Physiological responses of radiata pine roots to soil strength and soil water deficit

We investigated physiological responses of radiata pine (Pinus radiata D. D on) roots to soil strength and soil water deficit by measuring the osmotic potential (Psi (pi)) and yield turgor (Y) in the elongation zone of root se gments of seedlings growing (i) in polyethylene glycol 4000-containing root ing solution of different water potentials (Psi (pi)) and (ii) in soil of d ifferent soil strengths (Q) at the same soil matric potential (Psim). Root elongation rate (Deltal/Deltat) decreased progressively with decreasing Y, and was associated with decreased Y,and decreased turgor pressure (P). Osmo tic adjustment occurred at Psi (s) < -0.2 MPa. Over a range in <Psi>(s) of -0.01 to -1.0 MPa, Psi (pi) fell 0.3 MPa whereas P fell 0.7 MPa. Mean Yin t he solution experiment was 0.37 MPa and did not differ significantly with P si (s) (P = 0.10). Root elongation rate decreased exponentially as Q increa sed from 0 to 3.0 MPa, and was associated with an increase in P of 0.11 MPa as a consequence of Psi (pi), decreasing by the same amount. Mean Yin the soil experiment was 0.49 MPa and did not change significantly with Q (P = 0 .87).