# 1995; Van der Weerd et al 2001, 2002) Diffusive exchange within

1995; Van der Weerd et al. 2001, 2002). Diffusive exchange within compartments and exchange between compartments, passing membranes, affect the observed relaxation times (Van As 2007; Van As and Windt 2008). The observed T 2 (and T 1) of vacuolar water has been demonstrated to depend on the bulk T 2 in the vacuole (T 2, bulk), and the surface-to-volume ratio, S/V, of the vacuole (van der Weerd et al. 2001): $$1/T_2,\;\textobs = (H\; \times \; S/V)\; + \; 1/T_2,\;\textbulk$$ (6)The proportionality constant H is directly related to the actual tonoplast membrane permeability

for water (van der Weerd et al. 2002; Van As 2007). Equation 6 holds also for water in (xylem) vessels, where H now represents the

loss SBI-0206965 molecular weight of magnetization at the vessel wall (Homan et al. 2007), demonstrating that T 2 of vessel water is directly related to vessel radius. As long as the observed relaxation times are longer than TE, the A 0 maps represent the water density of all water in a pixel and the different tissue types can be discriminated on the basis of their LY411575 price respective T 2 values (cf. Fig. 2). This condition is easily met for vacuolated plant tissue, where most of the water is in the vacuole, which has relatively long T 2 values, depending on the size (Eq. 6) and represents most of the water in such cells (Donker et al. 1997; Van der Weerd et al. 2000). It is advisable to use as short as possible TE values to cover the shortest T 2 values. Most probably extra-cellular water and water in fibers, with short T 2 values, are hard to observe in MSE type images. In order to obtain A 0 maps Sitaxentan of water with real short T 2 values, alternative image sequences can be used (Van As et al. 2009; Van Duynhoven et al. 2009). Xylem and phloem flow An example that clearly illustrates how MRI can be used to obtain information from structures that are smaller than a pixel is MRI flow imaging (for some overviews, see

MacFall and Van As 1996; Köckenberger 2001; Van As 2007; Van As and Windt 2008). In general, spatial resolution will not be high enough to resolve individual phloem or xylem vessels. As a consequence, pixels that contain flowing water will also contain a significant amount of stationary water. When Torin 2 cost vessels are very small, as is the case in phloem tissue, the relative amount of flowing water per pixel can be as small as a few percent. The greatest challenge in measuring phloem water transport, therefore, is to distinguish displacement of a small amount of very slowly moving water from a (very) large amount of stationary water showing displacements due to random movement as a result of Brownian motion.