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Ecology |
Department of Biology, The College of Staten Island, City University of New York, Staten Island, New York 10314 USA
Received for publication February 5, 2004. Accepted for publication August 26, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: drought stress • endophytes • forage crop • Lolium perenne • Poaceae • storage • turfgrass
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). It is also a widely used turfgrass and many cultivars have been developed for use on golf courses and lawns (Fermanian et al., 1996
). As an outcrossing species, it is genetically variable within and between cultivars (Casler, 1995
; Huff, 1997
; Kubik et al., 2001
). Jung et al. (1996
, p. 633) noted that "few species can tolerate the range in environmental conditions and grazing managements" typically associated with ryegrass habitats. These conditions include repeated defoliation, widely fluctuating temperature, and soil moisture deficits. Broad adaptability to a variety of environmental conditions and stresses has undoubtedly contributed to the worldwide ecological success of perennial ryegrass.
As in many cool-season, perennial grasses, L. perenne is often infected by a clandestine, endophytic fungus (Latch and Christensen, 1982
; Lewis et al., 1997
). The endophyte, formerly in the genus Acremonium, is now classified as Neotyphodium lolii (Latch, Christensen, and Samuels) Glenn, Bacon, and Hanlin (Glenn et al., 1996
). It produces intercellular hyphae that are easily detected by microscopic examination of the host leaf sheaths and culms (Welty et al., 1986
; Bacon and White, 1994
; Christensen et al., 2002
). This particular endophyte is asexual and only transmitted vertically within host seeds (White et al., 1993
; Clay, 1998
).
Endophytic fungi are biotrophic, obtaining nutrients from within the plant without causing host cell death (Carlile et al., 2001
). While the fungal partner in the grass-endophyte symbiosis likely benefits from the use of carbohydrates made during host photosynthesis, it is not clear to what extent, if any, L. perenne benefits from the relationship (Ravel et al., 1995
; Barker et al., 1997
; Cheplick et al., 2000
; Cheplick and Cho, 2003
). This is in marked contrast to the mutualistic relationship that is consistently observed between the well-investigated symbiosis of tall fescue (Festuca arundinacea) and its fungal endophyte (Neotyphodium coenophialum) (Belesky et al., 1987
; Clay, 1990
, 1998
; Hill et al., 1991
; Bacon, 1993
; Clay et al., 1993
; Latch, 1997
; Leuchtmann, 1997
; Malinowski and Belesky, 2000
). In F. arundinacea and a few additional grasses including other Festuca spp., Brachypodium sylvaticum, and Lolium perenne, endophyte-infected plants show increased resistance to herbivores (Cheplick and Clay, 1988
; Breen, 1994
; Brem and Leuchtmann, 2001
) and greater growth and competitive performance relative to uninfected plants (Clay, 1990
, 1998
; Hill et al., 1991
; Clay et al., 1993
; Latch, 1993
; Malinowski et al., 1997
).
In all likelihood, the grass-endophyte symbioses observed in nature span a continuum of interactions from antagonistic to mutualistic (Wilkinson and Schardl, 1997
; Saikkonen et al., 1998
; Cheplick and Cho, 2003
; Faeth and Sullivan, 2003
). That is, the nature of the grass-endophyte symbiosis is contingent on environmental conditions, even for a single host species (Ahlholm et al., 2002
; Morse et al., 2002
). This is likely to result in considerable variation in the types of effects endophyte infection elicit on host survival, growth, and reproduction. Furthermore, host grasses have been shown to exhibit genotypic specificity in terms of endophyte-mediated effects (Belesky et al., 1987
; Rice et al., 1990
; Cheplick, 1997
, 1998
; Cheplick et al., 2000
; Hesse et al., 2003
). In this way, endophytes will influence phenotypic variation among host genotypes. Endophytes can also modify the evolutionary dynamics within the host population by differentially impacting the growth, survival, and reproduction of infected and uninfected individuals.
Tolerance of host plants to drought stress has been called the most thoroughly documented feature of abiotic stress tolerance in endophyte-infected grasses (Malinowski and Belesky, 2000
). While it is true that some studies have provided evidence that the endophyte of Festuca arundinacea can improve host tolerance to, and recovery from, drought (Arachevaleta et al., 1989
; Bacon, 1993
; West et al., 1993
; West, 1994
; Malinowski and Belesky, 2000
), other studies have not found a consistent benefit to endophyte-infected tall fescue under drought (White et al., 1992
; Elbersen and West, 1996
; Hill et al., 1996
; Buck et al., 1997
). For other species of Festuca infected by fungal endophytes, under low water availability infected plants showed greater growth and biomass production in F. arizonica (Morse et al., 2002
), but lower tiller and biomass production in F. pratensis (Ahlholm et al., 2002
), compared to uninfected plants. In Lolium perenne, the effects of endophytes on drought tolerance and recovery from drought stress have also been equivocal; it has been difficult to document any definitive benefits associated with endophyte-infected hosts (Lewis, 1992
; West, 1994
; Ravel et al., 1995
; Barker et al., 1997
; Eerens et al., 1997
; Cheplick et al., 2000
). This widespread variation in the nature of the grass-endophte symbiosis both within and between species has led some to challenge the general assumption that fungal endophytes are predominantly beneficial to their grass hosts (Saikkonen et al., 1998
; Faeth, 2002
; Faeth and Sullivan, 2003
).
Given the importance of soil water availability to the growth and productivity of cool-season grasses in pasture and turf communities (Watschke and Schmidt, 1992
; Frank et al., 1996
), and the possible role of endophytic fungi in host drought tolerance (Bacon, 1993
; West, 1994
; Cheplick et al., 2000
; Malinowski and Belesky, 2000
), the present study was designed to investigate the impact, if any, of endophyte infection on the ability of L. perenne to recover from severe drought stress. Genotypic variation in the effects of both drought and endophytes was explored by using infected and uninfected replicates of 10 host genotypes. Specific questions addressed were (1) are fungal endophytes beneficial or deterimental to host recovery from drought?, (2) do host genotypes show significant variation in the ability to recover from drought?, and (3) are growth and allocation traits of host genotypes differentially affected by endophytes?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), each represented by endophyte-infected and uninfected replicates, were derived from extra plant material retained from an earlier dought experiment (Cheplick et al., 2000
). Infected seeds of the cultivar were originally obtained from Loft's Seed Company, Somerset, New Jersey, USA. Cheplick et al. (2000)
described how half of the ramets of each genotype were subjected to treatment with the systemic fungicide Benomyl (E. I. Dupont de Nemours Co., Acme Division, PBL/Gordon Corp., Kansas City, Kansas, USA) in January 1998 to generate endophyte-free (–E) ramets. The other half of the ramets of each genotype remained infected (+E). Infection status was determined by microscopic examination of host leaf-sheath pieces stained with aniline blue in lactic acid and examined at 400x (Bacon and White, 1994
).
Seedlings of L. perenne emerging from individual seeds were assumed to constitute distinct genotypes. This is reasonable as molecular analyses have shown that cultivars (including the one used here) of perennial ryegrass are typically composites of many genotypes (Huff, 1997
; Sweeney and Danneberger, 1997
; Kubik et al., 2001
). In Huff's (1997)
characterization of genetic diversity using random amplified polymorphic DNA (RAPD), each of the 180 individual ryegrass plants examined, representing 11 cultivars, showed a unique combination of the 33 RAPD markers scored. Preliminary work on the 10 genotypes used in the present experiment has revealed similar variation in RAPD markers (Cheplick and Huff, unpublished data). In addition, statistically significant differences in quantitative growth and storage traits among these same genotypes have been reported previously (Cheplick et al., 2000
; Cheplick and Cho, 2003
).
Over winter, 1999–2000 –E and +E plants of the 10 genotypes were maintained in deep, tubular pots (6.5 cm diameter by 25 cm depth) within a cold incubator designed to simulate winter conditions (15°C, 12 h light; 5°C, 12 h dark). The growth medium was a 2 : 1 : 1 mixture of topsoil : peat moss : fine vermiculite. These plants served as stock cultures and supplied the ramets planted for the experiment. Because two years had elapsed since the original fungicide treatment had been used to generate –E ramets, infection status was again verified by microscopic examination of leaf-sheath tissues sampled from the stock cultures prior to initiating the following experiment.
The experiment
Individual ramets, trimmed to 5 cm height, were excised from the stock cultures and planted into separate tubular pots (6.5 cm diameter by 25 cm depth) containing the soil mixture noted earlier during the final week of June 2000. Topsoil, consisting of a blend that included compost and peat, was a major component of the soil mixture. It supplied sufficient mineral nutrients for all plants throughout the 12-wk experiment and pots were never fertilized. During this time, there was no evidence that soil nutrients (or light) were limiting to plant growth.
For each genotype there were 16 –E ramets and 16 +E ramets planted. All plants were maintained in the greenhouse and watered as needed until the drought treatment was initiated (for half the plants) on 18 August 2000. During this pre-drought period, mean (±1 SD) minimum and maximum temperatures recorded on 10 randomly selected dates were 16.9° ± 0.3° and 24.3° ± 1.6°C, respectively.
There were 320 plants, 32 replicates per each of 10 genotypes, in the experiment. A few days prior to the drought period, the number of tillers and the total summed lengths of all tillers (TTL) were recorded. TTL serves as a nondestructive estimate of pre-drought size that correlates with shoot dry mass (r = 0.75, P < 0.01; shoot dry mass [in milligrams] = 0.1987 [TTL in centimeters] + 5.9289; Cheplick et al., 2000
). It is later used as a covariate during data analysis.
All plants were watered on 18 August, after which water was withheld from half of the plants (N = 160) to impose the drought stress treatment. The remainder of the plants (N = 160) were watered as needed and functioned as the control group. Each genotype was represented by 8 –E and 8 +E plants subjected to drought, and 8 –E and 8 +E plants watered as needed, typically 2–4 times/wk.
There were three sequential drought periods, each lasting 11–14 d. After each drought period, all plants were watered. During the experimental droughts, 19 supplementary tubular pots containing ryegrass plants in the same soil mix that were treated identically to the droughted plants were used to record soil moisture conditions. Periodically, a soil moisture probe (Lincoln Irrigation Inc., Lincoln, Nebraska, USA) was inserted to 10 cm depth into each supplementary pot. Relative soil moisture readings were converted to percent soil moisture (Cheplick et al., 2000
). The resulting time course for soil moisture before, during, and after the three successive droughts is depicted in Fig. 1. Over the three drought periods, mean (±1 SD) minimum and maximum temperatures were 18.9° ± 2.5° (N = 10) and 28.0° ± 1.9°C (N = 10), respectively. On sunny days, plants experienced full sun (1200 to 1500 µmol · m–2 · s–1) for ca. 4 h/d. No self-shading occurred as tiller numbers were relatively low (see Results) and tubular pots were spaced within the holding trays by one 6.5 cm diameter empty cell between each pair of pots to eliminate shading by neighbors.
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Response variables
To assess short-term recovery following drought, the number of live, green tillers was recorded 1 wk and 4 wk following cessation of the drought stress. On the day that plants were harvested (7-wk post-drought), the shoots were clipped to 1 cm above the soil surface and the leaf area of all live, green tillers was determined with a leaf area meter (LI-3000, LI-COR, Lincoln, Nebraska, USA). In addition, a small 5-mm piece of leaf sheath was excised from each plant and placed into a vial containing 70% ethanol (Hignight et al., 1993
) for later verification of endophyte status by light microscopy (Bacon and White, 1994
).
Following harvest, roots were thoroughly washed free of soil material, bagged and dried to constant mass at 60°C in a drying oven. Clipped shoots, including both green and dried tillers, and tiller bases (1 cm) that remained, were separately retained and also dried to constant mass. Using an electronic balance, the dry mass of roots, shoots, and tiller bases were determined.
Data analyses
The primary factors examined in an analysis of covariance (ANCOVA) of the response variables were host genotype, infection status (–E or +E), and treatment (drought or control). The covariate was total tiller length recorded immediately prior to initiation of the drought stress. The GLM procedure of the Statistical Analysis System, Version 8.2 (SAS Institute, Cary, North Carolina, USA) was used for all analyses. If the covariate was significant, least-squares means (±1 SE) adjusted for the covariate were calculated and are presented in the figures. For variables where the covariate was not significant, a three-way ANOVA was performed using the same factors (and their interactions) noted above.
Two allocation variables were derived from the response variables. Root : shoot ratio is (root mass)/(shoot mass + tiller base mass). To conform to ANOVA assumptions (Underwood, 1997
), it was log10-transformed prior to analysis. Previous research has indicated that root : shoot ratios are sensitive to soil moisture deficits (Frank et al., 1996
). Relative allocation of dry mass to tiller bases is the ratio of tiller base mass to total dry mass, expressed as a percent (Cheplick et al., 2000
). It is a relevant indicator of regrowth after environmental stress in L. perenne because nonstructural carbohydrate reserves are stored in tiller bases (Hull, 1992
; Donaghy and Fulkerson, 1997
, 1998
; Volaire et al., 1998
; Cheplick and Cho, 2003
). Drought tolerance in endophyte-infected grasses may also be influenced by carbohydrate reserves (Malinowski and Belesky, 2000
). To conform to ANOVA assumptions, allocation to tiller bases was arcsine, square-root transformed before analysis (Underwood, 1997
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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At the time of harvest 7 wk after drought stress was relieved, genotype and endophyte significantly affected the leaf area of live tillers (Table 1). There was no interaction of drought with endophyte: the leaf area of +E plants was consistently lower than that of –E plants in both control and drought-stressed groups (Fig. 3, upper). The overall effect of drought was only marginally significant (Table 1), indicating that previously drought-stressed plants were able to re-establish much of their leaf area by tiller regrowth over the 7-wk recovery period. Similar to tiller numbers at 4 wk post-drought, for leaf area there were significant genotype x endophyte and genotype x drought interactions in the ANCOVA (Table 1).
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For root : shoot ratio and allocation to tiller bases, pre-drought size (the covariate) was insignificant in the ANCOVA model; therefore, results of a three-way ANOVA are depicted in Table 2. Genotype and drought significantly affected root : shoot ratio. Averaged over all plants, mean (±1 SE) root : shoot ratio was 0.93 ± 0.04 for drought-stressed vs. 1.45 ± 0.05 for control plants. There was also a significant genotype x drought interaction (Table 2). For most, but not all, genotypes root : shoot ratio was greatly reduced for plants following drought (Fig. 4). Genotype G had a high root : shoot ratio in both control and drought groups, whereas genotypes R and T had a relatively low root : shoot ratio (Fig. 3). A significant three-way interaction was also found for root : shoot ratio (Table 2). This indicated that the effects of endophytes on root : shoot ratio depended on host genotype as well as experimental treatment. These endophyte-mediated effects were minor or nonexistent for some genotypes (e.g., E, R, and T), but positive for others (e.g., H and K; Fig. 4). For genotypes B and L, +E replicates showed a reduced root : shoot ratio relative to –E replicates in previously drought-stressed plants (Fig. 4).
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Allocation of dry mass to tiller bases was significantly affected by genotype, drought, and their interaction (Table 2). Averaged over all plants, mean (±1 SE) allocation to tiller bases was 13.0 ± 0.3% for drought-stressed vs. 11.8 ± 0.3% for control plants. There was also a significantly genotype x endophyte x drought interaction (Table 2). Because it was suspected that some of the differences detected might be due to differences in total mass among treatments and genotypes, a new ANCOVA was performed on allocation to tiller bases (arcsine, square-root transformed), using total dry mass as the covariate. The effect of total dry mass was highly significant (F1,269 = 17.04, P < 0.0001), but the effect of drought was no longer significant (F1,269 = 0.13, P = 0.71). However, genotype was highly significant (F9,269 = 6.75, P < 0.0001), and there were significant interactions of genotype x drought (F9,269 = 3.09, P < 0.01) and genotype x endophyte x drought (F9,269 = 2.00, P < 0.05). To illustrate the three-way interaction, least squares means adjusted for total mass are shown for all 10 genotypes in control and drought treatments in Fig. 5. For genotypes B and H, +E plants showed a lower allocation to tiller bases, while the reverse was true for genotype T. Other genotypes (e.g., E, G, L, and N) showed an endophyte effect that varied between control and drought-stressed groups (Fig. 5). For 7 of 10 genotypes, –E plants had a greater allocation to tiller bases following the alleviation of drought stress.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Clay, 1990
, 1998
; Bacon, 1993
; Hill et al., 1991
; Latch, 1997
; Malinowski et al., 1997
; Clay and Schardl, 2002
), the present study showed that the Neotyphodium endophyte had an overall detrimental effect on the growth of Lolium perenne as assessed by tiller production, leaf area, and dry mass. This was true in benign conditions where water was continuously available and in the recovery growth of plants following severe drought stress. While the endophyte of Festuca arundinacea has been shown to improve host tolerance to, and recovery from, drought (Arachevaleta et al., 1989
; Bacon, 1993
; West et al., 1993
; West, 1994
; Malinowski and Belesky, 2000
), consistent positive effects of the endophyte of L. perenne on host performance during and following drought have not been reported (Lewis, 1992
; West, 1994
; Ravel et al., 1995
; Barker et al., 1997
; Cheplick et al., 2000
; Hesse et al., 2003
).
It is unclear why a beneficial effect of infection in L. perenne does not appear to be as consistent as for F. arundinacea because both are outcrossing species that are evolutionarily closely related (e.g., intergeneric hybrids are possible between Lolium and Festuca spp.; Jung et al., 1996
). In fact, the Plants Database of the Natural Resources Conservation Service, United States Department of Agriculture (http://plants.usda.gov) now lists Lolium arundinaceum (Schreb.) S. J. Darbyshire as an acceptable species name for tall fescue. Also, most studies to date have mostly used cultivars derived from plants introduced from Europe. West (1994
, p. 89) suggested that the inconsistency in endophyte effects on L. perenne was due to the "milder, maritime climates that perennial ryegrass is usually tested in, compared with the warm, continental environments where tall fescue prevails." However, it should be noted that L. perenne is predominantly diploid, while F. arundinacea is an allohexaploid (Sleper and West, 1996
), and that different Neotyphodium spp. occupy each grass species. Furthermore, there are studies in which N. coenophialum did not improve F. arundinacea drought tolerance or recovery (White et al., 1992
; West, 1994
; Elbersen and West, 1996
; Hill et al., 1996
). Undoubtedly, both population genetic and environmental factors, including adaptation to specific environments (e.g., Hesse et al., 2003
), contribute to the great variability in host responses to endophyte infection within and among grass species.
Morse et al. (2002)
described how infection with Neotyphodium was beneficial to the growth of Arizona fescue (Festuca arizonica) under low water availability, but detrimental when ample water was available. They suggested that the endophyte could have antagonistic as well as mutualistic effects on its host. In another study of the same grass species where water and soil nutrient levels were varied in the field, Neotyphodium endophytes decreased growth and seed production in the host under most conditions (Faeth and Sullivan, 2003
).
While endophytes have been shown to enhance early seedling growth of L. perenne (Clay, 1987
), Cheplick et al. (1989)
found greater biomass production of +E seedlings relative to –E seedlings only when soil nutrients were not limiting. Furthermore, there was no significant effect of endophytes on adult plants, regardless of soil nutrient conditions (Cheplick et al., 1989
). Under both intraspecific competition and interspecific competition with F. arundinacea, –E plants of L. perenne performed better in terms of dry mass than +E plants (Marks et al., 1991
). The authors concluded that endophyte infection in perennial ryegrass "may be detrimental under many circumstances" (Marks et al., 1991
, p. 202). In a previous study, endophytes did not generally influence the ability of L. perenne to recover from drought stress, although +E plants previously exposed to drought showed a significantly lower mean tiller mass than drought-exposed –E plants (Cheplick et al., 2000
). Endophytic hyphae in the present study showed excellent survival of drought conditions and were abundant in leaf sheaths of +E plants following recovery from drought. Asexual endophytes of one native grass (Festuca arizonica) have been described as parasitic (Faeth and Sullivan, 2003
); in L. perenne there is little doubt that under a variety of environmental conditions both stressful and benign, there is a cost to endophyte infection, manifested by the reduced growth and lower biomass of infected plants.
Genotypic specificity of endophyte effects and drought recovery ability
The effects of endophytic fungi on growth and allocation traits of L. perenne depended on host genotype, a result congruent with other studies of endophyte-infected grasses (Belesky et al., 1987
; Rice et al., 1990
; Cheplick, 1997
; Meijer and Leuchtmann, 2000
; Cheplick and Cho, 2003
; Faeth and Sullivan, 2003
). A significant genotype x endophyte interaction was detected for the number of tillers present 4 wk into the drought recovery period, and for leaf area and total dry mass at harvest (Table 1). This has implications for the evolutionary dynamics of grass populations in which the frequency of infection is <100%.
Natural selection would be predicted to favor genotypes that can produce more tillers and develop a greater photosynthetic leaf area or mass in a competitive pasture or turf community, regardless of endophyte infection. Infected genotypes in which endophytes are detrimental to host growth may be selectively disadvantaged, while those in which endophytes improve host growth may be selectively favored. However, the relationship between mean tiller size and photosynthetic production was not a simple one in this experiment, as supplemental analyses revealed. Mean tiller mass was highly variable among genotypes, ranging from 53 to 191 mg (F9,270 = 61.1, P < 0.0001). Those genotypes with many tillers did not amass more total leaf area than those with fewer tillers, because mean tiller mass was lower in the former (for leaf area regressed onto mean tiller mass for the 10 genotypes, r2 = 0.14 and 0.13 for –E and r2 = 0.05 and 0.07 for +E in drought and control conditions, respectively; all P > 0.05). Interestingly, the relationship of mean tiller mass to total leaf mass varied with endophyte status: there was no correlation when genotypes were uninfected (r2 = 0.08 and 0.09 in drought and control, both P > 0.05), but a highly significant positive correlation when genotypes were infected (r2 = 0.76 and 0.54 in drought and control, both P < 0.01; Fig. 6). This example shows how endophytes add a new level of complexity to any selective process that results in the sorting of genotypes over time.
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). For example, out of 38 populations of L. perenne in Germany, 18 had an infection frequency between 11 and 80%, while the remainder were 
10% infected (Oldenburg, 1997
). For 523 Lolium spp. populations examined throughout Europe, 86% had an infection frequency between 0 and 50%, and only a few populations were >90% infected (Lewis et al., 1997
).
The potential effect of endophyte infection on host growth and allocation can be influenced by environmental conditions (Cheplick, 1997
, 1998
; Saikkonen et al., 1998
; Ahlholm et al., 2002
; Morse et al., 2002
). In the present study, the effect of endophytes on root : shoot ratio of the 10 host genotypes varied between drought-stressed and control groups (Table 2, Fig. 4). As is typical for cool-season grasses (Frank et al., 1996
), root : shoot ratio was lowest for plants previously subjected to drought, mostly because root mass was reduced much more than shoot mass in this treatment. The ability to rapidly restore root growth following drought stress is likely to be important to the re-establishment of tillers and leaf area expansion as shoot growth resumes (Passioura et al., 1993
; Frank et al., 1996
; Volaire et al., 1998
). Endophytic fungi differentially impact the ability of L. perenne genotypes to produce roots during recovery from drought (Hesse et al., 2003
), but the effect on root : shoot ratio was not consistently negative or positive (Fig. 4). There were additional genotype and genotype x drought interaction effects (Table 2), indicating considerable plasticity for this allocation trait and genetic variation in the ability to restore root : shoot ratios during recovery from drought. In habitats where periodic drought occurs (e.g., Volaire et al., 1998
), there is the potential for selection of specific genotypes best able to recover from soil water deficits in a population containing a mixture of genotypes.
As an indicator of the relative amount of stored reserves, allocation of dry mass to tiller bases was significantly greater in L. perenne plants previously exposed to drought. There was also a significant genotype x drought interaction (Table 2). During soil water deficit, nonstructural carbohydrates can accumulate in tiller bases (Hull, 1992
; Frank et al., 1996
; Volaire et al., 1998
), and these reserves can be important to regrowth following alleviation of stress (Donaghy and Fulkerson, 1997
, 1998
). The long 7 wk of regrowth in the current study probably depleted reserves built up during drought and for some genotypes, allocation to tiller bases following drought remained lower than in the control (Fig. 5). For other genotypes, previously drought-stressed replicates were clearly able to restore the buildup of reserves in tiller bases to a level that matched or exceeded that of control replicates.
The ability of L. perenne genotypes to allocate resources to tiller bases was differentially modified by endophytic fungi. As with the earlier drought study (Cheplick et al., 2000
), there was no consistent direction to the effect of endophytes on allocation to tiller bases (Fig. 5). However, for the majority of the genotypes (7 of 10), –E plants showed an equal or greater allocation to tiller bases than +E plants after recovery from drought stress. This suggests a cost to endophyte infection for some genotypes, as expressed by a lesser buildup of storage reserves. Although endophytic hyphae are entirely contained within host leaf tissues and theoretically could contribute to host dry mass, Tan et al. (2001)
reported that the endophyte does not comprise more than 0.2% of the mass of infected tissues of L. perenne. Dry matter accumulation in the organs studied here likely represents a by-product of net carbohydrate production, i.e., available carbohydrates beyond those metabolized by the symbiont. Cheplick and Cho (2003)
reported that the total nonstructural carbohydrate levels of –E and +E plants were not significantly different for these same genotypes of L. perenne growing under greenhouse conditions. Nevertheless, they found a significant genotype x endophyte interaction, showing that host genotypes vary in the ability to store carbohydrates when endophyte infected. Future studies that carefully document alterations in whole-plant carbohydrate physiology (e.g., Prud'homme et al., 1992
) that are associated with the presence of endophytic hyphae and specific environmental conditions will better reveal the potential costs of endophyte infection to their host grasses.
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FOOTNOTES |
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2 cheplick{at}mail.csi.cuny.edu
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Bacon C. W. J. F. White Jr 1994 Stains, media, and procedures for analyzing endophytes. In C. W. Bacon and J. F. White, Jr. [eds.], Biotechnology of endophytic fungi of grasses, 47–56. CRC Press, Boca Raton, Florida, USA
Barker D. J. D. E. Hume P. E. Quigley 1997 Negligible physiological responses to water deficits in endophyte-infected and uninfected perennial ryegrass. In C. W. Bacon and N. S. Hill [eds.], Neotyphodium/ grass interactions, 137–139. Plenum Press, New York, New York, USA
Belesky D. P. O. J. Devine J. E. Pallas Jr W. C. Stringer 1987 Photosynthetic activity of tall fescue as influenced by fungal endophyte. Photosynthetica 21: 82-87[ISI]
Breen J. P. 1994 Acremonium endophyte interactions with enhanced plant resistance to insects. Annual Review of Entomology 39: 401-423[CrossRef][ISI]
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Casler M. D. 1995 Patterns of variation in a collection of perennial ryegrass accessions. Crop Science 35: 1169-1177
Cheplick G. P. 1997 Effects of endophytic fungi on the phenotypic plasticity of Lolium perenne (Poaceae). American Journal of Botany 84: 34-40[Abstract]
Cheplick G. P. 1998 Genotypic variation in the regrowth of Lolium perenne following clipping: effects of nutrients and endophytic fungi. Functional Ecology 12: 176-184[CrossRef][ISI]
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