Morphology of the perennial ryegrass (Lolium perenne; Poaceae) coleorhiza and emerging radicle with continuous or discontinuous hydration

This study was conducted using scanning electron microscopy to characterize morphological changes in the maturing perennial ryegrass (Lolium perenne L.) coleorhiza and emerging radicle during continuous hydration or hydration interrupted by dehydration. With continuous hydration, coleorhizae emerged and cell expansion led to a progressive increase in tissue size. Coleorhiza cells developed extensions like epidermal root hairs. Although coleorhiza cells appeared undamaged by radicle emergence, they began deteriorating when the radicle had reached a length of approximately 2 mm. In response to dehydration, coleorhiza cells shrank but did not rupture. If dehydration was initiated during an early stage of coleorhiza development, greater tissue shrinkage occurred at -150 MPa than at -4 MPa; otherwise, coleorhizae showed no differential response due to dehydration water potential. Upon rehydration, coleorhizae dehydrated at -4 MPa regained cell turgor within 24 hr, while coleorhizae dehydrated at -150 MPa did not. Loss of the coleorhiza (due to desiccation) did not preclude radicle emergence, which occurred upon rehydration. Radicles up to 2 mm in length were more tolerant of dehydration than were coleorhizae. These results suggest that the coleorhiza may be an expendable tissue during germination, as its likely protective and absorptive roles are lost following a single harsh dehydration event.