Change in Diffusional Water Permeability of Spinach Leaf Cell Membrane determined by Nuclear Magnetic Resonance Relaxation Time

The nuclear magnetic resonance spin–lattice relaxation times of spinach leaf protoplasts suspended in 0·6 M mannitol solution were measured in the presence (T1s) and absence (T1f) of Mn2+ ions to determine the water exchange time λ and water permeability coefficient Pd of membrane. For the spinach stored for 1 d at 2 °C, λ was determined to be 76·4 m s and Pd was 49 μm s−1. As to the spinach stored for 6 d, λ was 52·7 ms and Pd was 71 u s−1. Values of Pd obtained in this study had the same order of magnitude as literature values. Both the decrease in λ and the increase in Pd with storage time showed that cell membrane became freely permeable state with storage duration. It was thought that the aquaporin of PM28A, a major intrinsic protein of the spinach leaf plasma membrane, was not a major cause of the increase in water permeability of cell membrane after harvest because the decrease in the expression of PM28A was observed in 1 d.