Date Published: October 28, 2015
Publisher: Public Library of Science
Author(s): Bin Wen, Tzvetanka D. Dinkova.
Mexican sunflower is native to Mexico and Central America and was introduced into China early last century. Now it has widely naturalized and is exhibiting increasing invasiveness in South China. As this species often dominates bare ground, a habitat characterized by extreme fluctuation in temperature and water, it is reasonable to hypothesize that it has special adaptations to high temperature and water stress. Using laboratory experiments to simulate these stresses, this study investigated the response of Mexican sunflower seed germination to temperature and water stress, and compared these responses with those previously reported for another invasive, bamboo piper, which is confined to relatively cool and moist habitats in Xishuangbanna. As expected, Mexican sunflower seeds exhibited higher tolerance to these stresses than bamboo piper. Germination of Mexican sunflower seeds was highest at 15–30°C, but significant numbers of seeds germinated and formed seedlings at 10°C and 35°C, at which no bamboo piper seeds formed seedlings, indicating a wider temperature range for germination than the latter. Roughly half the seeds survived 240 h continuous heat treatment and up to 15 h daily periodical heat treatment at 40°C, while bamboo piper seeds were mostly killed by these treatments. About 20% of Mexican sunflower but no bamboo piper seeds germinated after heat treatment for 30 min at 80°C. Germination was completely inhibited in bamboo piper seeds at -0.6 mPa, while 20–60% of Mexican sunflower seeds germinated depending on PEG or NaCl as osmoticum. This higher tolerance in Mexican sunflower seeds accords with its stronger invasiveness in this area. This comparison between two plant invaders demonstrates that invasiveness is not an all-or-nothing situation, and that adaptation to local habitats is a critical determinant of successful invasiveness for an alien plant.
Mexican sunflower (Tithonia diversifolia (Hemsl.) A. Gray) in the family Asteraceae is a weed native to Mexico and Central America. It has been introduced widely into Asia, Africa, America and Australia, as an ornamental plant for its beautiful tall flowers, and as a green manure and erosion-control plant for its fast growth. Now it has become an aggressive invader in Southeast Asia, South Africa and many Pacific islands [1–4]. This species was found escaped in Yunnan, China, in the 1930s, but has behaved as an aggressive invader only from around 2000 [5, 6]. Now it is abundant in tropical China, including Fujian, Guangdong, Hainan, Guangxi, Yunnan and Taiwan .
Mexican sunflower seeds used in this study had a 100-seed weight of 442.48±3.29 mg, an initial moisture content of 9.00±0.38% (wet weight basis), and an initial germination of 92.33±1.84%. Almost all germinated seeds developed into normal seedlings in the experiments, so only germination percentage is reported in this study.
Following a previous study on bamboo piper seeds , this paper investigated germination of Mexican sunflower seeds under high temperature and water stress, and found that Mexican sunflower seeds have markedly higher abiotic tolerance than bamboo piper seeds. Although water stress conditions restricted seed germination in both species, a germination percentage of 20–60% was achieved by Mexican sunflower at -0.6 mPa (Fig 4) while no germination occurred in bamboo piper at this water potential . In addition, about 25% of Mexican sunflower seeds germinated in soil as dry as 12.5% moisture content (dry weight basis, Fig 5). Similarly for high temperature stress, a significant number of seeds germinated although high temperature also reduced seed viability and impaired seed germination in Mexican sunflower. After heat treatment for 30 min at 80°C, all bamboo piper seeds were killed  while 20% of air-dried Mexican sunflower seeds survived and developed into normal seedlings (Fig 2). Moreover, Mexican sunflower seeds can germinate at higher incubating temperatures. When incubated at 35°C, 20–40% of Mexican sunflower germinated and formed normal seedlings (Fig 3) but no seedlings formed for bamboo piper . Few bamboo piper seeds formed seedlings when the duration of continuous heat treatment at 40°C increased to 240 h, or the period of time for daily periodical heat treatment reached 12 h , while these treatments only partially impaired seed germination and seedling establishment in Mexican sunflower (Figs 6 and 7). Compared with bamboo piper, Mexican sunflower seeds germinated across a wider range of environmental conditions.