Frictional Potential Losses and Total Water Potential in Plants: a Re-evaluation

A theoretical treatment is given of isothermal potential lossesdue to frictional resistances against water flow in plants.It becomes apparent that in the past two main difficulties havebeen widely ignored: the proper choice of dimensions for fluxesand resistances deserves consideration, and the original vanden Honert concept seems to be especially prone to misinterpretation.A revised equation is presented for total water potential ata certain point in the plant, and some implications of thisapproach are outlined.     

A latitudinal gradient in climate effects on seabird demography: results from interspecific analyses

For an understanding of the effect of climate change on animal population dynamics, it is crucial to be able to identify which climatologic parameters affect which demographic rate, and what the underlying mechanistic links are. An important reason for why the interactions between demography and climate still are poorly understood is that the effects of climate vary both geographically and taxonomically. Here, we analyse interspecifically how different climate variables affect the breeding success of North Atlantic seabird species along latitudinal and longitudinal gradients. By approaching the problem comparatively, we are able to generalize across populations and species. We find a strong interactive effect of climate and latitude on breeding success. Of the climatic variables considered, local sea surface temperatures during the breeding season tend to be more relevant than the North Atlantic Oscillation (NAO). However, the effect of NAO on breeding success shows a clear geographic pattern, changing in sign from positive in the south to negative in the north. If this interaction is taken account of, the model explains more variation than any model with sea surface temperature. This superiority of the NAO index is due to its ability to capture effects of more than one season in a single parameter. Mechanistically, however, several lines of evidence suggest that sea surface temperature is the biologically most relevant explanatory variable.     

Mating systems and territory in Lapwings Vanellus vanellus

In a population of Lapwings Vanellus vanellus studied for 5 years near Bergen, southwester Norway, 23–41% of the males were polygynous, having two and sometimes three mates, whereas 50–77% were monogamous and 0–21% remained unmated. Bachelors held territories in or immediately adjacent to the study area each of the years. As predicted from the Polygyny Threshold model, primary females generally laid eggs earlier than monogamous females, but the overlap in laying dates was substantial between these two classes. The number of females and start of breeding on the territories were analysed in relation to nine variables pertaining to safety from predation and to food. Of these, territory size correlated most consistently with number of females (positively) and the laying of first egg (negatively). Numbers of females observed were not significantly different from numbers expected from territory size in 4 out of 5 years; however, earlier start of breeding on large territories indicated that females did not settle according to the Neutral Mate Choice model. The Female-biased Sex Ratio hypothesis was refuted, unmated territorial males being available throughout the breeding seasons.     

Leaf growth dynamics of two congener gymnosperm tree species reflect the heterogeneity of light intensities given in their natural ecological niche

Chamaecyparis obtusa var. formosana and Chamaecyparis formosensis are congener gymnosperm tree species native to Taiwan cloud forests; occupying different niches there. While the seedlings of C. formosensis occur predominantly under bright conditions in large forest gaps, seedlings of C. obtusa var. formosana are mainly found below the canopy of mature forests or in small gaps. It is well known that congener species occupying different niches typically differ in several ecophysiological and morphological traits, but the differences in growth dynamics of such species are still totally unclear, as the diurnal growth dynamics of gymnosperm leaves have not been investigated before. Modern methods of digital image sequence processing were used in this study to analyse the leaf growth dynamics of the two species. We found that both species show similar base–tip gradients and pronounced diurnal growth rhythms with maxima in the evening. Differences between the two species concerning their growth dynamics correlated closely with their ecological amplitudes and abundances. Chamaecyparis obtusa var. formosana grew faster than C. formosensis in low light intensity, typical for closed-canopy situations, and reacted quickly by increasing or decreasing growth rate when light intensity changed within a range typically found below small canopy gaps. In contrast to this, C. formosensis grew better in light intensities typical for open vegetation situations, but reacted slower towards changes of light intensity. Based on those results, the hypothesis can be developed that fluctuations of leaf growth dynamics reflect heterogeneities of the light environment within the niche occupied by a given species.     

Leaf Discs or Press Saps? A Comparison of Techniques for the Determination of Osmotic Potentials in Freeze-Thawed Leaf Material

Two standard methods for the determination of osmotic potentialsin plant organs, thermocouple hygrometry of press saps and offreeze-thawed leaf discs, were compared on leaves of ten species.Strictly parallel samples invariably showed lower (more negative)osmotic potentials for the leaf discs. Evidence is presentedthat osmotically-active solutes are not completely extractedduring the preparation of press saps and that the discs contain,on average, a smaller amount of apoplastic water than the saps.Hydrolysis of macromolecules in the discs was not responsiblefor the differences. The presence of the cutinized epidermisresults in a comparatively long equilibration time for leafdiscs, which may be reduced by appropriate measures. Key words: Thermocouple psychrometry, vapour pressure equilibration, leaf discs, press saps, solute extraction     

Copepod life cycle adaptations and success in response to phytoplankton spring bloom phenology

In a seasonal environment, the timing of reproduction is usually scheduled to maximize the survival of offspring. Within deep water bodies, the phytoplankton spring bloom provides a short time window of high food quantity and quality for herbivores. The onset of algal bloom development, however, varies strongly from year to year due to interannual variability in meteorological conditions. Furthermore, the onset is predicted to change with global warming. Here, we use a long-term dataset to study (a) how a cyclopoid copepod, Cyclops vicinus , is dealing with the large variability in phytoplankton bloom phenology, and (b) if bloom phenology has an influence on offspring numbers. C. vicinus performed a two-phase dormancy, that is, the actual diapause of fourth copepodid stages at the lake bottom is followed by a delay in maturation, that is, a quiescence, within the fifth copepodid stage until the start of the spring bloom. This strategy seems to guarantee a high temporal match of the food requirements for successful offspring development, especially through the highly vulnerable naupliar stages, with the phytoplankton spring bloom. However, despite this match with food availability in all study years, offspring numbers, that is, offspring survival rates were higher in years with an early start of the phytoplankton bloom. In addition, the phenology of copepod development suggested that also within study years, early offspring seems to have lower mortality rates than late produced offspring. We suggest that this is due to a longer predator-free time period and/or reduced time stress for development. Hence, within the present climate variability, the copepod benefited from warmer spring temperatures resulting in an earlier phytoplankton spring bloom. Time will show if the copepod's strategy is flexible enough to cope with future warming.     

Alternative Methods of Analysing Water Potential Isotherms: Some Cautions and Clarifications: I. THE IMPACT OF NON-IDEALITY AND OF SOME EXPERIMENTAL ERRORS

In recent years alternative ways have been proposed to transformmeasurements of leaf water potential, , and relative water content,R^*, in order to derive values of osmotic pressure at full turgidityin leaves and shoots, ^o(when 0). Two types of transformationsare usually considered: 1/ versus R^* and versus 1/R^*, and linearregression is used to fit the data in the region where turgoris thought to be zero. It appears that when ^o is estimated bylinear extrapolation of 1/Psi; versus R^* then apoplastic watermight not influence the accuracy of ^o but when the versus \/R^*transformation is used apoplastic water causes an underestimateof ^o. We examine the accuracy of the estimate of ^o obtainedfrom the two transformations when there are random errors in, systematic errors in , and when the osmotic solutions arenon-ideal. The 1/ versus R^* transformation generally producesthe best estimate of ⁰ by linear extrapolation.     

A Diagram for the Description of Water Relations in Plant Cells and Organs

A diagram is described which is based on the ideal relationshipbetween osmotic potential and volume as defined by Boyle's andVan't Hoff's laws. The new graphical presentation differs fromthe related earlier one by Hammel and co-workers in the choiceof relative water content (RWC), a term depending on the volumechanges of the vacuole, as the reciprocally plotted variable.The diagram is well suited for didactic purposes and may haveadvantages for the analysis of experimental data.