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.

ble. Kust and Smith (1961) reported that 'Vernal' alfalfa ideally would be harvested three times (at about 42-d Producers in the lower Midwest often plant grazing-type alfalfa intervals) before 1 September in Wisconsin as a compro-(Medicago sativa L.) cultivars assuming they can withstand frequent hay harvests. However, little research has examined the long-term mise among forage yield, quality, and stand persistence. effects of frequent hay harvests on grazing-type compared with hay-However, harvesting four or more times during the type alfalfa. Our objective was to determine how cultivar-type and growing season or after 1 September reduced yields the harvest frequency change long-term alfalfa yields, quality, and stand following year by 30 to 78%. Subsequent studies by persistence. 'Alfagraze' (grazing-type), 'Pioneer 5373' (modern, Matches et al. (1970), Brink and Marten (1989), Brown multipest resistant hay-type), and 'Cody' (older hay-type) alfalfa were et al. (1990), and Sheaffer et al. (2000) have shown that seeded on 1 Apr. 1994, near Mt. Vernon, MO. Cultivars were main alfalfa should be harvested every 30 to 35 d during the plots, and four, five, or six harvests before 15 September (42, 34, and growing season to maximize forage yield, quality, and 28 d between harvests, respectively) were subplots in a randomized stand persistence. complete block in a split-plot arrangement. Cumulatively, over 5 yr, Research from Georgia suggests that harvesting al-Pioneer 5373 produced 6% (2.8 Mg ha Ϫ1 ) more forage than Alfagraze and 10% (5.0 Mg ha Ϫ1 ) more than Cody. Alfalfa harvested four times falfa frequently during the growing season may be less yielded 7% (0.8 Mg ha Ϫ1 ) more per year than when harvested five detrimental for grazing-type cultivars than for traditimes, and 28% (3.1 Mg ha Ϫ1 ) more than when harvested six times.tional hay-types Bouton, 1991, 1992).Alfagraze and the hay-types yielded equally when harvested six times. Brummer and Bouton (1991) found that Alfagraze, a Alfagraze usually had equal or lower detergent fiber, and equal or cultivar selected under continuous grazing pressure, was greater crude protein (CP) concentrations than other cultivars. Harmorphologically different than modern hay-types, being vesting more frequently improved forage quality but had little impact more decumbent and having many, thin stems compared on plant density. We concluded that a grazing-type, like Alfagraze, with modern hay-types. They suggested that these mordoes not show a yield or persistence advantage over a modern hayphological traits made grazing-type alfalfa cultivars toltype under frequent hay harvest regimes in the lower Midwest. Howerant of frequent harvests. Subsequent research with ever, the grazing-type used in this study often had superior forage quality.

Leaf elongation rate (LER) in grasses is dependent on epidermal cell supply (number) and on rate and duration of epidermal cell elongation. Nitrogen (N) fertilization increases LER. Longitudinal sections from two genotypes of tall fescue (Festuca arundinacea Schreb.), which differ by 50% in LER, were used to quantify the effects of N on the components of epidermal cell elongation and on mesophyll cell division. Rate and duration of epidermal cell elongation were determined by using a relationship between cell length and displacement velocity derived from the continuity equation. Rate of epidermal cell elongation was exponential. Relative rates of epidermal cell elongation increased by 9% with high N, even though high N increased LER by 89%. Duration of cell elongation was approximately 20 h longer in the high-than in the low-LER genotype regardless of N treatment. The percentage of mesophyll cells in division was greater in the high-than in the low-LER genotype. This increased with high N in both genotypes, indicating that LER increased with cell supply. Division of mesophyll cells adjacent to abaxial epidermal cells continued after epidermal cell division stopped, until epidermal cells had elongated to a mean length of 40 micrometers in the high-LER and a mean length of 50 micrometers in the low-LER genotype. The cell cycle length for mesophyll cells was calculated to be 12 to 13 hours. Nitrogen increased mesophyll cell number more than epidermal cell number: in both genotypes, the final number of mesophyll cells adjacent to each abaxial epidermal cell was 10 with low N and 14 with high N. A spatial model is used to describe three cell development processes relevant to leaf growth. It illustrates the overlap of mesophyll cell division and epidermal cell elongation, and the transition from epidermal cell elongation to secondary cell wall deposition.Leaf growth in grasses is predominantly unidirectional, parallel with the longitudinal axis of the leaf. A 3 Abbreviations: SLW, specific leaf weight; LER, leaf elongation rate; PPFD, photosynthetic photon flux density. 549Volenec and Nelson (29) used high and low rates of N fertilization to further alter LER of two genotypes of tall fescue selected for contrasting leaf growth rates. Mean LER of these genotypes was 89% higher with high N, but length of fully elongated epidermal cells was unaffected by N treatment. The number of epidermal cells produced per file per day increased 90% with high N, suggesting that much of the increase in LER following N fertilization was due to increased cell division.Our long-term goal is to understand carbohydrate metabolism during the growth of grass leaves. Our objectives in the present study were to evaluate the effect of N on mesophyll cell division and rate and duration of epidermal cell elongation in leaf blades of two genotypes of tall fescue. Data for epidermal cell lengths utilized here have been reported previously (28, 29).Mesophyll cell division data reported here, and data for leaf elongation rate and structural ...

Leaf elongation rate (LER) of grasses depends on N supply and i s expressed mostly through cell production, whereas most N in mature leaf tissues is chloroplastic. Our objective was to evaluate a possible competition for N between cell production and chloroplast development processes, utilizing the gradient of cell development along the leaf growth zone of tal1 fescue (Festuca arundinacea Schreb.). Under the two contrasting N regimes, total N content was highest in the cell production zone, declined sharply as cells elongated, and remained relatively constant in more dista1 positions, at values close to those measured in mature tissues. A similar pattern was found for N in proteins and nucleic acids that were not soluble in 80% ethanol. Content of N compounds soluble in 80% ethanol was higher in the cell production and elongation zones than in mature parts of the leaf. NO,-N content was low in the cell production zone and increased in the cell elongation zone for high-N plants. l h e deposition rate of total N in the growth zone was much higher with plants in high N than in those shifted to no N. For both N regimes, most N was deposited during cell production and early cell elongation. Little N was deposited during cell maturation where ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was being actively synthesized. lhis suggests that synthesis of Rubisco, and probably other chloroplastic proteins, occurs largely from recycling of N that was previously incorporated into proteins during cell production. Thus, Rubisco content in mature tissues is more closely associated with N deposited during cell production than with N deposited during its biosynthesis.

Investigations were performed to better understand the carbon economy in the elongation zone of tall fescue leaf blades. Plants were grown at constant 21 degrees C and continuous 300 micromoles per square meter per second photosynthetic photon flux density where leaf elongation was steady for several days. Elongation occurred in the basal 20 mm of the blade (0-20 millimeters above the ligule) and was maximum at 9 to 12 millimeters. Eight 3-millimeter long segments were sampled along the length of the elongation zone and analyzed for water-soluble carbohydrates. Sucrose concentration was high in the zone of cell division (0-6 millimeters) whereas monosaccharide concentration was high at and distal to the location where cell elongation terminated (20 millimeters). Fructan concentration increased in the basal part, then remained constant at about 85% of the total mass of water-soluble carbohydrates through the remainder of the elongation zone. Data on spatial distribution of growth velocities and substance contents (e.g. microgram fructan per millimeter leaf length) were used to calculate local net rates of substance deposition (i.e. excess rates of substance synthesis and/or import over substance degradation and/or export) and local rates of sucrose import. Rates of sucrose import and net deposition of fructan were positively associated with local elongation rate, whereas net rates of sucrose deposition were high in the zone of cell division and those of monosaccharide were high near the termination of elongation. At the location of most active elongation imported sucrose (29.5 milligrams per square decimeter per hour) was used largely for synthesis of structural components (52%) and fructan (41%).