Thermal Time Basis Research Articles

A Thermal Time Basis for Comparing the Germination Requirements of Alfalfa Cultivars with Different Fall Dormancy Ratings

Fall dormancy plays important roles in the evaluation of alfalfa’s winter hardiness and in the selection of alfalfa breeding. A rapid and effective method to estimate the fall dormancy rating of alfalfa will shorten the breeding cycle. The purpose of this study is to test the correlations between the germination thermal time model parameters and the fall dormancy ratings and to evaluate the potential of the thermal-based fall dormancy methodology. Alfalfa cultivars with a series of fall dormancy ratings were used to study the responses of seed germination at six constant temperatures (5, 10, 15, 20, 25, 30 °C). The results showed that all cultivars had a relatively high germination percentage at all temperatures and the optimal temperature is 25 or 30 °C. Germination rate and base temperature significantly increased with the fall dormancy rating of alfalfa cultivars while thermal time (θT) decreased with the fall dormancy rating. The extremely significant linear regression relationships between the germination rate, base temperature (Tb), θT, and fall dormancy rating indicated that it is convenient and straightforward to predict the fall dormancy rating of unknown cultivars or lines using thermal time model parameters. This method can significantly shorten the selection and breeding cycles in alfalfa cultivation.

A decision support system (GesCoN) for managing fertigation in open field vegetable crops. Part I-methodological approach and description of the software.

Reduced water availability and environmental pollution caused by nitrogen (N) losses have increased the need for rational management of irrigation and N fertilization in horticultural systems. Decision support systems (DSS) could be powerful tools to assist farmers to improve irrigation and N fertilization efficiency. Currently, fertilization by drip irrigation system (fertigation) is used for many vegetable crops around the world. The paper illustrates the theoretical basis, the methodological approach and the structure of a DSS called GesCoN for fertigation management in open field vegetable crops. The DSS is based on daily water and N balance, considering the water lost by evapotranspiration (ET) and the N content in the aerial part of the crop (N uptake) as subtraction and the availability of water and N in the wet soil volume most effected by roots as the positive part. For the water balance, reference ET can be estimated using the Penman–Monteith (PM) or the Priestley–Taylor and Hargreaves models, specifically calibrated under local conditions. Both single or dual Kc approach can be used to calculate crop ET. Rain runoff and deep percolation are considered to calculate the effective rainfall. The soil volume most affected by the roots, the wet soil under emitters and their interactions are modeled. Crop growth is modeled by a non-linear logistic function on the basis of thermal time, but the model takes into account thermal and water stresses and allows an in-season calibration through a dynamic adaptation of the growth rate to the specific genetic and environmental conditions. N crop demand is related to DM accumulation by the N critical curve. N mineralization from soil organic matter is daily estimated. The DSS helps users to evaluate the daily amount of water and N fertilizer that has to be applied in order to fulfill the water and N-crop requirements to achieve the maximum potential yield, while reducing the risk of nitrate outflows.

Contrasting patterns of fatty acid composition and oil accumulation during fruit growth in several olive varieties and locations in a non-Mediterranean region

Olive growing has expanded considerably in the last few decades outside of the Mediterranean Basin to non-traditional regions in the Southern Hemisphere. When growing olive genotypes (i.e., varieties) outside of their area of origin, the importance of environmental factors such as temperature and genotype×environment interactions in determining olive oil production and oil quality has been suggested. In several Mediterranean varieties and one South American variety, we assessed the dynamics of fruit growth and oil accumulation along with the evolution of fatty acid composition at multiple locations over two growing seasons. Oleic acid content (%), the principal fatty acid present in olive oil, showed four contrasting patterns during fruit growth when modeled against thermal time from flowering using linear and bilinear regressions: (1) a sharp linear decrease for the varieties ‘Arauco’ and ‘Arbequina’; (2) a plateau followed by a late linear decrease of moderate slope for ‘Barnea’ and ‘Manzanilla Fina’; (3) a slow linear decrease for ‘Frantoio’; and (4) no decrease in ‘Coratina’. Linoleic acid (%) showed linear increases in ‘Arauco’ and ‘Arbequina’ that appear to be inversely related to the decreases in oleic acid, while bilinear patterns were found for many other varieties. Both the rates of fruit growth and of oil accumulation were more important in determining maximum fruit dry weight and oil concentration (%), respectively, than duration when expressed on a thermal time basis. Temperature during oil synthesis was negatively related to final oil concentration. Experiments under controlled conditions would greatly contribute to our understanding of how fruit growth as well as oil quantity and quality are influenced by environmental factors.

Use of Logistic Equation for Detection of the Initial Parasitism Phase of Egyptian Broomrape (Phelipanche aegyptiaca) in Tomato

The dynamics of the host–parasite relationship between tomato cv. Brigade and Egyptian broomrape is temperature-related. This relationship was utilized for the development of an equation on the basis of thermal time (as measured by growing degree days, GDD, C) to predict the parasitism dynamics of Egyptian broomrape in tomato. To obtain a reliable prediction from thermal time values, studies based on a wide range of temperatures are essential. Four temperature-regime treatments and five levels of infestation with Egyptian broomrape seeds were tested in a multiclimate greenhouse (phytotron) and a temperature-controlled greenhouse, respectively. The day/night temperature regimes were 20/12 C, 23/15 C, 26/18 C, and 29/21 C and the infestation levels were 0 (noninfested control), 1, 5, 10, and 25 mg of Egyptian broomrape seeds per liter of soil. As expected, increasing temperature or infestation levels resulted in faster appearance and higher rate of attachments, respectively. The relation between development of attachments and GDD was described as a three-parameter logistic curve. In both temperature-regime and infestation-level experiments, the development of attachments began 200 GDD after planting and the maximal number of attachments was recorded 800 GDD after planting. A significant reduction in the aboveground biomass of the tomato plants due to increased Egyptian broomrape biomass was recorded only for the 26/18 C and 29/21 C day/night treatments and the three highest infestation levels (5, 10, and 25 mg L−1soil). The ability to predict the start of parasitism can be used to develop a climate-based system for Egyptian broomrape control with herbicides.

Dependência térmica da germinação de sementes de Drymaria cordata (L.) Willd. ex Roem. & Schult. (Cariophyllaceae)

A temperatura representa um importante fator ambiental regulador da germinação de sementes. Procurou-se avaliar a resposta de sementes de D. cordata à temperatura, com base no modelo de graus-dia, testando-se assim a eficiência desse modelo em descrever o comportamento germinativo da semente em diferentes regimes térmicos. Testou-se também a resposta das sementes à luz, concluindo-se que a luz branca promove a germinação. As temperaturas mínima, ótima e máxima de germinação foram, respectivamente, 17,1, 26 e 33,4 ºC. Considerando-se que a velocidade de germinação de D. cordata variou com a temperatura numa relação aproximadamente linear, o modelo de graus-dia pode ser uma ferramenta válida para se estudar a dependência da temperatura da germinação dessas sementes. Uma possível aquisição de dormência durante a incubação isotérmica pode exigir a aplicação de outros modelos que descrevam melhor o comportamento germinativo de D. cordata em diferentes regimes térmicos.

Evaluation of Grain Filling Rate and Duration in Bread and Durum Wheat, under Heat Stress after Anthesis

AbstractBread and durum wheat genotypes were submitted to heat stress during the grain filling period, and relationships between grain weight and accumulated time from anthesis until maturity, using days after anthesis and growing degree days, were described by cubic polynomials. Maximum grain weight and the duration and rate of grain filling were estimated from the fitted curves. It was found that bread and durum wheat exposure to high temperatures significantly decreased grain weight and hastens physiological maturity (shortening the grain filling period). High temperatures significantly affected the rate (on a growing degree day basis) and duration (on Julian day units) of grain filling. The grain filling rate, on a thermal time basis, was positively associated with the final grain weight and the estimated maximum grain weight. The duration of grain filling does not appear to be a limiting factor for genotype grain weight stability, being mainly fixed by temperature. Grain weight of the controlled plants was positively correlated with the final and maximum grain weight of heat stressed plants. It was concluded that a high grain filling rate and a high potential grain weight are major traits that can be useful to improve heat tolerance of Triticum under Mediterranean environments.

Isotopic evidence for the provenance and turnover of organic carbon by soil microorganisms in the Antarctic dry valleys

The extremely cold and arid Antarctic dry valleys are one of the most environmentally harsh terrestrial ecosystems supporting organisms in which the biogeochemical transformations of carbon are exclusively driven by microorganisms. The natural abundance of (13)C and (15)N in source organic materials and soils have been examined to obtain evidence for the provenance of the soil organic matter and the C loss as CO(2) during extended incubation (approximately 1200 days at 10 degrees C under moist conditions) has been used to determine the potential decay of soil organic C. The organic matter in soils remote from sources of liquid water or where lacustrine productivity was low had isotope signatures characteristic of endolithic (lichen) sources, whereas at more sheltered and productive sites, the organic matter in the soils that was a mixture mainly lacustrine detritus and moss-derived organic matter. Soil organic C declined by up to 42% during extended incubation under laboratory conditions (equivalent to 50-73 years in the field on a thermal time basis), indicating relatively fast turnover, consistent with previous studies indicating mean residence times for soil organic C in dry valley soils in the range 52-123 years and also with recent inputs of relatively labile source materials.

Root penetration rate - a benchmark to identify soil and plant limitations to rooting depth in wheat

Data on wheat rooting depth was compiled from 36 agronomic experiments conducted in southern NSW from 1990 to 2004. Rooting depth was measured by direct soil coring and observation of roots using core-break or root washing techniques. Maximum rooting depth varied from 80 to 180 cm and was influenced by the depth of soil wetting, soil type and the duration of the vegetative phase (sowing to anthesis) as determined by interactions of sowing date, variety and seasonal conditions. The root penetration rate (RPR cm/day), defined as (maximum root depth measured at or after anthesis) / (days from sowing to anthesis), emerged as a simple but unifying parameter which could be used to estimate the potential rooting depth of wheat on different soils. RPR, expressed on a thermal time basis, was highly correlated with that expressed on a simpler time basis (r = 0.92). Incomplete wetting of the soil profile reduced maximum rooting depth and RPR in 12 of the 36 crops studied, and root penetration in the subsoil was clearly restricted in soil layers with less than 45 to 50% plant available water. Soil type influenced the RPR. The average RPR for wheat was 1.13 ± 0.04 cm/day on Red Kandosols (n = 11), 1.01 ± 0.07 cm/day on a Red Sodosol (n = 3) and 0.79 ± 0.03 cm/day on Red Chromosols (n = 10). The RPR was relatively constant across cultivars and sowing dates within these soil types, although there was some evidence for a reduction in RPR with later sowing independent of time or thermal time. We suggest that the RPR (cm/day) established for wheat on various soil types provides a useful tool for wheat growers to estimate the rooting depth and available water and nutrients in-season. It also provides a benchmark to indicate potential subsoil limitations to crop growth, and for researchers investigating opportunities to increase the maximum rooting depth of wheat through management or breeding.

A hydrothermal time model explains the cardinal temperatures for seed germination

AbstractTemperature (T) and water potential (y) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination (Tb), an optimum T at which germination is most rapid (To), and a maximum or ceiling T at which germination is prevented (Tc). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of Tb multiplied by the time to a given germination percentage (tg). Similarly, germination at reduced y can be characterized on a hydrotime basis, or tg multiplied by the y in excess of a base or threshold y that just prevents germination (yb). Within a seed population, the variation in thermal times to germination among different seed fractions (g) is based on a normal distribution of yb values among seeds (yb(g)). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds To. We report here that supra‐optimal temperatures shift the ψb(g) distribution of a potato (Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above To. A modified hydrothermal time model incorporating changes in ψb(g) at T > To describes germination timing and percentage across all T and ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.

Use of non-selective herbicides for wheat physiological and harvest maturity acceleration

Double cropping of wheat ( Triticum aestivum L.) and soybean ( Glycine max (L.) Merr.) in the southeast of Buenos Aires Province, Argentina, is risky because of the short frost-free period after wheat harvest and the probability of water deficits during the soybean cropping period. Soybean yield declines rapidly with delays in the sowing date. The objective of this work is to evaluate the potential to anticipate wheat harvest through the application of non-selective herbicides prior to physiological maturity. Two experiments were carried out in Tandil during the 1997 (wet year) and 1998 (dry year) growing seasons. Treatments consisted of herbicide (Glyphosate and Paraquat) applications to two wheat cultivars at different moments prior to physiological maturity. Herbicide applications produced greater grain moisture decreases than in the checks. The contact product exhibited greater drying rates than the systemic one in the water-deficient year. No differences in herbicide effect were observed between cultivars. Grains continued to accumulate dry weight after herbicide applications. In 1997, herbicide applications with grain moisture of 45% produced grain weight decreases close to 9% and a 7 days anticipation in harvest time. In the same year, herbicide applications with grain moisture of 40% decreased grain weight non-significantly and anticipation in harvest time was close to 5 days. In 1998, applications with both 40 and 45% grain moisture, did not significantly decrease grain weight and the anticipation in harvest time was close to 3 days. Considering both years and cultivars, harvest anticipation and grain dry weight decreases due to herbicide applications were better associated with the current rates of grain filling ( r=0.87 and 0.76 for harvest anticipation and grain dry weight decrease, respectively, on the basis of chronological time, and r=0.85 and 0.79, respectively, on the basis of thermal time) than with grain moisture during that time.

The fate of nitrogen from winter-frozen rapeseed leaves: mineralization, fluxes to the environment and uptake by rapeseed crop in spring.

For environmental purposes, very early sowing of winter rapeseed may reduce winter nitrate leaching thanks to the high N uptake capacities of rapeseed in autumn. However, freezing could lead to high losses of leaf nitrogen, amounting to more than 100 kg N ha-1 (Dejoux et al., 1999). Here we investigated the agronomic and environmental consequences of the decomposition of fallen leaves, based on field and laboratory studies with 15N labeled leaves (C:N=9). The potential kinetics of decomposition of leaves were measured by incubation in the laboratory. In the field, all leaves were removed at beginning of winter and replaced by labeled leaves, artificially frozen at −15°C , which were laid on the soil surface. Compared on a thermal time basis, decomposition proceeded as quickly in the field as in the incubations and was complete after 116 normalized days at 15 °C. The proportion of 15N derived from labeled leaves, absorbed again by the rape plants, was 28% at flowering and 24% at harvest. This high N recovery is assumed to result from the synchronization of leaves decomposition and active N absorption by rape in spring. Leaf N mineralization did not increase soil N mineral content at flowering or at harvest, but we observed a 40% loss of 15N. As no leaching was simulated, this loss was supposed to be gaseous. Such a high percentage could be explained by the fact that the decomposing leaves lay on the soil surface, and by climatic conditions conducive to such emissions. For environmental purposes, the quantity and nature of these gaseous N emissions have to be studied for other climatic conditions and types of leaves. As a proportion of N is reabsorbed, N fertilizer application rates could be reduced accordingly.

Intercepted Radiation at Flowering and Kernel Number in Maize

When water and nutrients are not limiting growth, kernel number per plant (KNP) can be calculated for maize (Zea mays L.) using linear models based on intercepted photosynthetically active radiation per plant (IPARP). These models do not include the concept of a threshold IPARP for KNP. Our objective was to study the response of KNP to IPARP in order to improve current models. Published information from several field experiments, performed with two maize hybrids in the temperate region of Argentina, was used for the analysis. Both linear and curvilinear (inverse linear and exponential) models based on IPARP explained more than 75% of the variability in KNP and kernel number per apical ear (KNA). Curvilinear models gave a better fit at low (<0.5 MJ plant−1 d−1) and intermediate (1.3‐2 MJ plant−1 d−1) values of IPARP than a simple two‐line linear model. Models allowed the estimation of maximum KNA and maximum KNP, and the minimum IPARP to reach them, but only curvilinear models allowed the estimation of a threshold IPARP (0.31‐0.37 MJ plant−1 d−1) to avoid plant barrenness. The inverse linear model gave a better prediction of maximum kernel number than the other two models. The exponential model allowed a better estimation of the initial value (2.16 MJ plant−1 d−1) for expressing prolificacy (ears plant−1) > 1. The relationship between KNP and IPARP did not improve when IPARP was expressed on a thermal time basis (photothermal quotient), probably because air temperature did not vary much among the situations explored in this work.

A thermal time basis for comparing the germination requirements of some British herbaceous plants

The effect of temperature on germination rate was assessed for seeds of 31 wild plant species and four cultivated species growing in the UK. The temperature at which seed first germinated ranged from 2 °C (Brassica rapa) to 11 °C (several species). As the temperature progressively increased, the percentage of seed that germinated rapidly increased to near 100%, whereas the duration for germination progressively decreased up to the thermal optimum. Above the thermal optimum, the duration for germination initially remained constant, and then rapidly increased until the thermal maximum was exceeded when no seed germinated. To assess the effect of temperature on germination rates, the reciprocal of the duration (1/duration) for 50% germination was calculated and regressed against temperature. For most species a linear regression accounted for >90% of the variation in germination rates for temperatures up to the thermal optimum. From the regressions, the base temperatures (Tb, below which no germination is expected) and the thermal constant (S, expressed in degree days) for 50% germination were calculated. The extremes were the alpine species Dryas octopetala, which had a high Tb (c. 12.0 °C) and low S (26 degree days), and Saxifraga tridactylites which had a low estimated Tb (c. −1.8 °C) and a high S (136 degree days). The estimated value of S increased with increasing percentage germination, and although S varied considerably between species with similar values of Tb, a weak inverse relationship between Tb and S was indicated. A thermal time approach was shown to be useful when analysing and comparing thermal requirements for germination of the seed of wild plant species. It showed, for example, that percentage germination often decreased as temperatures approached Tb, and that for some seed germination ceased at temperatures well above Tb.

Effects of a 4 °C increase in temperature on partitioning of leaf area and dry mass, root respiration and carbohydrates

1. Plants of Bellis perennis, Dactylis glomerata and Poa annua were grown from seed in controlled‐environment cabinets at either 16 or 20 °C; at the higher temperature all three species had increased total dry mass and leaf area when assessed on the basis of chronological time. On the basis of thermal time (summation of degree‐days above 0 °C; days °C) temperature decreased the dry mass in P. annua. 2. Partitioning was assessed as a change in the allometric coefficients relating shoot and root dry mass, leaf and plant mass, leaf area and plant mass, and leaf area and leaf mass. Of the 12 relationships examined only three were affected by temperature: there was increased partitioning towards the shoot relative to the root in D. glomerata and increased partitioning towards leaf area rather than leaf mass in D. glomerata and B.perennis. 3. Root respiration was unaffected by temperature of growth in D. glomerata and P.annua but was lower in B. perennis grown at elevated temperature. 4. Root respiration acclimated to temperature in P. annua and B. perennis (i.e. when measured at the same temperature, respiration was higher in plants grown at 16 °C). 5. Root soluble carbohydrate concentration was unaffected by temperature of growth in any of the species. Feeding sucrose to the roots for a short period had no effect on the rate of respiration of B. perennis or D. glomerata but increased root respiration of P. annua.

Respiration and germination rates of tomato seeds at suboptimal temperatures and reduced water potentials

Suboptimal temperature (T) affects germination rates (reciprocal of time to radicle emergence) on a thermal time basis; that is, the T in excess of a base or minimum temperature multiplied by the time to a given per cent germination [tg) is a constant. Respiration rates are also sensitive to T, and proportional relationships are often found between respiration rates and germination rates. Reduced water potential (ψ) delays seed germination on a hydrotime basis (i.e. the ψ in excess of a base water potential multiplied by tg is a constant). It was tested whether respiration rates prior to radicle emergence vary in proportion to T and ψ as expected from the thermal and hydrotime models. Respiration rates (C02 evolution) of cold-tolerant, rapidly germinating (PI 341988) and cold-sensitive, more slowly germinating (T5) tomato [Lycopersicon esculentum Mill.) seeds were evaluated over a range of T and ψ conditions. For both genotypes, respiration rates until the beginning of radicle emergence were related to T on a thermal time basis and increased approximately linearly with ψ above -2.0 MPa, consistent with the hydrotime model. Respiration rates were uniquely related to germination rates, regardless of whether germination timing was affected by T, ψ, or genotype. However, germination timing was unaffected when respiration rates were manipulated by varying 02 partial pressure. Thus, while both germination and respiration rates vary with T and ψ consistent with thermal and hydrotime models of biological time, respiration rates per se were not the limiting factor in germination timing of tomato seeds.

Hydrothermal time analysis of tomato seed germination at suboptimal temperature and reduced water potential

AbstractBoth temperature (T) and water potential (ψ) have consistent and quantifiable effects on the rate and extent of seed germination (radicle emergence). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of a base (Tb) multiplied by the time to a given percentage germination (tg). Similarly, germination at reduced ψ can be characterized on a hydrotime basis, or the ψ in excess of a base (ψb) multiplied by tg. Within a seed population, the variation in thermal times to germination for a specific percentage (g) is based upon the normal distribution of ψb values among seeds (ψb(g)). Germination responses across a range of suboptimal T and ψ might be accounted for by a general hydrothermal time model incorporating both T and ψ components. We tested this hypothesis for tomato (Lycopersicon esculentum Mill.) seeds of two genotypes differing in germination rates and tolerance of suboptimal T and ψ. For combinations of T (10−25°C) and ψ (0 to −0.9 MPa), a general hydrothermal time model accounted for approximately 75% of the variation in times to germination within the seed populations of both genotypes, and over 96% of the variation in median germination rates. However, ψb(g) distributions were sensitive to both the T and ψ of imbibition, resulting in a poor fit of the model to specific time course data. Analysis of germination timing separately for low and high ψ ranges within a given T resulted in specific models accounting for 88−99% of the variation in individual germination times and >99% of the variation in madian germination rates. Thus, for a given T and ψ range, the hydrotime model closely matched tomato seed germination time courses. Accumulated hydrothermal time accounted well for germination rates at ψ> −0.5 MPa across suboptimal T if ψb(g) was allowed to vary with T. Germination did not show a consistent response to T at ψ < −0.5 MPa, and estimated Tb values varied over different T ranges. Generalization of the hydrothermal time model across the entire range of suboptimal T and ψ was limited by physiological adjustments of the seeds to their current environment. The hydrothermal time model detected and quantified these adjustment processes that would otherwise not be evident from inspection of germination time courses. Temperature and water potential influence the time to germination via physiological mechanisms that reciprocally interact.

Influence of Temperature On the Development and Survival of Meloidogyne Javanica

Responses to temperature of a population of Meloidogyne javanica from Tanzania were examined. The minimum time for development from J2 to first J2 of the next generation was determined over a temperature range from 18-30°C. The rate of development increased linearly between 18°C and 27°C and, on a thermal time basis, the threshold or base temperature for development was estimated by extrapolation to be 12.9° C and the requirement for development c. 350°C days above base. These values are compared with those for M. hapla, a temperate species, and their effects on development time in relation to temperature are discussed. The effect of higher temperatures on egg viability was also tested. Short exposures to 70°C were lethal and exposure to 45°C and above killed a progressively increasing proportion of eggs. Exposure in the field in Tanzania of egg masses buried at different depths to solarization under clear polythene sheeting showed that at 15 cm depth all eggs were dead after 2-3 weeks with an average and a maximum soil temperature at 16.00 h of 43°C and 45°C respectively for the covered and 40°C and 42°C for the uncovered plots. Irrigating the plots before covering slightly decreased mean soil temperatures compared with the covered, dry plots and greatly increased egg survival.

Leaf area and crop growth in relation to phenology of barley in Mediterranean environments

In a barley/livestock farming system of northern Syria, high biomass production in addition to high grain yield is desirable. The aim of this study was to assess the effect of environment and phenology on growth and yield of barley in northern Syria. Leaf area duration (LAD), specific leaf area (SLA), crop growth rate (CGR) on a thermal time basis, and biological and grain yields were compared for entries representing three contrasting development patterns: early heading spring types (pattern A), medium early heading winter types (pattern B), and late heading spring types (pattern C). The experiment was conducted in 1988/89 at two sites: Tel Hadya (TH, 250 mm precipitation) and Breda (BR, 180 mm). Cold damage occurred in winter and, especially at BR, drought stress developed in spring.

The influence of pre-floral growth and development on the pathway of floral development, dry matter distribution and seed yield in oilseed rape (Brassica napus L.)

Patterns of floral development, dry matter distribution and seed yield were examined in winter oilseed rape plants subjected to different pre-floral growth environments. The duration of pre-floral growth and plant size at flower initiation, measured in terms of total mainstem leaf number, were manipulated by varying the temperature between seedling emergence and flower initiation. Exposure of seedlings to low temperatures from cotyledon expansion onwards markedly reduced the duration of pre-floral growth and the number of leaves on the mainstem. The subsequent development pattern of plants was largely dependent on the date of flower initiation and therefore vernalisation requirement. Indeed, the period of growth from flower initiation to maturity, considered on the basis of thermal time, was directly related to the duration of pre-floral growth and mainstem leaf number. The thermal durations of the bud development phase and flowering period in plants exposed to different pre-floral cold treatments but with a common date of flower initiation were similarly linked to these two parameters. Plants exposed to prolonged periods of low temperature treatment from cotyledon expansion onwards initiated fewer mainstem leaves during a relatively short pre-floral growth phase and their yield potential was limited by a reduction in branch and flower numbers. Plants maintained at higher temperatures produced more mainstem leaves during an extended period of pre-floral growth and supported a greater number of branches and flowers. However, this additional yield potential was not realised due to a reduction in seed numbers and mean seed weight. It appeared that seed yield of these plants was limited by increased competition between an excessive number of lower branches and flowers, a problem apparently created by excessive pre-floral growth. Minimal competition for available assimilates between the limited number of branches of plants with a shorter pre-floral growth phase and fewer mainstem leaves, resulted in lower levels of pod abortion, greater seed production and ultimately increased seed yields.

Pre-flowering Growth and Development of the Inflorescences of Maize

The growth and development of the main stem and the two uppermost axillary apices of maize were studied during the period from emergence until flowering. Plants were grown in the field under varying levels of applied nitrogen fertilizer at two times of sowing. The effects of daylength were isolated from those of temperature by making comparisons of growth and development on a thermal time basis. The growth and development of the terminal (male) apex and the two uppermost axillary (female) apices followed the same patterns, with apex volumes increasing curvilinearly with increase in number of leaf or husk primordia. The f?Gf?(relative growth rate of volume) of the terminal apex was, however, only approximately one-tenth of the axillary apices. There was no difference in growth and development between the first and second axillary apices before flowering: other factors, such as accumulation of dry weight, rather than primordia production, must be responsible for an axillary apex's potential to bear grain. Applied N, and to a lesser extent short days, increased the rates of growth and development of all the apices. For example, applied N increased the RGR (volume) of the apical domes, and the rate of production of spikelet primordia, by about 25%. All axillary apices and treatments showed a single relationship between number of spikelets and surface area of the ear: a favourable environment (e.g. high N) simply accelerated the progression of spikelet production and area expansion along this single path. We conclude that this path is probably determined genetically and that N and time of sowing influence potential yield of maize through effects on the surface area of the ear but not on the density of spikelets formed.