02734nam a2200277 a 450000100080000000500110000800800410001910000190006024501040007926000410018330000100022450000160023452019620025065000130221265000220222565000100224765000260225765000230228365000230230665000220232965300170235165300210236865300220238965300260241165300190243715594701999-12-14       1984    bl uuuu  m   00u1 u    #d1 aGOEDERT, C. O.  aSeed dormancy of tropical forage grasses and implications for the conservation of genetic resouces.  aReading: University of Readingc1984  a190p.  aPhD.Thesis.  aInvestigations are described on the characteristics of tropical grass seeds which affect their management, particularly in genebanks. Five tropical forage grass species were selected for study - Brachiaria humidicola, Brachiaria decumbens, Panicum maximum, Paspalum guenoarum and Andropogon gayanus. It is shown that germination tests currently provide the best basis for monitoring viability. However two problems are particularly prevalent in tropical grasses which hinder the interpretation of germination tests: first, freshly harvested 'seeds' often show low purity (i.e. a high proportion of immature and empty spikelets); and secondly, many of the true seeds often show pronounced dormancy (i.e. though viable they are incapable of germination without special stimuli). Investigation of the purity problems showed that it can be overcome with the use of X-ray equipment, but other methods are unreliable. Investigation of the dormancy problem led to the conclusion that advantage needs to be taken of the interaction of several dormancy-breaking agents. It is shown that simultaneous use of three of these are specially valuable-alternating temperatures, light and nitrate ions. In particular it is important to define the optimum characteristics of alternating-temperature regimes for maximum stimulation of germination. Techniques for doing this were developed using a two-way temperature-gradient plate. Aplitude and thermoperiod were both shown to be particularly important attributes and, although the optima for each species investigated differed in detail, in no case did any optimum diverge far from a regime in which seeds were subjected to diurnal cycles of 20h at 15º and 4h at 35ºC in the presence of 10-2 M KNO3 and intermittent illumination. The significance of these and other experimental results are discused in relation to seed physiology and to problems of the management of seed accessions in genebanks for genetic conservation.  adormancy  agenetic resources  aseeds  aDormência da Semente  aFisiologia Vegetal  aGramínea Tropical  aPlanta Forrageira  aFeed grasses  aPlant physiolofy  aRescurso genetico  aResource conservation  aTropical crops