Crop Science 43:258-265 (2003)
© 2003 Crop Science Society of America
TURFGRASS SCIENCE
Summer Root Decline
Production and Mortality for Four Cultivars of Creeping Bentgrass
Bingru Huang* and
Xiaozhong Liu
Dep. of Plant Sci. Rutgers Univ., New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu)
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ABSTRACT
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A better understanding of the seasonal dynamics of root activity for creeping bentgrass [Agrostis palustris Huds. = A. stolonifera var. palustris (Huds.) Farw.] would be beneficial for developing effective breeding and management programs to maximum summer turf quality. The objectives of this study were to examine dynamics changes in root production and mortality for four creeping bentgrass cultivars that differ in summer shoot performance. The study was conducted on a USGA-specification putting green in Manhattan, KS, during 1997 and 1998. The cultivars evaluated were ‘L-93’, ‘Penncross’, ‘Providence’, and ‘Crenshaw’. Grasses were mowed daily at 4 mm and irrigated on alternate days. Root production and mortality were monitored using the minirhizotron technique. In both years, total root length and number were highest in August and then decreased in September for all four cultivars. From July to September, the length and number of newly produced roots decreased while those of dead roots increased. The ratio of dead roots to live roots in length and number increased in late summer for all cultivars. While the differences in total root length and number among cultivars were not consistent between 2 yr, Penncross consistently had more dead roots, fewer new roots, and higher root mortality compared with the other cultivars. The results indicate that summer root decline of creeping bentgrass resulted from both decreased new root production and increased root mortality, which could be associated with high soil temperatures during the summer. Variation in root production and mortality may contribute to the differences in shoot summer performance of the four cultivars.
Abbreviations: DRL, dead root length • DRN, dead root number • NRL, new root length • NRN, new root number • RD, maximum rooting depth • RML, root mortality ratio in length • RMN, root mortality ratio in number • TRL, total root length • TRN, total root number
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INTRODUCTION
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TURF QUALITY of creeping bentgrass, a cool-season grass, often declines during the summer. The extent of summer bentgrass decline varies with cultivars (Wang et al., 1998; Liu and Huang, 2001). Accompanying turf quality decline are decreases in root biomass, number, and length (Beard and Daniel, 1965; Wang et al., 1998; Yelverton, 1999). Root systems of creeping bentgrass appear to be shortened during the summer (Carrow, 1996).
Changes in new root production and death of old existing roots may contribute to both the decline in turf and root quality of creeping bentgrass. Most studies on turfgrass roots evaluated only total root biomass, number, and length using the destructive sampling techniques such as soil coring (Ralston and Daniel, 1972; Kurtz and Kneebone, 1980; Wang et al., 1998; Yelverton, 1999). Root dynamic changes is less studied because root production and death are difficult to measure in situ in field conditions. The newly developed minirhizotron imaging technique is capable of distinguishing newly produced roots, existing roots, and decaying or dead roots (Upchurch and Ritchie, 1983; Ferguson and Smucker, 1989; Cheng et al., 1991). This nondestructive technique involves tracking the fate of individual roots that have grown against clear plastic tubes with a miniature video camera. Because the same roots are repeatedly examined across time, the technique eliminates spatial variation being confounded with temporal variation and permits a high frequency of root examination. Murphy et al. (1994) demonstrated that root density measured with minirhizotron was correlated with root length density measured by soil coring for both creeping bentgrass and annual bluegrass (Poa annua L.). This suggests that the minirhizotron method is suitable for studying root growth of turfgrasses throughout the growing season.
The objectives of this study were to investigate changes in root production and mortality of creeping bentgrass throughout the summer months for four creeping bentgrass cultivars, L-93, Penncross, Providence, and Crenshaw, and to determine whether decline in summer turf and root performance of creeping bentgrass is associated with changes in root production or mortality. These four cultivars differ in summer shoot performance. Our previous study found turf quality and canopy photosynthetic rate were highest for L-93, lowest for Penncross, and intermediate for Providence and Crenshaw during summer months of 1997 and 1998 (Liu and Huang, 2001).
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MATERIALS AND METHODS
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Plant Materials and Growing Conditions
Four creeping bentgrass cultivars, L-93, Providence, Crenshaw, and Penncross, were seeded in 2.05-m-wide by 7.35-m-long plots at a rate of 76 kg ha-1 during late September in 1996 on a USGA-specification putting green at the Rocky Ford Turfgrass Research Center, Kansas State University, Manhattan, KS. The green was covered with a fabric tarp from December 1996 to March 1997 to allow better canopy establishment. By June 1997, the canopy was completely closed. During the growing season from mid-June to mid-October in 1997 and from early May to late October in 1998, grasses were mowed daily, except Sundays, at a height of 4 mm. During this period, the green was irrigated on alternate days to replace 100% of the evapotranspiration rate of the previous 2 d. Turf received four applications of 216 kg ha-1 of N and 54 kg ha-1 of K and P in 1997 and 238 kg ha-1 of N and 60 kg ha-1 of K and P in 1998 to maintain adequate soil nutrients. In both years, Iprodione [(3-(3,5-dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamide) was applied at 3 kg ha-1 to control dollar spot and brown patch as a curative treatment as soon as initial symptoms were detected.
Experimental Design and Statistical Analysis
Cultivars were arranged in a randomized complete-block design with three replicates. Effects of cultivars, time of observation, and their interactions on various root parameters were analyzed as a repeated measure dataset using the analysis of variance according to the general linear model procedure of the Statistical Analysis System (SAS Inc., Cary, NC). Significance of cultivar differences and variation among times of observation was determined with the cultivar x time mean square as an error term using the standard F test. Differences between cultivar means at a given time of observation or differences between times for a given cultivar were separated by the least significance difference test at the 0.05 level.
Measurements
Soil temperature at the 5-cm soil depth was monitored at three locations in each plot with thermocouples, and data were collected with a CR-10 datalogger (Campbell Scientific Inc., Logan, UT). Root growth, production, and mortality were monitored from mid-June to late-October in 1997, and from late-May to late-October in 1998 using minirhizotrons (Upchurch and Ritchie, 1983). Two minirhizotron (clear butyrate) tubes (90 cm long and 5 cm in diameter), with each bottom plugged with a rubber stopper and sealed with waterproof silicon sealant, were installed at a 30° degree angle from the soil surface in each plot for root observation. On the upper side of each tube were etched frames (1 cm x 1.6 cm) that extended along the length of the tube, which allows the camera to return to exact locations in all tubes. It allows precise comparisons of individual viewing frames across time. The tubes were positioned in the soil with the upper end of each tube plugged with a rubber stopper level with the soil surface such that tubes did not interfere with mowing. Video images of roots visible against the upper surface of each tube were recorded using a high-magnification minirhizotron camera (BTX-100X, Bartz Technology Company, Santa Barbara, CA) and a Sony camcorder (Park Ridge, NJ). Root images were taken weekly in 1997 and twice per week in 1998 at 1.0-cm intervals along the upper surface of the tube, starting at 1.0 cm from the soil surface until a depth where no roots were found. Each image frame was 1.6 cm wide (across the tube) and 1.0 cm long (along the tube).
The majority of roots for creeping bentgrass were found in the upper 10 cm of soil. Therefore, image frames at 2 to 3, 5 to 6, and 8 to 9 cm were captured as TIF files onto a PC. Because the seasonal changes and cultivar variation showed the same pattern in all three depths, root data only at 5 to 6 cm, the intermediate depth, were reported in this paper. Roots were traced manually in each image frame and analyzed for root number and length using an image analysis program (ARCOS, Graphic Equation, Houston, TX). Root initiation and death per image frame were calculated based on appearance or disappearance of roots, respectively, by comparing the initial image with images recorded later in the same position of the tube. Roots that did not exist in an image but appeared as white and turgid in an image recorded later were considered as new roots. In contrast, roots that exhibited a dark color in a preceding image and were impossible to be discerned from the background color of the soil or disappeared in following images were defined as decaying, dead roots (Huck and Taylor, 1982). Roots that were repeatedly present in the same image frame were considered as existing roots. The length of all roots identified at a particular time is expressed as total root length, which is calculated as: Total root length = existing roots + new roots + dead roots. Root mortality was calculated as the percentage of total root length or number that were classified as dead. Maximum rooting depth was expressed as the soil depth below which roots were no longer found.
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RESULTS
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Soil Temperature
Daily maximum soil temperature fluctuated across time (Fig. 1). The monthly average was 22°C in June, increased to 32°C in July and August, and decreased to a 26°C and 22°C in October in both 1997 and 1998.
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Fig. 1. Daily soil temperature at the 5-cm soil depth from June to October in 1997 and 1998. The dotted line indicates the optimal temperature for root growth (18°C).
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Total Root Length and Total Root Number
Total root length (TRL) (Fig. 2) and total root number (TRN) (Fig. 3) increased from May in 1998 and June in 1997 to reach the highest in October. There was a decrease in September and a rapid increase in October. This growth pattern was observed in both 1997 and 1998.
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Fig. 2. Total root length for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the bottom of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Fig. 3. Total root number for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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During most of the experimental period in 1997, TRL and TRN were not significantly different among cultivars. In 1998, both TRL and TRN were higher for Penncross, Crenshaw, and Providence than L-93 during most of July and August. No difference in TRL and TRN were found among the first three cultivars during most of the experimental period.
New Root Production
All four cultivars exhibited vigorous root growth in May and June. This new growth was characterized by thick, white roots. New root length (NRL) (Fig. 4) for all four cultivars declined significantly from June in 1997 (cultivar average of 2.97 cm cm-2) and May in 1998 (3.25 cm cm-2) to the lowest level in August in both years. The average NRL in August was 1.41 cm cm-2 in 1997 and 1.53 cm cm-2 in 1998. New root length in October increased to an average of 2.0 cm in both years. An average of nine new roots per square centimeter of soil were found in May and June in 1997 and 1998, respectively (Fig. 5). By August, new root number (NRN) decreased to approximately three in 1997 and two new roots per square centimeter in 1998. From August through October there was an increase in new root length for all cultivars. New root length averaged 6 and 4 cm cm-2 in 1997 and 1998, respectively, in late October.
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Fig. 4. Length of newly produced roots for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top or bottom of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Fig. 5. Number of newly produced roots for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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During most of the experimental period in each year, particularly July and August, Penncross had lower NRL and NRN than L-93, Providence, and Crenshaw. The NRL in August 1997 averaged 1.62, 1.33, 1.60, and 1.12 cm cm-2 for L-93, Providence, Crenshaw, and Penncross, respectively. The NRL in August 1998 were 1.36 cm cm-2 for L-93, 1.38 cm cm-2 for Providence, 1.30 cm cm-2 for Crenshaw, and 0.95 cm cm-2 for Penncross. The differences in NRL and NRN among L-93, Crenshaw, and Providence did not show a consistent trend across time during the 2 yr of this study.
Root Mortality
Decaying, dead roots were characterized by a dark brown color that were impossible to be discerned from the background color of the soil or disappeared from a given image frame. Dead root length (DRL) (Fig. 6) and dead root number (DRN) (Fig. 7) were generally the lowest in June 1997 and May 1998 and reached the highest levels in August in both years for all four cultivars. Dead root length values averaged across all cultivars were 0.65 cm cm-2 in mid-June of 1997 and 0.74 in late May of 1998. Dead root length increased to an average of 4.5 cm cm-2 in August of 1997 and 1998. There were 
1 dead root cm-2 of soil in mid-June of 1997 and late May of 1998 and increased to 8 and 10 in August of 1997 and 1998, respectively.
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Fig. 6. Length of dead roots for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Fig. 7. Number of dead roots for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Penncross had higher DRL than L-93 and Providence in both 1997 and 1998 and than Crenshaw only in 1998 during most of July and August. Crenshaw had higher DRL than L-93 and providence in July and August of 1997 and mid-July and September in 1998. The DRL values averaged during July and August were 3.34, 3.56, 4.01, and 4.15 cm cm-2 for L-93, Providence, Crenshaw, and Penncross, respectively, in 1997; The corresponding figures were 3.31, 3.80, 3.97, and 4.53 cm cm-2 in 1998.
Dead root number was higher for Penncross than L-93, Providence, and Crenshaw in late July and August in both years, with the July and August average of 10, 8, 8, and 7 dead roots cm-2 of soil, respectively, in 1997, and 12, 9, 9, and 10 dead roots cm-2 of soil, respectively, in 1998. Differences in DRN among L-93, Providence, and Crenshaw did not show a consistent trend across the 2 yr of this study.
Root length mortality (RML) (Fig. 8) averaged across all four cultivars increased from 16 and 14% in mid-June of 1997 and 1998, respectively, to an average of 58 and 49% during August of 1997 and 1998, respectively. The level of root mortality in number (RMN) increased from 13% in mid-June of 1997 to an average of 41% during August of 1997 and from 26 to 46% in 1998 (Fig. 9).
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Fig. 8. Root mortality in length for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Fig. 9. Root mortality in number for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top of the figure are LSD values (P = 0.05) for cultivar comparisons at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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The levels of RML and RNN were significantly higher for Penncross than for the other three cultivars from late July to September in both years. No consistent difference in RML and RMN was observed among L-93, Providence, and Crenshaw during the 2 yr of this study.
Maximum Rooting Depth
Maximum rooting depth (RD) averaged across the four cultivars changed from an average of 25 cm in June to 21 cm in September in both 1997 and 1998 (Fig. 10).
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Fig. 10. Maximum rooting depth for ‘L-93’, ‘Providence’, ‘Penncross’, and ‘Crenshaw’ in 1997 and 1998. Vertical bars on the top are LSD value (P = 0.05) among four cultivars at a given day. The bar on the right side is for comparisons among dates for all four cultivars.
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Crenshaw maintained greater RD than Penncross and providence from June to September in 1997 and from June to August in 1998. A higher RD was detected for Crenshaw compared with L-93 in June and July of both years. During most of the study period, L-93 had a greater RD than Providence in 1997 and than Penncross in 1998.
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DISCUSSION
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Summer Root Decline
Total root length and number increased until late July and thereafter, plateaued or even decreased slightly in August and September. From September through October both root length and number increased, reaching their highest level on the last rating date of each year. Maximum rooting depth was highest in May, declined during July to September, and recovered somewhat in October. These results indicate that root growth of creeping bentgrass exhibits seasonal patterns, with slowing root growth and weakening rooting ability during summer. This is in agreement with the observations of other researchers. Using a large rhizotron, Karnok and Kucharski (1980) and Koski (1983) observed that total root length of creeping bentgrass was greatest during the months of March through June, with a second (but smaller) period of increase occurring during October. Murphy et al. (1994) reported that total number of roots excluding dead roots, which may indicate new root production, was greatest early in the growing season followed by a decline during July for creeping bentgrass and annual bluegrass. In our study, total number of roots with all dead roots being counted, which represents total root production or accumulation, increased from May or June to reach the maximum in October.
Previous research in seasonal rooting characteristics of turfgrass mainly examined total root biomass or length and did not distinguish root birth from death (Ralston and Daniel, 1972; Karnok and Kucharski, 1980; Kurtz and Kneebone, 1980; Koski, 1983; Murphy et al., 1994; Wang et al., 1998 Yelverton, 1999). In our study, the dynamics of root birth and death was monitored nondestructively using the minirhizotrons, which demonstrated that summer root decline of creeping bentgrass in terms of total root length or number was associated with decreased new root production and increased root mortality, but to a greater extent with increased root mortality. Few new roots were still produced during July, August, and September, although maximum root production was observed in May and June. This indicated that new root production did not cease during summer, but was minimal during warmest periods. The small amount of new root production during summer months could be related to short periods of soil cooling during the summer. Beard and Daniel (1965)(1966) found that root production occurred immediately following temperature drops and suggested that lower soil temperatures encourage or are required for new root growth.
Root mortality was 40 to 60% for both cultivars of creeping bentgrass during midsummer, implying that about one-half of the roots produced prior to the summer died during the hot periods. Agrostis tenuis Sibth. (= A. capillaris L.), a species closely related to creeping bentgrass, replaces about one-half of its root system each year; root production of A. tenuis is greatest in spring and fall periods, with little or no root growth occurring during the summer months (Sprague, 1933; Stuckey, 1941). The pattern of seasonal changes in root production and mortality with creeping bentgrass was similar to that of A. tenuis. The results of this study further demonstrate that root decline in creeping bentgrass during the summer was due to higher root mortality coupled with a decline in the rate of root production and growth. These changes could account for the observed shortening of rooting depth and decrease of overall turf quality.
Midsummer decline of root production and increased mortality of existing roots were related closely to changes in soil temperatures. Cool-season turfgrasses maintain optimal root growth at temperatures between 10 and 18°C (Beard, 1973). In this study, the daily maximum soil temperature was higher than 18°C from July to September. Several studies conducted in controlled environmental growth chambers reported that root production was greatly reduced and root death increased at high soil temperatures (Schmidt and Blaser, 1967; Ralston and Daniel, 1972; Xu and Huang, 2000). Xu and Huang (2000) reported that high soil temperature was more detrimental than high air temperature to root growth and activity.
Changes in root growth are related to variations in shoot growth. The decrease in root growth and increases in root mortality were accompanied by the decline in turf quality, photosynthesis, and leaf photochemical efficiency during summer (Liu and Huang, 2001). Root growth inhibition and death reduce the supply of water, nutrients, and root-produced hormones such as cytokinins to the shoots, which is probably a major factor leading to the decline in turf quality (Carrow, 1996). In contrast, the formation and elongation of roots is controlled, in part, by carbohydrate relations and plant hormones that are governed by shoot growth (Charlton, 1996).
Cultivar Variation
Cultivar differences in total root length and number were not consistent across the 2 yr of this study. Wang et al. (1998) also reported inconsistent cultivar variations for total root weight in creeping bentgrass, even though summer shoot performance consistently varied among cultivars. However, across the 2 yr of this study, L-93, Crenshaw, and Providence consistently showed more new root production and lower mortality rate than Penncross, with the greatest differences occurring during July and August. The differences among the three cultivars other than Penncross were not consistent across different years. This is consistent with previous studies (Huang et al., 1998) where root viability decreased with temperature increases, to a greater extent for Penncross than Crenshaw and L-93. The maximum rooting depths of the cultivars had a general ranking of Crenshaw > L-93 
Providence > Penncross.
Development of deep, extensive root systems (big root mass or length) is often emphasized as an important factor in plant growth and adaptation to environmental stresses. Our study demonstrated persistent production of new roots and maintenance of low root mortality during summer could contribute to creeping bentgrass adaptation to summer stress. Maintenance of high root viability or low root mortality could be used as selection criteria in breeding for improving summer performance of cool-season turfgrasses. Seasonal growth habits of roots and cultivar variations in rooting characteristics should be a consideration in developing effective turfgrass management practices, particularly the timing of fertilization that is critical in the culture of cool season turfgrasses.
Received for publication June 1, 2001.
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REFERENCES
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ABSTRACT
INTRODUCTION
