A gradual rather than abrupt increase in strength gives better root penetration of strong layers

The purpose of this paper is to test the hypothesis that gradual rather than abrupt increases in strength lead to increased root penetration of strong layers. This was tested experimentally in a model system with multi-layered wax discs with a total thickness of up to 6 mm that either increased in strength with depth, or decreased in strength with depth. Strength was varied by altering the proportions of hard paraffin wax and white soft paraffin. Multi-layered wax discs consisted of three layers, each 1.5 or 2 mm thick, to give a disc thickness of either 4.5 or 6 mm. ‘Test’ wax discs had a 40% hard wax layer above a 60% wax layer, which was above an 80% wax layer. This led to a gradual increase in strength as roots encountered the disc from the sand growing medium. In ‘control’ discs, the order was reversed so that 80% wax was encountered immediately below the sand. These treatments were used to challenge roots of the rice cvs. Azucena and Bala. Compared with the control, the test layers increased root penetration of Azucena from a mean of 5.9 root axes to 12.4, while penetration of Bala was increased from 0.5 to 5.6. These results show that there was increased root penetration when roots encountered a gradual increase in strength rather than a sudden increase in strength. This was explored further with numerical simulations of stress distributions at the tips of roots. These simulations indicated that gradual rather than abrupt increases in strength decreased stress concentration at the root tip, consistent with the experimental results.