Effects of severe water stress on partitioning of 14C-assimilates in tomato plants

Tomato plants (Lycopersicon esculentum Mill. cv Nikita) were grown hydroponically and subjected to severe water stress induced by addition of PEG-6000 to the nutrient solution. The PEG-treatment clearly impaired growth. Leaf photosynthesis decreased during the experiment. Moreover, the decrease in photosynthesis was associated with a decrease in dry weight of the shoot compared to the root. Also leaf area expansion, stomatal conductance and transpiration decreased. Water stress enhanced the transport of 14C-assimilates from the source leaf to the lower parts of the plant where the assimilates were incorporated in the lower stem, the leaves below the source leaf and the roots. It was observed that 14C was much more concentrated in the roots compared to the other plant parts

Statistical analyses of long-term variability of AGN at high radio frequencies

We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used. We have applied three different statistical methods to study the time scales: The structure function, the discrete correlation function and the Lomb-Scargle periodogram. We discuss also the differences and relative merits of these three methods. Our study reveals that smaller flux density variations occur in these sources on short time scales of 1-2 years, but larger outbursts happen quite rarely, on the average only once in every 6 years. We do not find any significant differences in the time scales between the source classes. The time scales are also only weakly related to the luminosity suggesting that the shock formation is caused by jet instabilities rather than the central black hole.Comment: 19 pages, 12 figures, Accepted for publication in A&

Two classes of radio flares in the blazar PKS 0420−-014

The two 5GHz VLBI (Very Long Baseline Interferometry) observations (1996 June and 1997 November) presented in this paper, combined with several formerb VLBI observations at 8.4GHz and 5GHz, suggest that the radio flares of the blazar PKS 0420$-$014 can be divided into two classes according to their geometric origins in 5 or 8.4GHz VLBI maps and the properties of light curves. One class of flares, which we call {\it core flares}, originate from the core. Core flares have large lags between the light curves at different frequencies, and will probably lead to the ejection of new jet components. The other class of flares, which we call {\it jet flares}, come from jet components. Jet flares vary simultaneously at different wavelengths, and may due to the Doppler boosting effect of rotating knots moving along a helical jet. The radio flare in 1991, accompanied by a simultaneous gamma-ray flare, was identified as a core flare.Comment: 9 pages, 3 figure

Molecular Gas in the Powerful Radio Nucleus of the Ultraluminous Infrared Galaxy PKS 1345+12

Millimeter CO(1-0) interferometry and high resolution, Hubble Space Telescope (HST) 1.1, 1.6, and 2.2 micron imaging of the radio compact galaxy PKS 1345+12 are presented. With an infrared luminosity of 2x10^{12} L_sun, PKS 1345+12 is a prime candidate for studying the link between the ultraluminous infrared galaxy phenomenon and radio galaxies. These new observations probe the molecular gas distribution and obscured nuclear regions of PKS 1345+12 and provide morphological support for the idea that the radio activity in powerful radio galaxies is triggered by the merger of gas rich galaxies. Two nuclei separated by 2" (4.0 kpc) are observed in the near-infrared; the extended southeastern nucleus has colors consistent with reddened starlight, and the compact northwestern nucleus has extremely red colors indicative of an optical quasar with a warm dust component. Further, the molecular gas, 3mm continuum, and radio emission are coincident with the redder nucleus, confirming that the northwestern nucleus is the site of the AGN and that the molecular gas is the likely fuel source.Comment: LaTex, 5 pages with 1 postscript and 1 jpg figure, ApJ Letters, in press (August 20, 1999

Xylem surfactants introduce a new element to the cohesion-tension theory

Vascular plants transport water under negative pressure without constantly creating gas bubbles that would disable their hydraulic systems. Attempts to replicate this feat in artificial systems almost invariably result in bubble formation, except under highly controlled conditions with pure water and only hydrophilic surfaces present. In theory, conditions in the xylem should favor bubble nucleation even more: there are millions of conduits with at least some hydrophobic surfaces, and xylem sap is saturated or sometimes supersaturated with atmospheric gas and may contain surface-active molecules that can lower surface tension. So how do plants transport water under negative pressure? Here, we show that angiosperm xylem contains abundant hydrophobic surfaces as well as insoluble lipid surfactants, including phospholipids, and proteins, a composition similar to pulmonary surfactants. Lipid surfactants were found in xylem sap and as nanoparticles under transmission electron microscopy in pores of intervessel pit membranes and deposited on vessel wall surfaces. Nanoparticles observed in xylem sap via nanoparticle-tracking analysis included surfactant-coated nanobubbles when examined by freeze-fracture electron microscopy. Based on their fracture behavior, this technique is able to distinguish between dense-core particles, liquid-filled, bilayer-coated vesicles/liposomes, and gas-filled bubbles. Xylem surfactants showed strong surface activity that reduces surface tension to low values when concentrated as they are in pit membrane pores. We hypothesize that xylem surfactants support water transport under negative pressure as explained by the cohesion-tension theory by coating hydrophobic surfaces and nanobubbles, thereby keeping the latter below the critical size at which bubbles would expand to form embolisms

A powerful ultra steep spectrum radio galaxy having an extremely low excitation emission line spectrum

In the course of our optical follow up of the Ooty sample of ultra-steep spectrum radio sources (USSRS), we have discovered an exceptionally interesting case of a classical double radio source of size ~14" and spectral index α = - 1.2 associated with a red galaxy at a modest redshift of z = 0.477. The galaxy exhibits gaseous wisps broadly coincident with the radio lobes. The optical spectrum is dominated by an emission line showing a very large rest-frame equivalent width of 350 Å, a spatial extent of ~10" at the position angle of the radio structure, and a tilt with a maximum rest-frame velocity difference of ~760 km s-1. These properties, as well as the presence of the optical wisps suggest that the system is a product of a merger involving a gas-rich disk and a massive elliptical. Even though the object is relatively bright (V= 21.3), additional spectral lines could only be detected after a persistent, deep integration. These very faint features, identified with Hβ, [O III] λ5007, and H, K and G-band, have lead to an unambiguous interpretation of the dominant emission line as being [O II] λ3727. The detection of faint lines may have considerable relevance to some cases of radio galaxies for which extremely large redshifts (z > 3) have been inferred recently. The red colour of the galaxy, and the equivalent widths of the stellar absorption features suggest that the continuum of 1411-192 is dominated by an old stellar population. The physical conditions of the gas are similar to those of LINERS, with shock-heating as a possible ionization mechanism. The association of an "ultra-soil" emission line spectrum with such a powerful radio galaxy is intriguing

Wavelet analysis of a large sample of AGN at high radio frequencies

We have studied the characteristic timescales of 80 AGNs at 22, 37 and 90 GHz examining the properties of the wavelet method and comparing them to traditional Fourier-based methods commonly used in astronomy. We used the continuous wavelet transform with the Morlet wavelet to study the characteristic timescales. We also gain information when the timescale is present in the flux curve and if it is persistent or not. Our results show that the sources are not periodic and changes in the timescales over a long time are common. The property of wavelets to be able to distinguish when the timescale has been present is superior to the Fourier-based methods. Therefore we consider it appropriate to use wavelets when the quasi-periodicities in AGNs are studied.Comment: 14 pages, 10 figures, A&A in pres

The ISOPHOT-MAMBO survey of 3CR radio sources: Further evidence for the unified schemes

We present the complete set of ISOPHOT observations of 3CR radio galaxies and quasars, which are contained in the ISO Data Archive, providing 75 mid- and far-infrared spectral energy distributions (SEDs) between 5 and 200 micron. For 28 sources they are supplemented with MAMBO 1.2 mm observations and for 15 sources with new submillimetre data from the SCUBA archive. We check the orientation-dependent unified scheme, in which the powerful FR2 narrow line galaxies are quasars viewed at high inclination, so that their nuclei are hidden behind a dust torus intercepting the optical-ultraviolet AGN radiation and reemitting it in the infrared. We find that (1) both the quasars and the galaxies show a high mid- to far-infrared luminosity ratio typical for powerful AGNs and (2) -- when matched in 178 MHz luminosity -- both show the same ratio of isotropic far-infrared to isotropic 178 MHz lobe power. Therefore, from our large sample investigated here we find strong evidence for the orientation-dependent unification of the powerful FR2 galaxies with the quasars.Comment: 16 pages, 7 figures, 3 tables, accepted by Astronomy & Astrophysic

Long-term radio variability of AGN: flare characteristics

We have studied the flare characteristics of 55 AGN at 8 different frequency bands between 4.8 and 230 GHz. Our extensive database enables us to study the various observational properties of flares in these sources and compare our results with theoretical models. We visually extracted 159 individual flares from the flux density curves and calculated different parameters, such as the peak flux density and duration, in all the frequency bands. The selection of flares is based on the 22 and 37 GHz data from Mets\"ahovi Radio Observatory and 90 and 230 GHz data from the SEST telescope. Additional lower frequency 4.8, 8, and 14.5 GHz data are from the University of Michigan Radio Observatory. We also calculated variability indices and compared them with earlier studies. The observations seem to adhere well to the shock model, but there is still large scatter in the data. Especially the time delays between different frequency bands are difficult to study due to the incomplete sampling of the higher frequencies. The average duration of the flares is 2.5 years at 22 and 37 GHz, which shows that long-term monitoring is essential for understanding the typical behaviour in these sources. It also seems that the energy release in a flare is independent of the duration of the flare.Comment: 11 pages, 9 figures, 2 tables, accepted for publication in A&

Optical and radio behaviour of the BL Lacertae object 0716+714

Eight optical and four radio observatories have been intensively monitoring the BL Lac object 0716+714 in the last years: 4854 data points have been collected in the UBVRI bands since 1994, while radio light curves extend back to 1978. Many of these data are presented here for the first time. The long-term trend shown by the optical light curves seems to vary with a characteristic time scale of about 3.3 years, while a longer period of 5.5-6 years seems to characterize the radio long-term variations. In general, optical colour indices are only weakly correlated with brightness. The radio flux behaviour at different frequencies is similar, but the flux variation amplitude decreases with increasing wavelength. The radio spectral index varies with brightness (harder when brighter), but the radio fluxes seem to be the sum of two different-spectrum contributions: a steady base level and a harder-spectrum variable component. Once the base level is removed, the radio variations appear as essentially achromatic, similarly to the optical behaviour. Flux variations at the higher radio frequencies lead the lower-frequency ones with week-month time scales. The behaviour of the optical and radio light curves is quite different, the broad radio outbursts not corresponding in time to the faster optical ones and the cross-correlation analysis indicating only weak correlation with long time lags. However, minor radio flux enhancements simultaneous with the major optical flares can be recognized, which may imply that the mechanism producing the strong flux increases in the optical band also marginally affects the radio one.Comment: 18 pages, 15 Postscript figures, 5 JPEG figures, accepted for publication in A&