| dc.description.abstracteng | Polyploidy, whole-genome duplication, enhances stress-tolerance to drastic environmental compared to their diploid progenitor by enabling more extensive adaptation as advantages of gene and genome duplication. Polyploidy acts as drivers of evolution and speciation in plants. Polyploidy in angiosperms is an influential factor to trigger apomixis, the reproduction of asexual seeds. Apomixis is usually facultative, which means that both sexual and apomictic seeds can be formed by the same plant. Environmental abiotic stress, e.g., light stress, can change the frequency of apomixis. Photoperiod stress in plants influences flowering, photosynthesis, growth, metabolite profiles, and production of reactive oxygen species (ROS). The light stress creates photodamage due to the inhibition of photosystem II (PSII) repair and alternation in the photosynthetic redox signaling pathways. Apomeiosis, the production of unreduced embryo sacs, versus meiotic development is influenced by ROS scavenging. The excess of ROS in reproductive tissue generates oxidative stress. In the archespor, oxidative stress might lead to DNA double-strand breaks (DSBs) and induction of meiosis as a DNA repair mechanism. Stress-adapted plants are able to maintain the metabolic network in ROS scavenging, including compatible solutes, antioxidants, and stress-responsive proteins. In polyploid plants, the higher stress tolerance reduces oxidative stress. Hence, in facultative apomictic polyploids, lowered stress levels could result in a decrease in proportions of meiotic ovules and favor apomeiotic development. The main aims of this research were to explore with prolonged photoperiods whether polyploidy alters proportions of sexual ovule and sexual seed formation under light stress conditions and to observed the extent of stress effect on photosynthesis in the leaves that appear together with the flower buds. I used three facultative apomictic, pseudogamous cytotypes of the Ranunculus auricomus complex (diploid, tetraploid, and hexaploid). Stress treatments were applied by extended light periods (16.5 h) and control (10 h) in climate growth chambers. Proportions of apomeiotic vs. meiotic development in the ovule were evaluated with clearing methods, and the mode of seed formation was examined by single seed flow cytometric seed screening (ssFCSS). I further studied pollen stainability to understand the effects of pollen quality on seed formation. In basal leaves, I analyzed the effect of extended photoperiod on photosynthesis efficiency as a proxy of stress conditions. The flower buds are covered by green sepals as photosynthetic tissue, and hence we expect the same photosynthetic performance and stress effects as in the basal leaves. Photosynthesis performance was measured by applying an extensive analysis of chlorophyll a fluorescence to record the parameters: PSII maximum efficiency (ɸPSII), the maximum quantum efficiency of PSII photochemistry (QY_max), relative electron transport rate (rETR), fluorescence induction curve (IC) of non-photochemical quenching (NPQ), and fast fluorescence transient curve (OJIP curve). Results revealed that under extended photoperiod, all cytotypes produced significantly more sexual ovules than in the controls, with the strongest effects on diploids. The stress treatment affected neither the frequency of seed set nor the proportion of sexual seeds nor pollen quality. Prolonged photoperiod did not enhance the photosynthesis efficiency (QY_max and ɸPSII) of three cytotypes of R. auricomus. Among cytotypes, diploids were the most sensitive to the extended photoperiod compared to polyploids as indicated by the alternation of non-photochemical quenching parameters (NPQ, qE, NPQE, and qN), specific energy flux parameters (ABS/RC, DI0/RC, and TR0/RC), and performance index on absorption basis (PI_Abs). In tetraploids, the fraction of light excess was quenched into photochemistry (qP), but another fraction exceeded the capacity of photon trapping (TR0/RC), hence dissipated as non-photochemical quenching (qL). The hexaploids presented a variation of photosynthesis performance among two clones which might relate to different habitats. These findings confirm our hypothesis that megasporogenesis is triggered by light stress treatments. Comparisons of cytotypes support the hypothesis that ovule development in polyploid plants is less sensitive to prolonged photoperiods and responds to a lesser extent with sexual ovule formation. Polyploids may better buffer environmental stress, which releases the potential for aposporous ovule development from somatic cells, and may facilitate the establishment of apomictic seed formation. The photosynthesis performance of R. auricomus relates to the mode of ovule formation, as diploids showed the highest sensitivity to prolonged photoperiod concomitant to the highest proportions of sexual ovules, followed by tetraploids. Hexaploids, however, exhibited a very large variance in the proportions of sexual ovules, which we also observed here in photosynthesis performance. I suppose that this variation is mostly referable to two different ecotypes | de |