School of Agriculture, Food and Ecosystem Sciences - Theses
Permanent URI for this collection
Search Results
1 - 10 of 15
-
Item
-
ItemTowards association studies in Pinus radiata D.Don - populations and wood property candidate-genes( 2006)In Australia and New Zealand Pinus radiata D. Don wood quality is receiving increasing interest from tree breeders. This is partly due to declining resource wood quality associated with more advanced generation breeds leading to increased rejection and product downgrading in processing. While log segregation and wood grading at mill-door yields immediate benefits to processors the underlying cause is not addressed. The only long-term solution is to include wood quality in breeding programs. Wood quality traits are costly and difficult to measure. Marker-assisted selection offers a potential solution and quantitative trait loci (QTL) mapping studies have been undertaken with the aim of facilitating this. It is becoming increasingly clear that in widely outcrossing species with long generation times and very large genomes these approaches will not work. The identification of the underlying genetic sites, or tightly linked marker sites, would rectify this and in model species linkage-map based cloning has been used extensively. This approach is also not practical in species such as P. radiata. Association testing combined with a candidate-gene approach is therefore widely believed to be one of the only methods remaining. This approach uses a priori information to select and then test the phenotypic effects of variants within candidate-gene loci. Implementation of these studies relies heavily on the results of other investigations, especially those that generate DNA sequence information. Also required is detailed knowledge of the genetic population structure, the patterns of nucleotide diversity and the patterns of linkage disequilibrium. On a more practical level suitable populations need to be identified while the current methods for the collection and handling of samples for molecular investigations are limiting. The selection of candidate genes is also a non-trivial process. For candidate-gene association studies to be successful in P. radiata all these factors need to be addressed. This formed the main aim of this thesis. A multi-pronged approach was used. Firstly, at the population level, the genetic resources available for association studies were identified and the underlying genetic population structure of these resources and the patterns of nucleotide diversity and linkage disequilibrium were investigated. Secondly, improved methods for the collection and isolation of genomic DNA were developed and thirdly, a small set of wood quality candidate-genes were selected and further characterised with the aim of identifying those with the most promise of harbouring causative variation for inclusion in future association studies. This was achieved by literature based review, linkage mapping onto wood property QTL maps and neutrality testing. Results include support for previous population genetic studies showing P. radiata to have a complex genetic structure compared to most pine species. This study also indicated significant levels of migration between the three mainland populations. Within the candidate-genes two, cinnamyl alcohol dehydrogenase and sucrose synthase, showed interesting patterns of population differentiation and/or nucleotide diversity while the results for one other gene, korrigan, did not agree with previous investigations.
-
Item
-
ItemNutritional modification of muscle long-chain omega-3 fatty acids in lambs : effects on growth, and composition and quality of meat( 1999)Three experiments were conducted to investigate the effects of dietary supplements rich in omega-3 fatty acid on muscle omega-3 fatty acid deposition. The consequential effects on growth performance of lambs, colour and lipid oxidative stability of muscle over refrigerated display, and the sensory properties of cooked meat were also examined. A mixture of lucerne chaff : oaten chaff was used as basal diet, offered in different proportions were fed to lambs ad libitum (Expt. 1) or at 90% ad libitum (Expts. 2 and 3). Such mixtures of roughage diet support slow growth and provide a feed quality pattern similar to late spring to late summer pasture. In Expt. 1, fish meal (7%), canola meal (8%) and soymeal (7%) as natural feed supplements were compared in lambs fed low quality roughage diet. In Expt. 2, fish meal (9%) and oilseed supplements either in unprotected form (rapeseed - 7%) or in protected form (ground canola seed - 6%) were examined in lambs on medium quality roughage diet. Lipids and the proteins in the ground canola seed were treated (RUMENTEK) with aldehyde to protect them from the rumen microbial activity. Fish meal (9%), fish oil (1.5%), fish oil (1.5%) with sunflower meal protein (9%),' and sunflower meal protein alone (10.5%) (a commercial product of a protein supplement from RUMENTEK) were compared in lambs fed medium quality roughage diet in Expt. 3. Long-chain omega-3 fatty acids (eicosapentaenoic acid + docosahexaenoic acid) in muscle longissimus thoracis was increased modestly and markedly with fish meal and fish oil alone or with sunflower meal protein diet, respectively. These long-chain fatty acids were deposited in the muscle structural phospholipid rather than in storage triglycerides. All the diets mentioned above also significantly reduced omega-6:omega-3 fatty acid ratio in meat which is another beneficial effect, as the dietary recommendation in many countries has been to reduce the ratio of omega-6:omega-3 in human diet. Soymeal diet increased modestly both the omega-3 and omega-6 fatty acid content of muscle longissimus thoracis resulting in no differences in the omega-6:omega-3 ratio of the meat. A supplement of protected canola seed significantly increased the precursors of omega-6 (linoleic) and omega-3 (linolenic) but not the long-chain analogues such as arachidonic acid (omega-6) and eicosapentaenoic, docosahexaenoic acid (omega-3), respectively. The marked increase in linoleic acid content was in both triglyceride and phospholipid fractions of muscle longissimus thoracis but the modest increase in linolenic acid content was only in triglyceride fraction of meat. Supplements of canola meal used in Expt. 1, unprotected rapeseed used in Expt. 2 and protected sunflower meal protein used in Expt. 3 did not alter the fatty acid composition of muscle longissimus thoracis compared with lambs fed the control diet in that particular experiment. The increased level of long-chain omega-3 fatty acid and/or omega-6 fatty acid with the lipid supplements discussed above did not significantly affect the meat colour stability and lipid oxidative stability of fresh and vacuum packaged meat over the storage at refrigerated display. This suggests that the conditions under which the animals are grown (grazing vs grain fed or feedlot) and the species of animal are important in determining the oxidative stabilities of meat by altering the levels of muscle vitamin E concentrations at slaughter. The level of inclusion of lucerne chaff in the basal diet is an important factor in improving the redness of meat indicated by the a*-value; a higher level of lucerne chaff intake is more likely to be associated with increased intake of vitamin E. Thus colour and lipid oxidative stabilities of meat can be improved in red meat animals that are on poor quality diets by the inclusion of lucerne chaff in their diet. The sensory properties of cooked meat evaluated in the present study were not affected by the significant increase in muscle long-chain omega-3 fatty acid or omega-6 fatty acid content with fish oil and protected canola seed supplements, respectively. Addition of protected sunflower meal as a protein supplement together with fish oil significantly lowered the ratings of flavour and overall acceptability of meat compared with the control lambs. The results demonstrate that the common `lamby' and `muttony' flavour and aroma attributes were not hidden by any of the dietary treatments. These two characters associated with the species flavour and aroma were recognised by the panellists as a distinct attribute. Dry matter intake was not adversely affected by any of the lipid supplements used in the present study. Feed conversion efficiency was highest with fish meal diet on both low and medium quality roughage diets. At medium quality roughage-based diet, Feed conversion efficiency was modestly improved by protected canola seed diet but other supplements providing either natural (unprotected rapeseed) or protected protein (protected sunflower meal) did not support significant differences compared with basal diet. The significant increase in liveweight gain with fish meal diet reflected a significant increase in hot carcass weight compared with all other supplemented lambs either on low or on medium quality roughage diet. Protected lipid and protein offered by protected canola seed diet significantly and moderately increased liveweight gain and hot carcass weight from control diet but not different from unprotected rapeseed diet. The greatest muscle deposition was with the fish meal diet and is attributed mainly to the increased amount of protein and energy absorbed from the small intestine of those lambs. In addition to energy and protein absorption, the alteration of omega-3 polyunsaturated fatty acids in muscle membranes may have a further influence in lean meat production. In terms of carcass gain and intramuscular fat deposition of fishmeal and fish oil fed lambs, the results also lead to a hypothesis that modifying omega-3 polyunsaturated fatty acid of muscle membrane phospholipids may have an influence in improved muscle deposition in lambs by improving the insulin action at skeletal muscle site.
-
ItemThe influence of pre- and post-slaughter factors on pork quality( 1998)A complex interaction of animal factors, pre-slaughter management of the pig and post-slaughter carcass processing can have a significant influence on pork quality. Pork quality attributes are dependent upon the concentration of glycogen in the muscle at slaughter and the rate and extent of post-slaughter muscle glycolysis. Acute stress just prior to slaughter can lead to rapid rate of glycolysis and therefore rapid accumulation of lactate and a low muscle pH and pale, soft, exudative (PSE) pork. On the other hand low muscle glycogen at slaughter can lead to insufficient lactic acid formation, high muscle pH and dark, firm, dry (DFD) pork. Major pork quality defects such as PSE and DFD have resulted in large economic losses for the Australian pork industry, where PSE pork alone has been estimated to cost the industry $24 million per year. This thesis comprises a review of the literature related to the biochemistry involved, particularly muscle glycogenolysis, in the conversion of muscle to meat and the influence of various animal, pre- and post-slaughter factors on pork quality. The experimental work comprises of six experiments which examine the relationships between on-farm management, pre-slaughter management and post-slaughter processing and their influence on muscle physiology and glycogenolysis and ultimate pork quality. 1. Determination of muscle glycogen and lactic acid concentrations in muscle samples removed from the live pig using a shot-biopsy procedure. A total of 14 male crossbred finisher pigs were used in this experiment. The shot-biopsy procedure was used to remove muscle samples from the live pig. The Longissimus thoracis and Biceps femoris muscle had similar glycogen (13.7 and 12.5mg/g wet weight respectively) and lactic acid (1.3 and 1.9mg/g wet weight respectively) concentrations. By determining resting muscle glycogen and lactic acid concentrations in the live pig, the results can be used as an indicator of the extent of glycogenolysis that occurs in the muscle pre-slaughter and in comparing the influence of pre-slaughter factors such as handling and mixing of unfamiliar pigs on muscle glycogen concentrations at slaughter. 2. The effect of on farm and pre-slaughter handling on pork quality. Thirty-six male crossbred pigs were used in a 2x2 factorial design which consisted of 2 on-farm handling treatments (positive and negative handling) and 2 abattoir handling treatments (minimal and negative handling just prior to slaughter). Pigs negatively handled on-farm had lower muscle glycogen concentrations at slaughter, but there was no difference in muscle paleness or % drip loss compared to pigs positively handled on-farm. Pigs negatively handled at the abattoir had lower muscle glycogen concentrations at slaughter, but muscle paleness, % drip loss and the incidence of PSE were similar compared to pigs minimally handled at the abattoir. The results from this experiment indicate that pigs negatively handled on-farm have the potential to respond adversely to handling at the abattoir and hence may result in inferior pork quality. 3. The effect of mixing pigs during lairage and negative handling just prior to slaughter on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 lairage treatments (unmixed pigs during lairage and pigs mixed during lairage) and 2 abattoir handling treatments (minimal and negative handling just prior to slaughter). Mixing of pigs during lairage induced a high level of fighting among pigs and hence the pigs had lower muscle glycogen concentrations at slaughter, lower % drip, darker pork, a higher incidence of DFD and a lower incidence of PSE pork compared with unmixed pigs kept together during lairage. Negative handling just prior to slaughter did not affect muscle glycogen concentrations at slaughter or pork quality indicators in pigs that were unmixed or mixed during lairage. The unmixed pigs had a higher incidence of PSE irrespective of the handling treatment just prior to slaughter. This experiment has demonstrated that mixing of unfamiliar pigs during lairage can have a major influencer on pork quality, increase the incidence of DFD pork and reduce the incidence of PSE . 4. The effect of negative handling pre-slaughter and delays in carcass processing post-slaughter on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 abattoir handling treatments (minimal and negative handling just prior to slaughter) and 2 carcass processing durations (normal - 45 min and delayed - 70 min processing ). Pigs negatively handled prior to slaughter had lower muscle glycogen concentrations at slaughter, higher surface exudate and higher incidence of PSE pork compared to pigs minimally handled just prior to slaughter. Delays in carcass processing resulted in paler pork in the leg although the effect was minimal. The results from this experiment suggest that the quality of preslaughter handling had a greater effect on the incidence of PSE compared to delays in carcass processing. 5. The effect of dietary magnesium aspartate on pork quality. Forty-eight male crossbred pigs were used in a 2x2 factorial design which consisted of 2 diet treatments (control diet and diet supplemented with magnesium aspartate (MgAsp)) and 2 abattoir handling treatments. Pigs fed the MgAsp supplemented diet had higher plasma magnesium concentrations and lower plasma noradrenaline concentrations at slaughter compared to pigs fed the control diet. Pigs fed the control diet also had higher % drip loss, paler pork and a higher incidence of PSE compared to pigs fed the MgAsp supplemented diet. Dietary MgAsp supplementation reduced the incidence of PSE to zero irrespective of the handling treatment prior to slaughter. Pigs fed the control diet and negatively handled at the abattoir had lower muscle glycogen at slaughter and higher % drip loss compared to pigs fed the control diet and minimally handled prior to slaughter. The results from this experiment indicate that dietary MgAsp supplementation is effective in reducing the effects of stress, improving pork quality and reducing the incidence of PSE pork. 6. The effect of different dietary magnesium supplements on pork quality. Fortyeight male crossbred pigs were used in this experiment and consisted of 4 diet treatments (control diet, magnesium aspartate (MgAsp) supplemented diet, magnesium sulphate (MgSO4) supplemented diet and magnesium chloride (MgC12) supplemented diet. The MgAsp and MgSO4 but not the MgC12 supplemented diet increased plasma magnesium concentrations in comparison with the control diet however no reduction in plasma catecholamine concentrations were observed. Pigs fed the magnesium diets (MgAsp, MgSO4 and MgCl2) had higher muscle glycogen concentrations at slaughter and lower % drip loss in comparison with pigs fed the control diet. The results further highlight the benefit of dietary magnesium supplementation in improving pork quality. Thus MgSO4 and MgC12, which are cheaper and more readily available sources of magnesium can be as effective as MgAsp in improving pork quality. 7. The effect of dose and duration of dietary magnesium aspartate supplementation on plasma metabolites in pigs. Sixteen crossbred males which were cannulated via the cephalic vein prior to the start of the supplementation period were used in this experiment. The pigs were allocated to one of four diets (control diet, 20g MgAsp, 40g MgAsp and 60g MgAsp respectively). Pigs fed the magnesium diets exhibited increases in plasma magnesium concentration in the first two days. Also the highest increases in plasma magnesium concentrations corresponded to the pigs fed the lowest magnesium supplemented diet. The lowest magnesium dose maintained plasma concentrations throughout the supplementation period whereas the other diets declined to pre-treatment concentrations. The results hence indicate, that a lower dose of dietary magnesium for a shorter duration is more appropriate to maintain increases in plasma magnesium concentration. The results in this thesis demonstrate the importance of positive on-farm handling, management of pigs during the lairage period, handling just prior to slaughter and carcass processing to consistently produce quality pork. The propensity to develop PSE may be already evident prior to transport from the farm, however, results indicate that the factor which determines whether the pork carcass will be PSE is the nature of handling the pigs receives in the last few minutes prior to slaughter. Finally the results from this thesis highlight the effectiveness of dietary magnesium supplementation in improving pork quality and reducing the incidence of PSE pork.
-
Item
-
ItemPhysiological and morphological responses of several genotypes of tropical and sub-tropical maize (Zea mays L.) to water and nitrogen deficit during the early vegetative phase( 1997)Intermittent drought combined with nitrogen stress during early growth of maize crops in the tropics limits maize production in developing countries. Plant morphological and physiological responses of different maize genotypes to two aspects of stress, water deficit and nitrogen stress, were examined during the growth of several maize genotypes. Eight maize genotypes including three inbred lines (Gn 160, Gn 106, Gn 175), three single crosses (Gn 160 x Gn 175, Gn 106 x Gn 175, Gn 106 x Gn 160) and two open pollinated cultivars (Tuxpeno Sequla Cycle 0, drought sensitive; Cycle 8, drought resistant) were examined for drought tolerance during early growth. Variables used for assessing tolerance were dry weight of shoot and root, leaf area, relative water content, water potential and stomatal conductance which were measured in both well-watered and the respective droughted treatments for each genotype. Under growth room conditions, the level of severe drought imposed caused a substantial reduction in growth of eight maize genotypes, relative to the respective well-watered conditions. The single cross Gn106 x Gn160 revealed drought tolerance traits in shoot and total biomass, which were higher than the other single crosses. It is concluded that this single cross is more drought-tolerant than the other single crosses during early growth stages. Tuxpeno Sequla Cycle 0 and Cycle 8 had similar shoot and total biomass under both water regimes, and did not show differences in drought tolerance during early growth. There was no clear indication of transmissability of drought tolerance from parent lines to their respective single crosses. The combined effects of drought and timing of N stress during early growth on morphological and physiological traits, which were altered during later growth and yield components, were investigated in two related tropical maize cultivars, Tuxpeno Sequla Cycle 0 (Tux. CO) and Cycle 8 (Tux. C8), which were selected for drought sensitivity (Tux. CO) and tolerance (Tux. C8), during flowering under glasshouse conditions. Total biomass at maturity and green leaf number below the ear at grain filling were similar between Tux. CO and C. 8 under different water regimes. Increased grain yield and harvest index in Tux. C8, compared with Tux. CO, resulted mainly from increased kernel number per row and rows per ear. Tux. C8 was not only higher in yield but also higher in N use efficiency than Tux. CO when experiencing moisture and N stress during early growth. Shoot biomass under moderate drought versus well-watered conditions was found to be more sensitive than root biomass in both Tux. CO and C8. Under drought compared with well-watered conditions, stem biomass was more sensitive than leaf biomass. Leaf area in the high-N treatments was more sensitive to water deficit than in low N treatments, as indicated by the fact that droughted plants in the early N application (NE) and split (NS) treatment produced lower leaf areas than well-watered plants, while in the control (NO) and late N (NL) treatments there was no significant difference in leaf area between drought and well-watered conditions. Early N application resulted in a shorter time from sowing to anthesis than later N application. Clearly N stress is the main limiting factor for growth and yield of maize under moderate level of water deficit. This study revealed that maize plants require N for growth and dry matter accumulation as early as immediately after seedling emergence even under low N input conditions (75 kg N per hectare). Increased grain N content and nitrogen harvest index for a split N application resulted from increased N content in the stem and leaf Even under low N input conditions i.e. the application equivalent to 75 kg N/ha in this experiment, N still needed to be supplied as a split application, in order to increase N use efficiency. Nitrogen harvest index under well-watered conditions was higher than that under drought, indicating that water deficit also had an effect on nitrogen harvest index of both cultivars (CO and C8) under different level of N treatments.
-
ItemProduction of heavy weight cryptorchid and wether lambs( 1996)Declining Iamb consumption remains a major problem facing the Australian prime Iamb industry. For Iamb to retain its market share and to remain a viable commodity in the future, leaner Iambs must be produced and marketed. Recent industry initiatives therefore focussed upon the production and marketing of Iambs to meet carcass specifications of 18-26 kg with a fat depth of 6 to 15 mm at the GR site. Emphasis was placed upon the production of lean carcasses weighing more than 22 kg, which are otherwise referred to as Elite Iambs. The development of production, management and marketing systems to produce and market Iamb carcasses of Elite Iamb specifications, both domestically and overseas, was the focus of this research program. The major objective of this experiment was to determine the potential of producing high and consistent quality meat from cryptorchid and wether Iambs meeting carcass specifications for Elite Iambs when slaughtered from six to eighteen months of age. One hundred and sixty three second cross Poll Dorset x Border Leicester/Merino cryptorchid and wether Iambs, born in May 1991 at Rutherglen Research Institute, Rutherglen, were grown and managed on annual pasture at an average liveweight of 50 kg prior to slaughter. Lambs were allocated to one of seven slaughter groups and finished every two months from 6 to 18 months of age and the influence of sex, age and nutrition on carcass composition, meat quality attributes and fat quality and consistency of cryptorchid and wether Iambs examined. Cryptorchid and wether Iambs slaughtered from 8 to 18 months of age produced carcasses meeting targeted specifications of greater than 22 kg with 6-15 mm of fat at the GR site. Lambs slaughtered at six months of age did not achieve the target liveweight of 50 kg at slaughter due to poor seasonal conditions which influenced the quality and availability of annual pasture in spring 1991. Cryptorchid Iambs had a lower dressing percentage and produced carcasses with a lower GR measurement than wethers at a cold carcass weight (CCW) of 23.5 kg. Cryptorchid Iambs exhibited growth rate advantages over wethers only when nutrition and seasonal conditions did not limit Iamb growth. When compared at the same fasted liveweight, cryptorchid Iambs deposited less internal fat than wethers. Cryptorchid carcasses also had a lower proportion of subcutaneous fat present on all primals, higher proportions of soft tissue and bone in the hindquarter and the EMA of the M. longissimus thoracis was significantly larger compared with wethers at a CCW of 23.5 kg. The proportion of CCW in the leg, chump and ribloin was not significantly influenced by Iamb sex. Nutritional management of lambs significantly influenced the proportion of subcutaneous fat on all primals. Meat quality was significantly influenced by Iamb sex however results obtained did not suggest that meat from cryptorchid Iamb carcasses was of inferior quality compared with wethers as only small differences in tenderness, intramuscular soluble collagen content, meat flavour, meat colour, intramuscular fat content and cooking loss were found. In addition, age and nutrition significantly influenced meat quality, however, consistent relationships were not found. The incidence of yellow soft subcutaneous fat was higher in cryptorchid Iamb carcasses than those from wethers with cryptorchids also producing subcutaneous fat with significantly higher levels of total unsaturated fatty acids and lower levels of stearic, palmitic and total saturated fatty acids compared with wethers. Age and nutrition also influenced the fatty acid composition of subcutaneous fat with Iambs fed lupins having significantly higher linoleic acid levels in subcutaneous fat. However, the magnitude of these differences did not explain differences found in fat quality and consistency between cryptorchid and wether carcasses slaughtered at different ages. Significant, though small, differences were found due to sex in both the triglyceride and phospholipid components of intramuscular fat. As higher levels of polyunsaturated fatty acids were found in the phospholipid component of intramuscular fat compared with cryptorchids, this may affect the keeping quality of meat from wether carcasses. Finally, although significant differences due to age were found in the fatty acid composition of intramuscular fat, these differences were not consistent with age and more likely reflected differences in nutritional management between slaughter groups. This study demonstrated that cryptorchid and wether Iambs turned off at heavy market weights on an improved plane of nutrition over a 12 month period produced carcasses meeting Elite Iamb specifications and yielded high quality, tender meat.
-
ItemThe effects of post-anthesis heat stress on wheat yield and quality( 1996)Post-anthesis temperature is a major determinant of wheat yield and quality. Post-anthesis temperatures in the moderately high range (ca 25-32C) are known to reduce grain yield but increase bread wheat quality, whereas very high (>32C) temperatures are known to significantly reduce both yield and quality. In Mediterranean and continental climates, such as Australia and the US., wheat is exposed to moderately high temperatures throughout most of the grain filling period, and very high temperatures may occur for an average 8% of grain growth. This thesis is primarily concerned with examining the effects of very high temperature on wheat yield and quality. Specifically, the study was designed to: 1) quantify the effects of short (3-5 day) periods of very high temperature on wheat yield and quality; and 2) determine the extent of genotypic variation in response of wheat yield and quality to very high temperature. Two varieties of wheat differing widely in heat tolerance were selected from 75 cultivars of wheat that were screened for tolerance to very high temperature. These two varieties (Oxley and Egret, heat sensitive and heat tolerant, respectively) were exposed to a variety of heat treatments in order to determine whether varietal differences in heat tolerance were maintained for heat treatments occurring at 3) different stages of grain growth and for 4) varying durations of heat stress. The 5) interaction of moderately high and very high temperatures was examined in order to determine whether cool temperatures following severe heat stress could alleviate the deleterious effects of very high temperature on yield and quality. In order to 6) examine the importance of acclimation to heat stress and to 7) establish a repeatable selection methodology, the impact of sudden increases to a high maximum temperature was compared with more gradual (6C h-1) rises to the same high temperature (40C). For each of the experiments 3 to 7 (above) results are presented for the effects of heat stress on: a) the accumulation of grain dry matter and water during grain growth; b) the accumulation during grain growth of total protein and its functionally-important fractions (SDS-soluble and SDS-insoluble polymer [glutenin], monomer [gliadin] and albumin/globulin), as determined by size-exclusion high-performance liquid chromatography and c) dough mixing behaviour using the 2-g mixograph. It is concluded that: 1) wheat genotypes vary widely in their responses of yield and quality to short periods of very high temperature; 2) the response to heat stress varies with the timing of stress: yield was reduced more by early than late-applied stress, whereas dough strength tended to decline most markedly in response to heat stress applied towards the end of grain filling; 3) both grain yield and dough strength declined linearly with increased duration of heat stress; 4) in a heat sensitive variety, moderately high and very high temperatures during grain filling each reduced grain yield and dough strength: cool temperatures following exposure to very high temperature did not reduce the effects of very high temperature on either yield or quality; 5) some varieties of wheat appear to acclimate rapidly to heat stress: a gradual (6C h-1) increase from ca 20-40C lessened the impact of heat stress on yield and quality when compared with a sudden increase over the same temperature range. These results are discussed with special reference to their implications for: 1) selecting and breeding for heat tolerance in wheat; 2) predictive modelling of the effects of very high temperature on wheat yield and particularly quality.
-
ItemPhysiology of fibre development in Eucalyptus globulus( 1995)Plantation Eucalypt forests are an important source of wood fibre for pulp and paper manufacture and opportunities exist to enhance the quality of wood grown in these forests by silvicultural and genetic management. However the ability to do this is limited by the relatively poor understanding of the processes of secondary xylem cell production and differentiation and the mechanisms that control these processes. Length is one important fibre dimension affecting paper properties and this project has investigated the within-tree variation in secondary xylem fibre tracheid length (fibre length) in Eucalyptus globulus Labill. Evidence is presented that cambial region gibberellins are a likely causal factor in secondary xylem fibre elongation in Eucalypts. The dimensions of the secondary xylem fibres of woody dicotyledons are determined in part by the dimensions of the fusiform cambial cells from which they are derived and in part by the processes that occur during differentiation. For Eucalyptus globulus, a strong positive correlation (r = 0.757, df = 7, P < 0.05) existed between fibre length and fusiform cambial cell length along the axis of the stem. There also existed a basipetal gradient of increasing secondary xylem fibre elongation during differentiation. Although not statistically significant, the estimated rate of fibre elongation and the time required for differentiating fibres to complete elongation also increased basipetally. Gibberellins GA1, GA4, GA8, GA9, GA19, GA20, GA29, GA44, GA81, indole-3-acetic acid (IAA) and abscisic acid (ABA) were identified in cambial region extracts of Eucalyptus globulus by comparing mass spectra and Kovats retention indices with those of authentic standards. The presence of brassinosteroid-like substances was also indicated by activity in the rice seedling leaf inclination assay. Using [17, 17- 2H2] GAs as internal standards, GA19, GA20 and GA44 were quantified at levels of 2-7 ng (g fresh weight)-1. Other GAs were present at levels < 1 ng (g fresh weight)-1. It seems likely that the early C-13 hydroxylation pathway of GA biosynthesis predominates in Eucalyptus cambial region tissues. Although it has yet to be definitively determined whether these endogenous cambial region GAs arise by in situ biosynthesis and metabolism or by transport from apical meristems and leaves, the presence of high levels of GA44 and GA19 indicate that cambial region tissues can likely perform both early and late oxidative steps in GA biosynthesis. Within-tree gradients in cambial region GA, IAA and ABA levels were also identified. However the interpretation of these gradients is difficult since they vary depending upon how they are expressed, i.e. hormone per g cambial region tissue that gives a concentration or hormone per square cm of stem surface that gives an absolute amount. This problem will be difficult to resolve without a greater understanding of the localisation of cambial region hormones in the various cell types in various stages of development that comprise the cambial region. Localised stem injection of Primo (trinexapac-ethyl), a late stage acylcyclohexanedione type inhibitor of GA biosynthesis, was used to reduce endogenous cambial region GA levels. High doses of Primo also reduced the elongation of secondary xylem fibres and the radial number of primary walled differentiating fibres undergoing elongation. Fibre elongation was positively correlated with endogenous cambial region levels of both GA1 (rs = 0.74, P < 0.01) and GA20 (rs = 0.72, P < 0.01). Correlations between endogenous IAA or ABA and fibre elongation were positive but weak and non significant. Gibberellins may thus be causal factors in secondary xylem fibre elongation in Eucalyptus. However a definite relationship between spatial and temporal variation in cambial region hormone levels and within-tree variation in secondary xylem fibre elongation has yet to be established.