NMR, Water and Plants

This Thesis describes the application of a non-destructive pulsed proton NMR method mainly to measure water transport in the xylem vessels of plant stems and in some model systems. The results are equally well applicable to liquid flow in other biological objects than plants, e.g. flow of blood and other body fluids in human and animals (Chapter 8). The method is based on a pulse sequence of equidistant πpulses in combination with a linear magnetic field gradient G.Following a general introduction and a survey of the properties of water in plants (Chapters 1 and 2), the basic NMR theory as well as reviews on the application of pulsed NMR to the determination of flow, diffusion and water content are presented in chapter 3.A mathematical treatment has produced analytical expressions for the shape of the signal S(t), based on a model in which the flowing fluid is thought to receive a ½π-τ-(π-τ-) _{<font size="-1">n</font>} pulse train: a ½πpulse upon entering the r.f. coil followed by a sequence of equidistant πpulses until the fluid leaves the coil; simultaneously, this movement. of the fluid along a magnetic field gradient applied in the direction of flow produces a phase shift of the nuclear magnetization with respect to the rotating frame of reference (Chapter 4). Although this model does not lead to perfect agreement between the experimental and theoretical signal shape S(t), it correctly predicts the effects of experimental parameters on S(t) via analytical expressions. The main results from this theoretical treatment in combination with computer simulations, which have been experimentally verified in glass capillary systems, are:- as long as T _{<font size="-1">2≥</font>} ½T _{<font size="-1">1</font>} , the mean linear flow velocity v can be found from the time t _{<font size="-1">max</font>} at which a maximum appears in the signal shape: v=C/t _{<font size="-1">max</font>} , where C is a calibration constant, depending on G, τand the flow profile. If T _{<font size="-1">2</font>} &lt;½T _{<font size="-1">1</font>} v can only be reliably determined when both T _{<font size="-1">1</font>} and T _{<font size="-1">2</font>} of the flowing fluid are known.- T _{<font size="-1">2</font>} and the amount of flowing water in the coil V, and consequently the volume flowrate Q, can be determined from the height of the maximum S(t _{<font size="-1">max</font>} ) and t _{<font size="-1">max</font>} . Depending on the value of T _{<font size="-1">2</font>} and the value of the ratio T _{<font size="-1">1</font>} /T _{<font size="-1">2</font>} , T _{<font size="-1">2</font>} and V are found from a semilog plot of either S(t _{<font size="-1">max</font>} ) vs. t _{<font size="-1">max</font>} (T _{<font size="-1">1</font>} >>T _{<font size="-1">2</font>} ) or ∂[S(t _{<font size="-1">max</font>} ) <strong>.</strong> t _{<font size="-1">max</font>} ]/∂t _{<font size="-1">max</font>} vs. t _{<font size="-1">max</font>} (T _{<font size="-1">1≈</font>} T _{<font size="-1">2</font>} ).Based on flow measurements in plant stem segments (Chapter 5) it has been suggested that T _{<font size="-1">2</font>} strongly depends on the vessel diameter for the narrow xylem capillaries. This behaviour of T _{<font size="-1">2</font>} can explain negative results in plant stems with small vessel diameter. Under the present experimental conditions the method has been successfully applied to Cucurbitaceae (cucumber, gherkin, pumpkin) and tomato plants.T _{<font size="-1">2</font>} measurements in wheat leaves have been shown to be insensitive to the presence of cell-bound paramagnetic ions (Chapter 7). The magnitude of T _{<font size="-1">2</font>} of two separate water fractions (covering -90% of the total water content) has been found to be inversely proportional to water content. Measurements of flow and water content have been combined for an intact gherkin plant (Chapter 5), demonstrating that the combination of both NMR methods results in a powerful non-invasive method to study important parts of the plant water balance simultaneously. The results strongly suggest that the method can be used as an early warning for development of stress phenomena in plants, due to drought and other factors. From the flow measurements it has been shown how in a plant system the values of T _{<font size="-1">2</font>} and T _{<font size="-1">1</font>} of the water in the xylem vessels can be determined and estimated, respectively.A comparison between the results obtained with NMR, heat pulse and weight balance flow measurements is presented in Chapter 6. A linear relationship between the linear flow velocity obtained by NMR and the volume flowrate determined by the balance method yields an effective cross-sectional area available for flow of ~50% of the cross-sectional area of the xylem vessels measured by using a microscope. NMR measurements alone yield a slightly lower value of the effective cross-sectional area. Compared with the NMR method, the heat pulse method monitors only relative changes in the flow velocity. A plot of the flow velocity obtained by the heat pulse method versus the volume flowrate obtained by the balance method exhibits some unwanted experimental scatter.Chapter 8 suggests some applications of the pulsed NMR flow method, also to other systems than plants, and defines important instrumental requirements for these applications