School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffects of post-anthesis stress on grain filling and malting quality of barley( 1995)Malting quality is variable from year to year in many countries with Mediterranean or continental climates. Environmental stresses are thought to contribute to that variability. However, little information is available on the relationships between environmental conditions during grain growth and malting quality. Two of the most frequent environmental constraints during grain filling in many cereal-growing areas of the world are high temperature and drought. Short periods (ca. 5 days) of very high maximum temperature (>35C) are quite common during grain growth and have been identified as a potential source of variation in malting quality. Therefore, the main objectives of the present study, were to determine the effect of short periods of high temperature and drought on grain growth and malting quality of barley. The study involved seven experiments, in which short periods (5 days) of high temperature and drought, combined or alone, were imposed during grain filling on the malting cultivar Schooner. Where possible, other malting cultivars were included (Chapters 2 and 3). The effects of short periods of high temperature on grain yield and malting quality of barley were first assessed under field conditions using portable chambers with thermostatically-controlled electric heaters. High temperature imposed for 5 days (17 to 21 days after anthesis) with a maximum temperature of 40C maintained for 6 h per day reduced grain weight by 13% in Schooner and 25% in Parwan. There was a reduction in starch content and an increase in nitrogen content in the heat treatments, but B-glucan content was not affected. High temperature reduced the amount of 'maltable' grain by reducing grain size and increasing screening percentage, and also reduced malt extract by 3-7%, which represents a large decrease for the malting industry. The other experiments in this thesis were carried out under controlled-environment conditions, in order to overcome difficulties of temperature and humidity control. Short periods of high temperature were imposed for 5 or 10 days at mid-grain filling on Schooner and Franklin, with or without drought treatments. Short periods of high temperature reduced grain weight by 5%, while drought reduced it by 20%. High temperature and drought together resulted in the greatest reduction (30%). There was a reduction in starch content and an increase in diastatic power and ?-glucan degradation under stress. However, malt extract was not significantly affected. To determine the importance of timing of short periods of high temperature and drought on grain weight and malting quality, a glasshouse experiment was carried out in which Schooner barley was exposed to these stresses at early, mid or late grain filling. Individual grain weight was most sensitive to high temperature and drought treatments imposed early in grain filling (10-15 days after anthesis) and was less sensitive to later treatments. Starch was reduced in amount and quality, especially with early stresses during grain filling. However, malt extract was not significantly affected. Finally, two experiments were carried out in the Canberra phytotron to study the effects of the temperature regime before and after heat stress on grain growth and quality. In the first experiment, the hypothesis that under a gradual increase in temperature, plants could develop some acclimation was tested. Plants experiencing either a sudden or a gradual increase did not exhibit any differences in grain weight or malting quality, but increasing the temperature in two steps (so that plants were exposed to 30 or 34C for 2 h before a 40C heat stress), appeared to have produced acclimation, since the reduction in grain weight under the two step treatment was about half that of either sudden or gradual increase in temperature. In the second experiment, the hypothesis tested was that grain growth would recover better from short stress under cool (21/16C) than warm (27/22C and 30/250 conditions following that heat stress. The reduction in yield caused by heat stress was not alleviated by the succeeding moderately high temperatures. The following conclusions were derived from this study: (i) the reduction in grain weight ranged from 5 to 35% in response to short periods of high temperature and drought during grain filling in barley. The magnitude of the reduction depended on duration and timing of exposure, (ii) the reduction in grain weight was accompanied by an increase in screening percentage corresponding to a large reduction in amount of 'maltable grain', (iii) grain composition was altered by these stresses, and in general, starch content was most affected. There was a strong and positive relationship between the reduction in grain weight and starch content per grain (R2=0.92, P<0.001). In all the experiments, there were reductions in the volumes of both A- and B-type starch granules; however, the reduction in grain weight was mostly closely related to the reduction in the volume of Atype starch granules. The stress-induced increase in nitrogen percentage was smaller than expected, probably because post-anthesis availability of nitrogen was less limited than under typical field conditions. Grain ?-glucan content tended to be reduced under drought but there was no clear trend under heat stress, and (iv) malt extract was not highly responsive in any of the high temperature or drought experiments. Malt extract was reduced by 3 to 7% in the field experiments (Chapter 2) and by 5% in a glasshouse experiment (Chapter 5) with short periods of heat stress. Although small relative to the grain yield reductions observed, such changes in malt extract are large for the malting industry. High temperature and drought affected several components of malting quality in opposing directions, for example the stresses reduced starch content, which would tend to reduce malt extract but also tended to decrease ?-glucan and increase diastatic power which would tend to increase malt extract. The net result of these opposing changes was generally a minor effect of heat stress and drought on malt extract, even though the main quality components contributing to malt extract often strongly responded to these stresses.
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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.