The cellular and metabolic events triggered by water uptake during seed germination. Germination is affected by both environmental factors (the availability of water, oxygen and light as well as the temperature) and intrinsic factors (dormancy, permeability of the testa to water and oxygen, and obstruction of radicle emergence by the endosperm). A rapid imbibition phase (phase 1) launches the resumption of basic metabolism. During this phase, known as 'physical' imbibition, a step-by-step activation of metabolic pathways results from the gradual increase in hydration (arrows). When the level of hydration exceeds 60%, the rate of hydration slows (phase 2) and new physiological mechanisms prepare cell expansion in the embryonic axes, culminating in the start of cell elongation. Osmotically active substances (solutes, such as sugars, amino acids, and potassium ions) are accumulated and acidification of the cell wall leads to a loosening of the bonds between cell-wall polymers. These events coincide with the activation of the H+ ATPase in the plasmalemma, which results in a further increase in water uptake that may coincide with weakening of the surrounding tissues (the endosperm) as the embryonic axes elongate and germination is completed. Completion of seed germination can be temporarily blocked by dormancy, which is in turn released by antagonistic interactions between the endogenous plant growth factors abscisic acid (ABA) and gibberellins (GAs) . Storage nutrients (lipids, proteins or starch) accumulated in the embryo's cotyledon and/or endosperm start to be mobilized before completion of germination and are used in the post-germination steps to sustain the young plant in its early growth stages, before it becomes autotrophic. If the cell cycle resumes during germination, the first cell division (mitosis) occurs in the postgerminative phase. The arrows indicate the particular hydration levels that are known to correlate with individual metabolic events. The sequence of events shown in this model results from studies in various species. Modified from .