What characteristic defines the Eragrosteae tribe?

The defining characteristic of the Eragrosteae tribe is indeed the presence of an open or contracted panicle inflorescence. This structural feature allows for the arrangement of flowers in a way that can be either widely spaced and dispersed (open) or more closely packed together (contracted). The morphology of the inflorescence plays a significant role in the reproduction and pollination strategies of the grasses within this tribe, adapting them to various environmental conditions.

In contrast, the other characteristics provided do not accurately represent the Eragrosteae tribe. For instance, having florets in clusters of four to six is not typical; members of this tribe generally do not have such specific clustering and tend to display different arrangements. Similarly, the idea of single flower spikelets doesn't apply here, as this tribe typically features complex arrangements of spikelets. Finally, glumes that cover whole spikelets are not a characteristic of Eragrosteae; many other grass tribes may exhibit such traits, but the Eragrosteae are defined by their specific inflorescence structure rather than by the morphology of their spikelets.

Overall, the open or contracted panicle inflorescence is a key identifying feature and is essential to understanding the taxonomy