Studies on the physiological, environmental and biochemical factors affecting the germinability of seeds of some forest tree species

This works is a study on ecological, physiological and biochemical factors that affect the germinability of seeds of nine important and endangered tree species in Sudan. The study included seeds of : Adansonia digitata (Tabeldi), Balanites aegyptiaca (Higleig), Cordia africana (Gimbeel), Diospyros mespiliformis (Gughan), Khaya senegalensis (Mahogany), Anogeissus leiocarpus (Sahab), Terminalia brownii (Darout), T. laxiflora (Subagh) and Ziziphus spina-christi (Sidir). Objective of this work was to study the phenomenon of dormancy in the seeds of the above mentioned species and to find the best and appropriate method to break it aiming at the provision of the right information and the suitable techniques for foresters in charge of afforestation programmes and for tree planters in general to contribute in the success of tree planting processes in the Sudan. A second objective of this study was a comparison of storage of seed of the nine species in an ordinary store 30+2 deg C and the cool store 12+1 deg C for 42 months (temporary storage) in order to make use of cool storage facilities available at the National and Regional Tree Seed Centres for seeds that are sensitive to room temperature storage for temporary storage 3-5 years before distribution. This is to conserve these valuable natural resources and to minimize the seed collection cost by avoiding the annual seed collection. In the beginning, seeds physical and physiological characteristics were explored through the determination of purity percentage, seed soundness, seed moisture content, number of seed /kg, seed viability and germination. Also, the kind of dormancy found in each seed type was identified and the presence of growth inhibitors in seeds of A. leiocarpus, T. brownii and T. laxiflora was detected. Seed and seedling structure were also studied. A number of pretreatments were successively applied for each kind of seed in relation to the type of dormancy found in each of them to know the optimum seed pretreatment. Certain phenomena associated with seed dormancy and their effects on germination were studied, these included seed polymorphism of K. senegalensis, B. aegyptiaca and Z. spina-christi and multiple seedlings of C. africana and Z. spina-christi. For seed storageseeds of the nine species were stored in ventilated containers (cotton sacks) at room temperature 30+2 deg C and in air tight containers (drums and jericans) in the cool store 12+1 deg C for 42 months. Germination of stored seed was checked every 6 months. This is in addition to the study of the biochemical changes in content of seed macrometabolites elementsproteins, ols and carbohydrates every 6 months and the correlation of these changes with seed germinability. The study came up with important facts on dormancy of seeds of the nine speciesseed dormancy, seed and seedling structure were found to be strongly correlated with ecological and environmental conditions prevailing in their habitats and this was explained for each kind of seed and seedlings. Optimum dormancy breaking methods were achieved for seeds of A. digitata, C. africana, Z spina-christi and K. senegalensis. Seed polmorphism was found to have a significant effect on lowering seed germination in K. senegalensis, B. aegyptiaca and Z. spina-christi seeds of T. brownii, T. laxiflora and A. leocarpus were found to have an after ripening period of 18 months in the store to break their dormancy, besides they have combined dormancy of two or more types and they also have serious problems associated with pollination and fertilization apparent from the high percent of empty fruits and obortive seeds in them. They also showed strong indications of the presence of growth inhibitors. Seeds of D. mespiliformis were found not to have any type of dormancy and the delayed emergence of the cotyledons from the seed coat was found to be due to the thick endosperm which is used for the development of a long strong root system for seedling anchorage during the rainy season and to reach the deeply found water during the dry season. As far as storage big differences were found in response of the different seed kinds to storage conditions. Seeds of B. aegyptiaca, K. senegalensis and D. mespiliformis positively responded to cool storage and adversely affected by room temperature storage. On the contrary, seeds of A. digitata, C. africana, T. laxiflora and Z. spina-christi were adversely affected by cool storage, they entered an induced dormancy phase expressed in the significantly lowered germination of their seeds in the cool store than those in the ordinary store. Seeds of T. brownii and A. leocarpus were not affected by any of the storage conditions. This study revealed that storage behaviour of dormant tropical seeds is quite different from that of non-dormant tropical seeds. In the former it is difficult to predict the seed age during storage from the germination test only, because of dormancy unless other measures are used like, viability test - (TTZ) or application of an appropriate dormancy breaking treatment before germination. Drop in seed essential metabolites (protein, oils, carbohydrates) was found highly correlated with viability loss of seeds sensitivity to room temperature storage (B. aegyptiaca, K. senegalensis, D. mespiliformis) and seed sensitive to cool storage (A. digitata, C. africana, T. laxiflora, Z. sspina-christi) while those which were not affected by any of the stores (A. leiocarpus, T. brownii and T. laxiflora) eigher showed negative or no correlation, and this may be due to the after ripening period (18 months) these seeds need to be kept in the store. Several recommendations were drawn from this work. In addition to recommended dormancy breaking technique and storage conditions it is recommended that further studies are needed on: nature and effect of growth inhibitors in Terninalia spp. and A. leocarpus, andd the study of their phenology (flowering and fruiting), more studies on dormancy of K. senegalensis seeds. Genetic studies on B. aegyptiaca in relation to seed polymorphism. It is recommended that seed pretreatment techniques at the National Tree Seed Centre should be improved and new techniques introduced either imported or innovated locally. Storage conditions at the center also needed to be improved by lowering the temperature of the cool store to around 5*C and provision of a stand bye generator