Elongation of the Grass Leaf and its Relationship to the Phyllochron

Abstract

The phyllochron (interval between appearance of successive leaves) in grasses is determined by timing of leaf initiation at the stem apex and duration of leaf tip elongation through the whorl of mature sheaths. Primordia in tall fescue (Festuca arundinacea Schreber) are initiated and enlarge to ⊄ 1.5 nun in length by production of cells that remain ⊄ 20 um long. As a young leaf continues to grow, epidermal cells beyond 1.5 nun from the base elongate to form a cell elongation zone of 7 to 40 mm, depending on genotype and environment, which pushes older tissue through the whorl. Leaf elongation rate is a function of length of the elongation zone and segmental elongation rate of positions within the zone. This zone constitutes a major sink for carbohydrate, N, and water deposition in the leaf. Ligule formation occurs in the cell division zone when the blade is ⊄20% of its final length. Blade and sheath elongation take place simultaneously thereafter with their relative proportion depending on position of the ligule in the elongation zone. Initiation of leaf elongation from a resting primordium at a given node is coordinated with ligule initiation in the next‐older leaf one node below and with cessation of cell division in the sheath and commencement of tiller elongation at the axillary bud two nodes below. Duration of leaf tip elongation through the whorl depends on elongation rate and sheath length. The importance of these factors in determining the phyllochron is often overlooked due to calculation procedures that assume a constant phyllochron when in fact the phyllochron often changes for each leaf.