Germination Performance of Yellow Cosmos : Understanding Its Invasion under Tropical Conditions

To further investigate the recently observed invasion of a new weed in the São Paulo areas, Brazil, we studied the germination performance and emergence characteristics of Bidens sulphurea. We measured the seed germination under different fixed temperatures (15, 20, 25, 30 and 35°C), photoperiods (18h/6h, 12h/12h, 6h/18h and 0h/24h [light/dark], respectively), light quality (white, blue, yellow, green, red and dark condition); and emergence in two seasons (winter and spring) in different sowing depths (0, 1, 3, 5, 7 and 9 cm). This species has the ability to germinate and emerge above 90% in a range of conditions of light, but it is very sensitive to variations in temperature, and several reductions on its germination and emergence are found bellow 20°C. The emergence of this species is severely controlled by seasonality and could occur up to 9 cm depth in Brazilian spring.


Introduction
Bidens sulphurea (yellow cosmos) is an annual weed with sexual reproduction, originated in Central America and disseminated for ornamental purposes (Kissmann & Groth, 1999).Its population is building-up rapidly and spreading around agricultural and urban sites on São Paulo state, Brazil.As well as Bidens pilosa (Kissmann & Groth, 1999), this species has the potential to be one of the main weeds in tropical zones.Indeed neither biological nor ecological information are available on the environmental factors influencing seed germination and seedlings emergency for this species.
The environmental conditions such as light, temperature, and the association of both (J.M. Baskin & C. C. Baskin, 2004) and the vertical distribution of the seeds on the soil (Souza, Pitelli, Simi, & Oliveira, 2009) are the main factors that influence seed germination and emergence in the tropics, promoting or breaking the dormancy of the seeds.Seed banks are the main source of weeds regeneration and potential for spreading in agricultural sites (Carmona, 1992).
Seeds, in general, germinate within a broad temperature range; the optimum temperature allows the highest seed germination performance (Roberto & Habermann, 2010).In addition, the maximum and minimum temperatures, above and below which seed germination may not occur, help to characterize the geographical and climatic region where seeds or fruits were harvested.
Weeds use efficiently the natural resources in nature, especially sunlight (Schmitt & Wulff, 1993).Consequently, they are also very well adapted to specific environments, showing highly heritable traits such as the high seed production (Ackerly et al., 2000;Masin, Zuin, Archer, Forcella, & Zanin, 2005), aggressiveness and success in spreading.
To further investigate the invasion "aggressiveness" of B. sulphurea, we measured the influence of temperature, photoperiod, quality of incident light and seeds sowing period at different depths, on seed germination and seedling emergence of this species.The main question addressed was testing the assumption that high spreading of B. sulphurea occurs because the species show high germination and the environmental changes related to light and temperature exercise small influence in this process.

Plant Material
Bidens sulphurea achenes (seeds) were randomly collected from mature capitula of healthy adult plants, in a few hours before beginning each experiments at the agricultural fiels on Faculdade de Ciências Agrárias e Veterinárias (FCAV-UNESP), Jaboticabal city, São Paulo state, Brazil.We adopted this procedure to avoid seed dormancy caused by storage (Finch-Savagel & Leubner-Metzger, 2006).

Effects of Temperature and Light on Germination
The three experiments were conducted independently in a germination chamber using a completely randomized design, being determined the effects of: 1-temperature (15, 20, 25, 30 and 35°C, under 12/12 photoperiod); 2-photoperiod (18/6, 12/12, 6/18 and 0/24 hours light/dark at 25°C); 3 -quality of incident light (yellow, blue, white, green, red and complete dark at 25°C), on germination of B. sulphurea.In all experiments 20 seeds were used in each box per repetition (4 repetitions), distributed in five columns in each box.For moisture maintenance and standardization, two filter paper sheets moistened with 10 mL of distilled water were used per treatment.Seed was considered germinated when the radicle visibly protruded through the seed coat (Reddy & Singh, 1992).
To test germination in the dark, boxes were wrapped in a double layer of aluminum foil.The light quality was obtained wrapping the box in a cellophane foil (yellow, blue, white, green and red) (Almeida & Mundstock, 2001).Germination was recorded daily during seven days, by removing germinated seeds.At the end of this period germination (G%) and germination rate (GR) were calculated (Maguire, 1962).All the evaluations were done in a dark room under green light (Felippe, Válio, Pereira, Sharif, & Vieira, 1983).

Influence of Seasonality on Seed Burial Depth
These experiments were conducted using two sowing seasons, the first one conducted on June 2009 (winter) and the second one on September 2009 (spring).
Fifty seeds were sowed in pots of 5 L at depths of 0 (soil surface), 1, 3, 5, 7 and 9 cm, in a randomized design with five repetitions.The soil (38% clay, 5% silt and 57% sand) was dried in the shadow and passed through a 5-mm sieve.Soil in the pots was moistened initially using a mist sprayer and then sub irrigated to maintain adequate soil moisture.Daily, for 21 days after sowing (DAS), seedlings were counted and removed (Souza et al., 2009).With the daily emergence data, for each sowing season, emergence rate was calculated (ER) (Maguire, 1962).Seedling emergence was defined as the coleoptile being visible at the soil surface (Reddy & Singh, 1992).

Statistical Analysis
The data were statistically analyzed using one-way ANOVA, and the means were compared by Tukey test at 5% level.For germination (G%) and emergence (E%) percentage, arcsin (sqr(x+0.5)/100)was calculated to perform the statistical analysis. www.ccsen

Effects
The    (monthly average 22.5ºC).These results are consistent with the stimulation of germination by temperature (Figure 1).
Seedling emergence of B. sulphurea was greatly influenced by seed burial depth mainly in June when the emergence bellow 7 cm depth was less than 1% and the ER was near zero (Figure 4B).The highest emergence was observed between 3 and 5 cm depth (18 and 22%) and the ER was 0.32 and 0.40.In September, as a result of temperature increase, the E% was above 50% (Figure 4C) even in the all depths and the ER increased in all depths too.In general the highest emergence in June was 24% and in September 84%.
In our study of temperature and light (Figures 1 and 2) germination percentage was higher than observed at the soil surface in September.This difference could be due to poor soil-seed contact or limited water availability on the soil surface than on the filter papers (Ghorbani et al., 1999).Seeds buried below 2 mm on the soil surface usually receive less than 1% of incident light (Egley, 1986).According to our study of light (Figures 2 and 3) B. sulpurea seeds germinated above 60% in dark conditions.In this case the observed variations on emergence at different sowing depths in both seasons are exclusively related to variations on soil temperature, suggesting that this species is light insensible but very sensible to temperature.

Conclusions
The results of this study highlight the B. sulphurea ability to germinate and emerge above 90% in a broad range of conditions of light, confirming our hypothesis, but this species is very sensitive to variations in temperature, and a severe reduction on germination and emergence is found bellow 20°C.The emergence of this species is severely controlled by seasonality and could germinate up to 9 cm depth in Brazilian spring.

Figure
Figure 2. B