WATER PERMEABILITY IN WHEAT ROOT PROTOPLASTS DETERMINED FROM NUCLEAR MAGNETIC RESONANCE RELAXATION TIMES

摘要

Diffusional water permeability coefficients (P-d) of wheat root protoplasts were determined by measuring NMR spin-lattice (T-1) and spin-spin (T-2) relaxation times in the absence and presence of added Mn2+ at various concentrations, These measurements enabled the determination of exchange times (lambda(a)) for water molecules between cells and their surrounding medium. It was found that lambda(a) determined from T-1 measurement was much larger than lambda(a) determined from T-2 measurement: ca. 135 ms versus 9.4 ms. Whilst lambda(a) determined by the two methods do not agree, they are each similar to individual values of lambda(a) from the literature using T-1 and T-2 measurements. The P-d value of 3.0 x 10(-5) m s(-1) determined from T-1 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 T-2 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, T-2 relaxation of protoplasts in the absence of added Mn2+ showed multi-exponential decay. These differences between the T-1 and T-2 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 T-1 rather than T-2 measurements ts used to obtain meaningful values of lambda(a). [References: 32]