MORPHOLOGICAL-GROWTH COMPARISONS OF FOUR HYBRID BERMUDAGRASS CULTIVARS

(CYNODON DACTYLON X CYNODON TRANSVAALENSIS )

IN FULL SUN AND A 20 % IRRADIANCE LEVEL

DECEMBER, 1997

International Sports Turf Institute, Inc.

RESEARCH REPORT No.202

by

S.I. Sifers1 and J.B Beard2

INTRODUCTION

Plant growth responses under shade conditions include anatomical, morphological, and physiological changes ( Beard 1970). Maintaining acceptable quality turfs under shade is a continuing problem. It is estimated that 25 % of all turfs are subjected to shade stress and have restricted root, shoot, rhizome, and stolon growth

( Almodares, et al. 1977-78; 1978-79) . There also is reduced wear, heat, cold, and drought resistance. The ability of turfgrasses to tolerate shaded conditions and their responses have been previously investigated ( Peacock and Dudeck 1993 ). The combination of low irradiance, decreased temperatures, increased humidity, and lower wind speeds all affect turfgrass development, such as, thinner leaves with less weight per unit area, reduced shoot density, less tillering and more a restricted root system ( Beard 1973).

There is a minimum irradiance level needed for photosynthesis to begin. Productivity in terms of amounts of synthesized carbohydrates increases with increased photosynthetic irradiance up to 116 to 233 W m-2. Futher increases do not increase dry matter productivity of cool-season grasses. In warm-season grasses photosynthetic productivity increases to a maximum of 390 to 465 W m-2, however not all warm-season grasses respond equally ( Dudeck and Peacock 1992 ). Among the warm-season turfgrasses, St. Augustinegrass is considered to have excellent shade tolerance, zoysiagrass good shade tolerance, bahiagrass, carpetgrass, and centipedegrass fair shade tolerance and bermudagrass poor shade tolerance ( Peacock and Dudeck 1993). Although bermudagrass has very poor shade adaptation there are differences among cultivars and not all cultivars have been assessed.

There is a need to characterize the comparative morphological-growth potential of the major commercially available dwarf hybrid bermudagrass cultivars for their responses to stress inposed by shade conditions both (a) at a low mowing height used on golf greens and, (b) at a mowing height used for golf fairways and sport fields . There is an immediate need to assess the existing hybrids and the newer vertical dwarf hybrid bermudagrass cultivars, such as Champion.

MATERIALS and METHODS

The bermudagrass cultivars to be comparatively assessed were Champion, Tifdwarf, Tifgreen, and Tifway. The sods were established in a sand root zone medium with a particle size that meets the specifications of the USGA Method for root zone construction. The containers were 150mm ( 6 in. ) in diameter by 175 mm (7 in. ) deep plastic pots in ten replications for each turfgrass cultivar. Following 120 days establishment in full sun, the pots were transferred on 6 August, 1997, to an area of dense shade created by post oak trees. A Li-Cor Integrating Quantum Radiometer Photometer was used to determine the photosynthetic active radiation (PAR) level existing at this site. The turfs in the shade were subjected to an average of 93 W m-2 or approximately 20 % of maximum benefical photosynthetic radiation.

The investigation was divided into two studies.

A non-limiting water level was maintained and nutrients were applied biweekly as a liquid foliar drench using 20N-20P2O5-20K2O at a 0.5 kg 100m-2 gm-1( 1 lb. N\1000 ft2 \ gm ) rate. Assessments were made at 7-day intervals following turf establishment and the turfs were then cut at the original heights with clippings collected for assessment.

At each interval, replicated assessments were made of the following:

(a) Leaf vertical extension rate. Determined by 5 random measurements of the growth above the container rim of each replication.

(b) Shoot/stolon biomass. Determined by mowing at the specified heights, collecting the clippings, then drying at 1050C (2200 F) for 24 hours, and weighing.

(c) Stolon number. Determined by counting the number of stolons extending exterior to the 470 mm circumference of the container.

(e) Visual turfgrass quality. Visual estimates were based on a composite of two main components: (a) uniformity of appearance and, (b) shoot density. This was assessed by two observers before mowing using a 1 to 9 scale with 9 = best and 1 = worst. A rating of 5.0 or better would indicate acceptable turf for fairways and 5.5 for greens.

At the conclusion of each study, destructive assessments of the shoot, mat, and root components were made to determine shoot density and biomass, mat/thatch depth and biomass, and root biomass. Cores of 855 mm-2 diameter were removed from each container, the shoots were separated for counting and retained for biomass determinations. Mat/thatch depth was measured and then saved for weighing. Roots were washed free of sand and saved for weighing. All plant material was dryed as previously noted, and then weighed.

An analysis of variance for all turf parameters in the test was conducted.

RESULTS - STUDY 1

Study 1. Green Cutting Height The assessments were terminated after 4 weekly assessments in full sun and after 6 weekly assessments in a 20 % irradiance. The full sun assessments reported are an average for the 4 weekly assessments. The degree of irradiance reduction was a very severe test for any turfgrass, and possibly created conditions found only in shade created by a very dense tree canopy or a building. Data for the above-ground assessments are shown at Table 1.

For golf greens, a minimum rate of vertical shoot and leaf growth is desirable if the density of the canopy and ball roll speed can be sustained. The Champion leaf extension rate per day was the least, both in full sun, and at each 20 % irradiance observation date. The leaf extension rate for all three cultivars approximately tripled during the first week at a 20 % irradiance level, and then the extension rate per day tended to decrease during the next 6 weeks. The leaf extension rate for Champion after 6 weeks at a 20 % irradiance level still ranked higher than that in full sun, while the Tifdwarf and Tifgreen extension rates ranked less than in full sun. Vertical leaf extention was observed at each assessment.

In full sun, the shoot/stolon biomasses of Champion and Tifdwarf were not different with Tifgreen being less. After 1 week at a 20 % irradiance level the Tifdwarf biomass was 143 % heavier than for Champion and 164 % more than for Tifgreen. At the the 2nd week assessment the above-ground biomass for Champion exceeded that for Tifdwarf by 137 % and exceeded that for Tifgreen by 153 %. All cultivars tended to decrease in biomass at each successive observation, which reflects the decreasing shoot density visually observed. At the 3 rd through the 6 th assessment dates Champion biomass was greatest of the three cultivars followed by Tifdwarf then Tifgreen.

Stolon number is a component of the capacity for lateral spread of the turf as well as the capability to recover from turf damage, with a high number usually reflecting the best potential. In full sun, the number of stolons per linear 470 mm for Champion was 4 times greater than the number for Tifdwarf, and 14 times greater than for Tifgreen. After the first week at a 20 % irradiance level the number of stolons for Champion were 1.8 times greater than for Tifdwarf, and 10 times greater than for Tifgreen. At the 6th week at a 20 % irradiance level the number of Champion stolons was 8.5 times greater than for Tifdwarf. No stolons were found extending from Tifgreen after 2 weeks at a 20 % irradiance level .

Visual turfgrass quality for all three cultivars in full sun exceeded the 5.5 acceptable standard for greens, with Champion being higher. At the 20 % irradiance level all cultivars rapidly lost much of their green color. Champion and Tifdwarf were acceptable only at week 1 with Tifgreen substantially lower. The uniformity of the turf canopy of Tifdwarf and Tifgreen became more open, but Champion remained dense throughout the study.

The assessments made upon termination of Study 1 are shown in Table 2. The Champion shoot density was 184 % greater than for Tifdwarf and 283 % greater than for Tifgreen. Visually there were no green leaves in the Tifdwarf and Tifgreen canopies and only a few green leaves in the Champion canopy.

The shoot/stolon biomass of Champion was 147 % and 170 % heavier than for Tifdwarf and for Tifgreen, respectively.

The Champion mat depth was 2 times greater and mat biomass weight was 5 times greater than for Tifdwarf, and the Champion mat depth was 3.3 times greater than for Tifgreen and the mat biomass was 4 times greater.

Roots extended to the bottom of the 175 mm deep containers for all three cultivars and were tan to brown in color with no white roots. This indicates that new root initiation had ceased. The root biomass of Tifgreen was 141 % greater than for Champion and 183 % greater than for Tifdwarf.

RESULTS - STUDY 2

Study 2.-Golf Fairway/Tee and Sport Field Cutting Height. The weekly assessments terminated after 4 weeks in full sun and after 5 weeks at a 20 % irradiance level. The full sun assessments reported are an average for the 4 weeks. Results for the above-ground assessments are shown in Table 3.

The Champion leaf extension rate per week was the least both in full sun and at each 20 % irradiance level observation date. In full sun the Champion leaf extension rate was 1.8 times less than for Tifdwarf, 2.8 times less than for Tifway, and 3.5 times less than for Tifgreen. The leaf extension rate for Champion approximately tripled and the other cultivars approximately doubled during the first week under the 20 % irradiance treatment, and then the extension rate per week generally tended to decrease during the next 5 weeks. The leaf extension rates for all cultivars after 5 weeks at a 20 % irradiance level were well below that in full sun. Although the leaves continued to extend, the leaf density and uniformity was less at each successive assessment date.

In full sun the Tifgreen shoot/stolon biomass weight was highest, the Tifdwarf and Tifway weights were next and similiar, with the Champion shoot/stolon biomass weight the least. After week 1 the Champion shoot\stolon biomass weight was 166 % higher than for Tifdwarf, and 125 % and 136 % more than for the Tifway and Tifgreen weights, respectively. From the 2nd week until termination, the above-ground biomass weight differences were minimal. All cultivars tended to decrease in biomass at each successive observation, which reflects the decreasing shoot density visually observed.

Stolon number is important at this cutting height and is a component in the capacity for lateral spread of the turf as well as the capability to recover from turf damage caused by golf club or sport shoe divoting. A high stolon number usually reflects the best potential. In full sun the number of stolon extensions for Champion per 470 mm was 1.7 times higher than for Tifdwarf, 2.6 times higher than for Tifway, and 4.6 times higher than for Tifgreen. After the first week at a 20 % irradiance level the Champion stolon extension numbers were 2 times greater than for Tifdwarf, 4 and 4.5 times more numerous than for Tifway and Tifgreen, respectively. No stolons were found extending from Tifdwarf at week 3 and only 2.3 at week 4. No external stolon extensions were found from either Tifway or Tifgreen after 2 weeks at a 20 % irradiance level.

Visual turfgrass quality for all four cultivars in full sun, and for one week at a

20 % irradiance level, exceeded the 5.0 acceptable standard for fairway\sports field turfs. In the following weeks the results were variable. All cultivars rapidly lost their green leaf color, and the turf canopy of Tifdwarf, Tifway, and Tifgreen became more open. The Champion cultivar remained dense throughout the study.

The assessments made at the termination of Study 2 are shown in Table 4. The Champion shoot density was 198 % greater than for Tifdwarf, 228 % greater than for Tifgreen, and 468 % greater than for Tifway. Visually there were no green leaves in the Tifdwarf, Tifway or Tifgreen canopies, with only a few green leaves in the Champion canopy. The shoot/stolon biomass weight of Champion was 14.7 times heavier than for Tifdwarf, 10 times heavier than for Tifgreen, and 3 times heavier than for Tifway. Mat depth and biomass weight is important on golf teeing grounds\fairways and sport fields to cushion damage from divoting by golf clubs or sport shoes, and to increase safety of participants. In the event damage occurs, the mat provides a reserve of meristems to assist in rapid regrowth of the damaged turf area. It also serves as a carbohydrate reserve when stresses occur to the above-ground shoots and stolons. The Champion mat depth was 8.6 times greater than for Tifdwarf, 6 times greater than for Tifgreen, and 3.4 times greater than for Tifway. The Champion mat biomass density was 118 % greater than for Tifdwarf, 160 % greater than for Tifgreen, and 202 % greater than for Tifway.

The roots extended to the bottom of the 175 mm deep containers for all four cultivars. Most roots were tan or brown, with some black in color. There were no white roots which indicates that new root initiation had ceased. Root biomass of the Tifgreen was 1.5 times greater than for Tifdwarf, 2.2 times greater than for Tifway, and 4.5 times greater than for Champion.

CONCLUSIONS

There were no cultivars, in either Study 1 or Study 2, that were adapted to proper growth in the severe 20 % irradiance conditions, which indicates that these bermudagrasses require more solar energy for continued growth than that available.

At both heights of cut there were large increases in vertical leaf extension rate for all cultivars during the first week in the 20 % irradiance level, and then all cultivars tended to decline until termination of the study. Stolon extension number for both heights of cut for all cultivars responded similarly. Other parameters generally tended to decline at each successive assessment period. A minimum of 5 weeks assessment was necessary to confirm these findings. Futher experiments should be conducted with all these cultivars in less severe shade or at higher irradiance level conditions to determine what energy level is necessary for growth or survival of these turfgrass cultivars.

The comparative morphological growth data reported for the assessments in full sun provided reconfirmation of the comparative cultivar responces previously published

( Beard, J. B, S. I. Sifers, and M. A. Brown. 1996).

REFERENCES

Almodares, A, J. B Beard, and A. C. Novosad (1978). A comparison of turfgrasses and cultural practices in shaded sites. Texas Turfgrass Research 1977-78, pp. 12-16.

Almodares, A. and J. B Beard (1979). Turfgrass shade research update. Texas Turfgrass Research 1978-79, pp. 25-28.

Beard J. B (1970). Turfgrass shade adaptation. pp. 273-283. In R. R. Davis (ed.) Proc. First Int. Turfgrass Res. Conf., Harrogate, England. July, 1969. Sports Turf Res. Inst., Bingley, England, U. K. .

Beard, J. B (1973). 'Turfgrass: Science and Culture' Prentice-Hall Inc.: New Jersey, U.S.A.

Beard, J. B, S. I. Sifers, and M. A. Brown (1996). Comparative morphological assessments of three low growing hybrid ( Cynodon dactylon x C. transvaalensis ) bermudagrasses. p 140. In Agronomy Abstracts , ASA, Madison, Wi.

Dudeck A. E. and C. H. Peacock (1992). Shade and turfgrass culture. pp. 269-284. In Turfgrass. V. Waddington, R. N. Carrow, and R. C. Shearman,(eds.) American Society of Agronomy, Madison, Wisc.

Peacock C. H. and A. E. Dudeck (1993). Responses of St. Augustinegrass cultivars                  ( Stenotaphrum secundatum ( Walt.) Kuntze ) to shade. In International Turfgrass Society Research Journal 7.Chapter 93. R. N. Carrow, N. E. Christians, R. C. Shearman ( eds.)