Tree and Forestry Science and Biotechnology ©2007 Global Science Books Germination Characteristics of Tree Seeds: Spotlight on Southern African Tree Species Simon A. Mng’omba1,2 • Elsa S. du Toit1* • Festus K. Akinnifesi2 1 Department of Plant Production and Soil Science, Faculty of Natural and Agricultural Sciences, University of Pretoria, 0002 Pretoria, South Africa 2 World Agroforestry Centre, SADC-ICRAF Agroforestry Programme, Chitedze Agricultural Research Station, P.O. Box 30798, Lilongwe 3, Malawi Corresponding author : * [email protected] ABSTRACT Many tree species of economic potential are propagated by seeds, but seed germination information is limited due to inadequate research. This frustrates genetic conservation, ecosystem restoration, domestication and biodiversity efforts. Deforestation, floods, drought and other disasters are endemic to many countries of Africa and these have a negative impact on the availability of many tree species which have a significant contribution as food and income sources. This review takes stock of seed germination and storage behavior of some important southern African tree species. Recalcitrant and orthodox seeds are the two extreme seed germination behaviors distinctly discussed, but information on seed germination and storage behavior of many southern African tree species is limited. Furthermore, knowledge on seed dormancy of recalcitrant or orthodox tree seeds is limited. Orthodox seeds tolerate relatively long storage periods, and hence have inherent primary seed dormancy. Seed dormancy for many recalcitrant tree seeds is not well explained, but such seeds are sensitive to desiccation injury and have a short storage period. Seed germination and storage behavior of many southern African tree species are not fully established with respect to secondary metabolites, fruit pulp composition, growth hormones and physical impediments. In this review, seed germination and storage behavior and different germination barriers will be the main focus. _____________________________________________________________________________________________________________ Keywords: hormonal balance, osmopriming, seed germination, seed testa CONTENTS INTRODUCTION........................................................................................................................................................................................ 81 IMPORTANCE OF TREE SPECIES ........................................................................................................................................................... 82 SIGNIFICANCE OF SEED GERMPLASM ............................................................................................................................................... 82 Seeds as vital propagules......................................................................................................................................................................... 82 Seeds for genetic improvement................................................................................................................................................................ 82 Seeds for genetic conservation ................................................................................................................................................................ 82 Seeds for molecular farming.................................................................................................................................................................... 83 SEED GERMINATION AND STORAGE BEHAVIOUR........................................................................................................................... 83 Recalcitrant seeds .................................................................................................................................................................................... 83 Intermediate seeds ................................................................................................................................................................................... 83 Orthodox seeds........................................................................................................................................................................................ 84 Seed viability in storage .......................................................................................................................................................................... 84 SEED DORMANCY ................................................................................................................................................................................... 84 Primary seed dormancy ........................................................................................................................................................................... 84 Secondary seed dormancy ....................................................................................................................................................................... 84 CHEMICAL COMPOSITION..................................................................................................................................................................... 85 Fruit pulp................................................................................................................................................................................................. 85 Plant hormones........................................................................................................................................................................................ 85 Phenolic compounds................................................................................................................................................................................ 85 SEED DETERIORATION ........................................................................................................................................................................... 86 SEED PRE-TREATMENTS ........................................................................................................................................................................ 86 Seed scarification..................................................................................................................................................................................... 86 Phytohormones........................................................................................................................................................................................ 87 Osmopriming........................................................................................................................................................................................... 87 STORAGE TECHNOLOGIES .................................................................................................................................................................... 87 Recalcitrant seeds .................................................................................................................................................................................... 87 Orthodox seeds........................................................................................................................................................................................ 87 CONCLUDING REMARKS ....................................................................................................................................................................... 87 REFERENCES............................................................................................................................................................................................. 88 _____________________________________________________________________________________________________________ INTRODUCTION and domestication (Berjak and Pammenter 2004). However, there has been little research on seed germination and sto- Tree seeds are stored as important genetic resources for rage behavior of many tree species, especially wild tree species biodiversity, ecosystem restoration, conservation species of southern Africa. It is envisaged that reliable but Received: 8 August, 2007. Accepted: 20 September, 2007. Invited Review Tree and Forestry Science and Biotechnology 1(1), 81-88 ©2007 Global Science Books cheap seed storage technologies could reduce forest genetic SIGNIFICANCE OF SEED GERMPLASM resource erosion or extinction ravaging many countries of southern Africa. This review takes stock of the knowledge Seeds as vital propagules gaps on seed germination and storage behavior of some im- portant tree species in southern Africa. Seed germination Seeds are still important starting materials for propagation and storage behavior and aspects of seed germination bar- of many vital tree species. Furthermore, the use of seeds as riers such as seed testa, pulp composition, hormonal bal- propagules has been considered the easiest and cheapest, ance and secondary metabolites (phenolic compounds) are and the most common means for many agroforestry and discussed in this review. timber tree species (Akinnifesi et al. 2007). Generally, this has been attributed to the fact that seeds are often easy to IMPORTANCE OF TREE SPECIES produce and handle (Black 1989). Comparatively, vegeta- tive propagation methods such as grafting, budding and air Biological diversity for tree species