CSU Study

CSU Research Study (cliff notes)
Mary Suplick 2000-2002

 

 

RESULTS AND DISCUSSION  (Last paragraph is most informative on drought resistance)

Chart abbriviations
HBG = Reveille    KBG  = Kentucky Bluegrass

Evapotranspiration (ET)

Mean Reveille ET of 5.0 mm d -1 was significantly less that of 6.1 mm d -1 found in Kentucky Bluegrass during the months of July through Sep during 2000 and 2001. While measured Kentucky Bluegrass mean ET was at the upper end of the range of those previously reported (3.6-6.3 mm d -1 ), throughout the duration of this study measured Kentucky Bluegrass ET was well correlated (R 2= 0.954) to that predicted by the Northern Colorado Water Conservancy District for Kentucky Bluegrass mowed at the same height (6.4 cm) (Fig. 2.1). As evaporative demand increased, the magnitude of difference between Reveille and Kentucky Bluegrass ET increased (Fig. 2.1). For example, regression analysis showed that Reveille ET was 85, 82, 81, and 80% that of Kentucky Bluegrass as predicted demand increased to 4, 6, 8, and 10 mm d -1 , respectively.  Our data is similar to that of Aronson et al. (1987) and Sheffer (1979) who found changes in the relative ranking of cool-season turfgrass ET as evaporative demand increased. Under non-limiting soil moisture conditions turfgrass water use is understood to be a function of climatic conditions and stand density. However, shoot characteristics which decrease shoot surface area and increase canopy resistance have been shown to moderate transpiration and increase drought resistance in turfgrasses (Burt and Christians, 1990; Kim and Beard, 1988). Relative to Kentucky Bluegrass , Reveille exhibited coarser leaf texture and slower shoot growth rate (as we will discuss below) that may have served to moderate transpiration.

Shoot Growth

Greenhouse Study

 

Mean Reveille clipping yield over the five-month period was 12% that of Kentucky Bluegrass , indicating an inherently slower rate of shoot growth. Interspecific differences in inherent leaf growth rates have been reported in cool season turfgrasses previously (Hull, 1992).  As discussed previously, an inherently slower rate of shoot growth is one characteristic by which turfgrasses may reduce transpiration.

Rooting Characteristics

Field Study

At each soil depth Reveille exhibited significantly greater root length density (RLD,) total root length (TRL) and total root mass (TRM)  than Kentucky Bluegrass (Table 2.1). Greatest RLD was observed within the 0-20 cm in both grasses, with Reveille distributing 82.9% and Kentucky Bluegrass 90.3% of their TRL at this depth. At the 20-40 cm depth Reveille produced 9.8% and Kentucky Bluegrass 7.3% of their roots. The remaining 7.3% of Reveille and 2.4% of Kentucky Bluegrass TRL were distributed within the 40-60 cm soil level.  Kentucky bluegrass roots were observed to be finer and more pliable than Reveille roots, which appeared thicker and coarser. Calculation of the mass to length ratio (TRM:TRL) for each grass supported this observation, with Reveille exhibiting a TRM:TRL of 0.0978 mg cm

-1 and Kentucky Bluegrass 0.0523 mg cm -1 .

Greenhouse Study

As in the field, Reveille exhibited greater RLD, TRL, and TRM than Kentucky Bluegrass . Differences were seen, however, in both grasses, in RLD distribution throughout the sand profile when compared to the field. In the greenhouse, Reveille root distribution from 0-20 cm was reduced to 65% and Kentucky Bluegrass to 79.4%, indicating greater distribution of roots deeper in the profile under sandy conditions when compared to the clayey field soil. At 20-40 cm, Reveille RLD was increased to 23.3% and Kentucky Bluegrass to 19%. Compared to field study, greater difference in RLD between two grasses was in the sand based soil profile in the greenhouse study where Reveille distributed 11.7 % and Kentucky Bluegrass 1.6% of their RLD.  Hybrid bluegrass MRE was 14.6 cm longer that of Kentucky Bluegrass . These results support those of Carrow (1996b) who noted that under field

conditions genetic rooting potential is rarely achieved due to physical and chemical stresses. Hybrid bluegrass and Kentucky Bluegrass TRM: TRL were 0.0859 and 0.0571 mg cm -1 , respectively, which were not significantly different from those under field conditions.  Fine root systems are generally believed to be more efficient investments than coarser root systems. However, as Eissenstat (1992) points out, this ignores other functions of roots that may permit coarse root to be more adaptive under adverse edaphic conditions such as greater longevity.

Drought Response

Leaf Water Content

Depletion of water from the top 20 cm of the soil profile progressed from field

capacity to approximately 23% volumetric soil water content over a 25-day period in Study I and a 30-day period in Study II (Fig. 2.2). During Study I Kentucky Bluegrass maintained leaf water content at approximately 72% for 12 days after irrigation ceased, after which time it began to decline rapidly (Fig. 2.3). Hybrid bluegrass maintained an inherently lower initial leaf water content of approximately 67% for 10 days after irrigation ceased, which

 

slowly declined to approximately 62% over the next 23 days, after which time it began to

decline dramatically.  Similar trends in leaf water content were observed in Study II (Fig. 2.3), however, because SWD of the top 0-20 cm of soil occurred more slowly, Kentucky Bluegrass was able maintain an approximate leaf water content of 70% for approximately 19 days, until it began declining. Hybrid bluegrass leaf water content began to decline 30 days after irrigation ceased. In both grasses and in both studies, initial decline of leaf water content coincided, approximately, with depletion of available water from the 0-20 cm soil level (28%) as previously determined for this field (Dahlin, 1992).

Turfgrass Wilting

In Study I Kentucky Bluegrass began exhibiting leaf wilt on day 12 of dry down and by day 25

showed > 45% leaf firing (Fig 2.4). In contrast, Reveille did not begin to exhibit leaf wilt until day 22 and by day 28 showed only 15% leaf firing. In Study II Kentucky Bluegrass again exhibited leaf wilting earlier than Reveille (at 22 vs. 33 days for Kentucky Bluegrass and Reveille, respectively). After 40 days of dry down Kentucky Bluegrass exhibited > 90% leaf firing while Reveille only exhibited 35% leaf firing. During both dry down cycles, Reveille gradually exhibited an increasingly grayish appearance as SWD progressed, whereas Kentucky Bluegrass progressed more quickly to leaf firing.

 

SUMMARY AND CONCLUSIONS

Our results indicate that Reveille exhibits a lower inherent ET rate than Kentucky Bluegrass , as well

 

as the ability to moderate its ET as evaporative demand increases relative to Kentucky Bluegrass , thereby reducing water loss. Hybrid bluegrass exhibited lower leaf water content and slower shoot growth than Kentucky Bluegrass under non-limiting soil moisture conditions, characteristics advantageous to reduce water use. Additionally, Reveille possesses a significantly deeper, more extensive root system than Kentucky Bluegrass , enabling water extraction from a greater volume of depth when surface soil water is depleted. We conclude that these factors combined contribute significantly to the reduced water use and greater drought avoidance observed in Reveille relative to Kentucky Bluegrass in these studies.