Noble Jamie Underwood

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Summer Dormancy and Endophyte Infection in Tall FescueJ.L. Underwood1, C.P. West1, D.P. Malinowski2, C.A. Guerber1, and B.C. Grigg11University of Arkansas2Texas AgriLife Research


Continuation of climatic change creates challenges for cool-season perennial grasses

Need for summer-dormant cool-season grasses in drought-prone transitional zones

Mediterranean-origin perennial grasses offers possibilities

Little is known about the role of endophyte in summer-dormant tall fescueJustification

Continuation of climatic change creates challenges for cool-season perennial grasses because of increasing temperatures, decreasing precipitation, abrupt rainfall patterns, and increasing frequency and severity of drought.This creates a need for summer-dormant cool-season perennial grasses in drought-prone transitional zones. Grasses originating from the Mediterranean offers possibilities because of their ability to senesce during the summer, even with irrigation and provides winter-active growth with peak growth occurring in late winter.However, little is known about the role of the endophyte in summer-dormant tall fescue.

(West, 1998)

Endophyte generally necessary for persistence south of this line

Summer-dormant fescueCool-season grasses in drought-prone transitional zones

This map shows the distribution of adapted areas for fescue growth. This transition zone is were tall fescue originating from continental Europe are best adapted. And anywhere south of this line is where endophyte is generally necessary for persistence. However, grasses from the Mediterranean may allow the area adapted for fescue to be expanded to regions of OK, TX, and CA. The mild winters allow for most of the growth of these Mediterranean types to occur from October until mid-April. These grasses use summer dormancy as a mechanism for avoiding drought and heat stress thereby improving persistence.

The south-west edge of the belt is where tall fescue has persistent problems of withstanding the harsh summer conditions. Additionally, there is no other predominant cool-season grass that is grown, other than wheat, with then incurs a yearly establishment cost. Thus, there is an opportunity for summer-dormant types to provide sustainable summer survival and to extent to region of production to more arid regions, such as in southern California and parts of Texas and Olkahoma. This leads to the question of the role endophyte plays on the survival of summer dormant populations. It has been well established that endophyte has a beneficial role in the summer-active cultivars, however, very little is known of the influence between endophyte and summer dormancy.

Summer activeIncompletely dormantCompletely dormant

Ability to survive summer stresses by ceasing growth & senescing vegetative tissues despite occasional summer rains (Volaire & Norton, 2006)

It is best to think of summer dormancy as on a sliding scale with varying degrees of summer dormancy. Summer dormancy is defined as the ability to survive summer stresses by ceasing growth and senescing vegetative tissues despite occasional summer rains.

This can be seen in this picture showing a summer-active plants vegetative regrowth in response to rainfall, whereas the summer dormant had none.

Plants with the least summer dormancy are referred to as summer active, where growth continues during the summer if there is adequate water.Incompletely dormant grasses exhibit reduced growth with partial senescence and little or no dehydration of leaf bases. Tall fescue has been found to exhibit varying degrees of incomplete dormancy.Completely dormant grasses exhibit cessation of growth and full senescence which no tall fescue cultivar has been found to exhibit. The mechanisms of summer dormancy are not well known; however, it is believe to be related to vernalization/flowering, photoperiod, and temperatures.

Determine influence of endophyte presence on expression of summer dormancy and survival in tall fescueObjectiveHypothesisSummer drought survival in summer-dormant fescues is more highly controlled by plant genotype than by endophyte presence.

Obj. 1 was to determine the influence of endophyte presence on expression of summer dormancy and survival in tall fescue

We predict that endophyte presence has no influence on the degree of summer dormancy expression and that summer dormancy expression is controlled genotypically. This was predicted because of past research by West et al. 2007, and research by Malinowski, in which no clear endophyte benefit on summer-dormant survival was found.

Materials and MethodsLocations: Fayetteville, AR and Vernon, TX

Summer-dormant tall fescue:Flecha (E-)Flecha MaxQ (E+)TX06V-BEF (E-)TX08V-BEI (E+)Summer-active tall fescueKentucky-31 (E-)Kentucky-31 (E+)

Irrigated and Non-irrigated

Agricultural Research and Extension Center here in Fayetteville on a Pickwick gravelly loam and at the AgriLife Research Center in Vernon, TX on a Miles fine sandy loam.The experiment was a randomized complete block with a split-strip design. There were 4 replications across 2 water treatments (which constituted the whole plots), irrigated and nonirrigated within which a 3 x 2 factorial was arranged in strips. There were 3 cultivars: TX06V-B-FA abbreviated by TX, Flecha abbreviated by FL, and KY-31 abbreviated by KY. TX and Flecha were incompletely summer dormant and KY-31 was summer-active and there was two endophyte treatments, endophyte-infected and endophyte-free, for each cultivar.

Materials and MethodsBiomass yields

Biomass yields were harvested at both locations. Aerial tissue, 7.5 cm and above, was harvest on the inner 36 plants of the 100 on a monthly basis from the summer of 2007 to the end of 2008.

Materials and MethodsBiomass yieldsTiller-base water content

Tiller-base water content was taken once a month from June to October in 2007 and 2008 only at Fayetteville, AR. Twelve tillers per plot were randomly excised and tiller-base water content was determined on the basal 2 cm minus leaf sheaths older than that of the youngest fully expanded leaf.The reason we are focusing of this tissue is because the tiller base is where the apical meristem is located and initiates new growth.


Materials and MethodsBiomass yieldsTiller-base water contentShoot senescence

The removed and aerial tissue from tiller-base water content was used to determine senescence. Material was separated into live and dead tissue, dried and weighed.


Materials and MethodsBiomass yieldsTiller-base water contentShoot senescencePlant survival

Plant survival was recorded at each biomass harvest for the inner 36 plants to monitor mortality for both locations.


Fayetteville Biomass

This graph plots changes in biomass yields,, among the three cultivars at Fayetteville for 2008 with months on the x-axis and kg DM ha-1 on the y-axis. Means are of the four replications across endophytes because no difference in endophyte were found. The analysis of variance was performed within dates with single degree of freedom contrasts. This analysis was completed for every measurement. Means with the same letter within dates are not different with an alpha=0.05. There was minimal growth in the beginning of 2008 until May where the dormant populations exhibited greater growth than the summer-active KY. During the summer, KY exhibited greater growth than the two incompletely summer-dormants, FL and TX, with FL being intermediate, which shows TX having greater dormancy than FL.And then you see the reverse occurring in the fall as the dormants release dormancy.

Vernon Biomass

This graph plots changes in biomass yields, kg DM ha-1, among the three cultivars at Vernon, TX. Means are of the four replications across endophytes. The same trends were seen for Vernon as in Fayetteville, but with more magnitude. Because of the warmer winter temperatures, there was considerably more winter and fall growth than in the humid environment. During the summer, summer-active KY showed the same trends as in Fayetteville with greater biomass production and FL as intermediate.

Fayetteville 2007

tiller-base water content dehydration and senescence were monitored at Fayetteville, AR in 2007 and 2008. 2007 is shown, for 2008 showed the same trends. Months again on the x-axis with g water per g dry weight on the y-axis for the top graph and % senescence on the bottom graph. Means shown are across irrigation treatments and endophyte. During summer, KY exhibited greater tiller-base water content than FL and TX, which is flipped in October as the dormant grasses release dormancy. For 23 Aug and Oct there was an endophyte x cultivar interaction with endophyte-infected exhibiting greater tiller-base water content in the summer-active KY. This followed previous research by Buck et al., 1997; however, no benefit to endophyte symbiosis was found in the summer-dormant populations for any measurement.

As tiller-base water content decreased, senescence increased with the summer-dormant populations having significantly greater senescence during the summer.

Survival 2008Water treatmentEndophyte Fayetteville, AR Vernon, TXPopulationstatus 3 June 1 July 1 Nov 3 June 3 July 1 Nov---------------------------% Survival----------------------------Non-KYE+888885747436IrrigatedE-777474757438FLE+999999989894E-999999979797TXE+100100100989998E-100100100999999IrrigatedKYE+989796474930E-979699737144FLE+999999939592E-10010099959591TXE+10010099999797E-99100100999994

Plant survival was monitored on a monthly basis at every biomass