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Changes of the free amino acids in unpollinated, cross-, and self-pollinated styles of clonal material fromOenothera missouriensis were observed between 0 and 48 hours after pollination. 1. The small amounts (0.1-0.5 × 10(-9) mol/mg dry weight) of the amino acids Gly, α-NH2-But, Ileu, Leu, Tyr, Phe, Orn, Lys, His and Arg show practically no changes in time and no differences among the 3 pollination conditions. 2. The active amino acids Thr, Ser, Pro, Ala, Val, Glu, Eth-NH2, γ-NH2-But and Asp with a higher initial level (0.4 - 10.5 × 10(-9) mol per mg dry weight) showed some dramatic changes and differences under the various pollination conditions. 3. Critical phases of oscillation of the amino acid pool were observed in unpollinated styles between 24 and 48 hours after anthesis. In both types of pollinated styles, striking changes were observed 12 hours after pollination. In the self-pollinated styles a maximum content of free amino acids was observed 48 hours after pollination. 4. In the unpollinated styles most of the free amino acids increased slightly with aging. Glu showed a distinct increase, which continued up to 80 hours after ripening of the style. 5. In cross-pollinated styles the content of Thr, Ala and Val increased slowly during the first 24 hours and then decreased with a minimum at 40 hours after pollination. Glu, Eth-NH2 and γ-NH2-But have a characteristic course with a maximum at 24 and a minimum at 36 hours after pollination. 6. In self-pollinated styles the high initial content of Pro, resulting from the input of pollen, decreased continually until 33 hours, but increased again after the complete inhibition of the pollentube growth. The amino acids Thr, Ser, Ala and γ-NH2-But too, have a parallel course first with an initial high at 18 hours followed by a maximum level at 42 hours after pollination. 7. A summarizing interpretation includes the following aspects: a) As a consequence of pollination, no new amino acids were activated or synthesized inOenothera styles. b) For cross- and self-pollinated styles an initial phase was distinguished, which lasted about 12 hours. It was similar for the active amino acids as well in the self- and cross-pollinated styles. Distinct differences were observed after this time. c) The maximum content of free amino acids in cross-pollinated styles corresponds with the penetration of the mass of the pollen tube tips to the base of the style. d) The main difference between self- and cross-pollinated styles is the behaviour of Pro and Glu in the pool of free amino acids. This supports the hypothesis that the main part of the free amino acids is used for metabolic purposes during pollen tube penetrance through the style. Specific consumption of amino acids in connection with the synthesis of proteins, which may be responsible for the incompatibility reaction, are overshadowed by the mass shifts of the metabolic amino acid pool.