Photosynthetic Changes in the Inducible CAM Plant Sedum telephium L Following the Imposition of Water Stress. II. Changes in the Activity of Phosphoenolpyruvate Carboxylase

In Sedum telephium, the switch from a weak-CAM to a full-CAMmode of photosynthesis in response to water stress, is accompaniedby a marked increase in the activity of the enzyme phosphoenolpyruatecarboxylase (PEPC) during the dark period of a diurnal cycle.Fractionation of the enzyme by non-denaturing polyacrylamidegel electrophoresis gives two active species; the activity ofthe more mobile species increases with the switch into a full-CAMmode of photosynthesis. Fractionation of the enzyme by denaturing electrophoresis andby gel filtration indicates that the molecular species particularlyactive in CAM is a monomeric protein, whilst the other readilyobservable species is a dimer. Sedum telephium, CAM, water stress, phosphoenolpyruvate carboxylase, night-time activation dimer, monomer     

Tropical zooplankton: phenotypic and spatial analyses of polymorphic Ceriodaphnia cornuta (Crustacea: Cladocera)

SUMMARY. 1. The predation-balanced polymorphism of Ceriodaphnia cornuta (a small-eyed horned form and a large-eyed unhorned form) in Lake Gatun, Panama, is re-examined approximately 12 years after the original study (Zaret, 1972a). We consider morphological variability, distribution and fecundity of the population inhabiting Laboratory Bay.
2. The two morphological characters emphasized by Zaret, eye size and horn length, are studied in detail. The C. cornuta population exhibits wide, yet continuous variation; coefficients of variation are 28% or eye diameter and range from 114% to 136% for horn and spine lengths. Phenotypic variation in the population is characterized by: (1) negative correlations between spine lengths and eye size; (2) positive correlations among spines from different body regions (anterior horn, lateral spines, and tail spine); and (3) weak correlations between body length and spine lengths.
3. While the horned and unhorned forms continue to coexist in Laboratory Bay. the frequencies of horned forms have increased in the open water areas since the time of the original study. The increase in horned form frequencies may be due to changes in fish predation intensity. Did vertical distribution patterns of the forms remain very similar to each other.
4. In contrast to previous laboratory findings, egg ratio calculations from field samples revealed similar clutch sizes for the horned and unhorned forms. The egg development time of the horned form, however, was found to be approximately 30% longer than that of the unhorned form, perhaps because of increased moulting difficulties produced by the presence of spines.     

Enzymic Activities Associated with the Ability of Aerial and Submerged Forms of Littorella uniflora (L.) Aschers to perform CAM

ABSRACT: Groenhof, A. C, Smirnoff, N. and Bryant, J. A. 1988. Enzymicactivities associated with the ability of aerial and submergedforms of Littorella uniflora (L.) Aschers to perform CAM.—J.exp. Bot. 39: 353-361. The submerged form of Littorella uniflora shows a full CAM modeof photosynthesis as shown by diel acid fluctuations and elevatedactivities (in comparison to non-submerged leaves) of the enzymesphosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme.Non-submerged plants exhibit no diel fluctuations of acidityand no changes in activity of NADP-malic enzyme or PEPC. PEPCactivity is low and NADP-malic enzyme is not detectable. Furthercharacterization of PEPC extracted from submerged plants duringthe light and dark periods of a diel cycle shows that the enzymeextracted in the dark is more active. In addition, the enzymeshows a decrease in K_m (PEP) and an increase in V_max in thepresence of glucose-6-phosphate, whilst in the presence of malateK_m (PEP) is increased and V_max decreased; this response to malateis only observed in the light and at pH 7.2. Molecular weightdeterminations using a Sephacryl S-300 column show that theenzyme extracted from plants during the dark period has an apparentmol. wt. of 375 KDa and the enzyme extracted from plants duringthe light period has an apparent mol. wt. of 307 KDa. Key words: Littorella uniflora (shoreweed), Crassulacean acid metabolism, PEP carboxylase, malic enzyme     

The Effects of Temperature on 86Rb Uptake by Two Species of Chlamydomonas (Chlorophyta, Chlorophyceae)

⁸⁶Rb uptake was examined in two species of unicellular greenalgae, Chlamydomonas nivalis isolated from snow, and a cellwall-less mutant of the temperate freshwater Chlamydomonas reinhardii.In C. reinhardii cells grown at 20°C and cooled rapidlyto 0°C, ⁸⁶Rb uptake was abolished. Cells cooled rapidlyto –5°C in the absence of ice accumulated ⁸⁶Rb veryrapidly but the time course of this uptake suggested non-selectiveaccumulation through a damaged plasmalemma. Cells grown at 8°Cwere viable, able to divide and motile; they showed no signsof cold-shock and ⁸⁶Rb uptake, albeit slow, was measurable at–5°C in the absence of extracellular ice. Cells ofC. nivalis grown at 20°C were damaged at sub-zero temperaturesalthough they did show an enhanced ⁸⁶Rb uptake at 0°C. Cellsgrown at 5°C were able to accumulate ⁸⁶Rb from media undercooledto -5°C in the absence of extracellular ice, and again showedenhanced uptake at 0°C. The process of acclimation to lowtemperature appears to differ in the two species. Key words: Chlamydomonas, temperature, ⁸⁶Rb uptake, membrane     

Development of a new piosphere in arid chenopod shrubland grazed by sheep. 2. Changes to the vegetation

The initial development of a piosphere created by stocking a new watering trough with ca. 200 sheep at a near-pristine site in arid chenopod shrubland near Whyalla, South Australia, was studied. The following variables were monitored for up to 8 years using fenced plots as controls:shrub mortality; forage biomass and defoliation of the dominant shrubs and short-lived grasses; linear dimensions of Rhagodia ulicina; the density of short-lived species; and the invasion of exotics. Patterns in these variables were looked for which reflected the attenuation of stocking pressure with distance from water; that is, ‘piosphere patterns’. Piosphere patterns were detected for many variables. Those with values increasing towards the trough were mortality of the main forage shrub Atriplex vesicaria. the density of three short-lived taxa, viz. the bassias (Sclerolaena obliquicuspis, S. patenticuspis. Maireana sclerolaenoides), Tetragonia tetragonoides, and annual Zygophyllum spp. (Z. crenatum. Z. ovatum), and growth of short-lived grass species when sheep stocking pressure was low. Variables with values decreasing towards the trough were grass biomass, percentage of A. vesicaria individuals in flower, the percentage foliation of A. vesicaria and of the forage shrub Maireana sedifolia, and shrub forage biomass of A. vesicaria and M. sedifolia. The rate at which these patterns were registered varied. After only 3 months, a piosphere pattern was evident for grass biomass. Marrubium vulgare, an introduced weed, invaded the trough site within the first 6 months. Almost no A. vesicaria shrubs had died after 2 years, but after 8 years there was a marked piosphere pattern in their mortality. The piosphere patterns in forage biomass and defoliation of A. vesicaria and M. sedifolia became more pronounced with time. Some variables did not exhibit a piosphere pattern, such as the dimensions of the shrub R. ulicina and the densities of successive cohorts of the forb Erodium spp., even though there was an overall effect of grazing in reducing their values. There was no mortality of two dominant shrubs, M. sedifolia and R. ulicina, during the first 8 years.     

Leaf anatomy in a Venezuelan montane forest

A survey has been made of the leaf anatomy of 43 woody species in three montane vegetation types of Isla Margarita, Venezuela, differing in levels of cloud cover (and hence wetness) and exposure to wind. The only character that varies significantly with increasing probability of periodic drought is specific leaf area, which can be related to the higher proportion of deciduous species in the drier habitat. Leaves become significantly smaller and thicker with increased exposure to wind, and have thicker outer epidermal walls and cuticles. Most characters, including stomatal density, guard cell length, palisade: non-palisade ratios, and the incidence of such features as hypodermis, sclerenchyma, crystals and secretory structures are highly variable and show few or no trends according to habitat. Some of these characters are apparently more constrained by phylogeny than by immediate ecological circumstances. As a contribution to the continuing debate on the significance of xeromorphy in tropical montane forest leaves, it is suggested that low light levels due to cloud cover can be discounted as an important determining factor; in the case of the Isla Margarita vegetation, the need to avoid excessive leaf temperatures may be of greater significance.     

Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Short-term (< 12 h) uptake experiments were conducted with6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K⁺ andNH₄⁺. At equal solution concentrations (0.5 mol m^–3) netK⁺ uptake was inhibited 30–35% by NH₄⁺ and NH₄⁺ uptakewas decreased 9–24%. Removal of NH₄⁺ resulted in completerecovery in K⁺ uptake rate, but NH⁴⁺ uptake rate did not recoverwhen K⁺ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m^–3. The relative effectof K⁺-NH₄⁺ interactions was not altered when Cl- was replacedwith SO₄^2–, but the magnitudes of the uptake rates wereless in the absence of Cl-. The V_max for NH₄⁺ uptake was reducedfrom 128 to 105 µmol g^–1 dry wt. h^–1 in thepresence of 0.5 mol m^–3 K⁺ and the K_m for NH₄⁺ doubledfrom 12 to 27 mmol m^–3 in the presence of K⁺. The resultsof these K⁺-NH₄⁺ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K⁺ coupled to NH₄⁺influx via an antiporter cannot be ruled out as contributingto the decrease in net K⁺ uptake. Key words: Nicotiana tabacum, K⁺, NH₄⁺, Uptake interactions     

Fusion <Emphasis Type="Italic">in vitro</Emphasis> of Membranes from Animal Cells

THE biological implications of membrane fusion are of general importance and several publications deal with the theoretical and practical aspects of this phenomenon^1–3. In this report, we describe experiments on the in vitro association or fusion of membranes. Because fusion was observed under physiological conditions it is possible that similar interactions also occur in vivo.