An ATP-synthesizing system in seeds

Using onion seed powder, a semi-in vitro system for ATP synthesis in seeds has been developed. The system requires AMP, phosphoenolpyruvate (PEP) and orthophosphate with apparent Km values of 0.8, 1.5 and 3.0 mM, respectively. ATP synthesis is pH-dependent with a sharp optimum at pH 6.4, it exhibits linearity with time up to 40 min, and with a seed powder concentration between 25 and 150 mg ml^-1. The system is stimulated by low concentrations (<25 mM) of K⁺ and Mg²⁺ but is inhibited by higher concentrations of K⁺ and Mg²⁺ as well as by low concentrations of Li⁺, Na⁺ and especially Ca²⁺. The maximal rate is about 5 pmol min^-1 mg seed powder^-1 in dry onion seeds. During seed imbibition the rate of activity increases by about 120% after 3 h, reaching a plateau which is steady up to 18 h, when the radicle emerges. A comparison of the ATP content in seeds during the early period of imbibition with the capacity of ATP synthesis at this stage reveals that the described system could provide, during germination, 100 times more ATP than that found in imbibed seeds. The system is shown to be present in ten different types of seeds.