Hypothalamic and pituitary periodicity demonstrated by isolated rat adrenal cells

Rats were maintained under standardized conditions of food and water ad libitum and a lighting schedule of 12 h light/we h dark for seven days. Animals were sacrificed at 0500 and 1700 h and tissues were removed and washed. Isolated intact adrenal and pituitary cells were prepared by collagenase treatment. Using a sequential incubation procedure, the release of pituitary ACTH by hypothalamic acid extracts (CRF) was assayed by stimulation of corticosteroid secretion from isolated adrenal cells. Maximum stimulation of adrenal steroids was achieved with hypothalamic extracts and pituitary cells from rats sacrificed at 1700 h, and minimum values were obtained at 0500 h. Intermediate levels were obtained when hypothalamic extracts at one time point were incubated with pituitary cells at the other. The results substantiate the late afternoon peak level of hypothalamic and pituitary secretion occurring prior to the peak activity pattern of the rat.     

Population ecology of halophyte seeds

Some aspects of the population biology of halophytes are considered in this review. Persistent seed banks have been reported for a number of inland- and coastal-salt marsh plant communities. Seeds of perennial grasses are often under-represented, while annuals and some perennial forbs may be over-represented in the seed bank. The persistent seed bank of annual halophytes appears adaptive, and provides multiple seed germination opportunities which may prevent local extinction when environmental stress increases. Somatic seed polymorphism provides a mechanism by which parent plants can respond to changing environments by partitioning their resources into reproductive units which have distinct germination responses. Parental effects may influence either seed morphology and/or physiological requirements of seeds when they are exposed to environmental stress. A prolonged germination period can provide plant populations with numerous opportunities to establish seedling cohorts. Early cohorts will have a selective advantage under moderate conditions because mortality will be low and plants will survive until maturity. However, fluctuations in salinity levels and tidal activity can cause high mortality in early cohorts in salt marsh habitats, providing later cohorts with an opportunity for establishment. Resource allocation to reproductive structures is related to plant size, which itself can be affected by both abiotic and biotic factors. Larger plants were found to produce more seeds than smaller plants in a population, but the mean seed weight was greater in small plants.     

Seed banks and seed population dynamics of halophytes

In this review I will describe the importance of seed banks and thepopulation dynamics of seeds on the distribution of species in salinehabitats. The main questions being examined in this review include: 1.Does the seed bank represent the flora of the entire salinity gradient or isit restricted to the species in each zonal community? 2. Is the size andspecies composition of the persistent seed bank regulated by the degree ofsalt stress in habitats along an environmental gradient? 3. Does thepopulation dynamics of seeds influence the temporal and spatial distributionof plant species in saline habitats? Seed banks may be transient orpersistent depending upon the physiological responses of species and thesoil environment in which the seeds are found. The formation of zonalcommunities in salt marsh environments is affected by changes in soilsalinity and flooding along an elevational gradient. Population dynamics ofseeds have been found to determine the spatial and temporal distributionof species along salinity gradients. The flora and relative density of speciesof zonal communities are significantly dependent upon the stress toleranceof species at different stages of development and the presence of transientor persistent seed banks. The occurrence of a seed bank is related to thesalinity tolerance of species at the germination stage of development, aseeds ability to tolerate hypersaline conditions and flooding, and whetheror not species are able to maintain a persistent seed bank until hypersalineconditions are alleviated.