By Bingru Huang, X. Liu, and Q. Xu
Turf quality of creeping bentgrass often declines on golf courses in warm climatic regions during summer months, which is typically accompanied or preceded by root shortening or death. This problem has been broadly defined as summer bentgrass decline.1,2
Many cultural and environmental factors may be associated with summer decline in turf quality and root growth. Dernoeden2 suggested that summer bentgrass decline may be more a physiological rather than a pathological problem. Indirect high temperature is one of the major factors causing loss of turf for creeping bentgrass. Mowing turf too short, such as the ultra-low mowing of today's putting greens, imposes additional stress on the turf by removing large amounts of leaf area that would otherwise be available for photosynthesis and carbohydrate production. Nonstructural carbohydrates in plants serve as energy reserves to be used under stressful conditions.5 Closely mowed turf may suffer from heat stress injury by depleting carbohydrate reserves due to the increased demand for carbohydrates (i.e., increased respiration) and decreased production of carbohydrates (i.e., decreased net photosynthesis).
This report summarizes results of our controlled-environment and field studies with the aim to better understand how carbohydrate metabolism is related to summer decline in turf quality and root activities of creeping bentgrass. Such information is important for developing effective management strategies to prevent or control summer bentgrass decline.
DECLINE IN TURF QUALITY AND ROOT GROWTH UNDER HEAT STRESS
Decline in turf quality and root growth for creeping bentgrass has been observed under high temperatures (above 80°F) in controlled-environment studies and during summer months in field plots.3,6,7,9,10 Creeping bentgrass cultivars vary in heat tolerance as demonstrated by differences in severity of turf quality decline with increasing temperatures. Our studies have identified L-93 to be more heat tolerant than Penncross, with less severe decline in turf quality and physiological activities under high-temperature conditions in controlled-environment studies and during summer months in the field.
 |
| Root growth of field-grown Crenshaw creeping bentgrass as shown using the mini-rhizotron technique in Manhattan, Kan. Photos show root growth observed on June 12, June 22, July 20, Aug. 10, Sept. 7, and Oct. 19, 1998. Root length and number of newly produced roots decreased while dead roots increased from July to September. |
Root production and mortality of three creeping bentgrass cultivars, Crenshaw, Penncross, and L-93, were monitored using the minirhizotron imaging technique in a USGA-specification putting green mowed at 0.125 and 0.156 inches in Manhattan, Kansas, during 1997 and 1998.4 For all cultivars, the length and number of newly produced roots decreased, while those of dead roots increased from July to September in both years. Root mortality rate exceeded root production rate, resulting in decline in total root length and number. L-93 maintained higher production of new roots and lower root mortality than Penncross during summer months, suggesting that cultivar difference in root production and mortality was associated with the differences in summer turf performance between heat-tolerant and heat-sensitive cultivars.
 |
|
| Scientists at Rutgers University continue their research to identify factors affecting summer bentgrass decline. The goal is to use the results of these physiological studies to develop cultural management strategies to limit the quality decline of this important putting green species. |
One of our recent studies found that root death and decreases in root metabolic activity, such as hormone synthesis (i.e., cytokinin production) and nutrient and water uptake, precede turf quality decline under heat stress in controlled-environment conditions.8 Root death occurred at 5 days of exposure to 95°F, followed by decline in cytokinin synthesis, nutrient and water content, and at last with turf quality declining at 20 days of heat stress. These results suggested that decreases in root activity and increased root mortality contribute to loss of turf quality in creeping bentgrass exposed to high temperatures. Improving heat tolerance of the root system is important for maintaining high-quality turf during summer months in warm climatic regions.
CARBOHYDRATE METABOLISM AND SUMMER BENTGRASS DECLINE
A field study was conducted in 1999 and 2000 in Manhattan, Kansas, to investigate whether summer decline in turf quality and root dieback are related to carbohydrate availability during summer months. Penncross and L-93 were examined in the study. Grasses were managed under USGA-specification putting green conditions with daily irrigation and were mowed at 4mm (0.175 inch).
Turf quality and the content of total nonstructural carbohydrate (TNC) and soluble sugars in shoots and roots, as well as carbon allocation to roots, exhibited seasonal variations as temperature changed across the seasons.12 Turf quality of both cultivars was highest in May, declined to the lowest level in August and September, and returned to a high level in October. Corresponding to seasonal variations in turf quality, the content of total nonstructural carbohydrates, sucrose, and fructans in both shoots and roots for both cultivars was highest in spring and fall and lowest during summer months in both years. Summer decline in carbohydrate content was more pronounced in roots than in shoots.
 |
| Photosynthetic and respiration rates of Penncross creeping bentgrass mowed at 5⁄32 and 1⁄8 inches during 1997 and 1998. Green lines indicate carbohydrate-producing photosynthetic rates, while red lines indicate carbohydrate-consuming respiration rates. Data show that as photosynthesis decreases during summer months, especially at the lower mowing height, respiration increases. The result is a net loss of available carbohydrates (shaded area) closely associated with the decline of turfgrass quality. |
In addition, the amount of carbon allocated to roots also decreased during summer months, particularly for heat-sensitive Penncross. Our studies conducted in controlled-environment growth chambers found that carbohydrate availability in shoots and roots decreased with increasing temperatures along with the decline in turf quality.3,6,7,9,10 Our results demonstrated that the decline in carbohydrate availability in shoots and roots, particularly in roots, and limited carbon allocation to roots during summer months contributed to the decline in turf quality and root dieback of creeping bentgrass under high-temperature conditions.
The decline in carbohydrate content in both shoots and roots during the summer may have resulted from an imbalance between carbon production (photosynthesis) and consumption (respiration).3,7,10 Our study also measured seasonal changes in turf quality, carbohydrate production through photosynthesis, and carbohydrate consumption through respiration using an infrared gas analyzer for creeping bentgrass mowed at 0.156 and 0.125 inch heights.
Turf quality declined more rapidly at the lower mowing height, which was attributed mainly to reduced leaf area, limiting photosynthesis. We found that canopy net carbon fixation rate decreased, whereas respiration or carbon consumption rate increased for both L-93 and Penncross during summer months. Carbon consumption rate exceeded carbon fixation rate in August and September when temperature was highest, particularly for grasses mowed at 0.125-inch height, where photosynthetic capability was most limited. The imbalanced carbon fixation and consumption, particularly for low-mowed turf, may lead to carbohydrate depletion and a decline in turf and root growth during summer months.
In summary, prolonged heat stress may cause carbohydrate depletion, leading to summer bentgrass decline. Close mowing that removes large amounts of leaves from plants can reduce total carbohydrate production, contributing to summer bentgrass decline. Cultural practices that could promote carbohydrate production, including raising mowing height, would be helpful in maintaining quality bentgrass greens during summer.
LITERATURE CITED
1. Carrow, R. N. 1996. Summer decline of bentgrass greens. Golf Course Management 64:51-56.
2. Dernoeden, P. H. 1998. Summer bentgrass decline complex may be more physiological than pathological. Turfax 6(2):4-5.
3. Huang, B., and H. Gao. 2000. Growth and carbohydrate metabolism of creeping bentgrass cultivars in response to increasing temperatures. Crop Sci. 40:1115-1120.
4. Huang, B., and X. Liu. 2003. Summer root decline: Production and mortality for four cultivars of creeping bentgrass. Crop Sci. 43:258-265.
5. Hull, R. 1992. Energy relations and carbohydrate partitioning in turfgrass. pp. 175-205. In D. V. Waddington, R. N. Carrow, and R. C. Shearman (eds.). Turfgrass. Amer. Soc. of Agron., Madison, Wis.
6. Liu, X., and B. Huang. 2000. Carbohydrate accumulation in relation to heat stress tolerance in two creeping bentgrass cultivars. J. Amer. Soc. Hort. Sci. 125:442-447.
7. Liu, X., and B. Huang. 2001. Seasonal changes and cultivars difference in turf quality, photosynthesis, and respiration of creeping bentgrass. HortScience 36:1131-1135.
8. Liu, X., and B. Huang. 2005. Root physiological factors involved in cool-season grass response to high soil temperature. Env. and Exp. Botany 53:233-245.
9. Xu, Q., and B. Huang. 2000. Growth and physiological responses of creeping bentgrass to changes in air and soil temperatures. Crop Sci. 40:1363-1368.
10. Xu, Q., and B. Huang. 2000. Effects of differential air and soil temperature on carbohydrate metabolism in creeping bentgrass. Crop Sci. 40:1368-1374.
11. Xu, Q., and B. Huang. 2001. Morphological and physiological characteristics associated with heat tolerance in creeping bentgrass. Crop Sci. 41:127-133.
12. Xu, Q., and B. Huang. 2003. Seasonal changes in carbohydrate accumulation for two creeping bentgrass cultivars. Crop Sci. 43:266-271.
Bingru Huang, Ph.D., Professor; Xiaozhong Liu, Ph.D. candidate; and Qingzhang Xu, Post-Doctoral Research Associate; Department of Plant Biology and Pathology, Rutgers University, New Brunswick, N.J.
