The effects of surface-sowing on root type, number and xylem radius were studied in relation to the seedling growth and survival of ryegrass and phalaris. Under optimal conditions in a growth cabinet, both species produced primary and lateral seminal roots, nodal roots and, in the absence of light, a subcoleoptile internode (s.c.i.). Phalaris had fewer lateral seminal roots and, in this species, internodal roots occurred along the s.c.i. Surface placement per se had no effect on any of the measured parameters of root development, but surface drying prevented nodal root primordia from extending, even when plants were otherwise supplied with water. This effect of surface drying on nodal root development was confirmed in a glasshouse study using undisturbed soil cores (30 by 60 cm) to simulate field conditions. Phalaris was less likely than ryegrass to produce nodal roots after surface sowing. The effective xylem radius (re) of the primary seminal root was 7.9 �m in ryegrass and 11.6 �m in phalaris. Calculations using the Poiseuille equation indicated that the axial resistance to water flow through these roots would greatly restrict seedling water uptake and growth in the absence of other roots. Lateral seminal roots and internodal roots had small vessels which could not significantly reduce the axial resistance to flow. Good seedling water relations in both species therefore depend on early development of the nodal roots which contain large xylem vessels (re > 16 �m). It was concluded that the effect of soil-surface drying on nodal root development was likely to account for some cases of poor vigour and survival of surface-sown grasses.
Shape of mole nose providing minimum axial resistance