Evaluation of Diverse Endophyte Strains for Effects on Tall Fescue Drought Tolerance

Mioko Tamura and C.P. West Crop, Soil & Environmental Sciences, Univ. of Arkansas, Fayetteville, AR

Results

DiscussionGreenhouse Trial

In the greenhouse trial (Table 1), comparisons can only be made within a run. In Run 1, strain 22 resulted in the highest survival rate, but not different from E-. High survival in strain 22 was associated with high water content, suggesting a slow rate of desiccation. Strain 22 also had the highest regrowth rate. Survival of E+ was unexpectedly low. In Run 2 (Table 1), no novel strains caused survival greater than E- or as well as E+, except strain 25. All novel strains showed regrowth numerically intermediate between E- and E+.

Field TrialIn the field trial, mild drought stress occurred in June, which was drier and warmer than normal (Table 2). July and August sampling periods were wetter than normal. Therefore, drought stress did not develop enough to rigorously test the novel strains for their effects on host growth and survival. Relative densities (Table 3) showed that plant density increased over the summer (values >1) due to the favorable growing conditions.Endophyte strains varied in hexose and sucrose (Table 4) concentrations. In the July sampling (reflecting June growing conditions), strain 22 tended to have the lowest hexose and highest sucrose levels. Correlations between carbohydrate levels and drought survival were prevented by growing conditions that were too favorable for inducing plant mortality.In conclusion, strain 22 showed some potential for superior drought tolerance enhancement of its host in the greenhouse trial; however, this could not be corroborated in the field because of unusually wet conditions. The field screening trial will be repeated in 2010.

Introduction Tall fescue, an important cool-season forage grass, is susceptible to summer drought stress in southeastern U.S., thereby reducing persistence. The plant naturally possesses a mutualistic fungal endophyte (Neotyph-odium coenophialum). The endophyte typically produces ergot alkaloids, which are toxic to animals; however, it is also shown to enhance host drought tolerance. Recent research has identified nontoxic, ‘novel’ strains of endophyte that lack ergot alkaloids. Such strains have potential use in tall fescue cultivars for protecting the plant from drought stress. We need to test such strains in an adapted tall fescue population to identify which ones benefit host drought tolerance. We hypothesize that beneficial strains would pro-mote greater soluble carbohydrate levels in the surviving tiller base as a mechanism enhancing viability of tiller meristems over endophyte-free plants.

Objectives Screen nontoxic endophyte strains for enhanced host drought tolerance

in greenhouse and field trials. Investigate the association between soluble carbohydrate accumulation

in tiller bases and persistence under drought in the field.

Methods A collection of novel endophyte strains (numbered 15-32), which lack

toxic alkaloid production were inoculated in cv. Kentucky-31.

Greenhouse Trial 10 nontoxic strains – separated into two runs of 5 strains each

2 controls: E- = endophyte-free; E+ = infected with toxic endophyte. PVC tubes (15 × 76 cm) filled with pasteurized sand,

transplanted with 3 plants/pot. 2 pots/strain/block; total of 5 blocks. Drought was imposed by withholding water for at least 4 wks. Blocks were sequentially rewatered to 80% capacity over 3-4 wks to

terminate drought over a range of severities (Fig. 1). Before rewatering, plants were clipped to 2-cm stubble, which was

sampled for tiller-base water content. Regrowth measured after 1 wk.

Field Trial (Fig. 2) 16 nontoxic strains and 2 controls, E- and E+; 4 replicates. Biomass: hand-clipped to 8-cm stubble in nonirrigated zone of plots. Tiller density: visually scored as 1-10 along a 40-cm length of row, with

10 = fully occupied row. Total hexose (glucose + fructose) and sucrose contents in 8-cm tiller

bases. Extracted ground samples with 80% ethanol (Zhao et al. 2008).For additional information, contact:Mioko Tamura, mtamura@uark.edu or Dr. Chuck West, cwest@uark.edu

Acknowledgement This research was supported by USDA-ARS Specific Cooperative Agreement 58-6227-8-042 through the Dale Bumpers Small Farm Research Center, Booneville AR.

ReferenceZhao, D., C.T. MacKown, P.J. Starks, and B.K. Kindiger. 2008. Interspecies variation of forage nutritive value and nonstructural carbohydrates in perennial cool-season grasses. Agron. J. 100:837-844.

Fig. 1. PVC pots in the greenhouse.

Table 1. Survival rate, water content, and regrowth length of tillers for each strain in the greenhouse experiment; means of 2 replications and 5 blocks.

Run 1 Run 2Endophyte Water Endophyte Water

strain Survival content Regrowth strain Survival content Regrowth% g g¯¹ DW mm % g g¯¹ DW mm

E - 29ab† 0.27ab 19b E - 30b 0.19ab 47bE + 20b 0.25ab 7b E + 57a 0.25a 79a17 19b 0.26ab 8b 23 23b 0.16b 61ab19 32ab 0.27ab 25ab 24 33b 0.20ab 78a20 20b 0.23b 15b 25 40ab 0.21ab 75ab21 32ab 0.28ab 22b 26 34b 0.23a 62ab22 47a 0.34a 37a 27 30b 0.20ab 67ab

LSD 22 0.11 15 LSD 20 0.06 29 † Means followed by the same letter are not significantly different by Tukey (α=0.05).

Table 4. Hexose (glucose + fructose) and sucrose concentrations in the field tiller bases for selected endophyte strains on the first day of each month indicated; means of 4 replications.Endophyte Hexoses Sucrose

strain July August September July August September –––––––––––––––––––––––––––––– mg g¯¹ ––––––––––––––––––––––––––––

E- 4.4b† 1.2ab 4.1 11.1b 15.4a 14.6E+ 5.3a 1.7a 4.2 9.7c 14.5ab 12.919 2.7c 1.3ab 4.2 11.2b 13.1b 12.422 2.7c 0.8b 4.4 12.0a 14.8ab 13.523 4.0b 1.2ab —— 11.5ab 14.7ab ——25 3.3c 1.3ab —— 11.5ab 14.4ab ——

LSD 0.6 0.6 1.4 0.7 2.1 2.2† Means followed by the same letter are not significant by Tukey (α=0.05).

Fig. 2. Field trial showing 2 reps.

Table 2. Monthly rainfall and mean temperature of June - August 2009

MonthMean

temperature Departure from

normal † Monthly rainfall

Departure from normal

—————⁰C ————— —————mm —————

June 24.4 1.6 65 -48

July 25.8 0.2 135 70

August 24.9 -0.2 165 70

76-cm

Nonirrigatedsampling zone

N. coenophialum; E. Bernard

Results

Table 3. Cumulative biomass yield (1 June – 1 September) and actual and relative tiller density of row segments; means of 4 replications. Endophyte Biomass Tiller density† Relative density†

strain yield on 1 September September/June kg ha-1

E- 3500‡ 9.5ab 1.45a § E+ 3375 8.8ab 1.06bc 15 3000 9.3ab 1.21b 16 2875 8.4b 1.23ab 17 2625 8.5ab 1.18bc 18 2125 8.4b 1.03bc 19 3250 8.8ab 0.97c 20 2125 8.8ab 1.18bc 22 2125 8.3b 1.15bc 23 2750 10.0a 1.12bc 24 3250 8.8ab 1.18bc 25 2500 9.6ab 1.09bc 26 2625 9.5ab 1.14bc 27 2875 9.3ab 1.10bc 28 2500 9.0ab 1.04bc 32 2625 9.8ab 1.13bc

LSD 1625 1.6 0.22 † Density was scored as 1 - 10 in 0.5 unit intervals; 10 defined as full cover. Relative density was calculated as ratio of September/June tiller density. ‡ There was no significant difference among strains for biomass yield by Tukey

(P>0.05). § Means followed by the same letter are not significantly different by Tukey (α=0.05).

15-cm

†Departure from normal, average value over 29 yr (1971-2000).