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Literature review Table 2.3: Organic seed endogenous and exogenous dormancy types (Modified from BASKIN & BASKIN (1998)). Endogenous Exogenous Type Cause Broken by Physiological Physiological inhibiting mechanism (PIM) of germination Morphological Underdeveloped embryo Morphophysiological Physical PIM of germination and underdeveloped embryo Seed/fruit coats impermeable to water Warm/cold stratification Appropriate conditions for embryo germination/growth Warm/cold stratification Opening of specialized structures Chemical Germination inhibitors Leaching Mechanical Woody structures restrict growth Warm/cold stratification The causes of non-deep physiological dormancy are factors relating to the covering structure (BASKIN & BASKIN, 1998). These factors include the physical barrier created by these structures, the resulting oxygen supply to the embryo, inhibitors within the covering structures and changes in the covering structures (BASKIN & BASKIN, 1998). Iris lorteti is an example of a species with seeds exhibiting non-deep physiological dormancy as a result of the physical restriction caused by their seed coats (BASKIN & BASKIN, 1998). It takes a force of 133.2 MPa, which can only be overcome by an embryo with sufficient growth potential, to break the seed coat of this species (BASKIN & BASKIN, 1998). The dormancy of such seeds can often be broken by a cold or hot stratification treatment (COPELAND, 1976). Such a treatment is performed by placing moistened seeds at low temperatures (3 to 10°C) for a certain period of time (COPELAND, 1976). In some cases (European ash seed) dormancy can only be overcome by stratification (COPELAND, 1976). In such cases the growth-stimulating substance produced during stratification breaks dormancy caused by inhibitory chemicals within 55
Literature review the embryo (COPELAND, 1976). Stratification has also been known to decrease the time to germination and increase growth rate in other species (COPELAND, 1976). Stratification may also decrease the sensitivity to external conditions, so that a seed might, for example, germinate at a less suitable temperature (COPELAND, 1976). The germination rate may also be improved by exposing seeds to different day/night temperatures (COPELAND, 1976). In some cases embryonic dormancy may be broken by exposure to a certain light intensity, wavelength and/or photoperiod (COPELAND, 1976). In some cases dormancy caused by inhibitory chemicals within the embryo can be broken by gibberellic acid (COPELAND, 1976). Other chemicals which are known to break non-deep physiological dormancy include potassium nitrate, kinetin and ethylene. The effects of endogenous physical dormancy also diminishes as seed age increases (COPELAND, 1976). In the case of morphological dormancy, germination is prevented at the time of maturity due to the morphological characteristics of the embryo. The embryo is underdeveloped or even undifferentiated at the time of dispersal and a period of growth, known as after-ripening, is required before the seed can successfully germinate (COPELAND, 1976; BASKIN & BASKIN, 1998). Morphological dormancy occurs in seeds with rudimentary and linear embryos. Most of the interior of these seeds is occupied by endosperm and the embryo may only be 1% of the seed volume or less (BASKIN & BASKIN, 1998). Morphophysiological dormancy is essentially a combination of the two dormancy types. In this case the embryo must grow to a species-specific critical size and the physiological dormancy of the seed must occur before germination can take place (BASKIN & BASKIN, 1998). The primary reason for dormancy of seeds with exogenous physical dormancy is the impermeability of their seed or fruit coats to water (BASKIN & BASKIN, 1998). Seeds with physical dormancy frequently have a palisade layer of lignified cells in the testa or pericarp (BASKIN & BASKIN, 1998). The breakdown of such hard seed coats in the natural environment occurs through the gradual processes of hydration and dehydration, exposure to hot and cold temperatures, scorching by fire and 56
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Propagation of Romulea species Pier
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Contents 2.7 GERMINATION PHYSIOLOGY
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Contents 5.3.1 Explants from seedli
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Abstract The suitability of various
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Declarations I Pierre André Swart,
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Declarations DETAILS OF CONTRIBUTIO
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Publications from this thesis ASCOU
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List of Figures Figure 2.13: Suther
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was significantly different from th
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List of Tables Table 2.1: Names of
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List of Abbreviations 2,4-D 2,4-Dic
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Die vlakhaas spits sy oor hy het di
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As die mens dan ook so sy dankbaarh
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Introduction where a great number o
Page 29 and 30: Introduction The subgenera Romulea
Page 31 and 32: 2 Literature review Leef van daad e
Page 33 and 34: Literature review Figure 2.2: Life
Page 35 and 36: Literature review Figure 2.4: Life
Page 37 and 38: Literature review The plants are sh
Page 39 and 40: Literature review flowers (DE VOS,
Page 41 and 42: Literature review often found on cl
Page 43 and 44: Literature review flowers of R. lei
Page 45 and 46: Literature review flattened upper s
Page 47 and 48: 2.2.16 Romulea tabularis Literature
Page 49 and 50: Literature review extinct, R. papyr
Page 51 and 52: Literature review R. leipoldtii occ
Page 53 and 54: Literature review Figure 2.8: Calvi
Page 55 and 56: Literature review Figure 2.14: Fras
Page 57 and 58: Figure 2.20: Nieuwoudtville weather
Page 59 and 60: Literature review Figure 2.26: Grah
Page 61 and 62: Literature review Figure 2.29: Diag
Page 63 and 64: Literature review Figure 2.30: A te
Page 65 and 66: Literature review Macronutrients ca
Page 67 and 68: Literature review The soils of the
Page 69 and 70: Literature review The embryo is the
Page 71 and 72: Literature review Phase 2 is seen a
Page 73 and 74: Literature review endogenous gibber
Page 75 and 76: Literature review tolerable level (
Page 77 and 78: Literature review these channels pe
Page 79: Literature review 2.8.7 Seed dorman
Page 83 and 84: Literature review germinated under
Page 85 and 86: Literature review (COPELAND, 1976).
Page 87 and 88: 2.9 BRIEF REVIEW OF IN VITRO CULTUR
Page 89 and 90: Literature review (DEBERGH, 1994).
Page 91 and 92: Literature review PIERIK (1997) sta
Page 93 and 94: Literature review The distinguishin
Page 95 and 96: Literature review The essential fun
Page 97 and 98: 2.9.3.4 Abscisic acid Literature re
Page 99 and 100: Table 2.6: Example of a matrix to e
Page 101 and 102: Literature review Young embryos req
Page 103 and 104: Literature review and the addition
Page 105 and 106: Literature review organ is then pro
Page 107 and 108: Literature review environmental str
Page 109 and 110: 2.11 IN VITRO FLOWERING Literature
Page 111 and 112: Literature review higher regenerati
Page 113 and 114: Subfamily Ixioideae (continued) Tri
Page 115 and 116: Literature review Table 2.8: Corm i
Page 117 and 118: 3 Investigation into the habitat of
Page 119 and 120: Soil sampling and analysis The firs
Page 121 and 122: Soil sampling and analysis Table 3.
Page 123 and 124: 4 Germination physiology 4.1 INTROD
Page 125 and 126: 4.2.2 Water content and imbibition
Page 127 and 128: Germination physiology (-N), phosph
Page 129 and 130: Germination physiology rosea seeds
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Germination physiology Figure 4.4:
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Germination (%) 50 40 30 20 10 0 77
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Germination (%) 100 80 60 40 20 0 c
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Germination physiology The relative
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Germination physiology seeds of R.
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5.2 MATERIALS AND METHODS In vitro
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In vitro culture initiation and mul
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5.2.3 Explant comparison In vitro c
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5.4 DISCUSSION In vitro culture ini
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6 In vitro corm formation and flowe
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In vitro corm formation and floweri
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6.3 RESULTS 6.3.1 Corm formation In
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Commercialization potential of Romu
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Commercialization potential of Romu
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BASKIN, C. C., and BASKIN, J. M. (1
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Literature cited DOWLING, P. M., CL
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Literature cited HILTON-TAYLOR, C.
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Literature cited KUMAR, A., SOOD, A
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Literature cited PIERIK, R. L. M. (
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Literature cited STEINITZ, B., COHE
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