The importance of regeneration to plant habitat specialization has been poorly studied. ten Brink et al. examine the adaptive association between germination ecology and specialization to either forest or open habitats in a comparative experiment using 17 congeneric species pairs with contrasting habitat preference. They find that seeds of forest and open habitat species respond differently to environmental cues. In a conceptual model they show that species from the two habitats are adapted to utilize different windows of opportunity in time (season) and space (habitat), and they suggest that phases in the plant life cycle other than the established phase should be considered important in adaptive specialization.
Heteromorphic seeds can show differences in dormancy and germination, but it is not known how this affects maintenance and regeneration of populations. Cao et al. bury different seed morphs of an annual halophyte, Suaeda corniculata, and examine germination dynamics at monthly intervals over 2 years. They find that black seeds have an annual dormancy/non-dormancy cycle that is absent in brown seeds, and they also exhibit an annual cycle in sensitivity of germination to salinity. Brown seeds form only a transient soil seed bank whilst black seeds form a persistent one. They conclude that differences in performance of the two seed morphs in the soil seed bank increase fitness of the species in unpredictable saline environments.
α -Amylase in grass caryopses (seeds) is usually expressed upon commencement of germination and is rarely seen in dry, mature seeds. However, Goggin and Powles show that selection for low dormancy in Lolium rigidum results in constitutive expression of an aluerone-localized α-amylase activity that is, unexpectedly, responsive to both gibberellin and glucose. This activity appears late during seed development and its presence in dry seeds confers resistance to the germination-inhibiting allelochemical benzoxazolinonenone. The incidental selection for expression of a constitutive, sugar- and hormone-responsive α-amylase activity during selection for low seed dormancy may therefore help to enhance seedling establishment under competitive conditions.
During the transition from endo- to eco-dormancy and subsequent growth, the bulb of onion (Allium cepa) changes from a sink organ to a source, but the mechanisms controlling these processes are not fully understood. Chope et al. carry out detailed analysis of whole-bulb physiology, biochemistry and transcriptional changes in response to sprouting and provide evidence that the monosaccharide-to-disaccharide ratio and zeatin riboside concentration are important factors in discriminating between sprouting and pre-sprouting bulbs. The results also suggest that commercial curing temperatures could be reduced without detrimental effects.
Germination timing, a major determinant of fitness, is extremely sensitive to environmental conditions but the physiological mechanisms regulating it are largely unknown. Barua et al. study Arabidopsis thaliana and find that some attributes, such as sensitivity to GA, predict germination responses to multiple seasonal cues while others, such as ABA sensitivity and seed-coat properties, predict responses to specific cues. The physiological mechanism associated with germination responses to specific environments can therefore influence the potential to adapt to novel combinations of environmental factors experienced during colonization or future climate change.
Jatropha curcas is a drought-resistant tree whose seeds are a good source of oil that can be used for producing biodiesel. Windauer et al. examine germination at different temperatures and water potentials and find that there is an extremely narrow thermal range within which dormancy problems are not encountered and that the seeds have a high sensitivity to water shortage. The use of a hydrotime analysis allows temperature effects on dormancy expression to be discriminated from those on dormancy induction.
Temperate woody perennials become dormant over winter; but dormancy is also a prerequisite for adequate flowering, thus limiting their cultivation at warmer latitudes. Julian et al. show that dormancy appears to mark a boundary between the development of the sporogenous tissue in stamen and the occurrence of meiosis for further microspore development in apricot, Prunus armeniaca. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.
Seed persistence in the soil under field conditions is issue for the maintenance of local plant populations and the restoration of plant communities. Saatkamp et al. compare seed survival in a burial experiment with data on various germination traits for a range of Mediterranean annual plants. Germination rate, light requirement for germination and germination in response to diurnally fluctating temperatures are found to be related to seed survival; however, survival differs according to season and this interacts with germination traits.