Effect of Light Quality (Red:Far-red Ratio) at the Apical Bud of the Main Stolon on Morphogenesis of Trifolium repens L.

A controlled environment experiment investigated whether thered:far-red (R:FR) ratio of light at the apical bud of the mainstolon could alter plant morphogenesis in clonal cuttings ofwhite clover (Trifolium repens L.) The apical bud included theapical meristem, five to six developing leaf primordia withassociated axillary bud primordia and stipules and the firstemerged folded leaf until development was greater than 0·3on the Carlson scale. Three light regimes were imposed on theapical bud by collimating light from R or FR light-emittingdiodes so that the R:FR ratio of light incident at the apicalbud was set at 0·25, 1·6 or 2·1, withoutsignificantly altering photosynthetically active radiation.The effect of these light regimes on white clover seedling growthwas also tested. At a low R:FR ratio seedling hypocotyl and cotyledon lengthswere significantly longer. However, with the cuttings, the lighttreatments did not alter node appearance rate or internode lengthof the main stolon, petiole length, area of leaves or totalshoot dry matter. There was one significant photomorphogeneticresponse in the cuttings, a delay of 0·5 of a phyllochronin the appearance of branches from axillary buds in the lowR:FR ratio treatment relative to the other treatments. Wherebranch appearance was delayed plants had fewer branches. Thisdifference could be ascribed solely to a delay in branch appearanceas there were no significant treatment effects on either theinitiation of axillary bud primordia within the apical bud,the probability of branching or on the rate of growth of branchesafter appearance. Because treatment of the apical bud inducedonly one of the many previously observed responses of whiteclover to a decrease in the R:FR ratio of light, we concludethat other plant organs must also sense the quality of incidentlight.Copyright 1994, 1999 Academic Press White clover, Trifolium repens, apical bud, light quality, red:far-red ratio, light-emitting diode, branching, axillary buds, photomorphogenesis     

Fine‐scale Population Genetic Structure of Two Dioecious Indian Keystone Species,Ficus hispida and Ficus exasperata (Moraceae)

Although Ficus (Moraceae) is a keystone plant genus in the tropics, providing resources to many frugivorous vertebrates, its population genetic structure, which is an important determinant of its long‐term survival, has rarely been investigated. We examined the population genetic structure of two dioecious fig species (Ficus hispida and Ficus exasperata) in the Indian Western Ghats using co‐dominant nuclear microsatellite markers. We found high levels of microsatellite genetic diversity in both species. The regression slopes between genetic relationship coefficients (f_ij) and spatial distances were significantly negative in both species indicating that, on average, individuals in close spatial proximity were more likely to be related than individuals further apart. Mean parent–offspring distance (σ) calculated using these slopes was about 200 m in both species. This should be contrasted with the very long pollen dispersal distances documented for monoecious Ficus species. Nevertheless, overall population genetic diversity remained large suggesting immigrant gene flow. Further studies will be required to analyze broader scale patterns.     

dam methylation and the initiation of DNA replication on oriC plasmids

Summary Plasmid DNA containing the replication origin of the Escherichia coli chromosome (oriC) has been shown to be inefficient as a template for DNA synthesis in vitro when isolated from dam mutants. here, we extend this study to hemimethylated oriC plasmids and to replication in dam-3 mutant enzyme extracts. The results show that: (1) hemimethylated oriC plasmids replicate with the same low efficiency as nonmethylated DNA; (2) DNA synthesis starts at oriC regardless of the methylated state of the template; (3) replication in dam-3 enzyme extracts is inefficient because this strain is deficient in DnaA protein; and (4) consistent with this observation, the copy number of the oriC plasmid pFH271 is reduced in the dam-3 mutant. However, we have found that low DnaA protein levels in dam-3 mutants are not sufficient to explain the reduced transformation efficiency of oriC plasmids. We suggest that there must exist in vivo inhibitory factors not present or present in low quantities in vitro which specifically recognize the hemimethylated or nonmethylated forms of the oric region.     

Cell swelling induced by the permeant molecules urea or glycerol induces immediate high amplitude thyrotropin and prolactin secretion by perifused adenohypophyseal cells

The permeant molecules, urea and glycerol, evoked a prompt secretory burst of TSH and PRL when added to the extracellular medium of acutely dispersed anterior pituitary cells. Secretion of both hormones was proportional to the concentration of urea or glycerol between 26 and 104 mM (r greater than 0.89, P less than 0.001). Equivalent concentrations of the impermeant molecule, mannitol, did not induce secretion. The acute TSH and PRL secretory responses to TRH, hyposmolarity, and permeant molecules were qualitatively indistinguishable. These data support our hypothesis that cell swelling and resultant plasmalemma expansion is a potent inducer of hormone secretion. Since the secretory response to permeant molecules was not reduced in a Ca2+-free medium containing 0.1 mM EGTA, an increase in Ca2+ transport across the plasmalemma to raise cytosol Ca2+ concentration does not appear involved.     

Influence of environmental factors on 2,4-dichlorophenoxyacetic acid degradation by Pseudomonas cepacia isolated from peat

A Pseudomonas cepacia, designated strain BRI6001, was isolated from peat by enrichment culture using 2,4-dichlorophenoxyacetic acid (2,4-D) as the sole carbon source. BRI6001 grew at up to 13 mM 2,4-D, and degraded 1 mM 2,4-D at an average starting population density as low as 1.5 cells/ml. Degradation was optimal at acidic pH, but could also be inhibited at low pH, associated with chloride release from the substrate, and the limited buffering capacity of the growth medium. The only metabolite detected during growth on 2,4-D was 2,4-dichlorophenol (2,4-DCP), and degradation of the aromatic nucleus was by intradiol cleavage. Growth lag times prior to the on-set of degradation, and the total time required for degradation, were linearly related to the starting population density and the initial 2,4-D concentration. BRI6001, grown on 2,4-D, oxidized a variety of structurally similar chlorinated aromatic compounds accompanied by stoichiometric chloride release.     

Cytogenetic analysis of 400 sperm from three translocation heterozygotes

Summary Sperm chromosome complements were studied in three men who carried reciprocal translocations. A total of 400 sperm were karyotyped after in vitro penetration of hamster eggs: 217 sperm from t(2;9) (q21;p22), 164 from t(4;6) (q28;p23) and 19 from t(7;14) (q21;q13). All possible 22 and 31 meiotic segregations were observed for t(2;9) and t(4;6); for t(7;14) only 22 segregations were observed. For alternate segregations, the number of normal sperm was not significantly different from the number of sperm carrying a balanced form of the translocation in any of the translocations, as theoretically expected. The percentage of sperm with an unbalanced form of the translocation was 57% for t(2;9), 54% for t(4;6) and 47% for t(7;14). There was no evidence for an interchromosomal effect in any of the translocations since the frequencies of numerical abnormalities (unrelated to the translocation) were within the normal range of control donors. The frequencies of X- and Y-bearing sperm did not differ significantly from 50%. Results from a total of 17 reciprocal translocations studied by sperm chromosomal analysis were reviewed.     

Intron sequence and structure requirements for tRNA splicing in Saccharomyces cerevisiae

Predicted single-stranded structure at the 3' splice site is a conserved feature among intervening sequences (IVSs) in eukaryotic nuclear tRNA precursors. The role of 3' splice site structure in splicing was examined through hexanucleotide insertions at a central intron position in the Saccharomyces cerevisiae tRNA gene. These insertions were designed to alter the structure at the splice site without changing its sequence. Endonuclease cleavage of pre-tRNA substrates was then measured in vitro, and suppressor activity was examined in vivo. A precursor with fully double-stranded structure at the 3' splice site was not cleaved by endonuclease. The introduction of one unpaired nucleotide at the 3' splice site was sufficient to restore cleavage, although at a reduced rate. We have also observed that guanosine at the antepenultimate position provides a second consensus feature among IVSs in tRNA precursors. Point mutations at this position were found to affect splicing although there was no specific requirement for guanosine. These and previous results suggest that elements of secondary and/or tertiary structure at the 3' end of IVSs are primary determinants in pre-tRNA splice site utilization whereas specific sequence requirements are limited.     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Effect of temperature

Photoinhibition of photosynthesis was induced in attached leaves of kiwifruit grown in natural light not exceeding a photon flux density (PFD) of 300 mol·m^-2·s^-1, by exposing them to a PFD of 1500 mol·m^-2·s^-1. The temperature was held constant, between 5 and 35° C, during the exposure to high light. The kinetics of photoinhibition were measured by chlorophyll fluorescence at 77K and the photon yield of photosynthetic O₂ evolution. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Photoinhibition followed pseudo first-order kinetics, as determined by the variable fluorescence (F _v) and photon yield, with the long-term steady-state of photoinhibition strongly dependent on temperature wheareas the observed rate constant was only weakly temperature-dependent. Temperature had little effect on the decrease in the maximum fluorescence (F _m) but the increase in the instantaneous fluorescence (F _o) was significantly affected by low temperatures in particular. These changes in fluorescence indicate that kiwifruit leaves have some capacity to dissipate excessive excitation energy by increasing the rate constant for non-radiative (thermal) energy dissipation although temperature apparently had little effect on this. Direct photoinhibitory damage to the photosystem II reaction centres was evident by the increases in F _o and extreme, irreversible damage occurred at the lower temperatures. This indicates that kiwifruit leaves were most susceptible to photoinhibition at low temperatures because direct damage to the reaction centres was greatest at these temperatures. The results also imply that mechanisms to dissipate excess energy were inadequate to afford any protection from photoinhibition over a wide temperature range in these shade-grown leaves.Abbreviations and symbols fluorescence yield correction coefficient - F _o, F _m, F _v instantaneous, maximum, variable fluorescence - K _D, K _F, K _P, K _T rate constants for non-radiative energy dissipation, fluorescence, photochemistry, energy transfer to photosystem I - PFD photon flux density - PSI, II photosystem I, II - _i photon yield of photosynthesis (incident light)