Investigations on the effects of harvest methods and storage conditions on yield, quality and germination of evening primrose (Oenothera biennis L.) seeds
Evening primrose has a good potential to become a commercial agricultural plant for GLA production, some disadvantages such as indeterminate inflorescence, high seed shattering during ripening, length of life cycle, lack of uniformity in seed germination are still problems to be overcome for this to happen. Although several studies have carried out to find ways of removing the seed shattering as well as reducing the flowering heterogeneity, seed shattering still posses the biggest problem in the production of evening primrose. This project was an agronomic procedure aimed at optimising the appropriate harvest time and harvest method that will lead to higher quality and quantity seed (in aspect of seed yield, oil and γ-linolenic acid percentage). Experience has shown that defoliation before harvest usually accelerates seed ripening and increases the homogeneity of seed maturation. The harvest method used in these experiments was defoliation by means of flame and by chemical. To date our knowledge is still limited about the effect of fertilizers (especially nitrogen) on seed yield and the percentage of oil of evening primrose seed. In this project, the effect of nitrogen under different harvest methods was studied by using some pot experiments.
The effect of storage conditions on the seed oil percentage and fatty acid composition of evening primrose were also investigated. In addition to these seed germination ability of evening primrose was also studied. For field experiments seed yield, thousand seed weight, oil percentage, raw protein percentage, fatty acid composition, plant dry matter, seed dry matter were measured. The parameters measured in the pot experiments were seed yield, thousand seed weight, oil and raw protein percentage, fatty acid composition, number of side shoots per pot, number of capsules per side shoot, number of capsules per main stem and total number of capsules per pot.
Both in field and pot experiments it was observed that harvest time had influence on the seed yield of evening primrose. Due to low maturity, a significant reduction in the seed yield of early harvested plants was observed. Similar pattern of seed yield was observed in plants that were harvested late. Contrary to early harvest high seed shattering is the main reason of low seed yield in the plants that were harvested at the late date. Oil and γ-linolenic acid percentage of the seed were significantly influenced by different harvest times. Low seed maturity at the early time and advance senescence of the seed during the over ripened period are the main reasons for the low percentage of oil. Regardless of the sowing time, in both field as well as pot experiments, it was observed that in most cases the maximum seed quantity and quality was achieved during the middle harvest time.
Despite the above interpretation of results, it is not easy to say how many days after flowering can be recommended as harvest time. The parameters such as plant variety, climate conditions, soil fertility, sowing time, etc. strongly influence harvest time. Generally, according to this study the average of the period after flowering to harvest in autumn and spring sown plants is different. In addition, nitrogen can influence this period. In total, 70-75 days after flowering is the recommended harvest time of the spring sown Oenothera biennis L. cv. Anothera in Germany. In autumn sown plants, the duration between flowering and harvest is longer than spring-sown plants (90-95 d after flowering). Study on the effect of nitrogen on the evening primrose in pot experiments showed that although nitrogen had negative influence on the percentage of seed oil, there was positive relationship between nitrogen and most measured parameters especially seed yield, linoleic and γ-linolenic acid percentage. There is a need to carry out further investigation on the effect of nitrogen as well as other plant nutrients on evening primrose in the field conditions.
In general based on obtained results it can be concluded that in both field and pot experiments no clear effect of defoliation was observed on the different measured parameters like seed yield, the percentage of oil and γ-linolenic acid. The observations of these experiments in most cases do not confirm the hypothesis in which defoliation reduces the heterogenity of seed ripeness and effects seed yield and seed quality. In most cases no interaction between method of harvest and harvest time as well as nitrogen fertilizer was observed. The hypothesis in which effect of defoliation depends on harvest time and nitrogen application is different was not agree with the results obtained.
the results of experiments, it can be concluded that there is not a big difference in the seed yield and seed oil quality of spring and autumn evening primrose. Thus, the hypothesis that the performance of spring-sown evening primrose is equal to autumn sown plants was confirmed with the obtained results.
results showed that the quality of newly harvested seeds of evening primrose gradually decrease during storage. The free fatty acid percentage and the peroxide value of stored seeds strongly increased with the storage time. It was observed that the increase was lower in the seed samples that were stored at low temperature than under room temperature and higher. A negative relationship between storage time and the percentage of oil was observed in this study. Based on the results obtained from the present study, it may be said that for oil quality and quantity the fresh seeds of evening primrose can be stored for a period of 4-6 months.
Temperature is the main factor affecting the germination of evening primrose seeds. A synergetic effect between light and temperature on evening primrose seed germination was observed in this study. Depending on harvest time a gradual increase in the seed germination percentage of newly harvested seed could be achieved by removing the physiological dormancy of embryo during storage. The reduction of germination with increasing storage time could be due to some stimulation of secondary dormancy in the embryo.
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