Creeping Bentgrass Cultivars Research Articles

Relative Fitness of Glyphosate-resistant Creeping Bentgrass Lines in Kentucky Bluegrass

Genetically transformed cultivars of creeping bentgrass (Agrostis stolonifera L. syn. Agrostis palustris Huds.) that are resistant to glyphosate have been developed by a collaboration of the Scotts and Monsanto companies. Prior to commercial release, we desired to determine if the transformed plants behave similarly to traditional creeping bentgrass except for the effects expected from the inserted gene, i.e., resistance to glyphosate. Therefore, studies were initiated on 23 June 2000 in Marysville, Ohio; 14 July 2000 in Middleton, N.J.; and 20 June 2000 in Gervais, Ore., to examine the relative lateral spread and competitive ability of several transformed lines of creeping bentgrass, non-transformed controls, and reference cultivars. Vegetative plugs of creeping bentgrass were transplanted into a mature stand of Kentucky bluegrass (Poa pratensis L.) or a uniform mixture of Kentucky bluegrass with perennial ryegrass (Lolium perenne L.). The plots were watered as needed to prevent moisture stress. Competitive ability of the transformed plants and reference cultivars were determined monthly by measuring the average diameter of the creeping bentgrass patch. On all observation dates, the transgenic lines, as a group, were smaller in average diameter (5.1-7.6 cm) compared to the reference cultivars (5.4-14.2 cm) and non-transformed control lines (5.9-10.2 cm). At the end of the observation period (Aug. 2001), no differences (P = 0.05) in lateral spread were observed between individual lines of transgenic bentgrass. Three lines of interest, ASR365, ASR368, and ASR333, had lateral spread rates that are similar to, or less than, that of their non-transformed parent and the conventional creeping bentgrass cultivars tested. Chemical names used: N-(phosphonomethyl) glycine (glyphosate).

Transient reporter gene (GUS) expression in creeping bentgrass (Agrostis palustris) is affected by in vivo nucleolytic activity.

Leaf and callus tissues of a creeping bentgrass cultivar (Penn A4) had high nuclease activities that degraded exogenously added plasmid DNA. When callus tissue was incubated for 24 h with heparin, spermidine, aurintricarboxylic acid or polyethylene glycol, only heparin and spermidine were effective as in vitro nuclease inhibitors, protecting exogenously added plasmid DNA from degradation. When beta-glucuronidase (GUS) reporter gene activity was evaluated in heparin-treated (0.6%), 14-month old callus following microprojectile bombardment, GUS activity increased 1000-fold compared to equivalent aged untreated Penn A4 callus. Similar enhancement from heparin pretreatment (0.6% or 1.2%) was not observed in 6-month old callus. This is likely due to much higher activities of nuclease in the younger callus.

Seasonal Changes in Carbohydrate Accumulation for Two Creeping Bentgrass Cultivars

Controlled‐environment studies suggest that turf quality decline of creeping bentgrass [Agrostis stolonifera var. palustris (Huds.) Farw.] under heat stress is associated with decreases in carbohydrate availability in plants. The study was designed to examine and compare seasonal changes of carbohydrate status and C allocation pattern for two creeping bentgrass cultivars, ‘Penncross’ and ‘L‐93’, that differ in heat tolerance under field conditions. The experiment was conducted from May to November in 1999 and 2000. Grasses were managed under putting green conditions with daily irrigation and mowing at a 4‐mm height. Turf quality of both cultivars declined from May and reached the lowest level in August and September, and recovered in October. The content of total nonstructural carbohydrates (TNC), sucrose, and fructans in shoots and roots exhibited a similar seasonal pattern as turf quality for both cultivars in both years. Reducing sugar content decreased during the summer, but did not recover in October, except in shoots of L‐93 in 2000. The decreases in carbohydrate content were more pronounced in roots than in shoots for both cultivars. Carbon allocation to roots also decreased during summer for Penncross. Cultivar differences in carbohydrate content were not consistent between fractions of carbohydrate and times of the year. The decline in carbohydrate availability, particularly in roots, and limited C allocation to roots during summer could contribute to quality decline under field conditions; however, cultivar variation in carbohydrate content was not related to the differences in turf quality decline between two cultivars.

Summer Root Decline: Production and Mortality for Four Cultivars of Creeping Bentgrass

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.

Summer Root Decline

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.

Interspecific Hybridization between Agrostis stolonifera and Related Agrostis Species under Field Conditions

Creeping bentgrass (Agrostis stolonifera L.) is a commercially important turfgrass used principally on golf courses. Weed control is one of the major problems encountered in golf course maintenance, largely because the major weed, Poa annua L., is another grass species with herbicide responses similar to creeping bentgrass. Development of creeping bentgrass cultivars expressing one of the herbicide resistance genes would provide an effective solution. The prospects of commercialization of transgenic cultivars of creeping bentgrass have raised questions about the potential for pollen‐mediated gene flow to related Agrostis spp. In a field study we have measured the frequency of interspecific hybridization between transgenic creeping bentgrass and four related species, A canina L., A castellana Boiss. and Reut, A gigantea Roth, and A capillaris L. Interspecific transgenic hybrids were recovered between creeping bentgrass and A capillaris and A castellana at frequencies of 0.044 and 0.0015%, respectively, which were considerably lower than intraspecific transgenic progeny recovery in the same experimental plots (0.631%). No interspecific transgenic hybrids were recovered with A gigantea or A canina

Geographic Distribution, Cultivar Susceptibility, and Field Observations on Bentgrass Dead Spot.

Bentgrass dead spot (BDS) is a disease of creeping bentgrass incited by Ophiosphaerella agrostis. This project was designed to determine the susceptibility of field-grown bentgrass cultivars to BDS and to gather information regarding the geographic distribution and field conditions favoring the disease. In a field cultivar evaluation trial, all major Agrostis spp. used on golf courses, including colonial, creeping, and velvet bentgrasses, were shown to be susceptible to an isolate of O. agrostis. Velvet bentgrass cvs. SR7200 and Bavaria were among the most and least susceptible cultivars, respectively. Among creeping bentgrass cultivars, L-93 generally was the most susceptible and Pennlinks, Penncross, and Crenshaw were among the least susceptible. Although recovery of BDS patches in the autumn was slow, Bardot colonial bentgrass and Crenshaw, Imperial, L-93, and Penn G-6 creeping bentgrasses showed the most rapid recovery prior to winter. Variation in the virulence of isolates and the potential for races of the pathogen, however, may affect cultivar susceptibility. The disease was most commonly found on 1- to 4-year-old golf greens and disease severity declined 1 to 3 years after it first appeared. BDS only was found on sand-based greens, collars, and tees and has not been observed in bentgrass grown on native soil. Between 1998 and 2001, O. agrostis was isolated from diseased leaves, roots, crowns, and stolons of creeping bentgrass and hybrid bermudagrass turf samples received from 13 states. The disease was most severe in sunny and exposed locations, especially on ridges, mounds, and slopes. In the mid-Atlantic region, BDS appeared as early as May and remained active as late as December. The disease was most active in July and August, and usually became inactive with the advent of frost in October.

Lack of Competitive Success of an Intraseeded Creeping Bentgrass Cultivar into an Established Putting Green

Intraseeding is a popular approach for converting established golf course putting greens to a more desirable cultivar of the same species without killing the existing turf. This study was conducted to determine the competitive success of an intraseeded creeping bentgrass (Agrostis palustris Huds.) cultivar in an established putting green using random amplified polymorphic DNA (RAPD) markers. Two replicate, United States Golf Association (USGA) specification greens, established with ‘Penncross’ creeping bentgrass, were arranged in a randomized complete block design with five conversion treatments as follows: control, scalping, coring, a trinexapac-ethyl application, and a glyphosate application. Vertical mowing and topdressing were followed by subsequent seeding with ‘Penn G-2’ creeping bentgrass (G-2) in two directions at 12.2 kg seed ha−1 Treatments were performed in October 1998, April 1999, and September 1999. Pretreatment samples taken from each plot on 22 Sept. 1998 and posttreatment samples taken on 28 May 1999 and 24 Mar. 2000 were evaluated for changes in population dynamics using RAPD markers. By the end of the study, both glyphosate-treated plots had completely shifted to G-2, while plots subjected to other treatments showed no evidence of the cultivar. The detection of G-2 in samples collected on 28 May 1999 suggested that a transient change occurred in the scalp (Exp. 2) treatment; however, evidence of G-2 was no longer evident on 24 Mar. 2000. Data indicated that the effectiveness of the intraseeding techniques and timings used in this study to convert putting greens to a new cultivar were quite limited. This may have been due to unsuccessful elimination of root competition from the existing turf. Until more effective intraseeding methods are developed, chemical renovation remains the most effective way to ensure the establishment of new cultivars.

Mowing Effects on Root Production, Growth, and Mortality of Creeping Bentgrass

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.

Tolerance to Quinclorac by Seedling Creeping Bentgrass

Annual grassy weeds often inhibit establishment of spring-seeded creeping bentgrass (Agrostis palustris Huds.) on golf courses. The objective of this experiment was to evaluate the safety of the annual grass herbicide quinclorac in spring-seeded creeping bentgrass in varying climatic regions of the United States. Experiments were initiated in Indiana, Iowa, and North Carolina in Spring 2000. Treatments included siduron at 6.72 kg·ha-1 a.i. applied immediately prior to planting (PRE), and quinclorac at 0.84 kg·ha-1 a.i. applied 7 days before seeding (DBS), PRE, and 14 or 28 days after emergence (DAE). Herbicides were applied to three creeping bentgrass cultivars at each location. Siduron reduced establishment of `Providence', `L93', and `Putter' creeping bentgrass in Indiana. Quinclorac applied PRE, 14 DAE, and 28 DAE caused short-term phytotoxicity, primarily in `Providence' in Indiana. Quinclorac applications did not significantly affect cover of `Providence', `L93', or `Putter' in Indiana or `L93', `Pennlinks', or `Penncross' in Iowa. All applications of quinclorac reduced cover of `L93', `Pennlinks', and `Penncross' in North Carolina. Though quinclorac applications resulted in no long-term damage when applied to creeping bentgrass seedlings in Indiana or Iowa, results from North Carolina indicate that caution should be exercised when using quinclorac on seedlings of creeping bentgrass. Chemical names used: 3,7-dichloro-8-quinolinecarboxylic acid (quinclorac); 1-(2-methylcyclohexyl)-3-phenylurea (siduron).

Lowering Soil Temperatures Improves Creeping Bentgrass Growth under Heat Stress

High soil temperature is a major factor limiting growth of cool‐season grasses. The objectives of this study with creeping bentgrass (Agrostis palustris Huds.) were to examine growth responses to lowering soil temperatures from the supraoptimal level under high air temperature conditions, and to determine the minimum reduction in soil temperature that could improve shoot and root growth. Two creeping bentgrass cultivars, L‐93 and Penncross, were exposed to the following air and soil temperature regimes in growth chambers and water baths: (i) optimal air and soil temperatures (20/20°C, control); (ii) lowering soil temperature by 3, 6, and 11°C from 35°C at high air temperatures (35/32, 35/29, 35/24°C); and (iii) high air and soil temperatures (35/35°C). Soil temperature was reduced from 35°C by circulating cool water (18°C) in water baths at variable rates. Turf quality, leaf chlorophyll content, shoot growth rate, and root/shoot ratio (R/S) ratio increased as soil temperature was reduced from 35 to 32°C to a greater extent for Penncross than for L‐93. Significant increases in tiller density, clipping yield, root number, and fresh weight were not observed until soil temperature was reduced to 29°C. When soil temperature was reduced to 24°C, turf quality, shoot growth rate, and R/S were maintained at the same levels as in the control regime. These results suggested that reducing soil temperature by 3°C or more is effective in improving turf quality and shoot and root growth of creeping bentgrass exposed to high air temperatures.

Seasonal Changes and Cultivar Difference in Turf Quality, Photosynthesis, and Respiration of Creeping Bentgrass

Summer decline in turf quality of creeping bentgrass (Agrostis palustris Hud.) is a major problem in golf course green management. The objective of this study was to examine whether seasonal changes and cultivar variations in turf performance are associated with changes in photosynthesis and respiration rates for creeping bentgrass. The study was conducted on a USGA-specification putting green in Manhattan, Kans., during 1997 and 1998. Four creeping bentgrass cultivars, `L-93', `Crenshaw', `Penncross', and `Providence', were examined. Grasses were mowed daily at 4 mm and irrigated on alternate days to replace 100% of daily water loss. In both years, turf quality, canopy net photosynthetic rate (Pn), and leaf photochemical efficiency (Fv/Fm) were high in May and June and decreased to the lowest levels in July through September. Whole-plant respiration rate (R) and canopy minus air temperature (▵T) increased during summer months. In October, turf quality and Pn increased, whereas R and T decreased. During summer months, turf quality was highest for `L-93', lowest for `Penncross', and intermediate for `Providence' and `Crenshaw'. Seasonal changes and cultivar variations in turf quality were associated with the decreasing photosynthetic rate and increasing respiration rate.

Dollar Spot Resistance among Blends of Creeping Bentgrass Cultivars

Disease is a major problem in creeping bentgrass (Agrostis stolonifera L.) on golf course putting greens. Genetic composition and uniformity of host populations can affect disease development. This study was conducted to determine the effects of blending creeping bentgrass cultivars on dollar spot (caused by Sclerotinia homoeocarpa F. T. Bennett) severity. ‘Penn A‐4’, ‘Crenshaw’, ‘L‐93’, ‘Mariner’, and ‘Penncross’ were planted in monostands or as two‐way or three‐way blends in a completely random design with three replications on a sand‐based putting green in the field at College Station, TX. Disease severity was determined by the number of infection centers and by total blighted area. Crenshaw was most susceptible, L‐93 most resistant, and all other cultivars were moderately resistant to dollar spot in monostands. Cultivar susceptibility in monostands to S. homoeocarpa was representative of each cultivars contribution to dollar spot severity in blends. In blends, L‐93 suppressed, whereas Crenshaw increased dollar spot. Penncross, Penn A‐4, and Mariner did not affect dollar spot severity across all blend combinations. Number of infection centers and infected area pooled across all monostands were similar when compared to all two‐ and three‐way blends indicating that dollar spot activity in blends was a compromise between resistance levels of individual cultivars. Two‐way blends of Penn A‐4, L‐93, Mariner, or Penncross with Crenshaw, however, resulted in 46 to 67% fewer infection centers and 71 to 98% less blighted area, when compared to the Crenshaw monostand. These data indicated that blending a resistant cultivar with a susceptible cultivar reduced dollar spot severity in the blended population.

Root Weight, Nonstructural Carbohydrate Content, and Shoot Density of High-density Creeping Bentgrass Cultivars

Limited information is available on the performance under temperate conditions in the United States of recently released cultivars of creeping bentgrass (Agrostis stolonifera L.) with high shoot density for use on golf course putting greens. Fifteen cultivars were established in Aug. 1996 on a greens mix with high sand content to compare their seasonal weights and total nonstructural carbohydrate (TNC) contents. The cultivars were maintained at 3.1 mm height of cut. Shoot density counts were taken during Apr., July, and Oct. 1998. Root weights and nonstructural carbohydrate levels were assessed monthly from June 1997 through Nov. 1998. A cultivar group contrast between the high shoot density cultivars (`Penn A1', `Penn A2', `Penn A4', `Penn G1', `Penn G2', and `Penn G6') and the standard cultivars (`Penncross', `Crenshaw', `Southshore', `DF-1', `Procup', `Lopez', `SR1020', and `Providence') revealed that the former averaged 342.9 and 216.1 more shoots/dm2 on two of the three sampling dates. Root dry weights did not vary significantly (P ≤ 0.05) among the cultivars. Performing a contrast between new high shoot density cultivars and standard cultivars revealed greater root dry weight in the former during Mar. and May 1998. Differences (P ≤ 0.05) in TNC were observed on two of the 18 sampling dates, but no trends were evident.

Supraoptimal Soil Temperatures Induced Oxidative Stress in Leaves of Creeping Bentgrass Cultivars Differing in Heat Tolerance

High temperature is a major factor limiting growth of cool‐season grasses during summer months. The objective of this study was to determine whether oxidative stress is involved in leaf injury induced by high soil temperatures in two creeping bentgrass (Agrostis palustris Huds.) cultivars, heat‐tolerant L‐93 and heat‐sensitive Penncross. Shoots and roots were exposed to four differential temperature regimes in growth chambers and water baths: (i) 20/20°C (control); (ii) 20/35°C (high soil temperature); (iii) 35/20°C (high air temperature); and (iv) 35/35°C (high shoot/soil temperatures). Turf quality, leaf photochemical efficiency (Fv/Fm), electrolyte leakage (EL), content of a lipid peroxidation product (malondialdehyde, MDA), and activities of the antioxidants superoxide dismutase (SOD) and catalase (CAT) were determined. Turf quality and leaf Fv/Fm ratio decreased, whereas EL and MDA contents increased under high soil temperature alone or in combination with high air temperature regimes in both cultivars, but to a greater extent in Penncross than in L‐93. Decreases in turf quality and Fv/Fm ratio and increases in EL and MDA were more pronounced at 20/35°C than at 35/20°C. The activities of SOD and CAT decreased with prolonged periods of high temperatures and to a greater extent for Penncross than for L‐93. The reductions in SOD and CAT activities were more severe at 20/35 than at 35/20°C. These results suggest that high soil temperature caused more severe oxidative damage to leaves than high air temperature by limiting antioxidant activities and inducing lipid peroxidation. This oxidative stress was associated with accelerated leaf senescence under high temperature conditions. Maintenance of antioxidant activities and low levels of lipid peroxidation was related to the better tolerance of creeping bentgrass to high soil temperature stress imposed on roots or high air temperature on shoots.

ROOT AND SHOOT PERFORMANCE OF THREE CREEPING BENTGRASS CULTIVARS AS AFFECTED BY NITROGEN FERTILITY1*

Cultivar selection and nitrogen (N) fertility significantly influence the performance of creeping bentgrass (Agrostis palustris Huds.) in warmer regions of the United States. This study was conducted to determine the effects of N on root and shoot growth of three creeping bentgrass cultivars. The effect of three N rates (195.3, 390.6, and 586.0 kg N/ha year) on the total root length density (TRLD), deep root length density (DRLD), visual shoot quality, shoot density, and root to shoot ratio (RSR) of ‘Crenshaw’, ‘L93’, and ‘Penncross’ creeping bentgrass were evaluated in the University of Georgia Rhizotron at Athens, GA. Over the 19 month study, cultivar type and N rate significantly affected root and shoot growth with slight interaction. Crenshaw and L93 showed greater TRLD, DRLD, and visual shoot quality than Penncross at the 390.6 kg N rate. RSR was significantly influenced by N rate but not cultivar type. Both L93 and Crenshaw possessed significantly higher RSR at the 195.3 kg N rate than the 586.0 kg N rate. *Contribution of the Georgia Agricultural Experiment Station, Athens, GA.

Morphological and Physiological Characteristics Associated with Heat Tolerance in Creeping Bentgrass

Growth of creeping bentgrass (Agrostis palustris Huds.) is limited by heat stress during summer. Understanding the morphological and physiological characteristics associated with heat tolerance of creeping bentgrass would facilitate breeding programs in improving those characteristics. The objectives of this experiment were to study responses of single‐leaf photosynthetic capacity, tillering, and root growth to heat stress for two creeping bentgrass cultivars differing in performance under heat stress and to determine the relative importance of these factors in heat tolerance. The cultivars L‐93 (heat tolerant) and Penncross (heat sensitive) were exposed to day/night temperatures of 20/15°C (control) and 35/30°C (heat stress) in growth chambers. Canopy net photosynthetic rate (Pn), single‐leaf Pn, and RuBP carboxylase (Rubisco) activity were reduced by heat stress for both cultivars. Canopy Pn of L‐93 was significantly higher (more than double at 64 d of treatment) than that of Penncross under heat stress. Cultivars were not different in single‐leaf Pn and Rubisco activity under either control or heat stress conditions. High temperature reduced plant density, tiller density, root number, and root fresh weight for both cultivars. L‐93 had higher plant and tiller densities, greater root to tiller ratio, and more and finer roots than Penncross under control and high temperature conditions. The better performance of L‐93 under heat stress was due largely to its morphological characteristics, including tillering and root growth, but was not related to single‐leaf photosynthetic capacity. Narrow leaves, small plants, dense tillers, big root system, and high root‐to‐shoot ratio could be used to select heat tolerant cultivars.

Growth and Carbohydrate Metabolism of Creeping Bentgrass Cultivars in Response to Increasing Temperatures

High temperature is a major factor limiting growth of creeping bentgrass (Agrostis palustris Huds.). Physiological causes of turf growth and quality decline with increasing temperature is not well understood. The objective of the study was to examine responses of growth and carbohydrate metabolisms to increasing temperatures in three creeping bentgrass cultivars. Sods of ‘Penncross’, ‘ISI‐AP‐89150’, and ‘SR 1020’ were grown in growth chambers and exposed sequentially for 20 d to each of the following temperatures: 20, 24, 30, 34, and 38°C. Evaluation and measurements were made at 10 and 20 d after each sequential temperature increase. Decreased root viability and root dry matter production of all cultivars was observed after a 10‐d exposure at 30°C and continued to decline with increasing temperatures. A decline in turf quality and leaf chlorophyll content (Chl) was observed at a 20‐d exposure to 30°C. Turf quality, Chl content, and root viability of SR 1020 were higher than those of Penncross after a 10‐d exposure at 30°C and 20 d at 34°C, and 10 d at 38°C, respectively. Canopy net photosynthetic rate (Pn) decreased with temperature in all cultivars. Dark respiration rates of whole plants (Rplant) increased with temperature up to 34°C, and then declined at 38°C. Daily carbon consumption to production ratio increased dramatically with temperature after 30°C, and Rplant exceeded Pn when temperature increased to 34 or 38°C in all cultivars. Plants grown at 30, 34, and 38°C had lower total nonstructural carbohydrate than those grown at 20 or 24°C. Results suggest that a decline in root activity of creeping bentgrass occurred before a decline in turf quality at temperatures above 30°C, and could be related to the imbalance between photosynthesis and respiration, and limited carbohydrate availability.

Root growth and nutrient element status of creeping bentgrass cultivars differing in heat tolerance as influenced by supraoptimal shoot and root temperatures

This study was designed to determine and compare root growth and nutritional responses of creeping bentgrass cultivars that differ in heat tolerance to differential, supraoptimal, shoot and root temperatures. Shoots and roots of ‘Penncross’ (heat sensitive) and ‘L‐93’ (heat tolerant) were exposed to four air/soil temperature regimes (20/20°C‐control, 20/35°C, 35/20°C, and 35/35°C) in water baths and growth chambers. Exposing roots to supraoptimal root temperature (35°C) while maintaining shoots at normal temperature (20°C) or particularly at 35°C reduced root fresh weight, root number, and contents of nitrogen (N), phosphorus (P), and potassium (K) in shoots and roots and accelerated root death for both cultivars. High root temperature had greater detrimental effects on root growth and nutrient element accumulation than high shoot temperature for both cultivars. A low root temperature at supraoptimal shoot temperature improved root growth, reduced root mortality; and increased N, P, and K contents in shoots and roots. Among the three nutrient elements, K was the most sensitive to changes in root temperature. L‐93 generally maintained higher fresh weight and number of roots and higher N, P, and K contents in shoots and roots, particularly K in roots, under high root (20/35°C) or shoot/root (35/35°C) temperatures. The results indicated that root growth and nutrient element accumulation, particularly of K, played an important role in creeping bentgrass tolerance to heat stress imposed on shoots by high air temperature or to roots by high soil temperatures. The enhanced root growth and nutrient element relations with a low root temperature at supraoptimal ambient temperatures could lead to the improved shoot growth in cool‐season grasses observed under these conditions.

142 Root Mortality and Nutrient Uptake of Creeping Bentgrass in Response to Differential Root and Shoot Temperatures

This study was designed to compare and determine root growth and nutritional responses of creeping bentgrass cultivars that differ in heat tolerance to deferential, supraoptimal shoot and root temperatures. Shoots and roots of `Penncross' (heat sensitive) and `L-93' (heat tolerant) were exposed to four differential air/soil temperature regimes (20/20-control, 20/35, 35/20, and 35/35 °C) in water baths and growth chambers. Exposing roots to supraoptimal root temperature (35 °C) while maintaining shoots at normal temperature (20 °C), or at 35 °C in particular, reduced root fresh weight, root number, the content of N, P, and K in shoots and roots, and accelerated root death for both cultivars. High root temperature had a greater detrimental effects on root growth and nutrient accumulation than high shoot temperature for both cultivars. Reducing root temperature at supraoptimal shoot temperature improved root growth, reduced root mortality, and increased N, P, and K content in shoots and roots. Among the three nutrient elements, K was the most sensitive to changes in root temperature. L-93 generally maintained higher root fresh weight and number, and N, P, K content in shoots and roots, particularly K in roots, under high root (20/35 °C) or shoot/root (35/35 °C) temperatures. The results indicated that root growth and nutrient accumulation, particularly K, played an important role in creeping bentgrass tolerance to heat stress imposed to shoots by high air temperature or to roots by high soil temperatures. Reducing root temperature under supraoptimal ambient temperatures enhanced root growth and nutrient relations, and thus could lead to the improved shoot growth in cool-season grasses as reported previously.