[1H]Spin-echo nuclear magnetic resonance in plant tissue. I. The effect of Mn(II) and water content in wheat leaves.

The effect of age-dependent Mn(II)-gradients, as observed by electron paramagnetic resonance (EPR), on the [1H]NMR spin-spin relaxation time (T2) was studied in wheat leaves. A non-exponential T2 spin-echo decay was always observed, revealing the presence of at least two different fractions of non- (or slowly) exchanging water in the leaves. No effect of the Mn(II)-concentration on T2 of the separate water fractions (covering approximately 90% of the total water content) has been found. From these observations we conclude that Mn(II) is present in bound form. The dependence of T2 on water content can be explained with a two-state model, demonstrating the occurrence of fast exchange within each of the two slowly exchanging water fractions.