Reaching natural growth: Sources of variation in plant traits between indoor and outdoor experiments

One of the main problems of indoor plant production that especially plant researchers are confronted with, is a clear difference between plants grown under indoor versus outdoor conditions. This reduce the comparability between indoor and outdoor experiments as well as the portability of findings from indoor experiments to real world conditions (Matsubara, 2018). Poorter et al., (2016) suggested multiple reasons why this may occur, with major effects coming from lower light quantities, higher plant density and shorter experiment durations in indoor compared to outdoor experiments. Other sources of variation have been pointed out, including age of the plants, leaf temperature, soil temperature, soil microorganism, lack of UV light and the light quality in indoor experiments (e.g. Hogewoning et al., 2010 b). In general, the artificial conditions in indoor growth facilities often produce higher specific leaf area, leaf nitrogen content and relative growth rate, as well as lower maximum photosynthesis, plant height and shoot dry weight, compared with outdoor experiments (Poorter et al., 2016). Light, as one of of the principal determinants of plant growth and development, is consider an important source of deviation between indoor and outdoor conditions. For example, the effect of either light quantity or quantity has been well described in plants from different species, by Arnott and Mitchell (1982). To compensate a growth limitation in plants due a possible lack of light in greenhouse or indoor growth facilities, additional lighting is well stablished in agriculture, especially in areas at higher latitudes with year-round lower levels of natural sunlight (e.g. Grammans et al., 2018). Poorter et al., (2016) suggest that an important difference between indoor and outdoor climates for plant growth is a significant lower daily light integral (DLI) radiation in indoor facilities compared with outdoor conditions. Especially in combination with a lack of light variation along the day may lead to plant growth in indoor conditions that deviates considerable from field grown plants. It was not until the development and mass production of light emitting diodes (LED) that dynamic and specifics wavelengths changes as well as fast fluctuations of light intensity became possible to be used in indoor plant growth facilities. Previous attempts in plant biological research to recreate sun-like lighting with conventional light sources used very complex and fault-prone setups (e.g. Thiel et al., 1995) which were thus never widely used or considered for commercial plant production. With the technical improvements in controlled environment capabilities, the use of indoor cultivation systems has increased worldwide. In indoor experiments several authors have demonstrated the positive effects of incorporating closer-to-natural environmental conditions in indoor facilities (e.g. Arve et al., 2017, Kaiser et al., 2020,), what can help without adding higher levels of complexity to reach either closer to natural plant growth under indoor conditions and thereby increase the quality of food production to taste, smell and look more natural, attributes that are desired by consumers (Arve et al., 2017) Due to the high degree of absorption of blue (B) and red (R) light by chlorophyll, and the higher electric efficiency of LED in these spectral ranges (Overdieck, 1978), these two wavelength ranges tend to be dominating in commercial LED lamp systems (Fujiwara and Sawada, 2006). Many studies have investigated the responses of plants to different B to R ratios. These studies revealed that independent of the light intensity, a required minimum percentage of B is need for plant growth (e.g. Miao et al., 2016), and suggestions to reproduce near to natural plant growth by correctly adjusting the B:R ratio in LED lamps has been done (Hogewoning et al, 2010 a), however without directly comparing indoor grown plants with an outdoor control. In the vast majority of studies related to light quality effects on plants, either low light levels (Macebo et al., 2011; Hogewoning et al., 2010 a; Hernandez and Kubota, 2016; Kim et al., 2004; Schuerger et al., 1997) or much higher than natural red to far ratios have been use (e.g. Bae and Cho, 2008; Hogewoning et al., 2010 a; Hernandez and Kubota, 2016; Hernandez et al., 2016; Kim et al., 2004; Shengxin 2016; Zhen and van Iersel, 2017). However, interactions between light quantity and quality have been reported previously (Furuyama et al., 2014), and modifications of the light spectra, especially in the red to far ratio, has shown to induce more natural like plant growth (Hogewoning et al., 2010 b). This highlights the requirement of finding light spectral combinations in LED lighting that results in the most natural like plant growth in indoor facilities. One challenge is that different species might react differently do changes in the applied light spectrum. Tests for the effect of a light spectrum on plant performance should thus be done across different plant species (as in this thesis) in order to reveal general patterns as well as species-specific responses. In principal, lamps with multi-channel LEDs enable the application of lighting that can mimic close to natural light quality and intensities changes during plant cultivation in indoor growth facilities (Bula et al., 1991). However, although the newest generation of LED lighting systems are equipped with 4 or more individually controllable spectral channels, growth facilities generally do not apply dynamic and natural changes in the light spectra on a standard base. The knowledge about the changes in light quality related to the solar elevation angle, latitude, as well as the presence or absence of clouds (e.g. Smit, 1982; Goldberg et al., 1977) has been so far reported mainly from an atmosphere-physical point of view, and has not been transferred to actual lighting systems used for plant culture in greenhouses or growth chambers. Additionally, it has been shown that light quality effects on plants can interact with other environmental factors, like temperature (e.g. Chiang et al., 2018). This highlights the importance of understanding the role of the light quality variation on plant development, especially in order to correctly predict the effect of climate crisis on plants from indoor experiments. Although it is known that the fluctuation of environmental factors has an effect on plant phenology and development, it is common practice to apply static environmental conditions in indoor experiments. Fixed day and night time climates may be oversimplified reductions of natural conditions and may lead to plant growth significantly deviating from field grown plants (Poorter et al., 2016). Especially, it is well-known that random and daily fluctuations of temperature and light, can affect plant performance in both positive and negative ways (e.g. Myster and Moe,1995; Kaiser et al., 2015; Kaiser et al., 2018). Several studies have measured the effect of light or temperature variations on plant performance under semi-controlled and controlled conditions, but again, simultaneous comparisons with outdoor grown plants are rare in the literature and normally just Arabidopsis thaliana has been used (e.g. Vialet-Chabrand et al., 2017; Annunziata et al., 2017; Annunziata et al., 2018). Nevertheless, from these studies it could be derived that changes in light quantity along the day may induce lower biomass but also higher maxium photosynthesis, especially per unit of leaf mass (Vialet-Chabrand et al., 2017), even though fast fluctuations in light intensity have been shown to reduce photosynthesis and productivity in the long term (Kaiser et al., 2018). Additionally, these studies have shown more evidence of the difference of plants grown under totally fixed climatic conditions compared with semi-controlled environments (i.e. greenhouses), highlighting the necessity of a better knowledge for a minimum requirement of environmental fluctuations for natural like growth in indoor experiments. To investigate more closer the potential causes for the differences in plant performance between indoor and outdoor plant experiments, and to enable more natural-like plant growth in indoor facilities, a joint project had been stablish between the University of Basel (Basel, Switzerland) and Heliospectra A.B. (Gothenburg, Sweden) within the research consortia PlantHUB (European industrial doctoral programme (EID) funded by the H2020 PROGRAMME Marie Curie Actions- People), coordinated and managed by the Zurich-Basel plant science center. The project consisted of 18 months of basic research at the University of Basel, followed by 18 months of applied research, software development and documentation at Heliospectra A.B. As a result of this collaboration, the present thesis aims to identify how climatic conditions (especially, light quality and fluctuation of light intensity, temperature and air humidity) need to be adjusted in growth chambers in order to reach the most natural like plant growth under indoor conditions. To avoid documentation about only species-specific reactions, several species from different functional plant types were always used. The work on this thesis was divided in 5 main modules that aimed to: 1) Understand and quantify the natural light quality changes along the day and along a whole season, assess the effect of cloudiness on the natural light spectrum, and correlated these findings to previous studies on light quality effects in trees (Chapter 1) 2) Investigate which light spectral combination of LED-lights can induce the most natural-like growth in plants grown in indoor chambers with constant climatic conditions (Chapter 2) 3) Identify the minimal degree of environmental fluctuations (of light, temperature and air humidity) necessary to reach natural-like growth in indoor grown plants (Chapter 3) 4) Understand the effect of asynchrony environmental fluctuations in indoor growth chambers, were potential interaction and/or synergies may occur depending of the degree of variability of each environmental variable (Chapter 4) 5) Test possible applications of light fluctuations to improve crop quality and develop software applications for optimized light control of multi-wavelengths LED assimilation lamps (Chapter 5 and Appendix

Fermi-LAT Search for Pulsar Wind Nebulae around gamma-ray Pulsars

The high sensitivity of the Fermi-LAT (Large Area Telescope) offers the first opportunity to study faint and extended GeV sources such as pulsar wind nebulae (PWNe). After one year of observation the LAT detected and identified three pulsar wind nebulae: the Crab Nebula, Vela-X and the PWN inside MSH 15-52. In the meantime, the list of LAT detected pulsars increased steadily. These pulsars are characterized by high energy loss rates from ~3 \times 10^{33} erg s$^{-1}$ to 5 \times 10$^{38}$ erg s$^{-1}$ and are therefore likely to power a PWN. This paper summarizes the search for PWNe in the off-pulse windows of 54 LAT-detected pulsars using 16 months of survey observations. Ten sources show significant emission, seven of these likely being of magnetospheric origin. The detection of significant emission in the off-pulse interval offers new constraints on the gamma-ray emitting regions in pulsar magnetospheres. The three other sources with significant emission are the Crab Nebula, Vela-X and a new pulsar wind nebula candidate associated with the LAT pulsar PSR J1023-5746, coincident with the TeV source HESS J1023-575. We further explore the association between the H.E.S.S. and the Fermi source by modeling its spectral energy distribution. Flux upper limits derived for the 44 remaining sources are used to provide new constraints on famous PWNe that have been detected at keV and/or TeV energies.Comment: Accepted for publication in Astrophysical Journal, 42 pages, 17 figure

PKS 1502+106: a new and distant gamma-ray blazar in outburst discovered by the Fermi Large Area Telescope

The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a rapid (about 5 days duration), high-energy (E >100 MeV) gamma-ray outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z=1.839) starting on August 05, 2008 and followed by bright and variable flux over the next few months. Results on the gamma-ray localization and identification, as well as spectral and temporal behavior during the first months of the Fermi all-sky survey are reported here in conjunction with a multi-waveband characterization as a result of one of the first Fermi multi-frequency campaigns. The campaign included a Swift ToO (followed up by 16-day observations on August 07-22, MJD 54685-54700), VLBA (within the MOJAVE program), Owens Valley (OVRO) 40m, Effelsberg-100m, Metsahovi-14m, RATAN-600 and Kanata-Hiroshima radio/optical observations. Results from the analysis of archival observations by INTEGRAL, XMM-Newton and Spitzer space telescopes are reported for a more complete picture of this new gamma-ray blazar.Comment: 17 pages, 11 figures, accepted for The Astrophysical Journa

PSR J1907+0602: A Radio-Faint Gamma-Ray Pulsar Powering a Bright TeV Pulsar Wind Nebula

We present multiwavelength studies of the 106.6 ms gamma-ray pulsar PSR J1907+06 near the TeV source MGRO J1908+06. Timing observations with Fermi result in a precise position determination for the pulsar of R.A. = 19h07m547(2), decl. = +06:02:16(2) placing the pulsar firmly within the TeV source extent, suggesting the TeV source is the pulsar wind nebula of PSR J1907+0602. Pulsed gamma-ray emission is clearly visible at energies from 100 MeV to above 10 GeV. The phase-averaged power-law index in the energy range E > 0.1 GeV is = 1.76 \pm 0.05 with an exponential cutoff energy E_{c} = 3.6 \pm 0.5 GeV. We present the energy-dependent gamma-ray pulsed light curve as well as limits on off-pulse emission associated with the TeV source. We also report the detection of very faint (flux density of ~3.4 microJy) radio pulsations with the Arecibo telescope at 1.5 GHz having a dispersion measure DM = 82.1 \pm 1.1 cm^{-3}pc. This indicates a distance of 3.2 \pm 0.6 kpc and a pseudo-luminosity of L_{1400} ~ 0.035 mJy kpc^2. A Chandra ACIS observation revealed an absorbed, possibly extended, compact <(4 arcsec) X-ray source with significant non-thermal emission at R.A. = 19h07m54.76, decl. = +06:02:14.6 with a flux of 2.3^{+0.6}_{-1.4} X 10^{-14} erg cm^{-2} s^{-1}. From archival ASCA observations, we place upper limits on any arcminute scale 2--10 keV X-ray emission of ~ 1 X 10^{-13} erg cm^{-2} s^{-1}. The implied distance to the pulsar is compatible with that of the supernova remnant G40.5-0.5, located on the far side of the TeV nebula from PSR J1907+0602, and the S74 molecular cloud on the nearer side which we discuss as potential birth sites

Fermi Large Area Telescope observations of PSR J1836+5925

The discovery of the gamma-ray pulsar PSR J1836+5925, powering the formerly unidentified EGRET source 3EG J1835+5918, was one of the early accomplishments of the Fermi Large Area Telescope (LAT). Sitting 25 degrees off the Galactic plane, PSR J1836+5925 is a 173 ms pulsar with a characteristic age of 1.8 million years, a spindown luminosity of 1.1$\times10^{34}$ erg s$^{-1}$, and a large off-peak emission component, making it quite unusual among the known gamma-ray pulsar population. We present an analysis of one year of LAT data, including an updated timing solution, detailed spectral results and a long-term light curve showing no indication of variability. No evidence for a surrounding pulsar wind nebula is seen and the spectral characteristics of the off-peak emission indicate it is likely magnetospheric. Analysis of recent XMM observations of the X-ray counterpart yields a detailed characterization of its spectrum, which, like Geminga, is consistent with that of a neutron star showing evidence for both magnetospheric and thermal emission.Comment: Accepted to Astrophysical Journa

Fermi Large Area Telescope Observations of Gamma-ray Pulsars PSR J1057-5226, J1709-4429, and J1952+3252

The Fermi Large Area Telescope (LAT) data have confirmed the pulsed emission from all six high-confidence gamma-ray pulsars previously known from the EGRET observations. We report results obtained from the analysis of 13 months of LAT data for three of these pulsars (PSR J1057-5226, PSR J1709-4429, and PSR J1952+3252) each of which had some unique feature among the EGRET pulsars. The excellent sensitivity of LAT allows more detailed analysis of the evolution of the pulse profile with energy and also of the variation of the spectral shape with phase. We measure the cutoff energy of the pulsed emission from these pulsars for the first time and provide a more complete picture of the emission mechanism. The results confirm some, but not all, of the features seen in the EGRET data.Comment: Accepted for publication in ApJ. 45 pages, 12 figures, 11 tables. Corresponding authors: O. Celik, F. Gargano, T. Reposeur, D.J. Thompso

Discovery of Pulsed γ\gamma-rays from PSR J0034-0534 with the Fermi LAT: A Case for Co-located Radio and γ\gamma-ray Emission Regions

Millisecond pulsars (MSPs) have been firmly established as a class of gamma-ray emitters via the detection of pulsations above 0.1 GeV from eight MSPs by the Fermi Large Area Telescope (LAT). Using thirteen months of LAT data significant gamma-ray pulsations at the radio period have been detected from the MSP PSR J0034-0534, making it the ninth clear MSP detection by the LAT. The gamma-ray light curve shows two peaks separated by 0.274$\pm$0.015 in phase which are very nearly aligned with the radio peaks, a phenomenon seen only in the Crab pulsar until now. The $\geq$0.1 GeV spectrum of this pulsar is well fit by an exponentially cutoff power law with a cutoff energy of 1.8$\pm 0.6\pm$0.1 GeV and a photon index of 1.5$\pm 0.2\pm$0.1, first errors are statistical and second are systematic. The near-alignment of the radio and gamma-ray peaks strongly suggests that the radio and gamma-ray emission regions are co-located and both are the result of caustic formation.Comment: 20 pages, 3 figures, 2 tables. Accepted for publication in Ap

Fermi Large Area Telescope Observations of the Crab Pulsar and Nebula

We report on gamma-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nancay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first gamma-ray peak leads the radio main pulse by (281 \pm 12 \pm 21) mus, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at E_c = (5.8 \pm 0.5 \pm 1.2) GeV, spectral index of Gamma = (1.97 \pm 0.02 \pm 0.06) and integral photon flux above 100 MeV of (2.09 \pm 0.03 \pm 0.18) x 10^{-6} cm^{-2} s^{-1}. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed gamma-ray photons are observed up to ~ 20 GeV which precludes emission near the stellar surface, below altitudes of around 4 to 5 stellar radii in phase intervals encompassing the two main peaks. The spectrum of the nebula in the energy range 100 MeV - 300 GeV is well described by the sum of two power-laws of indices Gamma_{sync} = (3.99 \pm 0.12 \pm 0.08) and Gamma_{IC} = (1.64 \pm 0.05 \pm 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via inverse Compton scattering from standard Magnetohydrodynamics (MHD) nebula models, and does not require any additional radiation mechanism.Comment: 17 pages, 9 figures, Accepted for publications in Astrophysical Journa

Detection of the energetic pulsar PSR B1509-58 and its pulsar wind nebula in MSH 15-52 using the Fermi-Large Area Telescope

We report the detection of high energy gamma-ray emission from the young and energetic pulsar PSR B1509$-$58 and its pulsar wind nebula (PWN) in the composite supernova remnant SNR G320.4-1.2 (aka MSH 15-52). Using 1 year of survey data with the Fermi-Large Area Telescope (LAT), we detected pulsations from PSR B1509-58 up to 1 GeV and extended gamma-ray emission above 1 GeV spatially coincident with the PWN. The pulsar light curve presents two peaks offset from the radio peak by phases 0.96 $\pm$ 0.01 and 0.33 $\pm$ 0.02. New constraining upper limits on the pulsar emission are derived below 1 GeV and confirm a severe spectral break at a few tens of MeV. The nebular spectrum in the 1 - 100 GeV energy range is well described by a power-law with a spectral index of (1.57 $\pm$ 0.17 $\pm$ 0.13) and a flux above 1 GeV of (2.91 $\pm$ 0.79 $\pm$ 1.35) 10^{-9} cm^{-2} s^{-1}. The first errors represent the statistical errors on the fit parameters, while the second ones are the systematic uncertainties. The LAT spectrum of the nebula connects nicely with Cherenkov observations, and indicates a spectral break between GeV and TeV energies.Comment: 14 pages, 6 figures, accepted for publication by Ap