Seed Self-Burial, Germination, and Seedling Survival for a Species with Hygroscopic Awns (Illinois)

Trypanospermic, or boring, fruits have one or more hygroscopic awns that twist and untwist in response to changes in relative humidity, with the effect of drilling the seed into the soil (Van Rheede van Oudtshoorn and van Rooyen 1999). In addition to assisting with self-burial, hygroscopic awns enable seeds to disperse, find suitable microhabitats for seed germination and seedling establishment, and avoid damage by fire and seed predators (e.g., Garnier and Dajoz 2001, Peart 1984). Studies have demonstrated that complete removal of hygroscopic awns results in immobile seeds and, in many cases, failure to germinate (e.g., Collins and Wein 1997, Sindel et al. 1993). Although these studies have investigated species that have from one to several awns, the deawning threshold to allow self-burial has not been tested for species with multiple awns (but see Peart 1979 for one-awn threshold). Chambers (2000) pointed out that aspects of seed morphology such as awns can influence seed entrapment, seed retention, and seedling establishment and therefore should be considered in restoration. Deawning may be an important factor in the successful establishment of species with hygroscopic awns in habitat restoration projects, since the removal of awns may hinder seed germination and seedling survival. Most seeds used in habitat restoration projects have undergone seed cleaning processes that separate pure seed from chaff and other vegetative debris such as awns.

Figure 1.

Beach three-awn grass (Aristida tuberculosa) has three awns of similar length (30–40 mm) twisted together basally into an 8–15 mm column. This species is an annual clump grass (30–70 cm) that grows in sandy soil habitats (sandy fields, hills, pinelands, maritime dunes, xeric pine-oak sandhills, and disturbed areas). It is critically impaired in Connecticut, Maryland, Michigan and New Hampshire; impaired in Indiana and Massachusetts; and vulnerable in Illinois, Iowa, Minnesota, Mississippi, New York, North Carolina, and Virginia (NatureServe 2008).

Photo by Mike Jeffords

The two objectives of this study are to determine the deawning threshold to allow for self-burial, and to investigate how deawning influences germination and seedling survival for beach three-awn grass (Aristida tuberculosa, Figure 1). We predict that progressive deawning will hinder seed self-burial, seed germination, and seedling establishment for this species. We selected beach three-awn grass for this study because 1) it has multiple awns; 2) it is a rare species in many states across its range in the USA (NatureServe 2008), and this information could aid in ex situ reintroduction projects in habitats such as oak savanna and dry sand prairie; and 3) previous research by Collins and Wein (1997) demonstrated that completely to partially buried awned seeds of beach three-awn grass will germinate.

We collected seeds for three trials at the Lost Mound Unit of the Upper Mississippi River National Wildlife and Fish Refuge, Illinois, USA (42.20441° N, –90.31374° W) in October 2006, and trials were conducted in 2007. In trial 1, three flats per treatment (72 cells/flat, 5.87 cm cell depth, 3.81 cm [top] and 2.13 cm [bottom] cell diameters) were used for the following treatments: 1) seeds with column/awns removed (Seed F); 2) seeds with column (AC); 3) seeds with one awn (1A); 4) seeds with two awns (2A); and 5) seeds with three awns (3A). Each well was filled to within 5 mm of the top with sandy soil (Torpedo sand, particle size < 9.5 mm). One seed was placed on the surface of the soil in each well, for a total of 1,080 seeds. Flats were then randomized in the growth chamber. Growth chamber conditions were as follows: temperature 26.7–30.6°C (day) and 18.3–21.1°C (night), with a 14-hour photoperiod. Every other day, the flats were misted with 20 sprays of water per flat (about 15 mL). Once a week, we counted the total number of seeds that self-buried and germinated (determined by presence of radicles). Counts were conducted...