School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemGenotype and environmental influences on phasic development in wheat (Triticum aestivum L.) and the expression of yield components, especially spikelet number per head( 1995)The variation in, and the influence of, certain environmental factors on preanthesis phases of development in wheat was examined with particular reference to the number of spikelets produced per head. When the pre-anthesis phase was divided into three phases; the Ieaf initiation, spikelet initiation and culm elongation phases, considerable cultivar variation was found in the durations and rates of the three phases. A cultivar was found that departed from the general negative correlation between rate and duration of spikelet initiation giving possible scope for breeding for increased spikelet number without altering the duration of spikelet initiation. Variation in the rate and duration of the three development phases was also found for a selection of diploid and tetraploid wheat. For these species, spikelet number was found to be more closely associated with the duration of spikelet initiation. This character could be of use in long-season wheat cultivars. When the effects of photoperiod and light intensity on wheat phasic development and spikelet number were compared, photoperiod was found to have more influence. The transfer of wheat cultivars between long and short photoperiods at double ridge and terminal spikelet determined that the rate of development was influenced by a "memory" effect; both prior and current photoperiods influenced the rate of development. It was also noted that initial exposure to long photoperiod could have a sustaining effect on wheat development. Subjecting wheat lines to increased temperature increased the durations of development, in terms of thermal time, indicating that the relationship was not linear. The durations of pre- and post-terminal spikelet phases were found to respond differently to temperature. A selection of 6 wheat cultivars, varying in time to anthesis were grown in the field and it was found that photoperiod responses exerted the major influence on the durations of development. Basic development responses and vernalisation were found to exert comparatively less influence on development. The importance of basic development responses were not discounted as a means for breeding wheat cultivars for specific environments.
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ItemCytology and fertility of wheat-rye (Secale) hybrids with induced pairing between homoeologous chromosomes( 1986)Studies were made of the influence of individual rye chromosomes and their arms on crossability, chromosome pairing and pollen and spikelet fertility in hybrids of hexaploid wheat cv. 'Chinese Spring' and its two homoeologous pairing mutants, ph2a and phIb , each with six wheat (cv. 'Holdfast') rye ('King II') chromosome addition lines and their telocentrics. Crosses were also made of the three 'Chinese Spring' parents each with seven rye (Secale) accessions, including different species. Studies were made of crossability, hybrid viability, pollen and spikelet fertility, and chromosome pairing in both the amphihaploids and amphidiploids from these crosses. Studies were also made of crossability, chromosome pairing and pollen and spikelet fertility in the F1 of crosses of the three 'Chinese Spring' wheats with both an octoploid and a hexaploid triticale. In these studies new information was being sought, both on genetic and evolutionary affinities between rye and hexaploid wheat and on approaches for the more efficient exploitation of genetic variation in the rye gene pool for wheat improvement.