Photoprotective Isoprenoids as Indicators for Stress Responses in Forest Trees

For long-lived forest tree species, intraspecific variation among populations in their response to environmental conditions can reveal their ability to cope with and adapt to climate change. Plants constantly adjust the pigment composition of the photosynthetic apparatus to environmental conditions. Under abiotic stress conditions (e.g. drought), plants induce isoprenoid-mediated photoprotective mechanisms such as non-photochemical quenching (NPQ) and increased biosynthesis of antioxidants to minimize photooxidative stress. This thesis investigated the intraspecific variation in the isoprenoid metabolism in Douglas-fir (Pseudotsuga menziesii) comparing Douglas-fir provenances, which represent populations from locations with contrasting environmental conditions. Furthermore, the influence of foliar photosynthetic pigments on leaf optical properties was studied using senescing sugar maple (Acer saccharum) leaves. First, I established a simple and cost-effective protocol for the rapid analysis of isoprenoids using high-performance liquid chromatography (HPLC) (Chapter 2). Three experiments were conducted to evaluate 1) the adjustments of the isoprenoid metabolism when photosynthesis is limited in response to environmental conditions (Chapter 3 and 4); 2) provenance-specific adjustments of photoprotective isoprenoids in response to drought (Chapter 3 and 4); and 3) the influence of senescence-associated changes in isoprenoid levels on leaf optical properties in sugar maple (Chapter 5). In chapter 3, photosynthesis and photosynthetic pigments in seedlings of two Douglas-fir provenances were compared under controlled drought conditions. In chapter 4, intraspecific variation in photosynthesis and photosynthetic pigments in response to changing environmental conditions were studied in mature trees of four provenances over the course of two years. Both experiments revealed that the more drought-tolerant interior provenances exhibit enhanced carotenoid-chlorophyll ratios, and larger pools of xanthophyll cycle pigments and β-carotene compared to coastal provenances from mesic habitats. This provenance-specific variation demonstrated the importance of the isoprenoid metabolism for the adaptation of provenances to drought. In chapter 5, the leaf optical properties of senescing sugar maple leaves were studied. The degradation of photosynthetic pigments as indicator for the progress of senescence was reflected by spectral reflectance measurements and digital image analysis. Isoprenoid metabolism may thus be a potential trait for selection of provenances for future forest management and indicator for remote-sensing of the plant physiological status.Ph.D