The order, rate, and extent with which plant tissues dehydrate during water stress and rehydrate remains largely unknown. In vivo magnetic resonance imaging was used at seven Tesla to measure changes in degree of water binding and relative water content (through T1 and N[H] changes) ofBlechnum unilateralestem regions as water stress developed and was relieved. Spin‐lattice relaxation times (T1s) ofB. unilateralestem regions were found to be positively linearly correlated with water potential. In the well‐watered state, the longest T1s were associated with the stem margin, followed by cortical parenchyma, pith parenchyma, and vascular bundles. During water stress, T1 values of the stem margin, cortical parenchyma, and pith parenchyma converged. After rehydration, T1s returned nearly to initial values. As indicated by a decrease in relative spin density (N[H]), the stem margin and cortical parenchyma began to lose water first during dehydration with the rate and extent of water loss being greatest for the stem margin. Later, water loss occurred from all regions; most rapidly and to a greater extent from the stem margin and vascular bundles. During water stress, N(H) of stem regions converged to a common value. Order of rehydration of stem regions was the same as order of dehydration. This study demonstrates that MR imaging can be used as a noninvasive tool for examining changes in the degree of water binding and water content of stem regions in live plants.