Photomorphogenesis and canopy dynamics. Phytochrome-mediated proximity perception accounts for the growth dynamics of canopies of Populus trichocarpa × deltoides'Beaupré'

The phytochrome family of signal-transducing photoreceptors provides plants with the capacity to perceive variations in the relative fluxes of red (R) and far-red (FR) radiation. This capacity has been proposed to be of ecological value in the perception of the proximity of neighbouring plants and the consequent induction of shade avoidance responses. The work reported here has evaluated this potential by determining quantitatively the effect of neighbour proximity on the growth of canopies of Populus trichocarpa×deltoides‘Beaupré’ trees, and relating the measured variables to the long-term vectoral radiation quality inside each canopy. The spectral distribution of radiation inside four canopies of Populus trichocarpa×deltoides‘Beaupre’ of different densities was monitored throughout the growing season. Spectral distributions inside the canopies were measured in 10° wedges at different heights and angles. The results are presented as PFD over 400–700 nm (PFD400–700) and PFD over 400–800 nm (PFD400–700). Results are also presented for the calculated phytochrome photoequilibrium (Pfr/P) and red:far-red ratio (R:FR). Data are presented as in-canopy angular and height profiles, and as diurnal and seasonal variations. PFD_400–700 and Pfr/P were found to be reduced inside each canopy, the reduction being greatest in the most dense canopy, and least in the most open canopy. At any height within each canopy, calculated Pfr/P decreased linearly with time throughout the growing season, until leaf senescence began. The reduction was greater in the denser canopies and was found to be similar for three consecutive field seasons. Linear relationships were found between plant stem growth rate, plant spacing and Pfr/P calculated from radiation propagated approximately horizontally within the canopies. The findings support the role of phytochrome in proximity perception in the natural environment and provide a quantitative basis for investigating the competitive interactions between plants growing in dense stands. The hypothesis is proposed that the dynamics of developing or regenerating canopies can be accounted for on the basis of phytochrome-mediated perception of the proximity of neighbouring plants.     

Effect of open-air fumigation with sulphur dioxide and ozone on phyllosphere and endophytic fungi of conifer needles

The phyllosphere microbial populations inhabiting the needles of three conifer species, Scots pine (Pinus sylvestris L.), Sitka spruce (Picea sitchensis L.) and Norway spruce (Picea abies (L.) Karst.), exposed to SO₂ and O₃, in an open-air fumigation experiment were analysed over a 3 year period using serial dilution after washing, direct plating and a fluorescein diacetate (FDA) enzyme assay. Total fungal populations ranged from 10² to 10⁵ colonyforming units (CPU) g^?1 fresh weight of needles. The dominant fungi isolated from needles varied with tree species and isolation technique; Aureobasidium pullulans (de Bary) Arnaud was most common on Scots pine and Norway spruce and white yeasts on Sitka spruce using the dilution plating method. However, direct plating of needle segments onto culture media indicated that Sclerophoma pythiophila (Corda) Hohnel was dominant on Scots pine and A. pullulans on Sitka and Norway spruce. Green needles of Sitka spruce were found to be endophytically colonized by Rhizosphaera kalkhoffii Bubak, but seldom by Lophodermium piceae (Fuckel) Hohn during extensive sampling in 1990. Statistical analyses revealed significant differences (P<0.05) between plots in the 3 year mean of the total fungal populations or the fungal biomass (FDA assay) on all three tree species. Differences between plots were also observed for a number of dominant component species. Data were also analysed for treatment effects. A significant effect of SO₂ treatment was observed on the total fungal populations on Sitka spruce (P<0.05) which were reduced markedly by the low-SO₂ treatment, while the O₃ treatment caused a significant increase in total fungal numbers on Scots pine (P<0.05). The FDA activity on needles of both Scots pine and Sitka spruce was noticeably higher in the 0₃-only treatment plot, but the overall O₃ effect was not significant. Treatment effects were also detected on the occurrence of component species. The serial dilution method revealed an SO₂ effect (P<0.05) of a reduction in the occurrence of pink yeasts on Sitka spruce and an O₃ effect (P<0.05) of an increase in the occurrence of S. pythiophila on Sitka spruce (P<0.01) but a decrease of Epicoccum nigrum Link and Cladosporium spp. on Scots pine. The direct-plating method revealed an SO₂ effect of an increase in S. pythiophila on Norway spruce (P<0.05). Ozone treatment caused a significant increase in the isolation of a black strain of A. pullulans on Norway spruce (P<0.05). Endophytic colonization of Sitka spruce needles by R. kalkhoffii was found to be increased on two occasions by O₃ exposure.     

Occurrence of unstable PEP carboxylase in C4 grasses in the genus Spartina Schreb.

Abstract The aims of this work were to discover the distribution within the C₄ grass Spartina anglica of a PEP carboxylase which is very unstable during and after extraction, and to determine whether this unstable form occurs in other members of the genus. In S. anglica, only the leaf contains an unstable PEP carboxylase. Within the leaf only the major one of two isoenzymes is unstable, and this is located in the mesophyll cells. The unstable isoenzyme is inactivated during extraction and storage unless protected by bovine serum albumin or Triton X-100, and is inactivated in assay mixtures at optimum pH in the absence of PEP. Evidence is presented that inactivation is not due to degradation or inhibition during extraction and storage. The enzyme from leaves of Spartina species taxonomically closely related to S. anglica is also very unstable during and after extraction, but that from less closely related species is much more stable.     

Archaeological Inference and Inductive Confirmation

The limitations of the hypothetico-deductive (H-D) method of inductive confirmation are described, and an alternate method, the hypothetico-analog (H-A) method is described in detail. The H-A method can be characterized as a modified and supplemented form of the simple H-D method, and is proposed as being more appropriate for archaeological inference. Aspects of the H-A method that are given particular attention include the establishment of boundary conditions for reference classes, plausibility considerations, multiple working hypotheses, bridging arguments, and criteria for selecting alternative hypotheses . [scientific method, archaeological inference, hypothetico-analog method, inductive confirmation]     

Effect of Temperature and External pH on the Cytoplasmic pH of Chara corallina

Cytoplasmic pH (pH_c) in Chara corallina was measured (from [¹⁴C]stribution)as a function of external pH (pH₀)and temperature. With pH₀near 7, pH_c at 25?C is 7.80; pH_cincreases by 0.005 pH units?C^–1 temperature decrease, i.e. pH_c at 5 ?C is 7.90. WithpH? near 5.5, the increase in pH_c with decreasing temperatureis 0.015 units ?C^–1 between 25 and 15?C, but 0.005 units?C^–1 between 15 and 5?C. This implies a more precise regulationof pH_c with variations in pH_o at 5 or 15 ?C compared with 25?C. The observed dp H_c/dT is generally smaller than the –0.017units ?C^–1 needed to maintain a constant H⁺/OH^–1,or a constant fractional ionization of histidine in protein,with variation in temperature. It is closer to that needed tomaintain the fractional ionization of phosphorylated compoundsor of CO₂–HCO₃^– The value of dpH_c/dT has importantimplications for several regulatory aspects of cell metabolism.These include (all as a function of temperature) the rates ofenzyme reactions, the _H+ at the plasmalemma(and hence the energy available for cotransport processes),and the mechanism for pH_c regulation by the control of bidirectionalH⁺ fluxes at the plasmalemma.     

A comparative study of the life style of two Jurassic irregular echinoids

The irregular echinoids Plesiechinus ornatus (Buckman) (Pygasteroida) and Galeropygus agariciformis (Forbes) (Cassiduloida) occur together in beds of the murchisonoe Zone, Bajocian, outcropping in the Cheltenham region of Gloucestershire. These species were largely restricted to different lithofacies within the carbonate shelf environment. Both adopted a hidden mode of life but achieved this by different techniques. Plesiechinus had fairly short spines and strongly muscular podia over the whole corona and was able to cover itself with coarse substrate particles. The oral tubercles are bilaterally symmetrical and are radially arranged. The oral spines are thought to have pulled sediment out from beneath the test, excavating a small depression for it. Galeropygus bore a dense covering of very small spines and its tube feet were differentiated into aboral respiratory podia and oral suckered podia. It had a preferred anterior direction of locomotion and is thought to have buried itself completely by excavating and ploughing into the substrate as it moved forward. Plesiechinus fed using only its lantern and postulated peristomial tube feet, whereas Galeropygus was a continugus sediment swallower and used its phyllode tube feet and peristomal lip spines in transferring particles towards the mouth.     

Sensitivity to metalaxyl of Phytophthora infestans populations in potato crops in south-west England in 1980 and 1981

A laboratory test was developed to assess the sensitivity of field populations of Phytophthora infestans to metalaxyl. Discs of potato leaf tissue were floated upon solutions of the fungicide at different concentrations and inoculated with spores. The extent of symptom development was noted after incubation under standard conditions for 5–6 days. In preliminary experiments growth of isolates of P. infestans obtained from culture collections was severely inhibited in discs treated at 2 μg/ml. By contrast the development of an isolate obtained from a crop in Eire in which blight control with metalaxyl had failed, and known to be markedly less sensitive in vitro, was unaffected in discs treated at 100 μg/ml. During the summer of 1980, 234 samples of P. infestans were obtained from 20 sites in south-west England, 10 of which had received sprays containing metalaxyl and 10 of which had not. All samples were sensitive to metalaxyl applied at 2 μg/ml. In 1981, 35 sites within the same area, 30 of which had received sprays containing either metalaxyl or ofurace (a related fungicide), were similarly surveyed. Most of the 79 samples of P. infestans examined proved sensitive and at all sites the amount of blight was small. However, at three sites, including one not treated with acylalanine fungicides, strains were found which were unaffected by 100 μg/ml metalaxyl in leaf disc tests. These findings are discussed in relation to the development of resistant blight in other areas and to the use of fungicide mixtures.     

The carbon canopy economy of the association between cowpea and the parasitic angiosperm Striga gesnerioides

The association between the parasite Striga gesnerioides and cowpea (Vigna unguiculata) was investigated using measurements of growth and gas exchange together with calculations of the carbon budget of the association. Striga gesnerioides has a very low photosynthetic capacity coupled with high rates of respiration. Even at photosynthetic light saturation shoots exhibit no net carbon gain. Thus S. gesnerioides is highly dependent on its host for carbon as well as for water and inorganic solutes. It is estimated that 70% of the carbon transferred from host to parasite is used in parasite respiration. Infected cowpea had a lower photosynthetic capacity, at times less than half that of uninfected plants. Infection with S. gesnerioides reduced the growth of cowpea by 75%. Calculations indicate that the loss of carbon from the host by export to the parasite is more important than reduced photosynthetic capacity of the host in accounting for the observed growth reductions.