Winterfat (Krascheninnikovia lanata) a native shrub has superior forage quality for livestock and wildlife, and is important in the structure and the function of the Northern Mixed Prairie of North America. Seedbeds in the Northern Mixed Prairie are characterized by high fluctuations in temperature and soil water, especially at the soil surface during the spring under unpredictable weather conditions. High seedling mortality is a major limitation for establishing winterfat from direct seeding. Objectives of this study were to: 1) quantify germination responses to temperature and water potential; 2) predict seed germination and seedling emergence using constructed threshold models; and 3) investigate physiological mechanisms and the ecological relevance of model parameters. The constructed thermal and hydrothermal time models predicted germination time in most controlled temperature and water potential regimes with the modification of model assumptions in winterfat. For the first time, it was proved that winterfat seeds have a subzero base temperatures (Tb) for germination, achieving 43 to 67% germination at 3oC. The estimated Tb was lower in the large seeds (-4.5oC) than in the small seeds (-3.5oC) and the difference between seed collections was also about 1oC. Lower Tb favors large seeds to accumulate more thermal time at a given temperature, especially in early spring or fall when temperatures are low. Basic assumptions of hydrothermal time model, such as the constancy of model parameters, are invalid in winterfat. Model parameters varied with water potential, temperature and seed size within a seed collection. The predictability of constructed models is acceptable for seedling emergence only at optimal conditions in the field. Adverse seedbed conditions such as high soil temperatures (> 15oC) and limited soil water (< -0.5 MPa) reduced predictability of seedling emergence with the hydrothermal time model. Pre- and post-germination events that affect seed deterioration, seedling mortality and seedling elongation may reduce the predictability of the hydrothermal time model. Small seeds required approximately twice as long as large seeds to reach 50% germination at -1 to -3oC. Greater cold tolerance in large seeds was correlated with greater membrane integrity, less cold imbibition damage, higher contents of soluble cryoprotective sugars, such as glucose, raffinose and sucrose during germination at low temperature. These sugars prevent from dysfunctions of cell membrane and enzymes at freezing temperatures.

Winterfat (Krascheninnikovia lanata) a native shrub has superior forage quality for livestock and wildlife, and is important in the structure and the function of the Northern Mixed Prairie of North America. Seedbeds in the Northern Mixed Prairie are characterized by high fluctuations in temperature and soil water, especially at the soil surface during the spring under unpredictable weather conditions. High seedling mortality is a major limitation for establishing winterfat from direct seeding. Objectives of this study were to: 1) quantify germination responses to temperature and water potential; 2) predict seed germination and seedling emergence using constructed threshold models; and 3) investigate physiological mechanisms and the ecological relevance of model parameters. The constructed thermal and hydrothermal time models predicted germination time in most controlled temperature and water potential regimes with the modification of model assumptions in winterfat. For the first time, it was proved that winterfat seeds have a subzero base temperatures (Tb) for germination, achieving 43 to 67% germination at 3oC. The estimated Tb was lower in the large seeds (-4.5oC) than in the small seeds (-3.5oC) and the difference between seed collections was also about 1oC. Lower Tb favors large seeds to accumulate more thermal time at a given temperature, especially in early spring or fall when temperatures are low. Basic assumptions of hydrothermal time model, such as the constancy of model parameters, are invalid in winterfat. Model parameters varied with water potential, temperature and seed size within a seed collection. The predictability of constructed models is acceptable for seedling emergence only at optimal conditions in the field. Adverse seedbed conditions such as high soil temperatures (> 15oC) and limited soil water (< -0.5 MPa) reduced predictability of seedling emergence with the hydrothermal time model. Pre- and post-germination events that affect seed deterioration, seedling mortality and seedling elongation may reduce the predictability of the hydrothermal time model. Small seeds required approximately twice as long as large seeds to reach 50% germination at -1 to -3oC. Greater cold tolerance in large seeds was correlated with greater membrane integrity, less cold imbibition damage, higher contents of soluble cryoprotective sugars, such as glucose, raffinose and sucrose during germination at low temperature. These sugars prevent from dysfunctions of cell membrane and enzymes at freezing temperatures.

The effects of global climate change on seed germination and plant regeneration have been reported in many species. However, there are no consistent trends in how seed quality and germination are affected by these conditions. Seeds of four native, one invasive, and two pairs of native/invasive species were collected from the USDA-ARS Prairie Heating and CO2 Enrichment Experimental plots in 2007 to 2009, located in the Mixed-grass Prairie near Cheyenne, WY. Field treatments include ambient (385 ppm, c) and elevated (600 ppm, C) CO2 concentrations, control temperature (t) and heating (1.5/3.0°C warmer day/night, T), and deep (ct-d) and shallow (ct-s) irrigation. Seed quality was evaluated and germination tests were conducted under alternating temperatures (10/0, 12.5/2.5, 15/5, 20/10, 25/15, 30/20, 35/25°C). Thermal time requirements (θ50) and base temperatures (Tb) for germination were determined using thermal time models. Elevated CO2 concentrations reduced seed fill and viability, germination and germination rate in Grindelia squarrosa. Heating increased seed viability in Koeleria macrantha from 56% to 79%. Heating, when combined with elevated CO2 concentrations, increased germination while CO2 alone decreased germination by about 14% in Bouteloua gracilis. Heating tended to enhance Tb and to reduce θ50 in Bouteloua gracilis, which may slow the initiation of germination but seeds germinate faster in spring. Elevated CO2 concentrations tended to increase but CT tended to decrease Tb in Centaurea diffusa, but not θ50. Heating tended to increase but all the other treatments tended to reduce Tb in Lanaria dalmatica, leading to a possible earlier start of germination. Irrigation treatments tended to show similar trend of effects on seed quality and germination as that in elevated CO2 concentrations. Species specific changes in seed quality and germination were observed, which may exert substantial cumulative effects on community composition in the long run. Invasive species may be more competitive under future climatic conditions compared with native species. However, the distribution and abundance of some native species, specifically Bouteloua gracilis, may be favored by climate change.

The primary aims of this dissertation were to (1) identify and characterize the water-gap complex in seeds of Geraniaceae, (2) investigate its role in physical dormancy (PY) break and (3) develop a new classification system for water-gap complexes in seeds of angiosperms. The winter annuals Geranium carolinianum and G. dissectum were selected as the main representative species for the study, and seeds of an additional 29 species from the Geraniaceae were used to compare the water-gap complex within the family. A new classification system for water-gap complexes in species with PY was developed by comparing the morpho-anatomical features of PY seeds and fruits of 16 families. The water-gap complex of G. carolinianum was identified as a micropyle-hinged valve gap complex, and only a slight morpho-anatomical variation was observed within the family. Ontogenetic studies of the seed coat of G. carolinianum revealed that the water-gap region of Geraniaceae develops as an entity of the micropyle. The timing of seed germination with the onset of autumn can be explained by PY-breaking processes involving (a) two-temperature-dependent steps in G. carolinianum, and (b) one or two moisture-dependent step(s) along with the inability to germinate under high temperatures in G. dissectum. Step-I and step-II in PY-breaking of G. carolinianum are controlled by chemical and physical processes, respectively. This study indicates the feasibility of applying the developed thermal time model to predict or manipulate sensitivity induction in seeds with two-step PY-breaking processes. The model is the first and the most detailed one yet developed for sensitivity induction in PY-break. Based on the morpho-anatomical features, three basic water-gap complexes (types I, II and III) were identified in species with PY in 16 families. Depending on the number of openings involved in initial imbibition, the water-gap complexes were subdivided into simple and compound. The new classification system enables the understanding of relationships between water-gap complexes of taxonomically unrelated species with PY.

Broccoli is a vegetable crop of increasing importance in Australia, particularly in south-east Queensland and farmers need to maintain a regular supply of good quality broccoli to meet the expanding market. However, harvest maturity date, head yield and quality are all affected by climatic variations during the production cycle, particularly low temperature episodes. There are also interactions between genotype and climatic variability. A predictive model of ontogeny, incorporating climatic data including frost risk, would enable farmers to predict harvest maturity date and select appropriate cultivar - sowing date combinations. The first stage of this research was to define floral initiation, which is fundamental to predicting ontogeny. Scanning electron micrographs of the apical meristem were made for the transition from the vegetative to advanced reproductive stage. During the early vegetative stage (stage 1), the apical meristem was a small, pointed shoot tip surrounded by leaf primordia. The transitional stage (stage 2) was marked by a widening and flattening to form a dome-shaped apical meristem. In the floral initiation stage (stage 3), the first-order floral primordia were observed in the axils of the developing bracts. Under field conditions, the shoot apex has an average diameter of 500 micro m at floral initiation and floral primordia can be observed under a light microscope. Sub-zero temperatures can result in freezing injury and thereby reduce head yield and quality. In order to predict the effects of frosts, it is desirable to know the stages of development at which plants are most susceptible. Therefore, the effects of sub-zero temperatures on leaf and shoot mortality, head yield and quality were determined after exposure of plants to a range of temperatures for short periods, at different stages of development (vegetative, floral initiation and buttoning). Plants in pots and in the field were subjected to sub-zero temperature regimes from -1 C to -19 C. Extracellular ice formation was achieved by reducing temperatures slowly, at a rate of -2 C per hour. The floral initiation stage was most sensitive to freezing injury, as yields were significantly reduced at -1 C and -3 C, and shoot apices were killed at -5 C. There was no significant yield reduction when the inflorescence buttoning stage was subjected to -1 C and -3 C. Although shoot apices at buttoning survived the -5 C treatment, very poor quality heads of uneven bud size were produced as a result of arrested development. The lethal temperature for pot-grown broccoli was between -3 C and -5 C, whereas the lethal temperature for field-grown broccoli was between -7 C and -9 C. The difference was presumably due to variation in cold acclimation. Freezing injury can reduce broccoli head yield and quality, and retard plant growth. Crop development models based only on simple thermal time without restrictions will not predict yield or maturity if broccoli crops are frost-damaged. Field studies were conducted to develop procedures for predicting ontogeny, yield and quality. Three cultivars, (Fiesta, Greenbelt and Marathon) were sown on eight dates from 11 March to 22 May 1997, and grown under natural and extended (16 h) photoperiods in a sub-tropical environment at Gatton College, south-east Queensland, under non-limiting conditions of water and nutrient supply. Daily climatic data, and dates of emergence, floral initiation, harvest maturity, together with yield and quality were obtained. Yield and quality responses to temperature and photoperiod were quantified. As growing season mean minimum temperatures decreased, fresh weight of tops decreased while fresh weight harvest index increased linearly. There was no definite relationship between fresh weight of tops or fresh weight harvest index and growing season minimum temperatures greater than 10 C. Genotype, rather than the environment, mainly determined head quality attributes. Fiesta had the best head quality, with higher head shape and branching angle ratings than Greenbelt or Marathon. Bud colour and cluster separation of Marathon were only acceptable for export when growing season mean minimum temperatures were less than 8 C. Photoperiod did not influence yield or quality in any of the three cultivars. A better understanding of genotype and environmental interactions will help farmers optimise yield and quality, by matching cultivars with time of sowing. Crop developmental responses to temperature and photoperiod were quantified from emergence to harvest maturity (Model 1), from emergence to floral initiation (Model 2), from floral initiation to harvest maturity (Model 3), and in a combination of Models 2 and 3 (Model 4). These thermal time models were based on optimised base and optimum temperatures of 0 and 20 C, respectively. These optimised temperatures were determined using an iterative optimisation routine (simplex). Cardinal temperatures were consistent across cultivars but thermal time of phenological intervals were cultivar specific. Sensitivity to photoperiod and solar radiation was low in the three cultivars used. Thermal time models tested on independent data for five cultivars (Fiesta, Greenbelt, Marathon, CMS Liberty and Triathlon) grown as commercial crops on the Darling Downs over two years, adequately predicted floral initiation and harvest maturity. Model 4 provided the best prediction for the chronological duration from emergence to harvest maturity. Model 1 was useful when floral initiation data were not available, and it predicted harvest maturity almost as well as Model 4 since the same base and optimum temperatures of 0 C and 20 C, respectively, were used for both phenological intervals. Model 1 was also generated using data from 1979-80 sowings of three cultivars (Premium Crop, Selection 160 and Selection 165A). When Model 1 was tested with independent data from 1983-84, it predicted harvest maturity well. Where floral initiation data were available, predictions of harvest maturity were most precise using Model 3, since the variation, which occurred from emergence to floral initiation, was removed. Prediction of floral initiation using Model 2 can be useful for timing cultural practices, and for avoiding frost and high temperature periods. This research has produced models to assist broccoli farmers in crop scheduling and cultivar selection in south-east Queensland. Using the models as a guide, farmers can optimise yield and quality, by matching cultivars with sowing date. By accurately predicting floral initiation, the risk of frost damage during floral initiation can be reduced by adjusting sowing dates or crop management options. The simple and robust thermal time models will improve production and marketing arrangements, which have to be made in advance. The thermal time models in this study, incorporating frost risk using conditional statements, provide a foundation for a decision support system to manage the sequence of sowings on commercial broccoli farms.

White spruce (<i>Picea glauca</i> (Moench) Voss), black spruce (<i>P. mariana</i> (Mill.) BSP), and jack pine (<i>Pinus banksiana</i> Lamb.) are dominant conifer trees within the boreal forest. Rising CO2 concentrations may create hotter and drier conditions in the Southern Boreal Forest of Canada, and have negative impacts on germination and regeneration of conifers. Conifers vary in their germination requirements and may have different responses to climate change. Experiments were conducted to access the germination potential, variability among collections, and to predict the ability of these conifers to germinate under future climatic conditions. Twelve collections of white spruce and black spruce and ten collections of jack pine seeds were collected from the Boreal Plain Ecozone of Saskatchewan. Seeds of all collections varied in their dormancy characteristics and dormancy breaking requirements because no single stratification or light treatment stimulated germination in all three species. Seed dormancy was greatest in white spruce and least in black spruce. Germination tests at 5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35°C were used to develop thermal time models. Each species had unique temperatures for optimal germination ranging from 20°C in white spruce, 20-25°C in black spruce, and 25-30°C in jack pine. The speed of germination under similar temperature regimes was fastest for jack pine, intermediate for black spruce, and slowest for white spruce. The base temperature for white spruce decreased (r=0.63, P=0.03) with increasing June precipitation while that of jack pine tended to increase with latitude (r=0.60, P=0.07) and April precipitation (r=0.58, P=0.08). No environmental variables correlated with germination of black spruce. The Canadian Global Climate Model, version 2, with emission scenarios predicted future temperature and precipitation at the sites where seeds were collected. Using the base temperature for germination as a guideline, temperatures suitable for germination in the spring are predicted to advance by a few weeks to a month and a half earlier with increased concentrations of CO2. Moisture availability may, however, control seed germination at these sites. Overall, jack pine and black spruce might better adapt to increasing temperature because of their high germination temperatures (>30°C). Variation in most germination parameters existed among collections, suggesting this variability can be used to select seed sources for reforestation or assisted migration in a changing climate.

Orchardgrass (<i>Dactylis glomerata</i> L.) and western wheatgrass (<i>Pascopyrum smithii </i>(Rydb.) L.) seeds have different degrees of dormancy that result in non-uniform seedling emergence in the field. Seed dormancy of the two species, in part, causes disagreement between germination tests in the laboratory and seedling emergence in the field. Experiments were conducted over two years in the laboratory and in the field to determine the effects of alternating temperatures on changes in seed dormancy and germination of orchardgrass and western wheatgrass. The two western wheatgrass cultivars (Walsh and LC9078a) had deeper dormancy than the two orchardgrass cultivars (Arctic and Lineta). Dormancy of both species was broken by temperatures with 10oC amplitude; this temperature variation was similar to that which occurred at a 1 cm depth in the soil. Optimal temperatures for germination of orchardgrass (10-25oC) were broader than those for western wheatgrass (15-20oC). Seedling emergence of orchardgrass was less sensitive to seeding date in the spring than western wheatgrass; seedling emergence of western wheatgrass increased as seeding date was delayed from early to late May if soil water was not limiting. The rate of seedling emergence increased with increasing temperature in both species, therefore, faster and more uniform seedling emergence can be expected from late spring seeding dates. Seeds were often exposed to light during germination tests in the laboratory while planting seeds in the soil usually prevented exposure of seeds to light. Seedling emergence of orchardgrass in the field was usually less than the germination percentage obtained in the laboratory because of light exposure during germination tests could break dormancy in orchardgrass seeds and the small seeds of orchardgrass had limited energy reserves for pre-emergence seedling growth. On the other hand, germination of western wheatgrass seeds was reduced by exposure to light during germination and seeds were larger than those of orchardgrass. Therefore, seedling emergence of western wheatgrass in the field was usually greater than germination tests would predict. The use of thermal time models to study seed dormancy changes and germination revealed the dual effects of temperature on these processes. The modified thermal time model takes the difference between germination and seedling emergence into account and can accurately predict seedling emergence in the field (R2=0.88 to 0.99). Thermal time models for predicting seedling emergence in the field can also be developed for other forages, however, cultivar- and species-specific parameters must be developed for the models.

Orchardgrass (<i>Dactylis glomerata</i> L.) and western wheatgrass (<i>Pascopyrum smithii </i>(Rydb.) L.) seeds have different degrees of dormancy that result in non-uniform seedling emergence in the field. Seed dormancy of the two species, in part, causes disagreement between germination tests in the laboratory and seedling emergence in the field. Experiments were conducted over two years in the laboratory and in the field to determine the effects of alternating temperatures on changes in seed dormancy and germination of orchardgrass and western wheatgrass. The two western wheatgrass cultivars (Walsh and LC9078a) had deeper dormancy than the two orchardgrass cultivars (Arctic and Lineta). Dormancy of both species was broken by temperatures with 10oC amplitude; this temperature variation was similar to that which occurred at a 1 cm depth in the soil. Optimal temperatures for germination of orchardgrass (10-25oC) were broader than those for western wheatgrass (15-20oC). Seedling emergence of orchardgrass was less sensitive to seeding date in the spring than western wheatgrass; seedling emergence of western wheatgrass increased as seeding date was delayed from early to late May if soil water was not limiting. The rate of seedling emergence increased with increasing temperature in both species, therefore, faster and more uniform seedling emergence can be expected from late spring seeding dates. Seeds were often exposed to light during germination tests in the laboratory while planting seeds in the soil usually prevented exposure of seeds to light. Seedling emergence of orchardgrass in the field was usually less than the germination percentage obtained in the laboratory because of light exposure during germination tests could break dormancy in orchardgrass seeds and the small seeds of orchardgrass had limited energy reserves for pre-emergence seedling growth. On the other hand, germination of western wheatgrass seeds was reduced by exposure to light during germination and seeds were larger than those of orchardgrass. Therefore, seedling emergence of western wheatgrass in the field was usually greater than germination tests would predict. The use of thermal time models to study seed dormancy changes and germination revealed the dual effects of temperature on these processes. The modified thermal time model takes the difference between germination and seedling emergence into account and can accurately predict seedling emergence in the field (R2=0.88 to 0.99). Thermal time models for predicting seedling emergence in the field can also be developed for other forages, however, cultivar- and species-specific parameters must be developed for the models.

White spruce (<i>Picea glauca</i> (Moench) Voss), black spruce (<i>P. mariana</i> (Mill.) BSP), and jack pine (<i>Pinus banksiana</i> Lamb.) are dominant conifer trees within the boreal forest. Rising CO2 concentrations may create hotter and drier conditions in the Southern Boreal Forest of Canada, and have negative impacts on germination and regeneration of conifers. Conifers vary in their germination requirements and may have different responses to climate change. Experiments were conducted to access the germination potential, variability among collections, and to predict the ability of these conifers to germinate under future climatic conditions. Twelve collections of white spruce and black spruce and ten collections of jack pine seeds were collected from the Boreal Plain Ecozone of Saskatchewan. Seeds of all collections varied in their dormancy characteristics and dormancy breaking requirements because no single stratification or light treatment stimulated germination in all three species. Seed dormancy was greatest in white spruce and least in black spruce. Germination tests at 5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35°C were used to develop thermal time models. Each species had unique temperatures for optimal germination ranging from 20°C in white spruce, 20-25°C in black spruce, and 25-30°C in jack pine. The speed of germination under similar temperature regimes was fastest for jack pine, intermediate for black spruce, and slowest for white spruce. The base temperature for white spruce decreased (r=0.63, P=0.03) with increasing June precipitation while that of jack pine tended to increase with latitude (r=0.60, P=0.07) and April precipitation (r=0.58, P=0.08). No environmental variables correlated with germination of black spruce. The Canadian Global Climate Model, version 2, with emission scenarios predicted future temperature and precipitation at the sites where seeds were collected. Using the base temperature for germination as a guideline, temperatures suitable for germination in the spring are predicted to advance by a few weeks to a month and a half earlier with increased concentrations of CO2. Moisture availability may, however, control seed germination at these sites. Overall, jack pine and black spruce might better adapt to increasing temperature because of their high germination temperatures (>30°C). Variation in most germination parameters existed among collections, suggesting this variability can be used to select seed sources for reforestation or assisted migration in a changing climate.

Broccoli is a vegetable crop of increasing importance in Australia, particularly in south-east Queensland and farmers need to maintain a regular supply of good quality broccoli to meet the expanding market. However, harvest maturity date, head yield and quality are all affected by climatic variations during the production cycle, particularly low temperature episodes. There are also interactions between genotype and climatic variability. A predictive model of ontogeny, incorporating climatic data including frost risk, would enable farmers to predict harvest maturity date and select appropriate cultivar - sowing date combinations. The first stage of this research was to define floral initiation, which is fundamental to predicting ontogeny. Scanning electron micrographs of the apical meristem were made for the transition from the vegetative to advanced reproductive stage. During the early vegetative stage (stage 1), the apical meristem was a small, pointed shoot tip surrounded by leaf primordia. The transitional stage (stage 2) was marked by a widening and flattening to form a dome-shaped apical meristem. In the floral initiation stage (stage 3), the first-order floral primordia were observed in the axils of the developing bracts. Under field conditions, the shoot apex has an average diameter of 500 micro m at floral initiation and floral primordia can be observed under a light microscope. Sub-zero temperatures can result in freezing injury and thereby reduce head yield and quality. In order to predict the effects of frosts, it is desirable to know the stages of development at which plants are most susceptible. Therefore, the effects of sub-zero temperatures on leaf and shoot mortality, head yield and quality were determined after exposure of plants to a range of temperatures for short periods, at different stages of development (vegetative, floral initiation and buttoning). Plants in pots and in the field were subjected to sub-zero temperature regimes from -1 C to -19 C. Extracellular ice formation was achieved by reducing temperatures slowly, at a rate of -2 C per hour. The floral initiation stage was most sensitive to freezing injury, as yields were significantly reduced at -1 C and -3 C, and shoot apices were killed at -5 C. There was no significant yield reduction when the inflorescence buttoning stage was subjected to -1 C and -3 C. Although shoot apices at buttoning survived the -5 C treatment, very poor quality heads of uneven bud size were produced as a result of arrested development. The lethal temperature for pot-grown broccoli was between -3 C and -5 C, whereas the lethal temperature for field-grown broccoli was between -7 C and -9 C. The difference was presumably due to variation in cold acclimation. Freezing injury can reduce broccoli head yield and quality, and retard plant growth. Crop development models based only on simple thermal time without restrictions will not predict yield or maturity if broccoli crops are frost-damaged. Field studies were conducted to develop procedures for predicting ontogeny, yield and quality. Three cultivars, (Fiesta, Greenbelt and Marathon) were sown on eight dates from 11 March to 22 May 1997, and grown under natural and extended (16 h) photoperiods in a sub-tropical environment at Gatton College, south-east Queensland, under non-limiting conditions of water and nutrient supply. Daily climatic data, and dates of emergence, floral initiation, harvest maturity, together with yield and quality were obtained. Yield and quality responses to temperature and photoperiod were quantified. As growing season mean minimum temperatures decreased, fresh weight of tops decreased while fresh weight harvest index increased linearly. There was no definite relationship between fresh weight of tops or fresh weight harvest index and growing season minimum temperatures greater than 10 C. Genotype, rather than the environment, mainly determined head quality attributes. Fiesta had the best head quality, with higher head shape and branching angle ratings than Greenbelt or Marathon. Bud colour and cluster separation of Marathon were only acceptable for export when growing season mean minimum temperatures were less than 8 C. Photoperiod did not influence yield or quality in any of the three cultivars. A better understanding of genotype and environmental interactions will help farmers optimise yield and quality, by matching cultivars with time of sowing. Crop developmental responses to temperature and photoperiod were quantified from emergence to harvest maturity (Model 1), from emergence to floral initiation (Model 2), from floral initiation to harvest maturity (Model 3), and in a combination of Models 2 and 3 (Model 4). These thermal time models were based on optimised base and optimum temperatures of 0 and 20 C, respectively. These optimised temperatures were determined using an iterative optimisation routine (simplex). Cardinal temperatures were consistent across cultivars but thermal time of phenological intervals were cultivar specific. Sensitivity to photoperiod and solar radiation was low in the three cultivars used. Thermal time models tested on independent data for five cultivars (Fiesta, Greenbelt, Marathon, CMS Liberty and Triathlon) grown as commercial crops on the Darling Downs over two years, adequately predicted floral initiation and harvest maturity. Model 4 provided the best prediction for the chronological duration from emergence to harvest maturity. Model 1 was useful when floral initiation data were not available, and it predicted harvest maturity almost as well as Model 4 since the same base and optimum temperatures of 0 C and 20 C, respectively, were used for both phenological intervals. Model 1 was also generated using data from 1979-80 sowings of three cultivars (Premium Crop, Selection 160 and Selection 165A). When Model 1 was tested with independent data from 1983-84, it predicted harvest maturity well. Where floral initiation data were available, predictions of harvest maturity were most precise using Model 3, since the variation, which occurred from emergence to floral initiation, was removed. Prediction of floral initiation using Model 2 can be useful for timing cultural practices, and for avoiding frost and high temperature periods. This research has produced models to assist broccoli farmers in crop scheduling and cultivar selection in south-east Queensland. Using the models as a guide, farmers can optimise yield and quality, by matching cultivars with sowing date. By accurately predicting floral initiation, the risk of frost damage during floral initiation can be reduced by adjusting sowing dates or crop management options. The simple and robust thermal time models will improve production and marketing arrangements, which have to be made in advance. The thermal time models in this study, incorporating frost risk using conditional statements, provide a foundation for a decision support system to manage the sequence of sowings on commercial broccoli farms.