Role of Obligatory Summer Dormancy in Cool-Season Perennial Grasses

for Improved Semiarid Grassland Ecosystems

 

A project sponsored by the Texas-Israel Exchange Fund, a subsidiary of the United States - Israel Binational Agricultural Research and Development Fund (BARD)

 

 

In Israel, native summer-dormant cool-season perennial grasses are an important source of winter forage for grazing livestock in rangelands.

In Texas, summer-dormant cool-season perennial grasses may complement forage from wheat pastures, especially during drought periods when wheat pastures look like that one above.                

Rationale

This research project is a continuation of our previous experiments on productivity and persistence of obligatory summer-dormant cool-season perennial grasses in semiarid environments of the Southern Great Plains of the USA. 

    Numerous ecotypes of cool-season perennial grasses originating from the Mediterranean Basin, including Israel, are obligatory summer-dormant, most likely in response to increasing day length and probably high temperatures. Obligatory summer dormancy is induced independently from soil moisture, even though it can be accelerated by initial drought period. Summer semi-dormant (also referred to as summer-active) and obligatory dormant cool-seasonal perennial grasses produce dormant regenerating buds at tiller bases, from which growth resumes. These surviving buds, activated in response to increased water availability (i.e., rainfall) and decreasing temperatures in autumn, enable perennial grasses to escape summer drought. In some species (e.g. Phalaris aquatica and Hordeum bulbosum), surviving buds are associated with corms produced at the base of flowering tillers (Fig.1). In these species, the onset of summer dormancy may be associated with timing of flowering. In other species (Dactylis glomerata ssp. hispanica, Lolium arundinaceum), only surviving shoot meristems give beginning to new tillers in autumn (Fig. 2).

Figure 1.   Surviving structures in Hordeum bulbosum (left) and Phalaris aquatica (right) are associated with corms produced at the base of flowering tillers in the early spring. 

 

Figure 2.   Tillers of summer-dormant types of Dactylis glomerata ssp. hispanica (left) Lolium arundinaceum (tall fescue) (right) do not develop corms and their survival depends only on protection of meristematic tissues from desiccation and death during summer drought. 

 

    Differences between dormancy types are gradual and depend on responsiveness to inductive and synergistic factors (i.e., day length, high temperature, drought), depth of dormancy induction, and kinetics of dormancy relaxation during summer. After dormancy relaxation, growth of regenerating buds is arrested during summer by lack of water. It is conceivable that obligatory and semi-dormant types differ in duration of dormancy (fast vs. prolonged relaxation from dormancy) and, therefore, in their drought tolerance and response to summer rainfall events. These particular adaptations suggest that in semiarid environments of the southern Great Plains and Israel, grasslands with obligatory summer-dormant cool-season grasses might be more persistent and sustainable than grasslands based on traditional (in the USA) summer semi-dormant cultivars. Obligatory summer-dormant cool-season grasses grow actively during winter and early spring; thus, they may better complement dual-use wheat than currently cultivated summer semi-dormant types. It has been estimated that 45-60 days of additional grazing and gain during early winter in the southern Great Plains may bring additional income of $62-$75 per ha. In Israel, better understanding of regulation of summer dormancy in adapted genotypes and introduction of new accessions of obligatory summer-dormant cool-season grasses will offer potential improvement in forage production and management of degraded natural grasslands.

    In 2004, we initiated a cooperative research program with Dr. Jaime Kigel of the Hebrew University of Jerusalem on mechanisms regulating summer dormancy in cool-season perennial grasses. Cooperation with Dr. Jaime Kigel gives us not only the opportunity to interact with an expert on summer dormancy in cool-season grasses, but also opens access to summer-dormant germplasm of tall fescue and orchardgrass native to Israel. These native populations may be used to develop summer-dormant cultivars of cool-season perennial grasses for the U.S. market.    

 

Objectives

    The overall objective of the project is to develop sustainable agroecosystems based on adapted and productive introduced cool-season perennial grasses in the southern Great Plains (Texas), and to improve degraded natural grasslands in Israel. Specific objectives are:

    1.    Identify the role of obligatory summer dormancy for persistence of cool-season perennial grass ecosystems (Texas and Israel);

    2.    Determine how environmental factors regulate the process of summer dormancy in cool-season perennial grasses (Texas and Israel);

    3.    Assess the value of obligatory summer-dormant cool-season perennial grasses as a component of managed grassland ecosystems (Texas).

 

 

Dr. Dariusz Malinowski and Dr. Jaime Kigel discuss effects of day length and temperature on induction of summer dormancy in cool-season grasses in a phytotron experiment at the Hebrew University of Jerusalem, Rehovot, Israel.

 

Dr. Dariusz Malinowski and Dr. Jaime Kigel collecting seed of summer-dormant tall fescue (right) in the Golan Heights, Israel. 

 

Left - Ashley Bain adjusts soil water content in pots with summer-dormant and summer-active cool-season grass species growing in a greenhouse experiment under full or limited irrigation at Vernon, TX.  

Right - Obligatory summer-dormant types of cool-season grasses cease growth during summer months even with adequate water supply (KA), while traditional, summer-active types continue to grow (CA, CU, and PF). 

 

 

Since we have initiated research on summer dormancy in cool-season perennial grasses (2000), Grasslands Flecha tall fescue became commercially available for producers in the Southern Great Plains who want to diversify their grazing programs and secure economically sound winter forage production.

A newly established stand of summer-dormant Grasslands Flecha MaxQTM tall fescue near Childress, TX in early April 2005.

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