Growth responses of seminal roots of wheat seedlings to a reduction in the water potential of vermiculite

We examined the elongation rate, water status and solute accumulation in the seminal roots of wheat seedlings (Triticum aestivum L.) that were growing in vermiculite with a water potential (Psi(w),) ranging from -0.03 to -1.10 MPa. The elongation rate of the primary seminal root was similar to that of the first pair of seminal roots but that of the second pair of seminal roots was lower at all values of Psi(w) tested. The elongation rate was highest in vermiculite with a Psi(w) of -0.03 MPa but did not decrease significantly until the Psi(w) was reduced to -0.15 MPa. Further reductions in Psi(w) reduced the elongation rate markedly. The Psi(w) of mature tissues was always similar to that of vermiculite. The osmotic potential (Psi(o)) decreased to the same extent as the decrease in Psi(w). Thus. the turgor pressure (Psi(p)) remained unchanged even in vermiculite with a low Psi(w) In elongating tissues. Psi(w) and Psi(o) were far lower than they were in mature tissues and, thus, reductions in turgor were not significant. Even when the Psi(w) of vermiculite changed, there were no consistent changes in terms of a difference in %P, between elongating plus mature tissues and vermiculite. There were also no consistent changes in levels of osmotica. calculated using the van't Hoff's law, in the elongating tissues but the levels in mature tissues increased in vermiculite with a low Psi(w). Our results suggest that (1) reductions in root elongation in vermiculite with a low Psi(w) were caused by reductions in the extensibility and/or increases in the yield threshold of cell walls and by reductions in the hydraulic conductivity of the tissues; and (2) a seminal root regulates its growth to keep turgor pressure unchanged.