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.

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.

Ovule and seed development of Lacandonia schismatica (Lacandoniaceae).

Ovule and seed development is described for Lacandonia schismatica, a species whose androecium is surrounded by the gynoecium. The ovule in each carpel is basal, anatropous, tenuinucellate, and bitegmic. The female gametophyte is formed by the micropylar megaspore cell, after a coenocytic stage of the four megaspore nuclei. The mature female gametophyte has the normal complement of seven cells and eight nuclei. We propose a new type of female gametophyte development on the basis of the coenocytic stage of the tetrad, the cellularization of the tetrad, and the survival of the micropylar spore. At seed dispersal time, the embryo has ~10-20 cells. Endosperm development is of the nuclear type. At maturity, endosperm cells show starch and protein inclusions as well as polysaccharides in their thick walls. The seed coat is formed from the outer integument; the inner one disappears. The exotesta contains tannin. The fruit (achene) wall is two-layered. The maturation of the fruits in a flower is synchronous, and they separate from the receptacle for dispersal.

Morph-biased male sterility in the tropical distylous shrub Erythroxylum havanense (Erythroxylaceae).

This study explores whether ecological factors, such as pollinators and pollen flow, or variation in pollen and ovule development account for the observed differences (approximately twofold) in the reproductive output of pin and thrum individuals of Erythroxylum havanense. The importance of ecological factors was assessed by means of comparison of the identity of pollinators and the rates of flower visitation, and by performing controlled hand pollinations and measurements of fruit set. In addition, we described the pollen and ovule development of thrum and pin individuals. Our results indicate that pollinators of E. havanense do not distinguish between floral morphs. The differences in fruit set between pin and thrum plants held even after hand pollination and, therefore, the observed differences in reproductive output between floral morphs of E. havanense cannot be explained in terms of asymmetrical pollen flow. There were no differences in the pattern of gynoecium development between the pin and thrum morphs, however androecium development showed marked differences between the morphs, and there was a resemblance between the developmental pathways leading to male sterility of the thrum morph of E. havanense with that of species with cytoplasmic male sterility (CMS).