Responser på høy luftfuktighet og betydning av abscisinsyre (ABA) på vekst, vannrelasjoner og genekspresjonsprofil

The aerial environment, which includes relative air humidity (RH), temperature, vapour pressure deficit (VPD) and CO2 availability, affects plant growth and development in many ways. The aerial environment determines plant gas exchange and influences plant function from physical trait development down to gene expression. The final regulation of plant gas exchange is through the stomatal pores, which are tightly controlled in order to maximise carbon assimilation while minimising excessive water loss to the atmosphere. Stomatal regulation is complex, and the full extent of signals involved in response to the aerial environment remains equivocal. High RH, or low VPD, plays a significant role in stomatal anatomy, movement and function, yet the sensing and signalling pathways associated with high RH and their specific effects on the regulation of transpiration are yet to be fully understood. Studies have shown that continuous growth in high RH leads to diminished stomatal function and response to stomatal closing stimuli, such as darkness and decreased RH. How this diminished function arises, and strategies to combat it are still under investigation, and the involvement of the phytohormone abscisic acid (ABA) is yet to be fully elucidated. The aim of this PhD was to investigate the effects of high RH on the growth and water relations of several important crop species: pea, tomato and barley, with a focus on stomatal morphology and function, as well as to analyse the interaction between RH and a potential mitigating factor (UV radiation), and determine the role of ABA in short- and long- term responses to high RH using ABA-deficient mutants. Previous studies have shown that periods of reduced RH, increased temperature, increased proportion of blue light or spraying with exogenous ABA can alleviate the reduction in stomatal functioning often found in plants grown in high RH. Exposure to UV radiation induces stomatal closure via both ABA-dependent and - independent pathways, thus UV radiation was tested as a potential mitigating factor to alleviate the stomatal effects of growth in high RH in pea plants (paper I). Further studies have indicated species-dependent effects of ABA on plant growth and water relations, thus the effects of growth in high RH of tomato and barley plants and their respective ABA-deficient mutants which have lesions in the same step of ABA biosynthesis were tested (paper II). Finally, the effects of long- and short-term exposure to high RH on WT and ABA-deficient flacca (flc) tomatoes and their differentially expressed genes (DEGs) were investigated to determine the effect of high RH on gene expression and how this was affected by ABA (paper III). Pea plants grown in high (90%) RH indicated higher susceptibility to leaf and DNA damage by night-time exposure to 0.15 W m-2 UV radiation than plants grown in moderate (60%) RH, which was brought about by a lower total antioxidant power and reduced flavonoid content in these leaves (paper I). Tomato and barley substantially differed in their response to growth in high RH, as well as the role of ABA therein (paper II). High RH alleviated somewhat the effects of ABA-deficiency in both species, though the mechanism was most likely through effects on water status in tomato, but not in barley. The differences between the two species may be linked to inherent differences between eudicots (tomato) and monocots (barley), but may also have arisen as an artefact of nitrate reductase (NR) deficiency in ABA-deficient barley (Az34), but not in flc. Finally, ABA concentration determined transpiration differences in response to RH in WT tomato plants, but this was driven by an ABA-independent mechanism, possibly a hydraulic response, in flc plants (paper III). However, the greater response of WT plants suggested that while hydraulics may induce stomatal and transpirational responses to RH, the presence and regulation of ABA has a greater effect. Furthermore, a lack of ABA resulted in the up- and down-regulation of thousands of genes in flc compared to WT plants and in response to changing RH, due to changes in water status, rather than ABA-deficiency per se. Despite turgor loss in moderate RH, flc plants did not undergo osmotic adjustment to regulate water loss, and showed increased ethylene (ET) biosynthesis and signalling compared to WT plants in moderate RH. In conclusion, this PhD work has shown species and genotypic differences in the effects of high RH on plant growth and hydraulic responses, with responses driven by different pathways in different species.Klimafaktorer som relativ luftfuktighet (RF), temperatur, lys og CO2 påvirker plantevekst og utvikling. Luftklima, og spesielt RF, bestemmer i stor grad gassutvekslingen og dermed transpirasjon hos planter. Reguleringen av gassutveksling skjer gjennom spalteåpningene, som har en viktig funksjon for å maksimere karbonassimilasjon og minimere vanntapet til atmosfæren. Spalteåpningsregulering er kompleks, og mange ulike typer signaler er involvert. Høy RF (>90%) har stor betydning for spalteåpningenes anatomi og morfologi, samt evne til bevegelse og funksjon. Forståelsen av hvordan signaleringen er aktivert under høy RF og dens spesifikke effekt på regulering av transpirasjon er imidlertid mangelfull. Studier har vist at vekst under kontinuerlig høy RH fører til nedsatt spalteåpningsfunksjon og dermed redusert lukkerespons på signaler som normalt fører til lukking, slik som tørkestress og mørke. Hvordan denne nedsatte funksjonsevnen oppstår og hvordan det er mulig å unngå en slik situasjon krever mer kunnskap, spesielt når det kommer til hvilken rolle plantehormonet abscisinsyre (ABA) har i prosessen. ABA er et av de viktigste signalene som styrer åpning og lukking av spalteåpninger. Målet med dette PhD-arbeidet var å undersøke effektene av høy RF på vekst og vannrelasjoner hos flere viktige matplanter: ert, tomat og bygg. Arbeidet har fokusert på spalteåpningsmorfologi og funksjon, samt å analysere samspillet mellom RF og ultrafiolett (UV) stråling, og fastslå rollen til ABA i både kort- og lang-tids responser på høy RF ved å benytte ABAmanglende mutanter som verktøy. Studier har vist at funksjonen til spalteåpninger utviklet under høy RF kan forbedres ved at de eksponeres for kortere perioder med lavere RF, økt temperatur, økt andel av blått lys daglig eller sprøytes med ABA. UV-stråling vil normalt indusere lukking av spalteåpninger gjennom både ABA-avhengig og -uavhengige prosesser, og derfor ble det testet om UV-eksponering kunne forbedre funksjonene til spalteåpninger hos ert utviklet under høy RH (artikkel 1). Studier har vist at effektene av ABA på plantevekst og vannrelasjoner er artsspesifikke, og dermed ble effektene av høy RF på tomat og bygg med deres tilsvarende ABA-manglende mutanter, som er mutert ved samme steget i ABA biosyntese undersøkt (artikkel II). I den siste artikkelen ble effektene av både lang- og kort-tids eksponering av høy RF på villtype og ABAmanglende flacca (flc) tomat og deres differensielt uttrykte gener undersøkt ved hjelp av RNA-sekvensering (artikkel III). Ert dyrket i høy RF (90%) viste økt følsomhet for skader på blad og DNA som et resultat av UV eksponering (0.15 W m-2) sammenlignet med planter dyrket i moderat (60%) RF. Økt skade oppsto på grunn av lavere total antioksidantkapasitet (FRAP) og redusert innhold av flavonoider (artikkel 1). Det ble funnet store forskjeller i respons på RF mellom tomat og bygg, og deres ABA-mutanter (artikkel II). Høy RF dempet effekten av ABA mangel hos begge arter, mest sannsynlig gjennom endringer i vannrelasjoner hos tomat, men ikke hos bygg. Forskjellen mellom artenes respons er muligens et resultat av innebygde forskjeller mellom tofrøbladet (tomat) og enfrøbladet (bygg) arter, men kan også henge sammen med at ABAmanglende bygg (Az34) mangler nitrat reduktase (NR) mens flc fortsatt har intakt NR. Det siste manuskriptet i avhandlingen viser at transpirasjonen sannsynligvis er drevet av endringer i mengde ABA hos villtype tomat, men at den er drevet av en ABA-uavhengig mekanisme, muligens hydraulisk respons hos flc (artikkel III) i respons på RF. Dessuten, gitt at responsen var sterkere i tomat villtype, ble det konkludert at hydrauliske mekanismer muligens induserer spalteåpnings- og transpirasjons-responser ved endringer i RF, men at tilstedeværelse og regulering av ABA har en sterkere effekt. Mangel på ABA (flc) førte til både opp- og ned-regulering av tusenvis av gener sammenlignet med villtype tomat. Den store forskjellen mellom flc og villtype oppsto på grunn av endringer i vannstatus, heller enn ABA-mangel per se. Til tross for tap av turgor under moderat RF har ikke flc planter evne til å gjennomgå osmotiske justeringer for å regulere vanntap, men viste økt biosyntese og signalering av etylen sammenlignet med villtype under moderat RF. Denne avhandlingen viser at effektene av høy RF på plantenes vekst og hydraulisk respons er art-spesifikke, og at responsene er drevet av ulike mekanismer i forskjellige arter. Det er derfor viktig å forstå ulike arters luftfuktighetsresponser for optimal vekst og stresstoleranse

Effects of UV radiation and air humidity on morphology, stomatal function and photosynthesis of Euphorbia pulcherrima

The combined effects of relative air humidity (RH) and UV radiation were tested on three cultivars of Euphorbia pulcherrima (Willd ex. Klotzch) at different ontogenetic stages in controlled environment growth chambers. In addition, the effects of UV radiation alone were tested on a fourth cultivar in a greenhouse compartment with natural background light. Growth chamber plants grown at 60 % or 90 % RH were either exposed to 0.15 W m-2 UV radiation for 40 minutes in the middle of the dark period (vegetative plants) or at the end of the light period (EOD, generative plants) or not exposed to UV. Vegetative ‘Christmas Feelings’ poinsettia responded strongly to RH. High RH increased plant height, shoot length, the number of leaves per shoot, leaf area, plant diameter and leaf petiole length, while decreasing leaf thickness and internode length. The effects of UV were minor and UV exposure resulted in a decrease in the number of side shoots on the main shoot, a decrease in leaf area and a decrease in petiole length. Generative ‘Infinity Red’ and ‘Bravo Bright Red’ poinsettia did not show an obvious stronger morphological response to either RH (60 % and 90 %) or UV alone, and the interaction effects indicate that the RH at which the plants were grown dictates the magnitude and direction of the UV response. Intraspecific differences were found between the two cultivars and the less compact cultivar ‘Infinity Red’ showed a stronger response to UV compared to the compact cultivar ‘Bravo Bright Red’. The stomatal responses of ‘Infinity Red’ and ‘Bravo Bright Red’ indicate stronger effects of RH than UV on both leaf and bract conductance measured in both light and dark conditions. Stomatal aperture size of ‘Infinity Red’ plants was affected by UV in the light, which caused a significant increase in stomatal aperture, while under dark conditions the effect of RH was much stronger and resulted in larger stomatal apertures under 90 % RH than 60 % RH. Photosynthesis in these cultivars showed no effect of RH or UV. Generative ‘Christmas Day’ poinsettia grown in a greenhouse compartment at constant RH (70 %) and either exposed to 7.5 minutes of 0.8 W m-2 UV EOD radiation or not exposed to UV indicated significant morphological responses to UV radiation, which resulted in a significant decrease in plant height, plant diameter, shoot length, internode length, leaf area and bract area, and also an increase in both leaf and bract thickness. No significant differences were found in time to flowering. Light response curves of ‘Christmas Day’ poinsettia indicate no effect of UV radiation on photosynthesis, though exposure to UV resulted in higher transpiration and stomatal conductance rates at all light intensities tested. In summary, the responses to UV radiation in poinsettia were dependent on ontogenetic stage, cultivar, and background climate such as RH and light conditions

UV radiation as a tool to control growth, morphology and transpiration of poinsettia (Euphorbia pulcherrima) in variable aerial environments

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Elevated air humidity increases UV mediated leaf and DNA damage in pea (Pisum sativum) due to reduced flavonoid content and antioxidant power

Growth in high relative air humidity (RH, >85%) affects plant morphology and causes diminished response to stomatal closing signals. Many greenhouses are prone to high RH conditions, which may negatively affect production and post-harvest quality. UV radiation induces stomatal closure in several species, and facilitates disease control. We hypothesised that UV exposure may trigger stomatal closure in pea plants (Pisum sativum) grown in high RH, thereby restoring stomatal function. The effects of UV exposure were tested on plants grown in moderate (60%) or high (90%) RH. UV exposure occurred at night, according to a disease control protocol. Lower stomatal conductance rates were found in UV-exposed plants, though UV exposure did not improve the rate of response to closing stimuli or desiccation tolerance. UV-exposed plants showed leaf curling, chlorosis, necrosis, and DNA damage measured by the presence of cyclobutane pyrimidine dimers (CPD), all of which were significantly greater in high RH plants. These plants also had lower total flavonoid content than moderate RH plants, and UV-exposed plants had less than controls. Plants exposed to UV had a higher content of cuticular layer uronic compounds than control plants. However, high RH plants had a higher relative amount of cuticular waxes, but decreased proteins and uronic compounds. Plants grown in high RH had reduced foliar antioxidant power compared to moderate RH. These results indicate that high RH plants were more susceptible to UV-induced damage than moderate RH plants due to reduced flavonoid content and oxidative stress defence.acceptedVersio

Elevated air humidity increases UV mediated leaf and DNA damage in pea (Pisum sativum) due to reduced flavonoid content and antioxidant power

Growth in high relative air humidity (RH, >85%) affects plant morphology and causes diminished response to stomatal closing signals. Many greenhouses are prone to high RH conditions, which may negatively affect production and post-harvest quality. UV radiation induces stomatal closure in several species, and facilitates disease control. We hypothesised that UV exposure may trigger stomatal closure in pea plants (Pisum sativum) grown in high RH, thereby restoring stomatal function. The effects of UV exposure were tested on plants grown in moderate (60%) or high (90%) RH. UV exposure occurred at night, according to a disease control protocol. Lower stomatal conductance rates were found in UV-exposed plants, though UV exposure did not improve the rate of response to closing stimuli or desiccation tolerance. UV-exposed plants showed leaf curling, chlorosis, necrosis, and DNA damage measured by the presence of cyclobutane pyrimidine dimers (CPD), all of which were significantly greater in high RH plants. These plants also had lower total flavonoid content than moderate RH plants, and UV-exposed plants had less than controls. Plants exposed to UV had a higher content of cuticular layer uronic compounds than control plants. However, high RH plants had a higher relative amount of cuticular waxes, but decreased proteins and uronic compounds. Plants grown in high RH had reduced foliar antioxidant power compared to moderate RH. These results indicate that high RH plants were more susceptible to UV-induced damage than moderate RH plants due to reduced flavonoid content and oxidative stress defence