The Role of Glumes and Gibberellic Acid in Dormancy of <i>Themeda triandra</i> Spikelets

Abstract Spikelets of Themeda triandra are dormant when mature and require an after‐ripening period in dry storage of approximately 12 months before full germination potential is realized. Successful germination of spikelets entails the splitting of the tough upper glumes by radicles. Dormany appears to result from a combination of embryo dormancy and mechanically resistant glumes. Glume removal from dormant spikelets increases germination while glume removal plus gibberellic acid increases germination even more. During the after‐ripening period, the growth potential of spikelets and caryopses increases as measured by their ability to germinate in the presence of the osmoticum polyethylene glycol 6000. The inhibition of germination by decreasing osmotic potential of the germination medium significantly interacts with the promotion caused by gibberellic acid indicating that both factors affect germination by altering the growth potential of the embryos. The increase in growth potential during after‐ripening is probably related to the synthesis of gibberellin‐like substances. It is hypothesized that dormancy breaking during after‐ripening occurs because the growth potential of embryos increases and this consequently increases the ability of radicles to split the upper glumes during germination.