nitrate, but the seminal roots required new lateral root growth to become active in nitrate uptake. The plants with confined nodal roots had a lower nitrate uptake than those with unconfined nodal roots 4d after wetting, indicating that the seminal root system was not able to compensate for lack of nodal roots. Insufficient nitrate was taken up after 4 d, by plants from either nodal root treatment, to increase the shoot N concentration significantly.