Altitudinal changes in the incidence of crassulacean acid metabolism in vascular epiphytes and related life forms in Papua New Guinea

Summary The occurrence of Crassulacean acid metabolism (CAM), as judged from ¹³C values, was investigated in epiphytes and some related plant species at a series of sites covering the approximate altitudinal range of epiphytes in Papua New Guinea. Comprehensive collections were made at each site and the occurrence of water storage tissue and blade thickness was also determined. Some 26% of epiphytic orchids from a lowland rainforest (2–300 m.a.s.l) showed ¹³C values typical of obligate CAM and possessed leaves thicker than 1 mm. A second group of orchids, mostly with succulent leaves, possessed intermediate ¹³C values between -23 and -26% and accounted for 25% of the total species number. Some species of this group may exhibit weak CAM or be facultative CAM plants. The remainder of the lowland rainforest species appeared to be C₃ plants with ¹³C values between -28 and -35%. and generally possessed thin leaves. Obligate CAM species of orchids from a lower montane rainforest (1175 m.a.s.l) comprised 26% of the species total and mostly possessed thick leaves. The remainder of the species were generally thin-leaved with ¹³C values between -26 and -35%. largely indicative of C₃ photosynthesis. Orchids with intermediate ¹³C values were not found in the lower montane rainforest. Obligate CAM appeared to be lacking in highland epiphytes from an upper montane rainforest and subalpine rainforest (2600–3600 m.a.s.l). However the fern, Microsorium cromwellii had a ¹³C value of -21.28%. suggesting some measure of CAM activity. Other highland ferns and orchids showed more negative °¹³C values, up to-33%., typical of C₃ photosynthesis. The highland epiphytic orchids possessed a greater mean leaf thickness than their lowland C₃ counterparts due to the frequent occurrence of water storage tissue located on the adaxial side of the leaf. It is suggested that low daytime temperatures in the highland microhabitats is a major factor in explaining the absence of CAM. The increased frequency of water storage tissue in highland epiphytes may be an adaptation to periodic water stress events in the dry season and/or an adaptation to increased levels of UV light in the tropicalpine environment.     

The relationship between turgor pressure and titratable acidity in mesophyll cells of intact leaves of a Crassulacean-acid-metabolism plant,Kalanchoe daigremontiana Hamet et Perr

Day/night changes in turgor pressure (P) and titratable acidity content were investigated in the (Crassulacean-acid-metabolism (CAM) plant Kalanchoe daigremontiana. Measurements of P were made on individual mesophyll cells of intact attached leaves using the pressure-probe technique. Under conditions of high relative humidity, when transpiration rates were minimal, changes in P correlated well with changes in the level of titratable acidity. During the standard 12 h light/12 h dark cycle, maximum turgor pressure (0.15 MPa) occurred at the end of the dark period when the level of titratable acidity was highest (about 300 eq H⁺·g^-1 fresh weight). A close relationship between P and titratable acidity was also seen in leaves exposed to perturbations of the standard light/dark cycle. (The dark period was either prolonged, or else only CO₂-free air was supplied in this period). In plants deprived of irrigation for five weeks, diurnal changes in titratable acidity of the leaves were reduced (H=160 eq H⁺·g^-1 fresh weight) and P increased from essentially zero at the end of the light period to 0.02 MPa at the end of the dark period. Following more severe water stress (experiments were made on leaves which had been detached for five weeks), P was zero throughout day and night, yet small diurnal changes in titratable acidity were still measured. These findings are discussed in relation to a hypothesis by Lüttge et al. 1975 (Plant Physiol. 56,613-616) for the role of P in the regulation of acidification/de-acidification cycles of plants exhibiting CAM.Abbreviations CAM crassulacean acid metabolism - FW fresh weight - P turgor pressure     

Hydrolysis of dietary flavonoid glycosides by strains of intestinal Bacteroides from humans.

Rutin and quercitrin are hydrolysed to quercetin, and robinin is hydrolysed to kaempferol, by faecal flora from healthy subjects. The enzymes required for these hydrolyses, namely alpha-rhamnosidase and beta-galactosidase, were produced by some strains of Bacteroides distasonis; other strains, however, synthesized beta-glucosidase. The last-named enzyme was also elaborated by Bacteroides uniformis and Bacteroides ovatus. All the enzymes were produced constitutively. A cell-free extract of B. distasonis containing beta-glucosidase displayed an enzymic activity of 1 mumol/10 min per 10 mg of protein.     

The role of laminin and the laminin/fibronectin receptor complex in the outgrowth of retinal ganglion cell axons

Chick embryo retinal ganglion cell (RGC) axons grow to the optic tectum along a stereotyped route, as if responding to cues distributed along the pathway. We showed previously that, in culture, RGCs from embryonic Day 6 retina are responsive to the neurite-promoting effects of the extracellular matrix glycoprotein laminin and that this response is lost by RGCs at a later stage of development. Here we report that, before axon outgrowth is initiated in vivo, laminin, is expressed along the optic pathway at nonbasal lamina sites that are accessible to the growth cones of RGC axons. The distribution of laminin within the pathway is consistent with its localization at the end-feet of neuroepithelial cells that line the route, and it continues to be expressed at these marginal sites during the first week of embryonic development. At later stages, concomitant with the loss of response by RGCs in culture, laminin becomes restricted to basal laminae at the retinal inner limiting membrane and pial surface of the optic pathway. Neurofilament-positive RGC axons bind a monoclonal antibody, JG22, which recognizes the laminin/fibronectin receptor complex, and continue to do so throughout embryonic development. We show that, in vitro, the JG22 antigen expressed by RGCs appears to function as a laminin receptor, by demonstrating that JG22 antibody blocks neurite outgrowth on a substrate of laminin. These findings are consistent with the possibility that laminin defines a transient performed pathway specifically recognized by early RGC growth cones as they navigate toward their central target.     

Genome relationship betweenPsathyrostachys huashanica andP. fragilis (Poaceae)

Hybrids between the Chinese endemic speciesPsathyrostachys huashanica Keng and the SW. Asian speciesP. fragilis (Boiss.)Nevski (all 2n = 14) developed normally but were completely sterile. Meiotic analyses revealed a high chiasma frequency indicating that the two species as well asP. juncea (Fisch.)Nevski share the same basic genome (called N). The hybrid nature of the plants was established through karyotype analysis and Giemsa C-banding.     

Population genetics implications of the premutation hypothesis for the generation of the fragile X mental retardation gene

Summary The population genetics implications of the premutation hypothesis for the generation of the fragile X mental retardation gene are explored. With some broad assumptions, the consequences of the model are that (a) 50% of mothers of probands carry the premutation; (b) 6.5% of mothers of probands receive the premutation from their mothers, 18.9% from their fathers, and 24.6% as a new mutation; (c) the incidence of carriers for the full mutation equals the incidence of affected males, whereas the incidence of carriers for the premutation is 1.35 times the incidence of affected males; (d) assuming mutation rates are equal in eggs and sperm, the mutation rate from normal to premutation alleles is 1.67x10^-4; (e) the expected segregation ratio in sibs of probands is 0.44, which corresponds to observed values. In addition, predictions using the premutation hypothesis of the expected segregation ratio in sibs of mothers of probands fits well with the data of Vogel and coworkers.