Summer Dormancy and Endophyte Infection in Tall Fescue

J.L. Underwood1, C.P. West1, D.P. Malinowski2, C.A. Guerber1, and B.C. Grigg1

1University of Arkansas, Fayetteville, AR, 2Texas AgriLIFE RESEARCH, Vernon, TX

ABSTRACT Summer dormancy is a drought escape mechanism for some Mediterranean-origin perennial grasses. Little is known of the influence of fungal endophytes (Neotyphodium spp.) on the expression of summer dormancy in their grass hosts or of the possible role of endophytes in host drought survival. Research was conducted to investigate interactions between summer dormancy potential and endophyte symbiosis as related to plant survival in tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] at Fayetteville, AR and Vernon, TX, USA. Field plots were established in adjacent trials, full irrigation and non-irrigation. Treatments in each trial consisted of three populations, each with or without endophyte, in four replicate blocks. Populations were TX06V-B-FA, ‘Grasslands Flecha’ (incompletely summer dormant) and the summer-active ‘Kentucky-31’. Shoot dry matter production, shoot senescence, tiller-base water content, and plant survival were measured. Incompletely summer-dormant populations achieved 99-100% survival even in the absence of endophyte at Fayetteville and greater than 90% survival at Vernon. No benefit from endophyte symbiosis was found in any measurement of summer-dormant populations. At Fayetteville, summer-dormant populations exhibited less growth (P<0.05), lower tiller-base water content (P<0.001 to P=0.065), and greater senescence (P<0.01) during the summers of 2007 and 2008 and greater growth from Nov 2007 to May 2008 (P<0.01) than Kentucky-31. The same dry matter production trends were seen for Vernon. Results suggest that endophyte symbiosis does not affect survival of summer-dormant populations of tall fescue. INTRODUCTION Understanding the mechanisms of overcoming drought and heat stress in cool-season perennial grasses is becoming increasingly important to sustaining low-cost, pasture-based livestock production. Summer-active cultivars of tall fescue currently recommended in the humid, southern U.S. exhibit greatly limited growth (quiescence) in response to drought and heat stress and risk severe stand depletion if not infected with the endophyte Neotyphodium coenophialum [(Morgan-Jones & Gams) Glenn, Bacon, & Hanlin comb. nov.]. In subhumid to semiarid regions of the U.S., there is potential for grasses using summer dormancy (as defined by Volaire and Norton, 2006) as a mechanism for avoiding drought and heat stress, thereby improving persistence. The influence of the endophyte on the expression of summer dormancy is unknown. To better understand physiological interactions between the host and endophyte in strategies for summer survival in tall fescue, research was conducted in a semiarid and humid environment. MATERIALS AND METHODS In 2007 and 2008, field trials were conducted in a humid environment at the Agricultural Research and Extension Center, Fayetteville, Arkansas, and in a semiarid environment at the Texas AgriLIFE Research Center at Vernon, Texas. Incompletely summer-dormant tall fescue populations, TX06V-B-FA (TX), originating in Israel, and the Mediterranean-type cv. Grasslands Flecha [Gentos SA (Buenos Aires, Argentina) and AgResearch Grasslands (Palmerston North, New Zealand)] (hereafter referred to as Flecha), and the summer-active Kentucky-31 (KY), each endophyte-infected and endophyte-free, were arranged in irrigated and

nonirrigated blocks. In the irrigated block, a drip system was used to maintain the soil matric potential at -0.03 MPa (field capacity, or 21.4% water by volume). Endophyte-infected plants of TX and KY contained their wild-type strains, whereas Flecha was infected with the novel AR542 strain. Cultivar effects were tested using orthoganol contrasts: a) summer active (KY) vs. the two summer dormants and b) TX vs. Flecha. Analysis of variance was performed with JMP 7.0 (SAS Institute Inc., Cary, NC) using the restricted or residual maximum likelihood (REML) method. Means were separated using Fisher’s protected least significant difference at α=0.05.

Biomass dry weight (DW) yield was determined at the beginning of June 2007 by hand-clipping a 90 x 90 cm area (36 plants) to a 7.5-cm stubble, dried at 60° C for 48 h, and weighted. Plant survival was determined by recording the number of plants showing at least one live, green tiller and expressing as a percentage of the 36 plants at each biomass sampling date. Tiller-base water content and senescence were recorded only at the Fayetteville location from June to October. One tiller per each of 12 random plants was harvested at soil level before biomass harvest. Water content was determined on the basal 2 cm minus leaf sheaths older than the youngest fully expanded leaf. Tissues were weighted for fresh weight then dried and reweighed. The removed tissue that was older than the youngest fully expanded leaf and the remaining aerial tiller portion of the water content sample were manually divided into live (green) and senesced tissue, dried, weighed, and percentage senescence was calculated. RESULTS AND DISCUSSION At Fayetteville, summer-dormant populations exhibited less growth in the summer of 2007 and 2008 and greater growth from Nov 2007 to May 2008 than KY (See Fig. 1 for 2008). At Vernon the same trends were seen, with summer-dormant populations exhibiting less growth than KY; however, the growth patterns were shifted earlier. In 2007, summer-dormant populations exhibited less growth during 1 June and 5 July and from 5 May to 3 July in 2008. Summer-dormant populations exhibited greater growth than KY from Aug 2007 to April 2008. From November 2008 to April 2008, TX exhibited greater growth than Flecha. This was also seen at Fayetteville on 4 Feb and 1 May in 2008. At Vernon in 2007, Flecha exhibited greater growth than TX during the summer. This was seen on 3 June 2008 at Fayetteville, showing that TX enters dormancy earlier and to a greater degree than Flecha. Summer-dormant populations exhibited lower tiller-base water content and greater senescence than KY during the summers of 2007 and 2008 (Fig. 2). In October 2007, the reverse occurred, in that summer-dormant populations exhibited greater tiller-base water content than KY. There was no endophyte effect on water content or senescence on any sampling date. In the two summer-dormant populations, endophyte infection was shown to reduce or cause no change in tiller-base water content. In a greenhouse trial, West et al. (2007) reported no significant endophyte effect on tiller-base water content in Flecha.

Table 1. Percentage of plant survival on three dates at Fayetteville, AR and Vernon, TX as influenced by irrigation, population, and endophyte status in 2008.

Figure 1. Dry matter production means in 2008 for Fayetteville, AR and Vernon, TX for each population. Means were averaged across endophyte treatments and four replications. Means with the same letter were not significantly different at α=0.05.

Figure 2. Tiller-base water content and percentage senescence means on a dry-weight basis in 2007 and 2008 for three populations at Fayetteville only. Means were averaged across irrigation blocks, endophyte treatments, and four replications. Means with the same letter were not significantly different at α=0.05.

At both locations, KY had lower survival (P<0.001 to P=0.007) than Flecha or TX (Table 1). In Fayetteville, significant mortality occurred in KY in the absence of irrigation (P≤0.001 to P=0.007) in response to drought stress, whereas Flecha and TX had high survival. Endophyte-infected KY tended toward greater survival than endophyte-free KY in the non-irrigated plots, but not significantly.

In Vernon, there was lower survival of endophyte-infected KY with irrigation (P= 0.006 to P=0.094) than of Flecha or TX because of disease infestation caused by above-normal rainfall in 2007. The lower survival of endophyte-infected than endophyte-free KY during the summer of 2008 cannot be explained. By the end of 2008, all KY stands had lower survival than Flecha or TX. The summer-dormant populations exhibited essentially 100% survival at Fayetteville and 90+% survival at Vernon throughout the duration of the study. CONCLUSION The semiarid environment of Vernon allowed for the differences in degree of dormancy among cultivars to be seen more clearly than in the humid environment at Fayetteville. Under severe drought stress, summer-dormant tall fescue populations reduced their growth earlier and exhibited lower monthly biomass yields, tiller-base water content, and greater senescence and survival than summer-active tall fescue. Endophyte symbiosis had no influence on the degree of summer dormancy expression or on the plant survival of summer-dormant populations, even under severe water deficit. The relative rankings of the populations in terms of summer growth were similar in all locations and water regimes. Reduced metabolic activity may be the overall cause of greater survival in dormant populations, compared with the summer-active grass. Acknowledgements- We are indebted to the USDA-ARS Dale Bumpers Small Farm Research Center for funding support (Agreement no. 6227-21310-008-38S). We appreciate the statistical expertise of Dr. Andy Mauromoustakos and the technical guidance of Dr. John Mattice. REFERENCES Volaire, F., and M. Norton. 2006. Summer dormancy in perennial temperate grasses. Annals of

Botany 98:927-933. West, C.P., F. Volaire, and F. Lelievre. 2007. Tiller survival after drought of 'Grasslands Flecha'

tall fescue as influenced by endophyte. p. 267-269. In A.J. Popay and E.R. Thom (eds.) Proc. 6th Int. Symp. Endophyte-Grass Interactions. Grassland Research and Practice Series no. 13, N. Z. Grassland Association, Christchurch, NZ.