Adaptive longitudinal growth of first-order lateral roots of a woody species (Spartium junceum) to slope and different soil conditions—upward growth of surface roots

First-order lateral roots originating in the upper part of the taproot of a woody species, usually termed surface roots, grow close beneath the soil surface, even on irregular or sloping ground. In slope condition, in fact, the surface roots can assume upward as well as downward growth. Existing knowledge on the controls over root direction does not fully explain these field observations.Two different soil types and sloping conditions were selected in field condition to explore the behaviour of the surface roots in the woody species Spartium junceum L. The root system 3D architecture was measured with a 3D digitizer and the angle of growth (0° = vertically downwards) and the radial direction (0° = horizontally downslope or northwards) of all root segments measured.Surface roots were more numerous in clay soil than in loam soil, independently from the slope inclination. They had initial angles larger than 90°, i.e. they grew upwards only in clay soil. The subsequent angles of growth maintained this value only in steep-slope condition, showing a clear soil type x slope inclination interaction. The initial angle of all first-order lateral roots decreased linearly with depth of origin on the taproot always in relation to the soil type, with this relationship being stronger in clay soil.These findings showed that the liminal angle (the preferred angle of growth) of surface roots was mainly affected by the soil type rather than the soil surface inclination. Thus, upward growth must stand in the plasticity of the plagiotropic response of these secondary laterals rather than in a strong internal control.