Cover illustration: Sprouting of wheat grain in the ear prior to harvest in response to rain is a serious issue for farmers in many parts of the world. Grain dormancy provides some resistance to sprouting but the mechanisms involved and whether they include effects on water movement into grains are still not clear. Questions that have not hitherto been resolved are: (i) how does water enter wheat grains, and (ii) does dormancy involve a restriction of water movement into grains? The upper panel of the figure shows wetted ears of a sprouting-susceptible, non-dormant, cultivar with roots protruding from the ear and two resistant, dormant, cultivars exposed to the same amount of rain but which show no evidence of sprouting. The second panel shows the increase in water content of the embryo and scutellum of non-dormant (uppermost line on the graph) and dormant grains during imbibition. The initial water uptake, Phase I, is rapid and there is no differentiation between non-dormant and dormant grains. Phase II, hydration to rupture of the grain coat, is very short in non-dormant grains which start to germinate immediately the embryo is hydrated. Phase II is substantially longer in dormant grains. Further increases in embryo water content were associated with the onset of germination (Phase III). The changes in water content in grain tissues can be visualized with magnetic resonance micro-imaging (MRmI). The third panel shows virtual sections, both longitudinal and transverse, of imbibed wheat grains where yellow represents tissues with a high water content and white represents tissues with a very high water content. Scutellar (thin yellow band) and embryo tissues in the longitudinal images on the left side show a high water content as early as 3-4 h after the start of imbibition. The transverse images on the right hand side of the panel, show that water is located in the grain coat but not the endosperm (upper right), the embryo tissues and micropyle (middle right), and in the micropyle at the base of the grain (lower right). By contrast with the embryo, scutellum, and grain coat, movement of water into the endosperm is much slower and was best visualized by imbibing grains in I2/KI solution which stains starch in the endosperm dark blue. The lower panel shows a longitudinal section through a grain imbibed in I2/KI for 18 h. (See Rathjen et al., pp. 1619-1631.)
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