Nonstructural Carbohydrates in Stem Base Components of Tall Fescue During Regrowth 1

Nonstructural carbohydrates in stem bases are important for regrowth of tall fescue (Festuca arundinacea Schreb.). The objectives of this study were to i) examine changes in concentration of nonstructural carbohydrates in individual stem base tissues during regrowth; ii) evaluate the relationship between leaf elongation rate (LER) and concentration of nonstructural carbohydrates; and iii) estimate the degree of polymerization (DP) fructans in tissues during regrowth. Plants were grown in a controlled environment chamber at 23°C with a photosynthetic photon flux density of 650 ~mol m-~ s-1 during a 14-h photoperiod. Plants were sampled immediately following defoliation and 2, 4, 7, 10, 14~ 17, 21, and 24 days later. Reducing sugars, nonreducing sugars, starch, and fructan in intact stem bases declined to approximately 50% of initial concentrations by Day 4, and returned to original concentrations by Day 24. Initial concentrations of starch and fructan were higher in younger (inner) sheaths, when compared to older sheaths. Concentrations of nonreducing sugars in all sheaths declined following defoliation, while concentrations of reducing sugars and starch did not change markedly. Fructan concentrations on Day 24 were 19 and 77% of initial concentrations in the oldest and youngest sheaths, respectively. Nonstructural carbohydrates ~accumulated in newly formed sheaths and were not restored in older sheaths. Nonreducing sugars and fructan in elongating leaf bases declined to approximately 30% of initial concentrations by Day 4. Maximum LER of 16 mm day-1 occurred between Days 2 and 7, which indicates that carbohydrates may not normally limit LER. The DP of fructan in old sheaths initially averaged 19 and decreased uring regrowth, while the DP of 15 of young sheaths remained constant. The DP of fructan in bases of elongating leaves was less than 6 during rapid growth. Bases of elongating leaves may be more appropriate than intact stem bases as indicators of the availability of nonstructural carbohydrates for regrowth because of their responsiveness to defoliation. Additional index words: Festuca arundinacea Schreb. , Fructan, Degree of polymerization, Leaf elongation, Leaf sheath, L9af meristem. ENERGY fo.r regrowth following harvest of forage grasses is provided by current phQtosynthate from residual leaf area, and/or nonstructural carbohydrates stored in roots or stubble (1,’3, 4, 5, 6, 17, 27). The primary site of storage of nonstructural carbohydrates in forage grasses is called the stem base or stubble (12, 20, 21), and is commonly defined the lower 5 cm of any given tiller. In vegetative forage rasses, the stem base is composed primarily of leaf sheaths enc~rchng bases of elongattng leaves, but depending upon species and sampling procedures, may include the terminal meristerfi and several unexpanded internodes of the stem~ (6, 14). High concentrations of total nonstructural carbohydrates (TNC) have been reported in leaf ~sheaths obtained from reproductive plants of timothy (Phleu~n pratense L.), and switchgrass (Panicum virgatum L.) (19). In addition, high TNC concentrations have been re" ~ Contribution from the Purdue Univ. Agric. Exp. Stn. , West Lafayette, 1N 47907. Journal Paper no. 1’02~8. Received 24 Apr. 1985. 2 Assistant professor, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907. ported in bases of elongati.ng. leaves of several grasses (9, 15, 25, 26). However, tt ~s unclear whether nonstructural carbohydrates in bases of elongating leaves are stored per se, serve as a substrate for biosynthesis, or are active in osmotic adjustment thereby maintaining positive turgor pressures in this meristematic tissue (2, 25, 26). The objectives of this study were to i) examine changes in concentration of nonstructural carbohydrates in individual leaf sheaths and bases of elongating leaves during regrowth following defoliation of tall fescue (Festuca arundinacea Schreb.); ii) relate changes in concentrations of nonstructurai carbohyd.rates in bases of elongating leaves to leaf elongation rates (LER); and iii) follow changes in the erage degree of polymerization (DP) of fructan tissues during regrowth. Intact stem bases were included for comparison. MATERIALS AND METHODS