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Crop Science 41:156-166 (2001)
© 2001 Crop Science Society of America

CROP QUALITY & UTILIZATION

Effect of Drought on Growth, Carbohydrates, and Soil Water Use by Perennial Ryegrass, Tall Fescue, and White Clover

H.D. Karstena and J.W. MacAdamb

a Agronomy Dep., 116 A.S.I. Building, The Pennsylvania State Univ., University Park, PA 16802
b Dep. of Plants, Soils, and Biometerology, Utah State Univ., Logan, UT 84322-4820

Corresponding author (hdk3{at}psu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In irrigated pastures of the semiarid, high-elevation western USA, perennial ryegrass (Lolium perenne L.) persistence is poor, and over time white clover (Trifolium repens L.) often dominates mixtures. Irrigation is often not available during autumn, when these perennial plants store carbohydrate reserves for spring regrowth. Our objective was to compare the effect of water stress on growth, carbohydrates, and soil water use of perennial ryegrass, white clover, and tall fescue (Festuca arundinacea Schreb.) in a greenhouse study. These three species were grown separately in a Kidman fine sandy loam, in 15-cm-diam, 1-m-deep pots and irrigated for 81 d (4 plants/pot). Paired pots were then either irrigated or subjected to water deficit (drought) for 30 d, followed by 10 d of recovery with irrigation. At 10-d intervals, four paired pots of each species were destructively sampled to determine leaf and storage organ dry matter and carbohydrate and simple sugar concentrations in storage organs. Root length density and soil water content were also sampled at 20-, 60-, and 90-cm soil depths. Leaf dry matter was lower in water-stressed plants than in irrigated plants by the end of the drought, but did not differ among species. After 10 d of recovery, storage carbohydrate concentration in droughted perennial ryegrass was lower than in white clover, and the ratio of simple sugars (droughted:irrigated) in perennial ryegrass was higher than in white clover. Tall fescue performed similarly to both species. Before the drought, grasses had similar, extensive root systems that withdrew more soil water from the 90-cm soil depth than did white clover. By the end of the 30-d drought, white clover had reduced soil water at all depths as much as the grasses. White clover survived drought and conserved carbohydrate reserves after 10 d of recovery better than did perennial ryegrass and similarly to tall fescue.

Abbreviations: DM, dry matter • DP, degree of polymerization • WSC, water soluble carbohydrates


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN MANY TEMPERATE pasture environments, perennial ryegrass dominates mixtures with white clover; however, in rotationally stocked perennial ryegrass-white clover pastures of the semi-arid (43-cm average annual precipitation), high-elevation (1200 m), intermountain western USA, white clover often dominates perennial ryegrass. In an irrigated field study in northern Utah, where two varieties of perennial ryegrass were sown with white clover and harvested mechanically to mimic a rotational grazing schedule, white clover dry matter averaged 55% of the total mixture yield over four harvest years (J. MacAdam et al., 2000, unpublished data). Poor persistence of perennial ryegrass is often attributed to lack of winter-hardiness, but may also be due to drought stress in autumn that is not apparent until spring growth is impaired. Compared with regions where perennial ryegrass is persistent, climatic conditions of the Intermountain West include moderate mean winter temperatures (-5 to - 2°C), high summer daytime temperatures (25–30°C), low summer nighttime temperatures (8–11°C), low humidity, and autumn routine droughts when irrigation water is not available. This combination of conditions is rarely reported in the literature on intensively managed, cool-season grass pastures and, with the exception of the cool summer night temperatures, may contribute to the poor persistence of perennial ryegrass.


   Tall Fescue
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tall fescue, another cool-season grass, appears to be more tolerant of the climate conditions typical of the Intermountain West (Jung et al., 1996; J. MacAdam et al., 2000, unpublished data), and was therefore included for comparison. In greenhouse and field drought studies under temperate conditions, tall fescue yielded more than perennial ryegrass and white clover, reportedly because leaf rolling prevented excessive stress (Johns, 1978; Johns and Lazenby, 1973). And compared with white clover, tall fescue's root system was more extensive below the 40-cm depth and extracted more soil water from 45- to 75-cm soil depth (Burch and Johns, 1978). Irrigated plots of tall fescue and white clover also yielded more than perennial ryegrass during the summer, in Armidale, New South Wales, Australia (at 1000 m, mean maximum daily temperature 28°C; Johns and Lazenby, 1973). Whether the Acremonium-fescue endophyte association contributed to tall fescue's drought tolerance is not known, as these studies were published before this association was understood. In the more humid United Kingdom, tall fescue cut at 5- and 6-wk intervals had a more extensive root system at a soil depth of 50 to 100 cm, and had extracted more water than perennial ryegrass (Garwood and Sinclair, 1979; Wilman et al., 1998). However, when cut at 3-wk intervals, the root system of tall fescue was smaller, and soil water extraction and aboveground dry matter yield were similar to perennial ryegrass (Garwood and Sinclair, 1979; Garwood et al., 1979).


   White Clover
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Research on white clover's response to drought provides some insight into how it might competitively dominate irrigated perennial ryegrass pastures in the Intermountain West. In comparison with perennial ryegrass and tall fescue during droughts in other temperate climates, white clover had a smaller root system, earlier leaf senescence leading to decreased leaf area during drought, and it yielded less than either grass species (Johns, 1978; Johns and Lazenby, 1973; Guobin and Kemp, 1992; Thomas, 1984; Whitehead, 1983). Under water stress, white clover did not reduce leaf conductance as much as forage grasses (Aparicio-Tejo et al., 1980; Burch and Johns, 1978; Johns, 1978, Thomas, 1984). Turner (1990b) and Hart (1987) have interpreted white clover's poor reduction of leaf conductance as a survival strategy. White clover leaves do not reduce conductance, but wilt and die, thereby reducing leaf area and transpiration. Osmotic adjustment in white clover stolons conserves the stolons until water is available, and the plant can regrow from the stolon (Turner, 1990a,b). Such a strategy could explain how white clover plants survive a drought better than perennial ryegrass and dominate a pasture over time. The higher optimum growth temperature for white clover (24°C) compared with perennial ryegrass (14–20°C; Mitchell, 1956) may also explain, in part, the higher yield of white clover relative to perennial ryegrass under irrigation during summers in Australia (Johns and Lazenby, 1973) and the Intermountain West.


   Carbohydrate Storage
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We hypothesized that white clover and perennial ryegrass have different patterns of utilizing storage carbohydrates during a drought, and these differences could result in different spring regrowth potentials. In cool-season grasses, the major storage carbohydrate, fructan, is stored in stem bases (Smith, 1967). Fructan concentrations increase in stem bases of cool- season grasses, particularly when temperatures decrease in autumn and following reproductive tiller production (Sullivan and Sprague, 1949; Brown and Blaser, 1965; Auda et al., 1966; Smith, 1968; Pollock and Jones, 1979). Fructan that accumulates in autumn is hydrolyzed during spring for production of reproductive tillers and after defoliation for regrowth (Sullivan and Sprague, 1949; Smith, 1967; Labhart et al., 1983; Gonzalez et al., 1989). In response to drought, concentrations of fructan and other water soluble carbohydrates (WSC) decrease in both perennial ryegrass and tall fescue basal tissue (Norris and Thomas, 1982; Suzuki and Chatterton, 1993; Spollen and Nelson, 1994; Volaire and Gandoin, 1996). A corresponding increase in sucrose and hexose concentrations during drought was attributed to fructan hydrolysis, which decreased osmotic potential, and improved the water status of the plants (Suzuki and Chatterton, 1993; Spollen and Nelson, 1994). By contrast, when white clover was exposed to drought the plant appeared adapted to storage carbohydrate reserve conservation (Turner, 1990a,b). During drought, white clover lost its leaves, and osmotic potential and pressure potential were maintained in the stolon, rather than leaves. When water was applied again, then white clover recovered (Turner, 1990a,b). In perennial ryegrass, Norris and Thomas (1982) found that when plants were cut after a drought, WSC concentrations decreased by 50%, presumably because of WSC utilization for plant regrowth. This use of WSC for regrowth would have reduced the osmotic effect of WSC and further depleted carbohydrate storage reserves.

Warm autumn weather can extend the grazing season in the Intermountain West beyond the time that irrigation water is available, and therefore perennial crops can experience late-season droughts. Since drought can cause fructan hydrolysis in grasses, an autumn drought could reduce the amount of fructan stored in perennial ryegrass and tall fescue, particularly if plants were grazed during the drought. However, if tall fescue is more tolerant of an autumn drought than perennial ryegrass, it might hydrolyze less fructan or be able to replenish a larger proportion of stem base fructan compared with perennial ryegrass after an autumn drought and before winter. By contrast, white clover is likely to shed its leaves, stop growing, and conserve a larger proportion of carbohydrates (starch) in the stolon. Our objective was to test this theory in the greenhouse by determining the effect of drought on growth, carbohydrate storage concentrations and patterns, rooting activity, and soil moisture use of white clover, perennial ryegrass, and tall fescue.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Establishment
On 30 Jan. 1997, `Pitau' white clover, `Moy' perennial ryegrass, or `Martin' tall fescue were seeded in a greenhouse in monospecific pots constructed of 15-cm diam, 1-m-deep polyvinylchloride (PVC) pipes with a 0.5-cm aeration hole at 15-, 30-, 45-, and 60-cm intervals from the bottom. Pots were filled with Kidman fine sandy loam, (a coarse-loamy, mixed, mesic Calcic Haploxeroll) at 1.25 g cm-3 bulk density over 7.5 cm of gravel. The field capacity of this fine sandy loam was previously determined in PVC pipes to be 0.234 kg kg-1 or 23.4%; the permanent wilting point for this soil was 0.05 kg kg-1 (D. Or, 1997, personal communication). Forty pots were planted with seeds of each species. After 1 wk, seedlings were thinned to four plants per pot.

During the first 3 wk of establishment, plants were grown at 17°C ± 4°C day and 9°C ± 4°C night temperatures, similar to air temperatures of Idaho and Utah intermountain valley in June, and under February photoperiods (average 10.5 h). Temperatures were then increased to typical August temperatures (29°C ± 4°C day/9°C ± 4°C night). Relative humidity averaged 35%. Supplementary lighting with high pressure sodium lamps (1000 W) was gradually extended over the 81 d of establishment to represent the day-length change from April to June and then decreased to August daylengths in the Intermountain West. Pots were irrigated during the establishment period according to field irrigation recommendations (4 cm water per week the equivalent of 800 mL per week per pot). During the first 8 wk (56 d) of establishment, pots were watered twice a week until plant transpiration increased and pots were then watered once a week. Soil water content was monitored in additional pots with time domain reflectrometer rods (data not shown) and by observing plant wilting response. During establishment and before the drought was imposed, fertilizer was applied in the irrigation water every third day so that all pots received the equivalent of 67 kg ha -1 N and 73 kg ha-1 P2O5 as recommended by the Utah State University soil test. Plants were defoliated 72 d after seeding to stubble heights typically recommended for grazing: 3.8 cm for perennial ryegrass and white clover and 7.6 cm height for tall fescue.

Nine days later, 81 d after seeding, all pots received 800 mL of water to mark Day 0 of the drought. This irrigation regimen created soil conditions that mimicked root-zone field conditions at the beginning of September. Pots were arranged in a randomized complete block design with two moisture conditions imposed over each of three species, replicated for six destructive sampling times and across four blocks in the greenhouse (2 water treatments x 3 species x 6 sampling dates x 4 blocks = 120 pots). Irrigated plants were irrigated at successive 6- or 7-d intervals. On Day 0, 10, 20, and 30 of the drought one pot from each replication of each of the six treatments was destructively sampled. Droughted plants were irrigated on Day 30 and a final destructive sampling occurred 10 d after rewatering. Because of time constraints, all four blocks could not be sampled in one day. Therefore, at each sampling interval (Day 0, 10, 20, 30, and 40) one or two of the four blocks were sampled in one day, for a period of 1 to 3 successive days. Daylength was decreased gradually during the 30-d drought treatment to represent September conditions in the Intermountain West. (13 h 7 min reduced to 11 h 22 min and average August and September temperatures were maintained at 29°C ± 4°C day/9°C ± 4°C night). To mimic a grazing event that would occur on average at 21-d intervals, all remaining plants were defoliated to 3.8 cm for perennial ryegrass and white clover and to a 7.6-cm stubble height for tall fescue on Day 10 of the drought.

Aboveground Tissues
Sampling
At each destructive sampling, white clover green leaves and petioles were separated from stolons, and grass leaf blades and sheaths above the stem base were separated from stem base storage tissues. Grass stem bases were defined as the tissue between the oldest leaf collar down to the uppermost root. Roots were cut away from the stolons and stem bases. Storage tissue was immediately placed in a plastic zip-closure bag, covered with crushed dry ice, then stored at -20°C and freeze-dried within 2 d. Leaf tissue was dried in a forced-air oven at 80°C to determine dry matter (DM).

Carbohydrate Analyses
Reducing sugars (fructose and glucose), sucrose, starch, and fructan concentrations were quantified for grass stem bases and white clover stolons according to the procedure of Volenec (1986). Fifty milligrams of dried tissue was extracted with 92% (v/v) ethanol to remove simple sugars (glucose, fructose, and sucrose). Extracts were prepared and separated by anion exchange chromatography using an inline pulsed amperometric detector (Dionex Corp., Sunnyvale, CA) as described by Chatterton et al. (1989). An HPLC-AS6 carbohydrate column containing inert styrenedivinylbenzene polymer was the separating medium. This analysis verified that fructose and glucose were extracted, and that fructan polymers were not present in the extracts. Simple sugars in the 92% ethanol extracts were assayed colorimetrically after reacting with 0.2% (w/v) anthrone in 78% (v/v) sulfuric acid with glucose as a standard (Koehler, 1952).

Following ethanol extraction, plant residues were resuspended in water and boiled. Starch was hydrolyzed with 500 units of amyloglucosidase (Sigma A7255, St. Louis, MO) in 200 mM acetate buffer pH 4.5 for 24 h at 55°C. Starch glucose was assayed by the glucose oxidase, glucose Trinder assay (Sigma 315-100), as tested and described by Brown and Volenec (1989). Fructan in the tissue residue was then subjected to acid hydrolysis by boiling tissues in 0.5 M H2SO4 for 15 min. The solution was neutralized with 1.0 M NaOH and the glucose released was assayed by the glucose oxidase, glucose Trinder assay (Sigma 315-100). Free and combined fructose was assayed with 0.2% anthrone in 76% sulfuric acid by the ketose-specific modification of the anthrone reaction described by Pollock and Jones (1979). Starch-derived glucose was subtracted from acid-hydrolysis glucose to determine fructan-glucose. Fructan concentration was calculated as the sum of fructose and fructan glucose x 0.9. All plant carbohydrate concentrations were determined on a dry matter basis. For purposes of estimating the mean degree of polymerization of the fructan molecules, it was assumed that there was one glucosyl residue per fructan molecule. Therefore, the fructose concentration in the fructan was divided by the glucose concentration.

Soil Moisture and Root Length Density
Sampling
After sampling aboveground material, a 3.5 cm-deep cross-section of the soil column was removed from each pot at depths of 20, 60, and 90 cm. Approximately 35 g of root-free soil was subsampled from each cross-section then dried at 100°C to a constant weight to determine soil water content. Soil was hand-sifted to collect all roots greater than 3 mm in length. Roots were rinsed with water and stored in 10% (v/v) aqueous isopropanol until total root length could be measured with a Comair laser scanner (Melbourne, Australia). Root length density was calculated for each depth as centimeters of root length per cubic centimeter soil volume.

Statistical Analysis
Means of absolute values for leaf and storage organ dry matter, storage organ sugar and carbohydrate concentrations, soil water content, and root length densities were analyzed by the GLM procedure of SAS (SAS Institute, 1998). Irrigation treatment, species, and destructive sampling date were main effects and all interaction terms were tested for significance. Within each destructive sampling, means were compared for each species and irrigation treatment combination (six paired comparisons) with Tukey's test so the experimentwise error rate would not exceed 0.05. In the figures presented, only means that are significantly different within each sampling date are labeled with different letters; all other comparisons were not significantly different according to Tukey's test. Soil water content and root length densities were similarly compared for each soil depth and sampling date combination with Tukey's test.

To compare species response to drought, measurements for aboveground characters of droughted plants were compared with those of irrigated control plants by calculating the ratio between pots paired by species and block in time. Ratios of droughted plants compared with irrigated plants were calculated for leaf dry matter, storage organ dry matter, sugar concentrations, and carbohydrate storage concentrations. Ratio data were analyzed by the GLM procedure of SAS. Within each destructive sampling time point, ratios of each species were compared with the other two species with pre-planned contrasts. Contrasts were declared to be statistically significant when P < 0.05; in some cases differences at the P < 0.1 level are also reported.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Leaf Dry Matter
Before the drought treatment began (Day 0), leaf DM did not differ among species (Fig. 1a) . Species x time interaction was significant, however, because irrigated white clover plants produced more leaf mass than irrigated perennial ryegrass on Day 30 and 40, perhaps because daytime temperatures prior to those time points were closer to the optimum growth temperature for white clover than for perennial ryegrass (Fig. 1a). Leaf DM did not differ between water treatments early in the drought period. However, by the end of the drought (Day 30) and after the 10-d recovery period (Day 40), the droughted plants produced less leaf dry mass than the irrigated plants (Fig. 1a). One week into the drought, leaves of water-stressed tall fescue plants were rolled. Leaf DM ratios decreased significantly over time to a mean of 0.5 or less after d 20 of the drought, indicating that the effect of the drought on leaf growth was similar for all species (Fig. 1b).



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  		Fig. 1. (A) Total leaf dry mass per pot; (B) Ratio of leaf dry mass; droughted:irrigated plants. Wi indicates when irrigated plants (controls) were watered. Wd indicates when droughted plants were watered. Bars represent + S.E. Different bold letters (a, b, and c) indicate species means that were significantly different (P < 0.05) within each harvest date. Different italicized letters (a and b) indicate species means that were significantly different (P < 0.1) within each harvest date

 
Among species, the leaf DM ratio was significantly higher (P < 0.1) only for white clover compared with tall fescue on Day 10 of the drought (Fig. 1a and 1b). Others have also found that white clover initially maintained more leaf area under water stress than cool season grasses (Burch and Johns, 1978; Johns, 1978; Thomas, 1984). By Day 30 in this study, leaf DM was significantly reduced in all three species. This contrasts with Burch and Johns (1978) and Johns (1978) who found that tall fescue maintained and yielded more leaf area than white clover under drought in Australia. The different findings may be due to low humidity and warmer temperatures used in this study, which might have produced more drought stress than occurred in the greenhouse studies in New South Wales, Australia (Burch and Johns, 1978; Johns, 1978). The tall fescue plants in the previous drought studies may also have benefitted from the Acremonium-fescue endophyte association which was not appreciated at that time. Further, older plants in the field study of Johns (1978) may also have been more drought tolerant.

Storage Organ Dry Matter
Before the drought began (Day 0), perennial ryegrass stem base DMs were twice those (P < 0.05) of white clover stolon DMs (Fig. 2a) . The stem base DMs of both irrigated grasses were greater than irrigated white clover stolon DMs on Day 10; and the irrigated tall fescue DM was significantly greater than that of white clover on Day 20. This DM difference appears to be an inherent species difference due to developmental and autumn storage carbohydrate differences between tillering-cool season grass stem bases and stoloniferous-white clover stolons. Stem base or stolon DM did not differ significantly within species between irrigated and droughted treatments during the drought. And, by the end of the drought (Day 30), all treatments and species were similar in storage organ DM. By Day 40, after 10 d of recovery, only irrigated tall fescue had significantly heavier stem bases than did the droughted tall fescue plants (Fig. 2a). Although total storage organ DM did not differ significantly between water-stressed species during the drought, differences in species responses to drought are apparent when storage organ DM ratios are compared (Fig. 2b).



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  		Fig. 2. (A) Stem base or stolon dry mass; (B) Ratio of stolon and stem base dry mass; droughted:irrigated plants. Wi indicates when irrigated plants (controls) were watered. Wd indicates when droughted plants were watered. Bars represent + S.E. Different bold letters (a and b) were significantly different within each harvest date, at P < 0.05. Different italicized letters (a and b) were significantly different within each harvest date at P < 0.1

 
Calculating the ratio of droughted to irrigated storage organ DM for each species reduces inherent variation due to species DM differences, and allows for comparison of the relative response of each species to drought. After 10 d of drought, the white clover stolon DM ratio was significantly higher than the perennial ryegrass stem base DM ratio (Fig. 2b). Ten days later, (Day 20), the white clover stolon DM ratio was significantly greater than the tall fescue stem base DM ratio (P < 0.1; Fig. 2b). At the end of the 30-d drought period, the DM ratios of stolons and stem bases did not differ significantly (P < 0.05) among the species, averaging 0.9 (Fig. 2b). After the 10-d recovery period, the mean storage organ DM ratios had decreased to a mean of 0.6, suggesting utilization of storage carbohydrates and other compounds for regrowth by all species. This pattern of storage organ DM ratio differences suggests that white clover plants initially conserved stolon DM better than perennial ryegrass and tall fescue conserved stem base DM during the drought (Fig. 2a). However, by the end of the 30-d drought and after the 10-d recovery period, storage organ DM was reduced similarly in all three species.

Overall, white clover plants exposed to drought produced about the same amount of leaf and storage organ DM as perennial ryegrass and tall fescue (Fig. 1a and 2a). This differs from drought studies conducted in the field and greenhouse where more mature, tall fescue or perennial ryegrass yielded more than white clover (Guobin and Kemp, 1992; Johns, 1978; Johns and Lazenby, 1973). Our results are probably different in part because they were not more mature plants growing in the field. However, in this experiment the similar aboveground production of white clover and perennial ryegrass is closer to what is observed in irrigated pastures in the Intermountain West. In a 1995 to 1999 irrigated Utah location where two varieties of perennial ryegrass were sown with white clover and harvested to mimic a rotational grazing schedule, white clover dry matter averaged 55% of the total mixture yield over four harvest years (J. MacAdam, unpublished data), verifying that these climatic and management conditions are unique in affecting relative performance of white clover and cool-season grasses. Since white clover has a higher optimum growth temperature than the cool season grasses, the cool night temperatures of the Intermountain West may significantly reduce white clover's metabolic activity, improving its performance during a drought.

Storage Carbohydrate Concentrations
Storage carbohydrate (fructan or starch) concentrations did not differ significantly among species or within treatments early in the drought. However, by the end of the drought (Day 30), and after the 10-d recovery, some differences had developed (Fig. 3a) . All perennial ryegrass and white clover plants were defoliated to 3.8 cm, and tall fescue to 7.6 cm (typical grazing height for each species) 9 d before the drought began (Day 9) and again on Day 10 of the drought. Defoliation may be responsible for a pattern that developed 10 d after the defoliation events (Days 0 and 20); the storage carbohydrate concentrations in the irrigated grass plants tended to be higher than in the white clover stolons (Fig. 3a). This pattern continued until the end of the drought (Day 30) when the irrigated perennial ryegrass stem bases had twice the concentration of storage carbohydrate as did white clover stolons; however after the 10-d recovery period (Day 40), the irrigated white clover and grass plants had similar storage carbohydrate concentrations (Fig. 3a).



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  		Fig. 3. (A) Storage carbohydrate concentrations, fructan plus starch; (B) Ratio of storage carbohydrates, fructan plus starch; droughted:irrigated plants. Wi indicates when irrigated plants (controls) were watered. Wd indicates when droughted plants were watered. Bars represent + S.E. Different bold letters (a and b) indicate means that were significantly different (P < 0.05) within each harvest date. Different italicized letters (a and b) indicate means that were significantly different (P < 0.1) within each harvest date

 
After the 10-d recovery (Day 40), the droughted cool-season grass stem bases had markedly lower storage carbohydrate concentrations than did the irrigated grasses (Fig. 3a). In the case of tall fescue, this corresponded with the significantly lower storage organ DM of the droughted tall fescue plants compared with the irrigated plants (Fig. 2a). The droughted perennial ryegrass plants had a significantly lower (P < 0.1) carbohydrate concentration than the droughted white clover stolons (Fig. 3a). Spollen and Nelson (1994) and Suzuki and Chatterton (1993) also found that under drought, the content of fructan decreased in tall fescue and perennial ryegrass leaf bases, presumably to release hexoses and sucrose for osmotic adjustment.

The ratio of storage carbohydrate concentrations in the storage organs of droughted compared with irrigated plants did not differ among the three species during the drought, and decreased to an average of 0.8 by the end of the 30-d drought (Fig. 3b). However, after the 10-d recovery period, the white clover ratio of storage carbohydrates in the droughted plants was twice that (P < 0.1) of the perennial ryegrass ratio (Fig. 3b). This was due to a decrease in the storage carbohydrate concentration in droughted perennial ryegrass (Fig. 3a) while in all other species, both droughted and irrigated, plant storage carbohydrates were replenished or did not change (droughted tall fescue). This supports the results of Norris and Thomas (1982) who found that the concentration of WSC in perennial ryegrass decreased by 50% when plants were defoliated after a drought.

There was high variation in the degree of polymerization (DP) of fructans of both perennial ryegrass and tall fescue and no statistical differences in DP between the watered or species treatments. However, the DP tended to be lower in the droughted perennial ryegrass plants than in the irrigated plants at Days 20 and 30 of the drought, while the DP of the droughted and irrigated tall fescue plants did not appear to differ (Fig. 4) . This pattern suggests that droughted perennial ryegrass plants may have utilized stored fructans as an osmoticum more during the drought than did tall fescue.



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  		Fig. 4. Degree of polymerization of fructans. Bars represent + S.E. Wi indiacates when irrigated plants (controls) were watered. Wd indicates when droughted plants were watered

 
Simple Sugar Concentration
As expected, the simple sugar concentrations were significantly higher in the droughted plants compared with the watered plants in tall fescue on Day 20, and in perennial ryegrass on Day 30 (Fig. 5a) . This is indicative of increases in osmotically active sugar concentrations in response to drought (Fig. 5a). This pattern of increase in the concentration of sucrose and hexoses during drought was also observed in leaf bases of tall fescue (Spollen and Nelson, 1994) and perennial ryegrass (Suzuki and Chatterton, 1993). It appears that the grasses hydrolyzed storage carbohydrates (fructans) towards osmotic adjustment in response to drought more than white clover. This corresponded with tall fescue's lower (P < 0.1) storage organ DM ratio (droughted:irrigated plants) compared with white clover on Day 20 of the drought, and is in agreement with other's observations that white clover conserves stolons well, but not leaves (Hart, 1987; Turner, 1990a,b). Further, Turner (1990a) also found that in white clover plants under water stress, the amino acid proline best explained an increase of pressure potential in clover stolons, and the proline concentrations increased about 12 times more than the WSC.



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  		Fig. 5. (A) Total simple sugars concentrations, glucose and fructose; (B) Ratio of simple sugars; droughted:irrigated plants. Wi indicates when irrigated plants (controls) were watered. Wd indicates when droughted plants were watered. Bars represent + S.E. Different bold letters (a and b) indicate means that were significantly different (P < 0.05) within each harvest date

 
When storage carbohydrate concentrations decrease over time, it is likely to be the result of conversion of polymers to simple sugars for cell maintenance or utilization for growth. The fructan concentrations of droughted perennial ryegrass decreased between Day 30 and 40 (Fig. 3a), and the simple sugar concentration also decreased (Fig. 5a). Leaf growth occurred in all droughted species during the recovery period (Fig. 1a, Day 30 and Day 40), but appeared to occur at the expense of replacement of storage carbohydrates in perennial ryegrass (Fig. 3a, Day 30 and 40).

The ratio of simple sugars (droughted:irrigated plants) increased across species during the drought to a mean of about 2 by Day 30, and was significantly higher in perennial ryegrass than tall fescue at the end of the drought (Fig. 5b, Day 30). Further, after the 10-d recovery period, the perennial ryegrass ratio of simple sugars (glucose, fructose, sucrose) was four-fold greater than the white clover ratio which decreased to less than 0.5 (Fig. 5b, Day 40, P < 0.05). This suggests that in perennial ryegrass the short-term response to defoliation after a drought is to initiate new growth (Fig. 1a) rather than polymerizing the carbohydrates back to storage (Fig. 3a). This utilization of carbohydrates after a drought for regrowth would agree with the significant regrowth that others have observed in cool-season forage grasses over a longer recovery period (compensatory growth), apparently due to utilization of osmotic adjustment sugars and active meristems (Frank et al., 1996; Horst and Nelson, 1979). However, if perennial ryegrass plants hydrolyze storage carbohydrates for regrowth in mid- to late autumn in the field and do not encounter lengthy enough growing conditions to replenish storage carbohydrates before winter, winter hardiness and spring reproductive tiller growth may be reduced. These data indicate that tall fescue and white clover did not break down storage carbohydrates for regrowth as readily as perennial ryegrass at the end of or after the drought, and therefore might have better winter survival and/or spring production. However, a field trial is needed to test adequately whether this would be the response of first year plants and mature plants in the field.

Root Length Density
On Day 0 of the drought, the root length densities of perennial ryegrass and tall fescue plants were 10 to 20 times greater than white clover at 20- and 60-cm soil depths, and 65 to 70 times greater than white clover at 90 cm (Table 1). Similarly, after 10, 20, and 30 d of drought and after the 10-d recovery period (Day 40) the root length densities of both droughted and irrigated perennial ryegrass and tall fescue plants at 20-, 60-, and 90-cm soil depths were greater than irrigated and droughted white clover plants, except that the root length density of white clover at 90 cm was not significantly different from that of perennial ryegrass on Day 30, or from that of tall fescue on Day 40 (Table 1).


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  		Table 1. Root length densities at 20, 60, and 90 cm in white clover, perennial ryegrass, and tall fescue droughted pots sampled at 10-d intervals throughout the study

 
White clover, a tap-rooted plant, had less fibrous root matter in the upper soil profile. This finding is in agreement with other studies that reported white clover seedlings and mature plants had smaller root systems than tall fescue and perennial ryegrass under humid, temperate, and irrigated conditions in greenhouse and field studies (Burch and Johns, 1978; Guobin and Kemp, 1992; Thomas, 1984; Whitehead, 1983).

The root length density of tall fescue did not differ from perennial ryegrass at 60 or 90 cm over the entire experiment (Table 1). The similarity in root length density between tall fescue and perennial ryegrass throughout the study was not expected, because perennial ryegrass is reputed to be more shallow-rooted than tall fescue (Garwood and Sinclair, 1979; Wilman et al., 1998). The differences others have documented in these longer term, field studies may also require more time and field conditions to develop, which we could not simulate in this greenhouse study. Wilman et al. (1998) and Garwood and Sinclair (1979) found that tall fescue's root system was larger than perennial ryegrass when the grasses were cut at 5- and 6-wk intervals. However, under a 3-wk cutting interval treatment, similar to the Utah rotational grazing schedule simulated in this study, the root system and soil water extraction of tall fescue, along with its drought tolerance, were reduced and more similar to perennial ryegrass (Garwood and Sinclair, 1979). Our plants were grown in PVC pots which elevated the soil temperatures compared with normal soil temperatures, and may also have altered the root growth patterns of these species.

Soil Water Content
When the drought began, the grass root systems were 10 to 20 times more dense at the 20- and 60-cm depths than the root systems of white clover, but the soil water content at 20 and 60 cm did not differ among the three species (Table 2). At 90 cm, the soil water contents in both the perennial ryegrass and tall fescue pots were significantly lower than in the white clover pots (Table 2). This was due to the 65 to 70 fold greater root length density of the grasses at 90 cm. When the drought began, the perennial ryegrass stem base DM was also significantly greater than the white clover stolon DM, suggesting that perennial ryegrass may have required more water to maintain its aboveground tissue. On Day 10 of the drought, soil water content at 90 cm was still significantly lower under tall fescue than white clover, as other studies also observed (Burch and Johns, 1978; Guobin and Kemp, 1992). However, in this experiment, even with a significantly smaller root system, white clover plants reduced soil water dramatically by Day 10, and had similar leaf DM compared with perennial ryegrass and tall fescue at the end of the drought (Table 2 and Fig. 1a). This agrees with earlier studies (Burch and Johns, 1978; Johns, 1978; Thomas, 1984; Hart, 1987; and Turner, 1990b) that reported that under early stages of drought white clover leaf area index and conductance were maintained until the leaves died, and then white clover's transpiration decreased. We did not quantify leaf area index or water use efficiency for these species, which might also explain some of the soil water content differences observed between the soil of the grasses and white clover.


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  		Table 2. Soil water contents at depths of 20, 60, and 90 cm in white clover, perennial ryegrass, and tall fescue droughted pots sampled at 10-d intervals throughout the study

 
During the first 10 d of the drought, the soil water content at all depths in the droughted white clover pots decreased by an average of 40%, and then declined more slowly from Day 10 to 20 when the plants were defoliated (Table 2). However, from Day 20 to 30, the soil water content in the white clover pots decreased again at all depths by an average of 47% (Table 2). By the end of the 30-d drought, the soil water content of the water-stressed white clover pots was the same as that of the water-stressed perennial ryegrass and tall fescue pots (Table 2). Even though the white clover root system was significantly smaller at all soil depths, all three plant species reduced the soil water at 20-, 60-, and 90-cm depths to the apparent permanent wilting point of 0.05 kg kg-1 (D. Or, 1997, personal communication) under drought.

If these plants had been growing in the same pot, white clover with its less-extensive root system, might have been at a disadvantage during periods of competition for water. The grass plants had reduced soil water to near the wilting point by 10 or 20 d (Table 2). However, observations in the field indicate that white clover is highly competitive with grasses in the climate of the Intermountain West. The two strategies that may be used by white clover to support this competition are aggressive replenishment of storage carbohydrates following drought, and steady extraction of available soil water to depths of at least 90 cm.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
After a 30-d drought and 10-d recovery simulating autumn conditions of the Intermountain West, storage carbohydrate concentrations in droughted plants relative to irrigated controls were higher in white clover stolons than perennial ryegrass stem bases. Perennial ryegrass stem bases also had a higher ratio of simple sugars at the end of the recovery period than white clover or tall fescue. This suggests that perennial ryegrass plants made more carbohydrates available for regrowth during the simulated autumn conditions than either white clover or tall fescue. In the field, such a response to drought of a new seeding of perennial ryegrass or mature plants might deplete plant storage carbohydrates and predispose perennial ryegrass to winter-kill or poor spring production compared with white clover. Tall fescue did not hydrolyze fructan to sugars to the extent that perennial ryegrass did after the drought, and therefore in the field, it may be better adapted to surviving the winter and coexisting with white clover in the Intermountain West. Tall fescue and perennial ryegrass root length densities were similar and greater than those of white clover. However, white clover, with its smaller root system, reduced soil moisture relatively rapidly in the first 10 d of drought, and to wilting point by the end of the 30-d drought. The results of this greenhouse study cannot completely describe the response of these species or more mature plants of these species to an autumn drought in the field. However, further investigation in the field into whether an autumn drought in the Intermountain West reduces perennial ryegrass carbohydrate reserves more than white clover and tall fescue appear warranted. Whether spring recovery from an autumn drought differs between these species also appears worthy of a field investigation. Although further field investigations are needed, at present we predict that autumn irrigation and tall fescue instead of perennial ryegrass would improve the grass-legume mixture balance and yield of rotationally grazed pastures in the Intermountain West.


   ACKNOWLEDGMENTS
 
Thanks to Philip Harrison and N. Jerry Chatterton for their assistance with anion exchange chromatography analysis, Jeffery Volenec for laboratory method advice, Susan Buffler and the Forage Crop Physiology, Utah State University undergraduate research students for their assistance, and Marvin Hall for his helpful comments on this paper.

Received for publication January 11, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Tall Fescue
 White Clover
 Carbohydrate Storage
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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