Large changes occur in the ascorbate system during the development of Vicia faba seed and these appear closely related to what are generally considered to be the three stages of embryogenesis. During the first stage, characterized by embryonic cells with high mitotic activity, the ascorbic acid/dehydroascorbic acid ratio is about 7, whereas in the following stage, characterized by rapid cell elongation (stage 2), it is lower than 1. The different ascorbic/dehydroascorbic ratio may be correlated with the level of ascorbate free radical reductase activity, which is high in stage 1 and lower in stage 2. Ascorbate peroxidase activity is high and remains constant throughout stages 1 and 2, but it decreases when the water content of the seed begins to decline (stage 3). In the dry seed, the enzyme disappears together with ascorbic acid. Ascorbate peroxidase activity is observed to be 10 times higher than that of catalase, suggesting that ascorbate peroxidase, rather than catalase, is utilized in scavenging the H(2)O(2) produced in the cell metabolism. There is no ascorbate oxidase in the seed of V. faba. V. faba seeds acquire the capability to synthesize ascorbic acid only after 30 days from anthesis, i.e. shortly before the onset of seed desiccation. This suggests that (a) the young seed is furnished with ascorbic acid by the parent plant throughout the period of intense growth, and (b) it is necessary for the seed to be endowed with the ascorbic acid biosynthetic system before entering the resting state so that the seed can promptly synthesize the ascorbic acid needed to reestablish metabolic activity when germination starts.

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S L" M M A R YQuiescent centre cells, which are known to have their cycle extended mostly in G, and to divide rarely, are stimulated to undergo DNA synthesis in root meristems of Allium cepa L. by treatment with ascorhic acid. The effect of ascorbic acid is dramatic: 21 "" of all the cells in the quiescent centre remain in the G, phase while 79",, enter S phase. From a mean value of 2204 X l()''//m'' in water-grown roots, the \olume of the quiescent centre drops to a mean value ot 167 x lO' /(m' following treatment with ascorbic acid. These results indicate that ascorbic acid stimulates not only the activity of the quiescent centre cells but also cell proliferation in the entire root meristem. The rate of cell progression througli tlie cycle may be related to cellular ascorbic acid content.