Measurement of soil matric potential under 'Williams' Bon Cretien' pear comparing regulated deficit with normal irrigation

Abstract

Soil matric potential (SMP) was measured under close planted 'William Bon Cretian' pear (Pyrus communis L.) after a period of withholding irrigation and during a period of regulated deficit irrigation (RDI). Trees were trickle irrigated with 20% replacement of pan evaporation over the planting square (Eps) after an initial period without irrigation, and compared with trees constantly irrigated at 80% Eps. Midway between 2 pairs of trees in each treatment (henceforth the 20% and 80% trees), gypsum blocks were installed in a grid pattern at depths of 10, 25, and 40 cm in the tree line and at 30 and 60 cm on the west and east sides of the tree line, to measure soil matric potential (SMP). The blocks were measured prior to, and 12 h, after irrigation. Consistent with previous results under RDI, shoot growth decreased and yields increased on the 20% relative to the 80% trees. Measured SMPs thus represent suitable levels for RDI. After withholding irrigation, SMPs at the 10 and 25 cm depths ranged between -1.25 and -2.61 MPa in the central position, but were less negative away from the tree line (-0.4 to -2.08) and at the 40 cm depth (-0.35 to -0.91). When averaged over all measurement dates during RDI, pre- and post-irrigation SMPs were more negative at all sites under the 20% compared with the 80% treatments. The pattern of wetting under the 20% treatment was restricted to the central tree line, with some skewing towards the east. Within this soil volume SMP fluctuated between irrigations, and prior to irrigation ranged between -0.3 and -0.7 MPa. As it was likely that only a small fraction of the available subsoil moisture was utilised, available water under the 20% treatment probably was confined to this small volume of soil.