Development of open and closed leaf sheaths in Festuca trachyphylla and Festuca rubra (Poaceae)

Abstract

The development of the leaf sheaths in two species of Festuca, subgenus Festuca, Festuca trachyphylla (Hackel) Krajina with open leaf sheaths and Festuca rubra L. s.l. with closed leaf sheaths is compared to determine the developmental basis of this taxonomically important character. In both species serial cross sections of seedlings, scanning electron microscopy, and dissections of whole plants showed that a short tubular leaf base is formed early during the second plastochron of leaf development. Later during the same plastochron, ligule formation delimits sheath and blade; at this stage, the tubular portion occupies about one-half (200 – 500 μm) of the incipient leaf sheath. Differential extension growth of the distal open portion and proximal closed section of the sheath results in the primarily open or closed sheaths that characterize the mature leaves of the two species. The longitudinal splitting of the tubular leaf sheath in a position opposite the midvein in F. rubra is anticipated by a zone of small-diameter cells within the ground tissue, but no other structural correlates of sheath mechanical strength and persistence were detected. Our observations indicate that subtle shifts in the distribution of growth during leaf expansion result in the contrasting character states exemplified by these two species. Key words: Festuca, grass leaf development, leaf sheath, Poaceae.