RESEARCH ARTICLE Seed germination ecology of Ageratum houstonianum: A major invasive weed in Nepal 1 1 2 3 4 Anju Lamsal , Mohan P. Devkota , Deepa S. Shrestha , Srijana Joshi , Anil ShresthaID * 1 Department of Botany, Tribhuvan University, Amrit Campus, Kathmandu, Nepal, 2 National Gene Bank, Nepal Agricultural Research Council, Lalitpur, Nepal, 3 International Centre for Integrated Mountain Development, Lalitpur, Nepal, 4 Department of Viticulture and Enology, California State University, Fresno, California, United States of America * [email protected] Abstract a1111111111 a1111111111 In recent years, spread of invasive alien plant species (IAPS) has been a major concern in a1111111111 Nepal. One such IAPS is Ageratum houstonianum, an Asteraceae, that is a prolific seed pro- a1111111111 a1111111111 ducer and difficult-to-control in farmland and various ecological regions causing crop yield and biodiversity losses. However, very little information is available on the germination biol- ogy and ecology of this species. Therefore, experiments were conducted to assess the effect of water stress, pH level, and light requirement on seed germination, and the effect of seed burial depth on seedling emergence. Water stress was simulated by polyethylene gly- OPEN ACCESS col solutions ranging from 0±5.56 MPa and pH solutions ranging from 4 to 9 were prepared Citation: Lamsal A, Devkota MP, Shrestha DS, using hydrochloric acid and sodium hydroxide. Germination tests were conducted in petri Joshi S, Shrestha A (2019) Seed germination ecology of Ageratum houstonianum: A major dishes lined with filter paper and placed in a controlled environment chamber set at 20Ê C. invasive weed in Nepal. PLoS ONE 14(11): Light requirement comparisons were made by having petri dishes wrapped with aluminum e0225430. https://doi.org/10.1371/journal. foil or left unwrapped. Seedling emergence was evaluated by placing seeds at depths rang- pone.0225430 ing from 0 to 20 mm in the soil. Results indicated that this species was moderately drought- Editor: Craig Eliot Coleman, Brigham Young tolerant because germination ceased beyond 0.51 MPa. Greater germination occurred at University, UNITED STATES neutral to acidic than at alkaline pH levels. The seeds were positively photoblastic because Received: August 22, 2019 no germination occurred under dark condition. No seedlings emerged from seeds placed Accepted: November 4, 2019 more than 2 mm deep in the soil, indicating that this is a primarily surface germinating spe- Published: November 21, 2019 cies. These findings will help predict future invasions and in development of management strategies for this IAPS. Copyright: © 2019 Lamsal et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are Introduction within the manuscript and its Supporting Invasive species are non-native and can include plants, animals and microbes that have been Information files. relocated deliberately or accidentally as a result of economic activities [1]. Invasive alien plant Funding: The authors received no specific funding species (IAPS) have attracted extensive attention among ecologists, agronomists, foresters, for this work. conservationists, farmers, and entrepreneurs because of their negative ecological impacts Competing interests: The authors have declared and associated economic loss at various scales [2]. For example, such species generally alter that no competing interests exist. ecosystems function by changing species composition, fragmenting natural areas, creating PLOS ONE | https://doi.org/10.1371/journal.pone.0225430 November 21, 2019 1 / 14 Seed ecology Ageratum houstonianum degradation, and by reducing biodiversity [3]. Therefore, IAPS have become the focus of intense research and management worldwide [4]. It has been reported that Ageratum houstonianum is one such problematic IAPS in Nepal with widespread distribution in the mid-hills and the plain areas along roadsides, protected national parks, agricultural fields, and fallow lands [5]. The authors further state that the local community have identified it as the worst invasive species in agroecosystems. However, there are no reports, other than distribution, on the ecology of this species in Nepal. A. houstonianum belongs to the Asteraceae family and it is native to Central America and Mexico. It was brought to Europe from these regions as an ornamental plant [6]. In recent years, it is reported to have a presence in all continents and pose serious threat to different ecosystems including community forests all over the world [7]. This species is a short-lived (annual or biennial) herbaceous plant growing 20±100 cm tall. Although the primary means of propagation is by seeds, it has been reported that it can also propagate vegetatively by stem portions [8]. An understanding of the biology and ecology of IAPS is essential for the development of management options, and this species is no exception. However, very lim- ited information is available on the germination ecology of this species. In comparison, there is more information available on the germination of another congener Ageratum spe- cies, A. conyzoides. For example, a study evaluated the effect of temperature and water stress on seed germination of A. conyzoides [9]. Similarly, a lot more information is available on the biology and ecology of other Asteraceae family species such as Artemesia [10, 11] and Conyza [12] because of their ornamental value, invasiveness, or resistance to several herbi- cides globally. Information on environmental factors affecting the germination of A. houstonianum specif- ically could prove to be very helpful in understanding its ecology and development of manage- ment techniques not only in Nepal but globally because of its worldwide presence. Therefore, the objective of this research was to develop information on the germination and emergence of A. houstonianum. Studies were conducted to determine the effects of varying levels of mois- ture and pH, and presence and absence of light on seed germination, the depth of seed burial on seedling emergence, and the ability of the seeds to germinate on immediate dispersal from the mother plant. Materials and methods Seed collection Seeds of A. houstonianum for the germination and emergence studies were collected on November 24, 2017 and January 16, 2018 for the first and second round of the study, respec- tively from the Ranibari Community Forest (27Ê43'52.81"N, 85Ê19'8.81"E, 1,303 masl) located in the northwestern part of Kathmandu, Nepal. Mature A. houstonianum plants growing in open areas with mature and healthy inflorescence at this site were randomly selected for seed collection. Seeds were collected from 20 plants randomly in a 10 m radius area by shaking the inflorescence after inserting them inside polythene bags. The seeds from different plants were mixed to make composite samples. The seeds were then transported to the lab and stored under room temperature (approximately 25ÊC). Seeds for estimating dormancy requirement were collected on July 4, 2018 and July 22, 2018, for the first and second round of the experi- ment, respectively from Kirtipur (27Ê67'29"N, 85Ê28'32"E, 1,330 masl), Kathmandu, Nepal using similar protocols. A preliminary germination test on the collected samples were done by putting some seeds on moist paper towels, under normal room conditions described above, to test for seed viability. More than 90% of the seeds germinated, thus indicating excellent seed viability. PLOS ONE | https://doi.org/10.1371/journal.pone.0225430 November 21, 2019 2 / 14 Seed ecology Ageratum houstonianum The authors confirm that no permission was needed to collect the weed seeds, as well as that the studies did not involve endangered or protected species. Moisture stress at seed germination studies Experiments were conducted in summer 2018 in a seed germination chamber (Model 67094A Indosaw Double Chamber Seed Germinator, Osaw Industrial Products Pvt. Ltd., Ambala Cantt., Haryana, India) in a laboratory of the National Gene Bank of the Nepal Agricultural Research Council (NARC), Khumaltar, Lalitpur, Nepal. Solutions of various water potentials (0, -0.149, -0.51, -1.09, -1.88, -2.89, -4.12, and -5.56 MPa) were prepared using polyethylene glycol (PEG 6000; Qualigens fine chemicals Besant Road, Mumbai, India). The PEG solutions were prepared according to a published protocol [13] and the solutions were calibrated accord- ing to the protocols used by Shrestha et al. [14]. Twenty-five seeds of A. houstonianum were placed on Petri dishes lined with Whatman No. 1 filter paper. Although the upper limits of the water potentials selected are more negative than those for other studies that assessed water potential levels of up to -1.0 MPa on some Asteraceae species [9, 12], the treatments were selected to explore drought tolerance extremes as some species, e.g. Echinochloa colona seeds germinated up to -5.5 MPa [14]. Ten ml of the different water potential solutions were added to each Petri dish with a pipette. The Petri dishes were then sealed with parafilm and placed in the seed germination chamber set at a constant temperature of 20 ± 2ÊC. Light intensity inside the chamber was measured with a Solar Power Meter (Amprobe Solar-100, Everett, WA, USA). The light intensity was measured at five locations