Plant attributes and their relationship to the germination response to different temperatures of 18 species from central Mexico.

Germination responses of non-dormant seeds to temperature and thermal requirements are affected by the geoclimatic origin of the species, along with specific attributes such as life form, life cycle or seed size. We evaluated the relationship of these attributes and temperature to germination in 18 species that inhabit a convergence area of two biogeographic realms. Seeds were sown at different constant temperatures. Base temperature (Tb ) and thermal time for 50% germination (θT(50) ) were determined. For Tb , θT(50) and seed size, we performed a cluster analysis and then applied a discriminant analysis (DA). DA was also performed using geoclimatic origin, life form and life cycle as grouping variables. Seed that did not germinate were transferred to the benefit temperature for germination. Finally, ethylene was applied to the remaining seeds that did not germinate. Temperature significantly affected final germination. Tb varied between 5 and 13 °C in 15 species and 19.0-21.5 °C in the remainder; θT(50) was 7-30 °Cd in eight species and 50-109 °Cd in the remainder. Cluster analysis showed three groups, and DA evidenced the relevance of Tb and θT(50) for this separation. Differences in life cycle were related to θT(50) . The geoclimatic origin was not significant. Thermoinhibition or thermodormancy were found in some species. Tb overlaps with environmental temperature of the growth season. Thermal traits for germination mainly reflect the species' life cycle, which is related to the main differences in reproductive performance among annuals and perennials. Local adaptation might mask the effect of geoclimatic origin of a species.

Ecological longevity of Polaskia chende (Cactaceae) seeds in the soil seed bank, seedling emergence and survival.

Soil seed banks are essential elements of plant population dynamics, enabling species to maintain genetic variability, withstand periods of adversity and persist over time, including for cactus species. However knowledge of the soil seed bank in cacti is scanty. In this study, over a 5-year period we studied the seed bank dynamics, seedling emergence and nurse plant facilitation of Polaskia chende, an endemic columnar cactus of central Mexico. P. chende seeds were collected for a wild population in Puebla, Mexico. Freshly collected seeds were sown at 25 °C and 12-h photoperiod under white light, far-red light and darkness. The collected seeds were divided in two lots, the first was stored in the laboratory and the second was use to bury seeds in open areas and beneath a shrub canopy. Seeds were exhumed periodically over 5 years. At the same time seeds were sown in open areas and beneath shrub canopies; seedling emergence and survival were recorded over different periods of time for 5 years. The species forms long-term persistent soil seed banks. The timing of seedling emergence via germination in the field was regulated by interaction between light, temperature and soil moisture. Seeds entered secondary dormancy at specific times according to the expression of environmental factors, demonstrating irregular dormancy cycling. Seedling survival of P. chende was improved under Acacia constricta nurse plants. Finally, plant facilitation affected the soil seed bank dynamics as it promoted the formation of a soil seed bank, but not its persistence.

Effects of seed burial on germination, protein mobilisation and seedling survival in Dodonaea viscosa.

Ecological restoration of disturbed areas requires substantial knowledge of the germination of native plants and the creation of novel methods to increase seedling establishment in the field. We studied the effects of soil matrix priming on the germination of Dodonaea viscosa seeds, which exhibit physical dormancy. To this end, we buried both pre-scarified (in H2SO4, 3 min) and non-pre-scarified seeds in the Parque Ecológico de la Ciudad de México. After seeds were unearthed, they were post-scarified for 0, 2, 6 and 10 min and their germination percentages compared to the germination of a control batch of laboratory-stored seeds. For both control and unearthed seeds, the protein pattern was determined in the enriched storage protein fraction in SDS-PAGE gels stained with Coomassie blue. Percentage germination increased as the scarification time increased. Pre-scarification significantly increased percentage germination of post-scarified seeds in relation to the control and non-pre-scarified seeds. In seeds unearthed from the forest site, the buried pre-scarified seeds had relatively high percentage germination, even in the absence of post-scarification treatment. A 48-kDa protein was not found in unearthed, pre-scarified seeds nor in the control germinated seeds, indicating that mobilisation of this protein occurred during soil priming. Burying seeds for a short period, including the beginning of the rainy season, promoted natural priming, which increased protein mobilisation. Functionally, priming effects were reflected in high percentage seedling survival in both the shade house and the field. Seed burial also reduced the requirement for acidic post-scarification.

Seed anatomy and water uptake in relation to seed dormancy in Opuntia tomentosa (Cactaceae, Opuntioideae).

BACKGROUND AND AIMS: There is considerable confusion in the literature concerning impermeability of seeds with 'hard' seed coats, because the ability to take up (imbibe) water has not been tested in most of them. Seeds of Opuntia tomentosa were reported recently to have a water-impermeable seed coat sensu lato (i.e. physical dormancy), in combination with physiological dormancy. However, physical dormancy is not known to occur in Cactaceae. Therefore, the aim of this study was to determine if seeds of O. tomentosa are water-permeable or water-impermeable, i.e. if they have physical dormancy. METHODS: The micromorphology of the seed coat and associated structures were characterized by SEM and light microscopy. Permeability of the seed-covering layers was assessed by an increase in mass of seeds on a wet substrate and by dye-tracking and uptake of tritiated water by intact versus scarified seeds. KEY RESULTS: A germination valve and a water channel are formed in the hilum-micropyle region during dehydration and ageing in seeds of O. tomentosa. The funicular envelope undoubtedly plays a role in germination of Opuntia seeds via restriction of water uptake and mechanical resistance to expansion of the embryo. However, seeds do not exhibit any of three features characteristic of those with physical dormancy. Thus, they do not have a water-impermeable layer(s) of palisade cells (macrosclereids) or a water gap sensu stricto and they imbibe water without the seed coat being disrupted. CONCLUSIONS: Although dormancy in seeds of this species can be broken by scarification, they have physiological dormancy only. Further, based on information in the literature, it is concluded that it is unlikely that any species of Opuntia has physical dormancy. This is the first integrative study of the anatomy, dynamics of water uptake and dormancy in seeds of Cactaceae subfamily Opuntioideae.

Comparison of light-regulated seed germination in Ficus spp. and Cecropia obtusifolia: ecological implications.

The major components of annual seed deposition in the rain forest at Los Tuxtlas, Veracruz, México are seeds of the pioneer tree species Cecropia obtusifolia and those of some species of Ficus. Cecropia obtusifolia Bertol. forms a relatively persistent viable soil seed bank, whereas seeds of Ficus are seldom found in the soil. Both genera require light for seed germination; however, the species differ in their germination responses to far red (FR) light under laboratory and field conditions. Seeds of C. obtusifolia did not germinate in low red/far red (R/FR) or pure FR, whereas seeds of the Ficus species did. This suggests that Ficus seeds do not become dormant under the light conditions (low R/FR ratio) beneath the leaf canopy of the rain forest. This difference may explain why the species differ in their presence in the soil seed bank.

Effects of litter from a tropical rainforest on tree seed germination and establishment under controlled conditions.

The effects of litter cover on the germination of seeds of three tropical rainforest, pioneer tree species (Cecropia obtusifolia, Heliocarpus appendiculatus and Piper auritum) was investigated. Germination of seeds of all tree species was partially or totally inhibited when the seeds were covered with litter, whereas uncovered seeds or seeds covered with paper instead of litter germinated. Seed size was directly correlated with the capacity of the germinated seeds to emerge through the litter. In soil experiments in the greenhouse, seedling emergence from the soil was inhibited by the presence of litter on the soil surface.

Ecological significance of light controlled seed germination in two contrasting tropical habitats.

The effects of temperature, photoperiod, phytochrome photoreversion and the response to a R/FR ratio gradient were investigated in seeds of four species from two contrasting tropical habitats; two species from a rain forest (Cecropia obtusifolia and Piper umbellatum) and two from a high altitude lava field covered by low vegetation (Buddleja cordata and Chenopodium ambrosioides). In the rain forest seed species the photoblastic response seems to be adapted to light quality changes due to canopy destruction, on the other hand, the lava field seed species seem to be adapted to instantaneous light stimulus such as would be produced by the sudden exposure of a buried seed to the soil surface light environment.

Ecophysiological characteristics of the seed of the tropical forest pioneer Urera caracasana (Urticaceae).

Urera caracasana (Jacq.) Griseb is a small, fast-growing evergreen pioneer tree which colonizes openings in the tropical rain forest of 'Los Tuxtlas', Veracruz, Mèxico. Annual seed production by 10 trees was estimated to range from 0.4 x 10(5) to 1.6 x 10(6) seeds per tree. Fifteen species of resident and migratory birds were observed to visit the plants and disperse the seeds. Most seeds imbibed in petri dishes placed on the forest floor beneath a small opening in the canopy, where the red/far-red ratio of the light was around 1.0, germinated within 2 weeks. Germination beneath the forest canopy, where the red/far-red ratio of the light was approximately 0.2, was much slower and did not exceed 40%. A minimum of 4 h daily exposure to unfiltered natural light was required for rapid germination in the forest. In the laboratory rapid germination at 25 degrees C required a minimum of 4 h white light. However, if temperature during the light period was 35 degrees C, 30 min exposure to white light daily was sufficient to induce germination. Seeds that failed to germinate when imbibed for 60-360 days in petri dishes placed on the forest floor beneath the forest canopy, germinated rapidly in the laboratory when held at 25 degrees C and exposed daily to 12 h white or red light. Seeds imbibed for 120 days or more beneath the forest canopy were also induced to germinate by 12 h daily exposure to far-red light when this was combined with a 35/25 degrees C day/night temperature regime. Seeds buried in vermiculite-filled nylon mesh bags disappeared rapidly with few remaining after 2 months. Seeds that survived remained viable and germinated when incubated in petri dishes at 25 degrees C. Initially, buried seeds required light for germination. However, after 17 months' burial, seeds germinated in darkness when transferred to the laboratory and incubated on agar at 25 degrees C.