Crop Science 42:1241-1250 (2002)
© 2002 Crop Science Society of America
TURFGRASS SCIENCE
Mowing Effects on Root Production, Growth, and Mortality of Creeping Bentgrass
Xiaozhong Liu and
Bingru Huang*
Dep. of Plant Science, Rutgers Univ., New Brunswick, NJ 08901
* Corresponding author (huang{at}aesop.rutgers.edu)
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ABSTRACT
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Understanding the response of root growth for creeping bentgrass {Agrostis palustris Huds. [ = A. stolonifera var. palustris (Huds.) Farw.]} to different mowing regimes will help develop effective management practices to maintain high quality turf. The objective of this study was to examine the effects of mowing height on root elongation, production, and mortality for two creeping bentgrass cultivars throughout the growing season, particularly during summer months. ‘Crenshaw’ and ‘Penncross’ grown in a USGA-specification putting green were mowed daily at two mowing heights: 3 and 4 mm. In comparison with mowing at 4 mm, mowing at 3 mm reduced cumulative new root length (CNRL), total root length (TRL), and maximum rooting depth (RD), but increased dead root length (CDRL), root turnover rate in both length and number, and root mortality at 3- and 9-cm soil depths in both cultivars. The decline in new root production and increase in root mortality occurred earlier in the season and were more pronounced at the 3-cm soil depth than at the 9-cm depth for both cultivars. The effects of low mowing on root growth were more pronounced during summer months than in the spring. Lowering mowing height during summer would be more detrimental for root growth than maintaining a higher mowing height, which could lead to turf quality decline of creeping bentgrass.
Abbreviations: CDRL, cumulative dead root length • CNRL, cumulative new root length • RD, maximum rooting depth • TL, the ratios of dead/new roots in length • TN, the ratios of dead/new roots in number • TRL, total root length • TRN, total root number
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INTRODUCTION
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CREEPING BENTGRASS is a widely used cool-season turfgrass on golf course greens in the transition zone and cool climatic areas. Currently, the typical mowing height of a creeping bentgrass green is 
4 mm. Lowering mowing heights to increase putting green speed is a common practice, but it can increase turfgrass susceptibility to environmental stresses, including heat stress (Beard and Daniel, 1965; Beard, 1973). Extensive research has found that low mowing reduces turf quality and growth of creeping bentgrass, especially during summer (Salaiz et al., 1995; Carrow, 1996; Beard, 1997; Huang et al., 1998).
Roots have been investigated much less than the aboveground parts of plants, because soil limits their accessibility for observation. The limited research on effects of mowing on rooting of turfgrass species has examined mainly total root number (TRN), biomass, or length by destructive sampling of the root system (Juska and Hanson, 1961; Beard and Daniel, 1965; Younger and Nudge, 1974; Krans and Beard, 1975; Parr et al., 1984; Salaiz et al., 1995; Yelverton, 1999). Previous research generally has found that root growth is reduced by lower mowing height, but little information is available on mowing effects on root production, turnover, or mortality. Yet, root production, turnover, and mortality are critical components influencing plant adaptation to various environmental stresses.
Root production and death are difficult to measure in situ in field conditions. Until recently, most root studies have been conducted by destructively sampling with a soil coring technique, which has several disadvantages (Fogel, 1985; Vogt et al., 1986; Eissenstat and Caldwell, 1987). Two of the most serious are (i) the inability to detect simultaneous initiation and death of roots and (ii) the inability to separate sampling and spatial variation from temporal variation. The minirhizotron imaging technique allows nondestructive monitoring of root production and mortality or life span (Upchurch and Ritchie, 1983; Ferguson and Smucker, 1989; Cheng et al., 1991; Hendrick and Pregtizer, 1993a,b). The 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 examined repeatedly, the technique eliminates confounding spatial variation with temporal variation and permits a high frequency of root examination. Its greatest advantage is that it provides greater information on the dynamic changes and demographics of roots. Until now, only one study in turfgrass has used minirhizotrons, which evaluated TRN in creeping bentgrass and annual bluegrass (Poa annua L.) without distinction of dynamic changes of root production and mortality (Murphy et al., 1994). This study suggests that the minirhizotron method is suitable for studying root phenology and profile distribution of turfgrasses and gives a reasonable measurement of root quantity throughout the growing season.
The objective of the current study was to examine mowing effects on new root production, root turnover rate, and mortality using the minirhizotron imaging technique for two creeping bentgrass cultivars throughout the growing season, particularly during summer months. Two cultivars were studied to determine whether the effects of mowing on the seasonal patterns of root growth and mortality are consistent between cultivars.
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MATERIALS AND METHODS
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Plant Materials and Growing Conditions
The experiment was conducted in 1997 and 1998 on a USGA-specification putting green at the Rocky Ford Turfgrass Research Center, Manhattan, KS. Two creeping bentgrass cultivars, Crenshaw and Penncross, were seeded in 215-cm-wide by 735-cm-long plots at a rate of 76 kg ha-1 in late September 1996. 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 except from early May to late October in 1997 and 1998, grasses were mowed daily, except Sundays, at 4 mm. During the entire experimental period, the green was irrigated daily to replace 100% of the evapotranspiration rate on the previous day. Grasses were syringed (wetting turf canopy with a small amount of water) on hot summer days. Turf received total N of 216 kg ha-1 yr-1 in 1997 and 238 kg ha-1 yr-1 in 1998 to maintain adequate soil nutrient status. In both years, Iprodione [3-(3,5-dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamide] at 3.1 kg ha-1 was applied several times during the study period to control diseases such as dollar spot (caused by Sclerotinia homoeocarpa F.T. Bennett) and brown patch (Rhizoctonia solani Kühn) as a curative treatment as soon as initial symptoms were detected.
Treatment and Experimental Design
Two mowing heights were imposed on both cultivars from early May to late October in 1997 and 1998. Grasses were mowed at 3 mm (low mowing) or 4 mm (high mowing) for 6 d wk-1.
The experiment was a randomized complete block (split plot) design with repeated measures. Cultivars are the main plots (each 735-cm-long and 205-cm-wide) and mowing heights are subplots (each 368 cm long and 205 cm wide). Both cultivar and mowing height treatments were replicated three times in three randomly assigned plots. The main effects of cultivar, mowing, and time of measurement, as well as their interactions were determined by analysis of variance according to the general linear model procedure of the Statistical Analysis System (SAS Inc., Cary, NC). A significant interaction between mowing height and time of measurement was detected for all measured parameters in each cultivar. Therefore, differences between mowing treatments were separated by the F-protected LSD test at the 0.05 level at a given date of measurement for each cultivar.
Measurements
Root growth, production, and mortality were monitored from mid-June to late-October in 1997, and from late-May to late-October in 1998 with the minirhizotron imaging technique (Upchurch and Ritchie, 1983). Two minirhizotron (clear butyrate) tubes (90-cm-long and 5 cm in diameter), with bottom plugged with a rubber stopper and sealed with waterproof silicon sealant, were installed at a 30° angle from the soil surface in each plot for root observation. On the upper side of each tube were etched frames (1 by 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 tube was positioned in the ground with the upper end of the tube plugged with a rubber stopper leveled with the soil surface such that tubes did not interfere with mowing management. Video images of roots visible against the surface of the tube were recorded using a high-magnification minirhizotron camera (BTX-100, Bartz Technology Company, Santa Barbara, CA) and a camcorder (Sony Electronics, Inc., Park Ridge, NJ). Root images were taken weekly in 1997 and biweekly in 1998 at 1-cm intervals along the upper surface of the tube, starting at 1 cm from the soil surface until a depth where no roots were found.
Majority of roots were found in the upper 10 cm of soil. Therefore, root images at 2- to 3- and 9- to 10-cm soil layers were captured as TIF files onto a PC and analyzed using an image analysis program (ARCOS, Graphic Equation, Houston, TX). Calibration of the system was done using known length and number of fine string pieces similar in diameter to roots of creeping bentgrass. A calibration image was created by taping the strings in random orientation to the outside of a minirhizotron tube. Images of the calibration strings were recorded using the minirhizotron camera in the same way as an image of roots growing against the side of the tube was recorded. Using this image, the calibration function of the software was used to create a calibration curve of pixels vs. true length (cm) or number of roots. This procedure worked well because the image of the calibration strings similar to creeping bentgrass roots was taken at the same focal distance on the camera as the images of the roots would be, and the calibration was performed in the same way as the root measurements were. The calibration was performed at the beginning of data collection and used the same calibration for all tubes.
The length and number of roots on the first image recorded at each depth for each tube was traced and measured manually. Root initiation and death per image frame recorded later in the same position of the tube were then calculated based on appearance or disappearance of roots, respectively, by comparing with the initial image. Roots that did not exist in an image but appeared in an image recorded later were considered as fresh, new roots. In contrast, roots that were present in an image but absent in an later image were considered dead roots. Roots that were repeatedly present in an image were considered as existing roots. The length of all roots produced at a particular time is expressed as TRL, which is calculated as:
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Root turnover rates in length and number were calculated as the ratio of cumulative dead roots to cumulative new roots. Root mortality was calculated as the percentage of dead roots of cumulative total roots. 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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Total Root Length
Total root length (Fig. 1)
at the 3- and 9-cm soil depths for both cultivars increased rapidly from May to June and then reached a maximum level in August in 1997 and 1998. Total root length at the 3- and 9-cm depths were reduced by the lower mowing height during most of the growing season for both cultivars in both years.
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Fig. 1. Effects of mowing height on total root length of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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New Root Growth
In 1997, CNRL (Fig. 2)
was significantly lower at low mowing than high mowing for Penncross at both the 3-cm and 9-cm depths beginning in late June, and for Crenshaw at the 3-cm soil depth beginning in mid-July and at the 9-cm depth beginning in late August.
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Fig. 2. Effects of mowing height on cumulative new root length of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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In 1998, CNRL was significantly lower at low mowing than high mowing for Crenshaw at both the 3- and 9-cm depths beginning in July (Fig. 2). However, no significant differences in CNRL were found between the two mowing heights for Penncross at either depth.
Mowing effects on CNRL for both cultivars were more pronounced in late summer than late spring in both years.
Cumulative Dead Root Length
In 1997, CDRL (Fig. 3)
was significantly greater at low mowing than at high mowing beginning in July at 3 cm and in August at the 9-cm depth for Penncross, and beginning in late August at both depths for Crenshaw.
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Fig. 3. Effects of mowing height on cumulative dead root length of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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In 1998, CDRL was greater at low mowing than high mowing beginning in late June to July at both soil depths for Penncross and at 3 cm for Crenshaw. Significant differences in CDRL were found only in September and October at the 9-cm depth for Crenshaw.
Root Turnover and Mortality
The ratios of dead to new roots in length (TL; Fig. 4)
and in number (TN; Fig. 5)
were greater at low mowing than at high mowing for Penncross and Crenshaw at both the 3- and 9-cm depths in 1997 and 1998. The TL values were <1.0 at either high or low mowing, with the exception of Penncross at the 9-cm depth in late 1997 and Crenshaw at the 3-cm depth in 1998.
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Fig. 4. Effects of mowing height on ratios of dead/new roots in length of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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Fig. 5. Effects of mowing height on ratios of dead/new roots in number of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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The TN values were <1.0 at high mowing for both cultivars at both depths in both years. However, at low mowing, TN at both the 3- and 9-cm depths increased to >1.0, beginning in July-August for Penncross and September for Crenshaw in 1997. In 1998, TN ratios at the 3-cm depth were >1.0, whereas those at the 9-cm depth were <1.0 for both Penncross and Crenshaw during the entire growing season.
Root mortalities in length (Fig. 6)
and number (Fig. 7)
at either the 3- or 9-cm depth were greater at low mowing than at high mowing for both cultivars in both years. The differences in root mortality in either length or number between the two mowing heights were greater for Penncross than Crenshaw at both soil depths in 1997 and at the 9-cm depth in 1998.
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Fig. 6. Effects of mowing height on root mortality in length of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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Fig. 7. Effects of mowing height on root mortality in number of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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Maximum Rooting Depth
Maximum rooting depth was significantly lower at low mowing than at high mowing in August for Penncross and July through September for Crenshaw in 1997 (Fig. 8)
. In 1998, RD was lower at low mowing than high mowing from June to September for both cultivars.
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Fig. 8. Effects of mowing height on maximum rooting depth of ‘Crenshaw’ and ‘Penncross’ in 1997 and 1998. Asterisk (*) indicates significant difference between mowing treatments (P = 0.05) at a given day of treatment.
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DISCUSSION
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Total root length of both cultivars mowed at 3 mm or 4 mm increased in spring and reached the maximum level during the summer months in both 1997 and 1998. This pattern reflected new root production early in the growing season and root dieback during the summer. This is in agreement with the general root growth pattern of cool-season grasses (Beard, 1973).
Total root length and RD were less for a 3-mm mowing height compared with 4 mm in both years. This result was consistent with previous reports that low mowing reduces root growth (Juska and Hanson, 1961; Beard and Daniel, 1965; Salaiz et al., 1995; Yelverton, 1999). The reduction of TRL generally can be attributed to limited new root production, increases in dead roots, or the combination of both. In the present study, low mowing both reduced new root production and increased root mortality for both cultivars. Low mowing also increased root turnover ratio in length or number. The decreases in TRL and number at the lower mowing height compared with those at the higher mowing could have been due mainly to limited new root production in 1997 and increased root dieback in 1998. Grasses were planted in fall 1996 and minirhizotron tubes were installed underground in spring 1997 when newly-generated roots in spring were observed through the minirhizotron. Therefore, the reduced TRL and number at low mowing in 1997 may be mainly due to decreases in new root production. Although roots were well established in 1998, low mowing still may have affected new root production, but also limited growth of existing roots and, thus resulted in more root dieback.
Limited new root production, increased root dieback, and increased turnover rate resulting from low mowing occurred earlier during the season at the 3-cm than at the 9-cm depth, particularly for Penncross. These earlier effects near the surface could have been related to soil temperature differences. Temperatures during summer months increased by 3°C within the surface 5 cm of soil and by <2°C at the 10-cm depth when mowing height was lowered from 4 mm to 3 mm (2001, unpublished data). Increasing soil temperature is a major factor limiting root production and leading to root death (Xu and Huang, 2000).
The effects of low mowing on root elongation, production, and mortality were related closely to those effects on shoot growth. Turf quality and canopy photosynthetic rate of Penncross and Crenshaw decreased when mowing height was lowered from 4 to 3 mm (2001, unpublished data). The plant canopy is the source of carbohydrate supply for root growth (Salisbury and Ross, 1992). The reduction in root production and increase in root mortality at low mowing could be due mainly to a limited carbohydrate supply from shoots to roots, especially during summer months when carbohydrate availability is low in both shoots and roots at high temperatures (2001, unpublished data). Therefore, low mowing, particularly during summer when air and soil temperatures are high, could result in carbohydrate depletion and, thus, lead to depressions of root growth and physiological function. Root growth, in turn, affects shoot growth and metabolism. Root dieback caused by low mowing may lead to water deficit, nutrient deficiency, cytokinin deficiency, and a hormone imbalance, especially during summer stress periods, and thus influence turf quality. However, the mechanisms by which mowing affects root physiological changes are not clear, and deserve further study.
In summary, lowering mowing height of creeping bentgrass is not recommended for golf course management, especially during summer months. The negative effects of low mowing were reduction in new root production and rooting depth and increases in root mortality and turnover rate, which could contribute to a decline in turf quality.
Received for publication April 16, 2001.
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ABSTRACT
INTRODUCTION
