Water permeability in wheat root protoplasts determined from nuclear magnetic resonance relaxation times

Abstract

Diffusional water permeability coefficients (P'd) of wheat root protoplasts were determined by measuring NMR spin-lattice (T1) and spin- spin (T2 relaxation times in the absence and presence of added Mn2+ at various concentrations. These measurements enabled the determination of exchange times (λ'a) for water molecules between cells and their surrounding medium. It was found that λ'a determined from T1 measurement was much larger than λ'a determined from T2 measurement: ca. 135 ms versus 9.4 ms. Whilst λ'a determined by the two methods do not agree, they are each similar to individual values of λ(a) from the literature using T1 and T2 measurements. The P(d) value of 3.0 x 10-5 m s-1 determined from T1 measurements is lower than the osmotic water permeability (P'f) of wheat root cells (P'f = 8.6 x 10-5 m s-1) obtained from pressure probe measurements and can be accounted for by the presence of the water selective channels. The T2 measurements, on the other hand, give rise to P(d) value of at least four times greater than the P(f) value determined from pressure probe measurements. Furthermore, T2 relaxation of protoplasts in the absence of added Mn2+ showed multi-exponential decay. These differences between the T1 and T2 measurements might be attributed to the presence of endogenous paramagnetic ions in the cells and their binding to cell membranes. In the absence of the detailed knowledge of the distribution of paramagnetic ions in plant cells, it is proposed that NMR T1 rather than T2 measurements be used to obtain meaningful values of λ(a).