Comparison of the effect of excessive light on chlorophyll fluorescence (77K) and photon yield of O2 evolution in leaves of higher plants

High-light treatments (1750–2000 mol photons m^–2 · s^–1) of leaves from a number of higher-plant species invariably resulted in quenching of the maximum 77K chlorophyll fluorescence at both 692 and 734 nm (F _M, 692 and F _M, 734). The response of instantaneous fluorescence at 692 nm (F _O, 692) was complex. In leaves of some species F _O, 692 increased dramatically in others it was quenched, and in others yet it showed no marked, consistent change. Regardless of the response of F _O, 692 an apparently linear relationship was obtained between the ratio of variable to maximum fluorescence (F _V/F _M, 692) and the photon yield of O₂ evolution, indicating that photoinhibition affects these two variables to approximately the same extent. Treatment of leaves in a CO₂–free gas stream containing 2% O₂ and 98% N₂ under weak light (100 mol · m^–2 · s^–1) resulted in a general and fully reversible quenching of 77K fluorescence at 692 and 734 nm. In this case both F _O, 692 and F _M, 692 were invariably quenched, indicating that the quenching was caused by an increased non-radiative energy dissipation in the pigment bed. We propose that high-light treatments can have at least two different, concurrent effects on 77K fluorescence in leaves. One results from damage to the photosystem II (PSII) reaction-center complex and leads to a rise in F _O, 692; the other results from an increased non-radiative energy dissipation and leads to quenching of both F _O, 692 and F _M, 692 This general quenching had a much longer relaxation time than reported for pH-dependent quenching in algae and chloroplasts. Sun leaves, whose F _V/F _M, 692 ratios were little affected by high-light exposure in normal air, suffered pronounced photoinhibition when the exposure was made under conditions that prevent photosynthetic gas exchange (2% O₂, 0% CO₂). However, they were still less susceptible than shade leaves, indicating that the higher capacity for energy dissipation via photosynthesis is not the only cause of their lower susceptibility. The rate constant for recovery from photoinhibition was much higher in mature sun leaves than in mature shade leaves, indicating that differences in the capacity for continuous repair may in part account for the difference in their susceptibility to photoinhibition.Abbreviations and symbols kDa kilodalton - LHC-II light-harvesting chlorophyll-protein complex - PFD photon flux density (photon fluence rate) - PSI, PSII photosystem I, II - F _O, F _M, F _V instantaneous, maximum, variable fluorescence emission - absorptance - _a photon yield of O₂ evolution (absorbed light) C.I.W.-D.P.B. Publication No. 925     

The pollination ecology of two nectarless Cimicifuga sp. (Ranunculaceae) in North America

The pollination ecology of Cimicifuga rubifolia Kearney and C. elata Nutt., endemic nectarless species in Tennessee and the Pacific Northwest, respectively, was studied. Both species are pollinated by bumblebee workers, which run over the flowers pressing their bodies against them. Large syrphids act as copollinators. Extremely high pollen/ovule ratios are explained as optimal sex allocation in response to pollen-tolling by the pollen-collecting pollinators. Facultative geitonogamy in the self-compatible C. elata seems to be evolved in response to very low pollinator density in the habitat. Because of the pollinator behavior, geitonogamy virtyally always precedes xenogamy. C. rubifolia encounters higher pollinator visit frequency than C. elata , and self-incompatibility prevents selfing. Since it is nectarless, C. rubifolia is inferior in competion for bumblebees with most other entomogamous plants. It appears to have a commensalistic relationship with its constant associates, Impatiens pallida (Balsaminaceae) and/or Polymnia canadensis (Asteraceae), which attract large numbers of bumblebees. C. rubifolia is pollinated by bumblebees encountering them when scouting around the nectar-flower stand. This conclusion is based on: 1) Negative correlation for distance between nectar-flowers and Cimicifuga and the fruit set of the latter, 2) non-skewed flowering period, 3) extremely extended flowering on both the ramet and population levels, only limited by flowering period of nectar-flower associates and by pollinator abundance, and 4) fruit set rates independent of visual display size, and also of flower density in the habitat (i.e., the pollination rate was directly proportional to the density).     

Insulin-like growth factor I enhances the formation of type I collagen in hydrocortisone-treated human osteoblasts

We have studied the effect of insulin-like growth factor I (IGF-I) on the formation of osteocalcin and type I collagen in isolated human osteoblasts. IGF-I at and above 0.1 nM stimulated the formation of type I collagen as measured by the type I procollagen carboxyterminal peptide (PICP), in human osteoblasts, incubated for 72 hrs in serumfree conditions. The secretion of osteocalcin was not affected by IGF-I while 1,25(OH)₂ vitamin D₃ significantly enhanced the formation of osteocalcin. When human osteoblast-like cells were incubated with hydrocortisone (1 M), a significant decrease in the release of both PICP and osteocalcin was seen. Addition of IGF-I to human osteoblasts also treated with hydrocortisone normalized the PICP-formation but did not affect the suppressed osteocalcin-formation. These data indicate that IGF-I reverses selective effects of hydrocortisone on bone.     

Paraheliotropic leaf movement in Siratro as a protective mechanism against drought-induced damage to primary photosynthetic reactions: damage by excessive light and heat

Damage to primary photosynthetic reactions by drought, excess light and heat in leaves of Macroptilium atropurpureum Dc. cv. Siratro was assessed by measurements of chlorophyll fluorescence emission kinetics at 77 K (-196°C). Paraheliotropic leaf movement protected waterstressed Siratro leaves from damage by excess light (photoinhibition), by heat, and by the interactive effects of excess light and high leaf temperatures. When the leaves were restrained to a horizontal position, photoinhibition occurred and the degree of photoinhibitory damage increased with the time of exposure to high levels of solar radiation. Severe inhibition was followed by leaf death, but leaves gradually recovered from moderate damage. This drought-induced photoinhibitory damage seemed more closely related to low leaf water potential than to low leaf conductance. Exposure to leaf temperatures above 42°C caused damage to the photosynthetic system even in the dark and leaves died at 48°C. Between 42 and 48°C the degree of heat damage increased with the time of exposure, but recovery from moderate heat damage occurred over several days. The threshold temperature for direct heat damage increased with the growth temperature regime, but was unaffected by water-stress history or by current leaf water status. No direct heat damage occurred below 42°C, but in water-stressed plants photoinhibition increased with increasing leaf temperature in the range 31–42°C and with increasing photon flux density up to full sunglight values. Thus, water stress evidently predisposes the photosynthetic system to photoinhibition and high leaf temperature exacerbates this photoinhibitory damage. It seems probable that, under the climatic conditions where Siratro occurs in nature, but in the absence of paraheliotropic leaf movement, photoinhibitory damage would occur more frequently during drought than would direct heat damage.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosyntem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - PLM paraheliotropic leaf movement; all data of parameter of variation are mean ± standard error     

Bromine oxidation of 1,2-O-isopropylidene-α-d-Glucofuranose and sucrose

Sucrose and $\text{1,2-O-}\text{isopropylidene-α-}\text{d}\text{-glucofuranose}$ (1) were oxidised with bromine in aqueous solution at pH 7 and room temperature. The resulting keto derivatives were converted into their more-stable O-methyloximes, which were characterised by spectroscopic and chromatographic methods. Oxidation of 1 occurred at C-3 and C-5, with a preference for C-5. In the sucrose derivatives isolated after oxidation, those having a keto group in the glucopyranosyl moiety preponderated. The axial fructofuranosyl aglycon protects position 3 in the glucopyranosyl group and oxidation occurs only at C-2 and C-4. Small amounts of sucrose oxidised at C-3 in the fructofuranosyl moiety were also found.     

Isolation and characterization of the dut gene of Escherichia coli. II. Restriction enzyme mapping and analysis of polypeptide products

Restriction endonuclease mapping of previously constructed dut plasmids has been carried out using the enzymes PvuI, PvuII and SacI. Various dut plasmids were also tested in the "maxicell" protein-synthesizing system. They all show two protein bands in common, one of Mr 16000 in agreement with the size previously reported for the purified dUTPase subunit (Shlomai and Kornberg, 1978). With the information obtained the structural gene for dUTPase can be assigned to a 950-bp SacI-PvuII fragment of the E. coli genome. Studies, described in the preceding paper, on the overproduction of dUTPase by bacterial strains carrying different dut plasmids strongly suggest that the dut gene is transcribed in the direction from the SacI site towards the PvuII site and that the SacI site is located within the dut control region. The second protein band observed in the "maxicell" experiments has an Mr of 23500. Its identity is unknown but it may represent a precursor of dUTPase or the product of a separate gene located between dut and pyrE.     

Hydrocarbon chain dynamics in lipid bilayers

A model is proposed for hydrocarbon chain dynamics in lipid bilayers. In the upper and middle parts of the chain all motion occurs by concerted rotations around at least two carbon carbon bonds at a time, preserving a structural with kinks (that is gauche^±trans gauche^? conformations) as the only deviations from the all-trans chain. At the end, independent rotations around carboncarbon bonds play a larger and larger part. This gives a reasonable interpretation of deuterium NMR data.