Kõrgendatud CO2 ja O3 kontsentratsioonide mõju fotosünteesi parameetritele ameerika haava lehestikus: päevased, sesoonsed ja aastatevahelised erinevused

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Inimtegevuse tagajärjel on suurenenud süsinikdioksiidi kontsentratsioon ([CO2]) ja osooni kontsentratsioon ([O3]) maalähedases atmosfäärikihis. Lisaks sellele prognoositakse kliima muutumist heitlikumaks ning esineda võib nii põua- kui liigniiskuse perioode. Kõrgem [CO2] üldjuhul soodustab taimede, sealhulgas puude kasvu, kuna [CO2] tõus põhjustab netofotosünteesi (Pn) suurenemist. Kõrged osooni kontsentratsioonid on puude kasvu ja metsa produktsiooni seisukohalt aga kahjulikud kuna osoon on tugev oksüdeerija, mis kahjustab fotosünteesiaparaati ning põhjustab Pn vähenemist. Pn väärtus näitab, kui palju lehe pinnaühik ajaühikus süsihappegaasi seob ning lehestiku pinnaga korrutatult iseloomustab see süsiniku hulka, mida taim ajaühikus kasvuks kasutada saab. Käesoleva doktoritöö eesmärk oli välja selgitada, kuidas mõjutavad kõrgendatud [CO2] ja/või [O3] kiirekasvuliste lehtpuude fotosünteesi iseloomustavaid parameetreid ning millest need mõjud sõltuvad. Töö tulemusena selgus, et Pn-i tundlikkuses kõrgendatud CO2 ja/või O3 kontsentratsioonidele esinesid nii päevased, sesoonsed kui ka aastatevahelised erinevused. Samuti muutis Pn-i tundlikkust keskkonnastress (põud ja kõrged temperatuurid). Keskkonnastress leevendas osooni negatiivset mõju, aga suurendas CO2 positiivset mõju Pn-le. Üldiselt oli CO2 ja/või O3 mõju Pn-le sügisel suhteliselt suurem kui suvel. Samuti selgus, et CO2 positiivne efekt Pn-le on ajas pigem kasvanud kui kahanenud. Kõrgendatud osooni negatiivne mõju Pn-le on aga jäänud 11 aasta jooksul praktiliselt samasuguseks. Kokkuvõtvalt näitas käesolev uurimistöö, et oluline on eelkõige mitme faktori koosmõju mõistmaks taimes toimuvaid muutuseid globaalselt muutuvas kliimas. Samaaegselt esinevate faktorite kombinatsioonid võivad taimede kasvu ja arengut mõjutada palju enam kui üksik faktor. Kuna muutused netofotosünteesis (aga ka õhulõhede juhtivuses) mõjutavad nii süsiniku sidumist kui vee tarbimist taimede poolt, siis võivad need muutused avaldada märkimisväärset mõju kogu ökosüsteemi süsiniku- ja veeringele. Seetõttu tuleks käesolevas doktoritöös kirjeldatud interaktiivseid mõjusid võtta arvesse ökosüsteemide produktsiooni ja aineringet kirjeldavates mudelites.The consequences of human activities have rapidly increased the concentrations of the main greenhouse gases, atmospheric CO2 ([CO2]) and tropospheric ozone ([O3]). In addition, it is predicted, that the extreme weather conditions can be more frequent: both draught and water-logging can occur. Elevated [CO2] is generally beneficial for plants, because CO2 is a substrate for photosynthesis and causes increases in light-saturated net photosynthesis, Pn. Tropospheric ozone is known to have a negative effect on plant growth and productivity. As a strong oxidant, ozone causes damage to photosynthetic apparatus and decreases Pn. The value of Pn shows the amount of CO2, entering each unit of leaf area per unit of time and Pn, multiplied by leaf area, determines the carbon gain of foliage during time unit. The objective of this thesis was to find out how and why the long-term effects of elevated [CO2] and/or [O3] on photosynthetic responses vary in fast growing hardwood trees. Our findings demonstrate that the photosynthetic responses to increasing [CO2] and/or [O3] are changing in diurnal, seasonal and interannual scales and depend on environmental constraints such as drought and high temperature. Drought and high temperature stress alleviated the negative impact of ozone and increased the positive impact of CO2 on Pn. We found that the relative effects of elevated [CO2] and/or [O3] on Pn were generally more pronounced in autumn compared to summer. These results also provide novel evidence that the [CO2] effect has been increasing rather than decreasing in time, but the negative ozone effect has remained the same over the 11 years of the study. This study highlights the importance of multiple factors in determining the future responses of trees to climate change. The key conclusion of this study is that exposure to combined factors can influence trees even more than exposure to a single factor. As changes in photosynthesis (but also in stomatal conductance) are likely to affect both the ability of plants to sequester carbon, and plant water use, these changes can affect ecosystem carbon- and hydrological cycles. Consequently, interactions discovered in this thesis should be taken into account in models that predict changes in productivity of forest ecosystems and the feed-backs from these changes on climate

Mechanisms for minimizing height-related stomatal conductance declines in tall vines

The ability to transport water through tall stems hydraulically limits stomatal conductance (g(s)), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height-related decrease in g(s), regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines in g(s) with height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximum g(s) along 15-m-long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood-to-leaf-area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching -1.9 MPa, far below the wilting point (-1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8-12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate that g(s) of distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.Peer reviewe

Will Photosynthetic Capacity of Aspen Trees Acclimate After Long-Term Exposure to Elevated CO2 and O3?

Photosynthetic acclimation under elevated carbon dioxide (CO2) and/or ozone (O3) has been the topic of discussion in many papers recently. We examined whether or not aspen plants grown under elevated CO2and/or O3 will acclimate after 11 years of exposure at the Aspen Face site in Rhinelander, WI, USA. We studied diurnal patterns of instantaneous photosynthetic measurements as well as A/Ci measurements monthly during the 2004–2008 growing seasons. Our results suggest that the responses of two aspen clones differing in O3 sensitivity showed no evidence of photosynthetic and stomatal acclimation under either elevated CO2, O3 or CO2 + O3. Both clones 42E and 271 did not show photosynthetic nor stomatal acclimation under elevated CO2 and O3 after a decade of exposure. We found that the degree of increase or decrease in the photosynthesis and stomatal conductance varied significantly from day to day and from one season to another

Rising atmospheric CO₂ explains 26–52% of the recent delay in autumnal senescence in important forest and crop species

There is strong evidence to suggest that global warming is leading to an extended growing season by altering the timing of autumnal events such as bud set and leaf abscission 1,2,3, with important impacts on ecosystem productivity and global carbon cycling. However, while temperature is an important driver of spring phenological events, the relationship between temperature and autumn phenology is weak4. Here, we present results from three open-air field experiments in which elevated atmospheric CO2 concentration [CO2] at the concentration likely to exist in 2050, extended the growing season of: (1) three abundant North American forest trees; (2) the world’s most extensively grown broad-leaved crop (soybean); and (3) two European poplars. Across experiments and over multiple years, elevated [CO2] delayed autumnal declines in leaf area, chlorophyll concentration, photosynthesis and normalized vegetation difference index (NVDI) by 2-7 days for soybean and 5-15 days for trees. These findings indicate that [CO2] alters growing season length and the rise in atmospheric [CO2] over the past 30 years could explain 26-52% of the extended growing season now ascribed to warming3