Discrepancies between water potential isotherm measurements on Pinus ponderosa seedling shoots: xylem hysteresis and apoplasmic osmotic potentials *

Abstract. Sap expression, air drying and a combined technique were used to measure the water potential isotherm of Pinus ponderosa Laws, seedling shoots with the pressure chamber. Discrepancies between water relations parameters derived from these techniques can be partially explained by air entry into air drying tissues, hysteresis in the xylem water potential isotherm and dilution of apoplasmic solutes during sap expression.     

Derivation of Plant Cell Wall Water Content by Examination of the Water-Holding Capacity of Membrane-Disrupted Tissues10

Plant cell lumina are several orders of magnitude larger thancell wall pores. If the membranes of the plant sample are disruptedand the tissue dried out, a value should be reached at whichthe cell lumina have drained but the cell walls remain relativelyhydrated. The water content of membrane-disrupted tissues atlow may, therefore, serve as a good approximation of plantcell wall water content (CW). The relationship between and water content of membrane-disruptedtissues was measured for four grass spaces over the range of0 to –40 MPa. It was found that most water loss occurredabove a of – 10 MPa. CW was estimated from the watercontent of membrane-disrupted tissues at lower and it was foundthat there was very little intra-or intraspecific variabilityin CW estimates when water content was expressed as a percentagedry weight of the tissue. This is in contrast to the high variabilityin CW estimated from the analysis of inverse water potential—watercontent (IP—WC) curves measured with the thermocouplepsychrometer. Applications of the membrane-disruption methodof estimating CW of plant tissue are discussed. Key words: Cell wall water, thermocouple psychrometer, membrane-disruption     

Partitioning Water Potential and Specific Salt Effects on Seed Germination of Four Grasses

In this study water potential (     

The ecology and phylogeny of oomycete infections in Asplanchna rotifers

1. Recently, the potential for parasites to influence the ecology and evolution of their zooplankton hosts has been the subject of increasing study. However, most research to date has focussed on Daphnia hosts, and the potential for parasites to influence other zooplankton taxa remains largely unstudied. 2. During routine sampling of zooplankton in a eutrophic lake, we observed that the rotifer Asplanchna girodi was often infected with a parasitic oomycete. Epidemics of this parasite occurred frequently, with three separate events in a single year. Prevalence at peak infection ranged from 29 to 41% and epidemics lasted from 17 to 56 days. Our data indicate that high densities of the host population are required for epidemics to occur. 3. Our morphological and molecular analyses suggest that this parasite is in the genus Pythium. Most Pythium spp. are plant pathogens, but our study supports recent work on Daphnia, suggesting that Pythium spp. are also important parasites of zooplankton. 4. As the parasite in this study was recalcitrant to cultivation, we developed an alternative method to verify its identity. Our approach used quantitative PCR to show that the ribosomal sequences identified increased with increasing density of infected hosts and, thus, were associated with the parasite. This approach should be generally applicable to other plankton parasites that are difficult to cultivate outside their hosts. 5. Infections significantly reduced host fecundity, lifespan and population growth rate. As a result of the virulence of this parasite, it is likely to influence the population ecology and evolution of its Asplanchna host, and may be a useful model system for studies on host–parasite coevolutionary dynamics.     

Effect of Matric-Priming Duration and Priming Water Potential on Germination of Four Grasses

In this study, a matric-potential control system was used todetermine the effect of matric-priming duration and primingwater potential on the germination response of Bouteloua curtipendula(Michx.) Torr., Cenchrus ciliaris L., Eragrostis lehmannianaNees, and Panicum coloratum L. Seeds were primed at water potentialsof –1·5 to –7·7 MPa for up to 14 d.Optimum germination generally occurred in treatments primedat high water potential for the shortest period. Germinationof seeds primed at lower water potential and for longer periodsexhibited a negative germination response relative to the control.Seeds were not redried after the priming treatment. Seed-wateruptake measurements suggest that a reduction in the lag timeof imbibition accounted for at least some germination-rate enhancementin the positive-priming treatments Key words: Germination, matric-priming, imbibition