Chilling injury in plants—the role of membrane lipid fluidity

Abstract. Many authors have associated chilling injury in plants with changes in the mobility of membrane lipids but have not proposed specific mechanisms for this association.
This paper explains how the mobility of membrane lipids can affect membrane thickness, membrane permeability, the electric field, cation concentration and water ordering near a membrane and hence change the conformation (and thus activity) of a membrane-bound enzyme. The complications in such a model due to protein-lipid interactions and lateral phase separations are also discussed.     

Flow variability maintains the structure and composition of in‐channel riparian vegetation

1. Naturally variable river flows are considered to be important for structuring riparian vegetation. However, while the importance of floods for the ecology of riparian vegetation is well recognised, much less is known about the importance of small fluctuations in river flows. 2. We investigated the effect of water supply diversion weirs on the riparian vegetation of upland streams. These weirs remove within‐channel fluctuations in flow but do not prevent large floods downstream. We surveyed the in‐channel and banktop vegetation of five streams, three of which were regulated by weirs and two of which acted as controls. 3. Unexpectedly, we observed greater species richness within the channel downstream of the weirs. This was because of increased numbers of exotic and terrestrial (‘dry’) plant species. Grass cover was also greater downstream of the weirs. There were no significant differences in the banktop vegetation between the upstream and downstream sites of the regulated streams. 4. Our results highlight the role of within‐channel flow variability in maintaining the composition of vegetation within the stream channel. We suggest that greater species richness does not necessarily indicate a less‐disturbed environment. Rather, a greater number of ‘dry’ species is indicative of the impacts of flow regulation. 5. Small fluctuations in river flows are probably necessary to protect the ecosystem structure and function of regulated streams. It is recommended that variable within‐channel flows be provided in regulated streams.     

Comparative Endocrinology of Piscine Hypothalamic Hypophysiotropic Peptides: Distribution and Activity

Fishes display a variety of anatomical relationships betweenbrain and pituitary to a degree unique among vertebrates. Thisgroup is pivotal for understanding evolution of functions ofhypophysiotropic peptides. We review information concerningoccurrences, distributions and physiological activities of threeidentified peptides in fish brain, and biological propertiesof fish brain extracts. Thyrotropin releasing hormone may bepresent universally in piscine central nervous tissue; however,this peptide has not been clearly demonstrated to have hypophysiotropicactivity in fishes. Somatostatin also has been shown to occurin fish brains; studies of actions of this substance are virtuallyabsent. Gonadotropin releasing hormone is apparently of broadoccurrence in fishes; its hypophysiotropic activity is wellestablished for several teleostean species. Anatomical relationshipsbetween brain and pituitary are particularly varied among elasmobranchs.Investigations involving additional elasmobranch representatives,as well as other fishes, are needed before generalizations canbe made. Widespread extrahypothalamic distribution of hypophysiotropicpeptides in lower vertebrates and neurotransmitter (or related)functions of neurones containing these peptides provide a basisfor proposals concerning evolution of hypothalamic control ofthe pituitary gland.     

Cell elongation in the soybean root: The influence of inhibitors of RNA and protein biosynthesis

A possible requirement for RNA and protein synthesis duringcell elongation of intact seedling tissue was studied usingthe soybean seedling foot with the elongating zone being delineatedby India ink marks at 2 and 7 mm back of the root tip. In contrastto most excised plant tissues, there was marked net synthesisof RNA and protein during cell elongation of the intact root.AD and CH were potent inhibitors of cell elongation in the soybeanroot. CH essentially eliminated protein synthesis, whether measuredby net accumulation of protein or by ¹⁴C-leiicine incorporation,while completely inhibiting cell elongation after a short lag.AD, on the other hand, only partially inhibited protein synthesiswhile causing almost total inhibition of cell elongation aftera lag. The capacity of the tissue to synthesize protein in thepresence of AD was correlated with the maintenance of functionalpolyribosomes, thus suggestive that m-RNA associated with theregulation of cell elongation is more unstable (i.e., a shortermean life) than total root m-RNA. FU did not inhibit cell elongation,protein synthesis or the level of functional polyribosomes.The requirement for RNA synthesis during cell elongation ofthe seedling root, as in excised plant tissues, appears to berestricted to the AMPrich species of RNA presumed to be m-RNA. ¹This research was supported by NIH grant GM 10157. ²Purdue University AES paper No. 3359. ³Present address: Dept. of Botany, National Taiwan University,Taipei, Taiwan.     

Performance of sampling strategies in the presence of known spatial patterns

In crop protection and ecology accurate and precise estimates of insect populations are required for many purposes. The spatial pattern of the organism sampled, in relation to the sampling scheme adopted, affects the difference between the actual and estimated population density, the bias, and the variability of that estimate, the precision. Field monitoring schemes usually adopt time‐efficient sampling regimes involving contiguous units rather than the most efficient for estimation, the completely random sample. This paper uses spatially‐explicit ecological field data on aphids and beetles to compare common sampling regimes. The random sample was the most accurate method and often the most precise; of the contiguous schemes the line transect was superior to more compact arrangements such as a square block. Bias depended on the relationship between the size and shape of the group of units comprising the sample and the dominant cluster size underlying the spatial pattern. Existing knowledge of spatial pattern to inform the choice of sampling scheme may provide considerable improvements in accuracy. It is recommended to use line transects longer than the grain of the spatial pattern, where grain is defined as the average dimension of clusters over both patches and gaps, and with length at least twice the dominant cluster size.     

Within plant interspecific competition does not limit the highly invasive thrips,Frankliniella occidentalis in Florida

1. Species invasions are often linked to reductions in biodiversity, and competitive superiority is often cited as the main reason for the success of an invasive species. Although invaded ecosystems are often examined, few have studied areas in which an invasive species has failed to successfully invade. 2. The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is a damaging pest and tospovirus vector that has invaded most of the world, and competitive superiority is considered one of the main reasons for this species' success. 3. However, a recent study demonstrated that competition between larval F. occidentalis and a native thrips species may be limiting F. occidentalis abundance in much of the eastern United States. Frankliniella occidentalis also has a limited abundance in central and southern Florida, which is dominated by the endemic F. bispinosa (Morgan). The potential for interspecific competition to limit F. occidentalis abundance in Florida was assessed. 4. The effects of competition between F. occidentalis and F. bispinosa on adult reproduction on a common host (Capsicum annuum L.) were quantified, using a response surface experimental design and a combination of linear and non‐linear competition models. 5. Evidence of symmetric competition between these thrips species was found, but contrary to expectations, F. occidentalis reproduced more in dense interspecific populations than F. bispinosa. These results suggest that, unlike most of the eastern US, interspecific competition is not important in limiting F. occidentalis abundance in central and southern Florida.