School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemReproductive development of wheat under different thermal and photoperiodic environments( 1995)The overall objective of the thesis was to advance knowledge concerning phenological development in wheat. Specifically, it examines the variability of response to the main environmental factors. These are mean temperature, vernalising temperature, and photoperiod. Responses were examined by changing the environmental factors in various combinations, and the generality of the responses was gauged by including different cultivars in each study. The thesis includes some simple mathematical descriptions of the responses. The thesis has seven chapters describing and analysing specific experiments. Each chapter has its own introduction, results, discussion and conclusions. Particular chapters examine (i) if thermal amplitude affects wheat development independently of mean temperature, (ii) whether there is variability in the sensitivity to mean temperature among different cultivars and phenophases in relation to cardinal (base and optimum) temperatures, (iii) whether genetic variability in response to vernalisation and photoperiod can be described with numerical parameters, and whether these parameters change with development, (iv) whether rate of change of photoperiod can affect wheat development independently of absolute photoperiod, and finally (v) whether the interactions between temperature x photoperiod are important modifiers of development. The durations of the developmental phases the seedling stage (Haun stage < 1) to terminal spikelet initiation and from then to anthesis showed no evidence of systematic change due to thermal amplitude (ranging from 0 to 14 C, around an average temperature of 19 C) in any of four cultivars examined. Final leaf number and phyllochron were not significantly affected by thermal amplitude. The same four cultivars were then subjected to a range of average temperatures between 10 and 25 C. The duration of the stage from seedling growth to anthesis was reduced as temperature increased towards 19 C. Further increase in temperature did not alter duration in the cultivars Condor, Rosella and Cappelle Desprez, but increased duration in Sunset. Rate of development towards anthesis generally increased curvilinearly with temperature, so the response was reassessed in greater detail by subdividing the full period to anthesis into three phases. All responses in all cultivars could then be described numerically within the linear constraints of the thermal time concept. Base and optimum temperatures increased as development progressed towards anthesis. Averaging across cultivars, base temperature rose from -1.9 to +8.1 C for the phases before and after terminal spikelet initiation, respectively. Optimum temperature also increased. Cultivars differed substantially in each of these parameters. The progressive increase in optimum temperature with phasic development was apparently the main reason why linear fits for the three phases appear curvilinear for the full phase to anthesis. Final leaf number was negligibly changed by temperature, but phyllochron was significantly reduced as temperatures increased to 19 C. Cultivars differed in their base temperature for leaf appearance but had a similar optimum temperature of approximately 22 C. It is concluded that cardinal temperatures not only change with phase of development, and are specific for each genotype, but also that they can be different for developmental processes that are occurring at similar times. A model partitioning the response to vernalisation into three parameters, viz. optimum vernalisation (Vo), vernalisation sensitivity (Vs) and basic length (Lb) was proposed to analyse the responses to vernalisation in the cultivars Odin, Robin, Rosella and Condor. Vernalisation lasted from 0 to 70 d after seed imbibition and significantly reduced the time to anthesis in all cultivars, changing all three parameters in each of the pre-anthesis phenophases considered. All cultivars exhibited quantitative responses to all levels of vernalisation during the vegetative phenophase to double ridge. However, for the reproductive phases, Odin failed to reach anthesis if treated with less than 2 weeks vernalisation, indicating that vernalisation affects development beyond the vegetative phase. There were significant progressive reductions in final leaf number with longer periods of vernalisation. For the most sensitive cultivars, Rosella and Odin, the number of leaves appearing after double ridge was reduced by vernalisation. However, the number of leaves appearing after double ridge was only partially associated with the length of the reproductive phase. In the sensitive cultivars, phyllochron was shorter early in plant development than later, the change occurring at about leaf 6. In a parallel study, the vernalisation period was interrupted by a 3 d period of 18 C to investigate whether a moderate temperature can produce devernalisation. Partial devernalisation occurred in Rosella and Odin. In a field experiment, photoperiod was extended artificially in five treatments up to terminal spikelet initiation viz.; natural photoperiod (rate of change of photoperiod=2.3 min d-1 ), two faster rates of change (9.8 and 13.1 min d-1 ) and two constant photoperiods of 14.0 and 15.5 h. After terminal spikelet initiation, the two constant photoperiods were extended to 15.0 and 16.5 h, respectively, and treatments were randomly re-allocated. The rate of development from seedling emergence to terminal spikelet initiation responded to increases in photoperiod in both cultivars but there was no effect of rate of change of photoperiod. Phyllochron did not alter during plant development or in response to the photoperiod regimes. Finally, the effects on development of photoperiod (9, 12, 15, 17, 19 and 21 h) and temperature (21/17 and 16/12 C) in combination were studied. Again, four cultivars (a non-segregating awned selection of Sunset, Sunsetaw, Condor, Rosella and Cappelle Desprez) were used. Increases in both photoperiod and temperature always reduced the time to heading, but genotypes differed substantially in the magnitude of their responses to the individual environmental variables, and also in their responses to the different combinations. The interaction effects were sometimes greater than the individual effects. A model of the response of wheat development to temperature was proposed which includes the effects of photoperiod not only on thermal time but also on base temperature. Differential responses to short photoperiods were evident amongst genotypes, indicating that more than one degree of sensitivity to photoperiod might be possible for a single cultivar. Final leaf number on the main culm increased with shortening photoperiod, but was unaffected by temperature as observed previously. Although time to heading was always linearly related to final leaf number, the results suggest that photoperiod acted at least partially independently on the timing of heading and on final leaf number. The responses to photoperiod x temperature during three phenophases (pre-double ridge, from then to terminal spikelet initiation, and from then to heading) were assessed using a mathematical description which partitioned the response of each cultivar and phenophase into one or two photoperiodic sensitivities (Ps and Ps2), an actual maximum length (Lma) of the phase, which occurs at the critical photoperiod (Pc), a potential maximum length (Lmp) and a basic length (Lb) of the phase that occurs at the optimum (Po) or longer photoperiods. The duration of the early phase to double ridge was quantitatively affected by photoperiod and could be described by a single sensitivity value (Ps) which differed in magnitude between cultivars. The Po also differed amongst cultivars, and was longer at the higher temperature, while Lb during this phase showed a significant cultivar x temperature interaction. The duration of the phase from double ridge to terminal spikelet initiation was quantitatively responsive to photoperiod in all cultivars, and the response was affected by temperature. However, the responses of these two phases were different, as judged by their parameters. In this phase, Condor, Rosella and Cappelle Desprez showed a 3 to 5 fold greater sensitivity to very short photoperiods (Ps2) than to longer photoperiods (Ps). The response to photoperiod between terminal spikelet initiation and heading was also significantly affected by photoperiod, but its magnitude was different amongst cultivars. Sunsetaw showed a simple quantitative trend, while Condor and Rosella, which also had quantitative responses, responded in a more complex fashion with a much stronger sensitivity to very short photoperiods (< 12 h, Ps2) than to longer photoperiods (Ps). Cappelle Desprez had a qualitative response for very short photoperiods. It was concluded that (i) differences among cultivars in response to . photoperiod can be conveniently partitioned into different parameters for describing photoperiodic sensitivity, (ii) these parameters appear to be unrelated, allowing for speculation that plant breeders could manipulate them independently for customising cultivars for particular environments, (iii) the parameters were sensitive to temperature, suggesting that it would be inappropriate to extrapolate the response to photoperiod from one thermal environment to another, and (iv) the length of the late reproductive phase from terminal spikelet initiation to heading was not only significantly affected by photoperiod, but was even more sensitive to photoperiod than the early phase to double ridge. This thesis concludes with a chapter that discusses the relationships between the results from individual studies and identifies avenues for future work.