Abstract
Seed germination and seed longevity under different environmental conditions are fundamental to understand the ecological dynamics of a species, since they are decisive for its success within the ecosystem. Taking this into account, seed germination and seed storage behavior of a pioneer species of tropical dry forest (Tecoma stans) were studied in the laboratory, to establish the effect of different environmental conditions on a local tree population. Two seed lots collected in July 2011, from Cali (Colombia), were evaluated under three alternating temperatures (20/30, 20/25, 25/30 ºC; 16/8 h) and four light qualities (12-hour white light photoperiod, darkness, and 15 minutes of red light -R and far red light -FR). Final germination was recorded for all treatments; for white light treatment the daily germination was recorded to calculate mean germination rate, mean germination time, and two synchronization indices. To assess the effect of light quality on physiological variables, a destructive germination test was carried out. For this test, another seed lot was evaluated under the same light conditions using an alternating temperature of 20/30 °C - 16/8 h, recording germination during six days for every treatment. In addition, seeds were stored at two different moisture contents (7.7, 4.1 %) and three storage temperatures (20, 5, -20 ºC), during two time periods (one and three months); a seed germination test was conducted for each treatment. Four replicates of 35 seeds per treatment were used for all experiments. Germination was high (GP > 90 %) with all alternating temperatures under white light, whereas under R, FR, and darkness germination was evenly successful at low temperatures, but at higher temperature, half of the seeds entered into secondary dormancy (GP= 45-65 %). However, mean germination rate and synchronization under R and FR decreased significantly in comparison to white light treatment and, consequently, mean germination time increased. Seed storage behavior of this species is orthodox due to the high germination (GP > 90 %) obtained under all treatments. In conclusion, T. stans seeds have a negative germination response at high incubation temperatures in the absence of white light, entering into a secondary dormancy. In contrast, an environment with a lower temperature and without white light delays the germination, but at the end seeds are able to reach the same germination values. This seed dependence on incident light in limiting conditions suggests a physiological mechanism on the seed tissues of this species, probably mediated by phytochromes. Finally, the orthodox seed storage behavior of T. stans is a reason to include this species in ex situ seed conservation programs for restoration and recovery of the tropical dry forest; however, long-term studies should be conducted in order to evaluate the maintenance of this characteristic throughout longer periods of time. Rev. Biol. Trop. 66(2): 918-936. Epub 2018 June 01.
Highlights
Tropical dry forest is a form of vegetation located in lowland territories with certain climatic conditions
Effect of temperature on physiological variables: Seed germination rates were high in all alternating temperatures and in both seed lots, with Germination percentage (GP) values close to 100 % (Table 2)
The high germination (> 90 %) obtained at three different alternating temperature (i.e. 20/25, 25/30 and 20/30 °C for 16/8 h, with a 12 hour photoperiod of white light) shows that T. stans seeds display a high degree of flexibility to variations in the environmental temperature, a result found in populations of T. stans introduced to Brazil (Socolowski et al, 2008)
Summary
Tropical dry forest is a form of vegetation located in lowland territories with certain climatic conditions In dry forests, for example, the average temperature required for a large number of seeds of tree species to germinate is 26.9 ± 0.2 °C; in the case of shrub species, it is 26.5 ± 0.8 °C (Baskin & Baskin, 2014). In Colombia, this plant is found mainly in the Andes and the Caribbean but is cultivated in tropical and subtropical regions (Gentry, 2009) This species produces large quantities of seeds dispersed by the wind (Pelton, 1964), mainly in dry seasons (RojasRodríguez & Torres-Córdoba, 2012). T. stans is typical of dry forests in a relatively early successional state or in places dominated by subxerophytic scrub vegetation and in welldrained and/or rocky soils, it may be found in areas of pre-mountain and mountain rainforest (Pelton, 1964)
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