Reproductive traits in four spontaneous Pappophorum vaginatum populations in arid Argentina

Highlights

• Variability in reproductive characteristics were shown in Pappophorum vaginatum.

• Sexual reproduction may be primarily important in P. vaginatum.

• Selection programs to obtain improved germplasm of P. vaginatum must be made.

Abstract

Arid zones, where native rangelands are common, are essentials for millions of people livelihood. These areas support 50% of the world's livestock and are where 44% of the world's food is grown. In rangelands of Central Argentina, Pappophorum vaginatum is basically the unique, warm-season perennial grass species palatable to grazing livestock. Our major objective was to determine variability in reproductive characteristics among four spontaneous, overgrazed populations (i.e., P1, P2, P3, P4) of that species to identify promissory materials for domestication. Studies were conducted during three consecutive growing seasons within the southwestern part of the Phytogeographical Province of the Monte, in southwestern Buenos Aires, Argentina. Measured reproductive characteristics were related to flowering initiation, seed production and natural reseeding potential. Significant differences were found for flowering initiation and natural reseeding potential, but not for viable anthecia per plant among the four populations of P. vaginatum. This species fructified from the beginning to the end of the studied growing seasons with a great anthecia production per plant. The light weight of these anthecia and their awns would favor a great wind dispersal and most likely the establishment of new seedlings of P. vaginatum. This suggests that sexual reproduction might have a relatively greater importance than asexual reproduction (i.e., tillering) in the persistence of the overgrazed P. vaginatum in the plant communities of the studied region. Selection of plant materials with a late flowering initiation will allow to extend the forage production of a better quality. The variability among and within populations found on this study supports the idea that would be promissory to start selection programs to obtain improved germplasm to reincorporate to grasslands of the south of the Phytogeographical Region of the Monte (Argentina) not only to increase livestock production but also to recover and maintain biodiversity.

Introduction

Grasslands cover 40.5% of the total land surface of the world (White et al., 2000) and have been lived in and used by people throughout human history (Gibson, 2009). In arid zones, where native rangelands are common, live approximately 2.5 billion people who depend on these ecosystem services for their livelihoods (UN EMG, 2010). According to Gaur and Squires (2018) these arid zones support 50% of the world's livestock where 44% of the world's food is grown. The three greatest threat to native grassland are agriculture, fragmentation and invasion of non-native species (Gibson, 2009). Thus, it is crucial to optimize the use of resources and knowledges, like those informed by Kassahun et al. (2008), Smith et al. (2010a), Tälle et al. (2016), Dubeux Jr. et al. (2017), Al- bukhari et al. (2018), Muir et al. (2018), Koncz et al. (2020) and Belesky et al. (2020), in order to harmonize rangeland utilization and biodiversity conservation.

Production of meat by livestock in 75% of continental Argentina, characterized by arid and semi-arid zones, is based on grazing of native vegetation (Busso et al., 2018). Thus, rangelands are important as a production factor (because they are the main food source for animals) and a biodiversity reservoir since they provide an ecological or environmental service with the subsequent social-economical impact (Mujica, 2010a, b).

Pappophorum Schreb is a grass genus native to the American continent with 10 species, 7 of which live in arid and semiarid zones of Argentina and boundary regions (Pensiero, 1986). It is considered mainly autogamous because of the presence of cleistogamy (Campbell et al., 1983; Pensiero, 1986). Rangelands of central Argentina are characterized by the scarcity of warm-season, native perennial grasses, palatable to livestock grazing (Busso et al., 2004). Within this region, at the South of the Phytogeographical Province of the Monte, one important species is Pappophorum vaginatum Buckley (Poaceae). This is basically the unique palatable perennial grass species during the warm-season for grazing livestock (Giorgetti et al., 1997). This situation exposes P. vaginatum to overgrazing and is currently considered as a decreasing species (Torres et al., 2013a). Within Argentina P. vaginatum can be found not only in the Phytogeographical Region of the Monte but also in those of the Pampas, Espinal and southeast of Chaco (Pensiero, 1986).

A valuable contribution for the recovery, maintenance and increase of biodiversity is the domestication of native and naturalized species with forage value, and the successful introductions (Mujica, 2010a, b). In addition, establishment of improved forages, among other technologies, can substantially increase the stocking rate of semiarid areas in Argentina (Garbulsky and Deregibus, 2004). Thus, much research has focused on the study of introduced species adapted to semiarid conditions since several decades (Ferri, 2014). However, the native germplasm of the semiarid region of Argentina is of a special interest and value in terms of genetic improvement. This is due to its evolutionary history and adaptation to the climatic and edaphic constraints of such region (Martin, 2005). Besides, several studies to a global scale have reported that the native species are the most adequate for ecosystem restoration in arid and semiarid regions (Waters and Shaw, 2003). Nevertheless, such species must go through successive steps of evaluation previous to their utilization (Tyler et al., 1987). This includes studies from their characterization and initial evaluation either at the greenhouse or at the field as isolated plants to comparative yield and grazing studies.

Studies of variability in characteristics of agronomic interest and elucidation of the strategies of adaptation to biotic and abiotic disturbances on the native or naturalized species are of critical importance. Genetic improvement should focus in combining the effects of natural selection to the local environment with those coming from artificial selection on agronomic characteristics, specially those linked to the control of transplanting and the production of forage and seeds (Mujica, 2010a, b). In this sense, various studies were made on native, perennial, warm-season forage grasses. For example, some of these studies were related to (1) elucidate strategies of competitive ability on P. vaginatum (Torres et al., 2013a, b) and Trichloris crinita (Lag.) Parodi (Greco and Cavagnaro, 2005); (2) study the phenotipic variation for various characteristics of agronomic interest in populations of T. crinita (Kloster et al., 2016), Setaria lachnea (Pensiero et al., 2011) and P. vaginatum (Entio et al., 2014b, c), and (3) evaluate the genetic variability on T. crinita (Cavagnaro et al., 2006). The strategic incorporation of forage, native species was also studied to recuperate degraded areas (Quiroga et al., 2009) and increase animal stocking rate (Passera et al., 1992).

More specifically, the variability in seed germination and on characteristics related to the initial seedling growth were studied in populations of P. vaginatum (Entio et al., 2014b; Entio et al., 2014c). Additional studies were made on reproductive characteristics on populations of Setaria lachnea such as the variability in (1) germination, seed weight and production, and cleistogamy (Pensiero et al.,1995), and panicle emergence and fecundity (Pensiero et al., 2011); and (2) flower phenology, number of panicles and percentage fructification (Exner et al.,2010).

One important mechanism which contributes to plant species persistence on rangelands is natural reseeding which depends on viable seed contribution into the soil seed bank. This was proved at the field under grazing conditions; for example, with the variety Aguapé of Lotus tenuis (Entio and Mujica, 2011). The soil seed bank can work as such as long as the disseminules preserve their viability (Baker, 1989). In addition, persistance of the soil seed bank is critical because it is a reserve of accumulated genetic adaptability for the population over which can act natural selection (Simpson et al., 1989). Perennial grasses at the Monte form transient soil seed banks; however, such transience appears to be little associated with losses due to germination (Marone et al., 2003). The main factor of seed losses can be attributed to predation by granivores (Marone et al., 2000). Anyhow, Entio et al. (2014a) suggested a positive effect of ants in the establishment of new seedlings of P. vaginatum from seeds obtained from plants at the stage of seed dispersal.

The hypothesis of this study is that there is variability in reproductive characteristics important for domestication (i.e. flowering initiation, viable seed production, and potential of natural reseeding) among spontaneous populations of P. vaginatum in the semiarid region of the Province of Buenos Aires. This will allow to identify promissory materials with later flowering initiation and greater viable seed production and potential of natural reseeding to select in programs of genetic improvement to use in grasslands restoration. In this way, an increase in the abundance of P. vaginatum either by artificial sowing, natural reseeding or both, will be important not only to increase forage availability for livestock grazing but also for the recovery and maintenance of biodiversity. The objective of this study was to analize the variability of various reproductive characteristics on four spontaneous native populations of P. vaginatum within the semiarid region of the Buenos Aires Province to identify promissory materials for domestication.