Mark Howieson and Nick Christians
Mowing is a common cultural practice essential to maintain dense, uniform, and appealing turfgrass. Mowing is defoliation, and therefore, removes leaf tissue that plants use to capture light energy from the sun. Captured light provides energy for the production of organic molecules, such as proteins and carbohydrates. Regrowth and initiation of new leaf tissue after defoliation is important to develop a new photosynthetic surface required for the production of carbohydrates. Carbohydrate and nitrogen reserves provide energy and raw materials for shoot and leaf regeneration. The most abundant carbohydrates in creeping bentgrass are fructose, glucose, sucrose, and fructan. Fructose and glucose can be consumed to provide energy or incorporated into reserve carbohydrates when energy is abundant. Sucrose is transported via the phloem from source to sink tissues and it is broken down into one fructose and one glucose. Fructan is a reserve carbohydrate found in creeping bentgrass and is composed primarily of fructose molecules. The objective of this study was to determine how fructose, glucose, sucrose, and fructan concentrations change over time in response to defoliation.
Creeping bentgrass seeds were planted in 14 x 20 inch flats filled with sand and grown in a greenhouse. High-pressure sodium lamps provided a 16-hour photoperiod. The sand was fertilized weekly with a modified Hoagland's solution that supplied 0.75 lbs of nitrogen per 1000 ft2 per month. Grass was clipped three times a week at a height of 0.5 inch with an electric reel mower prior to the initiation of the study.
There were four mowing height treatments. Grasses were clipped at 0.50, 0.35, 0.20 inches and not mown. Following mowing treatments, approximately 1.5 grams of stubble tissue were collected every 24 hours for 72 hours. A subsample was dried at 68ºC for three days for dry weight analysis, the remaining tissue was frozen and ground in liquid nitrogen. Ground samples were stored at -20ºC until further analysis.
Carbohydrates were extracted by placing the frozen tissue into 8 mL of boiling 80% ethanol for one hour. The supernatant was poured off and 5 mL of boiling water was added to the grass tissue. The tissue was extracted in the water for 1 hour and the aqueous supernatant was poured off and pooled with the ethanolic supernatant. The pooled supernatants were evaporated to dryness using a rotary evaporator and the insoluble residue was dissolved in 5 mL of water.
Carbohydrate concentration was determined by high performance liquid chromatography (HPLC) using a Sugar Pak I column and a refractive index detector. The mobile phase was 0.1 mM CaEDTA in water. The flow rate was 0.5 mL/minute and the column was heated to 90ºC. Inulin, sucrose, fructose, and glucose were used as standards.
The study was in a completely randomized design with three replications per treatment. Fisher's LSD (p=0.05) was used to separate means.
Fructan concentration declined in grass clipped at 0.35 and 0.50 inches for 48 hours after mowing (Figure 1). Fructan concentrations 72 hours after mowing were similar to those observed at the start of the study. Grasses clipped at 0.20 inch had the lowest fructan concentration at all sampling times and fructans did not accumulate 48 hours after mowing as they did in grasses mown at 0.50 and 0.35 inches (Figure 1). No differences were observed in sucrose concentration among the four mowing treatments (Figure 2). Grasses mown at 0.20 inches consistently had the lowest glucose and fructose concentrations over the 72-hour study (Figure 2 and 3).
These results emphasize the importance of not removing more than 1/3 of shoot tissue in a single mowing. Grasses mown at 0.20 had approximately 60% of above ground tissue removed and remained chlorotic for several weeks after mowing. These plants consumed large quantities of carbohydrates to recover from mowing stress. Grasses mown at 0.35 inches had 30% of above ground tissue removed by mowing. These plants were able to recover rapidly from the mowing stress by consuming stored carbohydrates to provide energy for regrowth. Future research will examine the quantities and activities of enzymes responsible for synthesis and catabolism of fructan, sucrose, glucose, and fructose. This research will further our understanding of how mowing affects carbohydrate metabolism.
Figure 1. Fructan concentration over time of creeping bentgrass after defoliation
Figure 2. Sucrose concentration over time of creeping bentgrass after defoliation
Figure 3. Glucose concentration over time of creeping bentgrass after defoliation
Figure 4. Fructose concentration over time of creeping bentgrass after defoliation
 |
ISU Turfgrass:2003 Turfgrass Report | College of Agriculture |
Contents ©1995-2003, ISU Horticulture Department