Cumulative Thermal Time Research Articles

A method for using monthly average temperatures in phenology models for grapevine (<i>Vitis vinifera</i> L.)

In recent years, there have been increasing efforts to link phenology models with seasonal climate predictions in so-called Decision Support Systems (DSS) to tailor crop management strategies. However, temporal discrepancies between phenology models with temperature data gathered on a daily basis and seasonal forecasting systems providing predictability on monthly scales have limited their use. In this work, we present a novel methodology to use monthly average temperature data in phenology models. Briefly stated, we modelled the timing of the appearance of specific grapevine phenological phases using monthly average temperatures. To do so, we computed the cumulative thermal time (Sf ) and the number of effective days per month (effd). The effd is the number of days in a month on which temperatures would be above the minimum value for development (Tb). The calculation of effd is obtained from a normal probability distribution function derived from historical weather records. We tested the methodology on four experimental plots located in different European countries with contrasting weather conditions and for four different grapevine cultivars. The root mean square deviation (RMSD) ranged from 4 to 7 days for all the phenological phases considered, at all the different sites, and for all the cultivars. Furthermore, the bias of observed vs predicted comparisons was not significantly different when using either monthly mean or daily temperature values to model phenology. This new methodology, therefore, provides an easy and robust way to incorporate monthly temperature data into grapevine phenology models.

Designing high-yielding wheat crops under late sowing: a case study in southern China

Cropping of rice and wheat (Triticum aestivum L.) in rotation contiguously in the same field is a fundamental pillar of double-cropping systems in southern China. Yields of such cropping systems are increasingly challenged as climate change (CC) drives increases in autumnal rainfall, delaying rice harvesting and subsequent sowing of wheat. Here, our purpose was to identify prospective traits of wheat crops enabling adaptation to later sowing and successively truncated growing seasons caused by CC. To identify traits that maintained or improved yields, we constructed 4,096 hypothetical genotypes underpinned by step-wise variations in parameters regulating phenology, growth and yield components. We then assimilated biophysical response surfaces through genotype (G) by environment (E) by management (M) analyses (G×E×M) using six locations spread across the breadth of southern China. We showed that later sowing reduced cumulative radiation interception, cumulative thermal time and crop capture of growing season rainfall. The culmination of these factors shortened crop duration and decreased biomass accumulation and retranslocation after anthesis, reducing grain number and penalising yields. Genotypes that had greater radiation use efficiency, longer juvenile phases and greater grain filling rates were more effective in alleviating yield losses with delayed sowing. However, not even the highest yielding genotype × management combination could entirely alleviate yield losses with delayed sowing. Our results suggest that CC and increasingly frequent extreme climatic events may reduce wheat yields in such cropping systems in the absence of other adaptation.

Predicting junglerice (Echinochloa colona L.) emergence as a function of thermal time in the humid pampas of Argentina

Junglerice (Echinochloa colona) is one of the most important annual weeds affecting crops in Argentina. A predictive seedling emergence model based on thermal time was developed and validated. Monitoring of seedling emergence was performed weekly during the growing season in a soybean field over four years. Cumulative thermal time, expressed in growing degree days (GDD), was used as the independent variable for predicting cumulative emergence. The variations in mean air temperature between late August and early September have determined a period with a conserved pattern over the years. That period had a close linear relationship (r2 = 0.99) with the beginning of seedling emergence. A double-logistic model fitted junglerice seedling emergence better than Gompertz, Logistic or Weibull functions. Model validation showed a good performance in predicting the seedling emergence (r2 = 0.99). Based on findings of this study it is possible to predict junglerice emergence by air temperature and, thus, to contribute reliably to the rational management of this weed.

Development of a Critical Nitrogen Dilution Curve Based on Leaf Area Duration in Wheat.

Precise quantification of plant nitrogen (N) nutrition status is essential for crop N management. The concept of critical N concentration (Nc) has been widely used for assessment of plant N status. This study aimed to develop a new winter wheat Nc dilution curve based on leaf area duration (LAD). Four field experiments were performed on different cultivars with different N fertilization modes in the Yangtze River basin and Yellow River basin in China. Results showed that the increase in LAD with increasing cumulative thermal time took the shape of an “S” type curve; whereas shoot N concentration decreased with increasing LAD, according to a power function. Both LAD and shoot N concentration increased with increasing N application. The new LAD based Nc dilution curve was determined and described as Nc = 1.6774 LAD−0.37 when LAD > 0.13. However, when LAD ≤ 0.13, Nc was constant and can be calculated by the equation when LAD = 0.13. The validation of Nc dilution curve with dataset acquired from independent experiments confirmed that N nutrition index (NNI) predictions based on the newly established Nc dilution curve could precisely diagnose N deficiency at different plant growth stages. The integrated N nutrition index (NNIinte), which was obtained by the weighted mean of NNI, was used to estimate shoot N concentration, shoot dry matter, LAD, and yield using regression functions. The linear relationships between NNIinte and these growth variables were well correlated. These results provided enough evidence that the new LAD–based Nc dilution curve could effectively and precisely diagnoses N deficiency in winter wheat crops.

Agronomic performance and seed quality attributes of Camelina (Camelina sativa L. crantz) in multi-environment trials across Europe and Canada

Camelina (Camelina sativa L. Crantz) is considered a relatively new oilseed Brassicacea in both Europe and North America, even though its history as a crop dates back to the Bronze Age. Camelina has recently received renewed interest from both the scientific community and bio-based industries around the world. The main attractive features of this species are: drought and frost tolerance, disease and pest resistance, a unique seed oil composition with high levels of n-3 fatty acids, a considerably high seed oil content, and satisfactory seed yields, in particular under low-input management and in limiting environments. Aiming at evaluating the feasible introduction of recently released camelina breeding lines under different environmental conditions and their productive potential a multi-location trial was set up. The agronomic performance of nine improved genotypes of camelina was evaluated in a wide range of environments in Europe (Greece, Italy, Poland) and in five locations across Canada, in two consecutive growing seasons (2015 and 2016). Sowing time was optimized for each location according to the different climatic conditions. Camelina proved to be a highly adaptable species, reaching seed yields of about 1MgDMha−1 under the most limiting conditions (i.e., low precipitation, poor soil quality, extremely high temperature at flowering). Growing environments characterized by mild temperatures and adequate rainfall (>170mm, during the growing season) resulted in higher average seed yields. The length of the growing cycle varied greatly between different locations (80–110d), but the cumulative thermal time was quite stable (∼1200 GDD, growing degree days). The advanced breeding line 787–08, which possesses up to 30% larger seed compared to the mean seed size of all other test entries, proved to be the most promising genotype across all locations in Europe and Canada, combining high seed yields (1.1–2.7MgDMha−1) with improved yield stability. To the best of our knowledge, for the first time, camelina lines with improved oil composition (i.e., increased oleic and α-linolenic and lower linoleic acid contents) for feed, food and industrial applications were identified (789–02 and 887).

How variable are non-linear developmental responses to temperature in two perennial forage species?

Developmental responses to temperature are critical to yield formation in crops and perennial grassland species. However, their characterisation over a broad range of temperatures relevant to climate change studies has been limited in these species. The present study sought to determine the non-linear developmental responses to temperature of two major grassland species, tall fescue (Festuca arundinacea) and alfalfa (Medicago sativa), and to assess i) whether a coordinated response occurred between different developmental processes, ii) if genotypes from contrasting origins differed in their responses, and iii) to quantify how a lack of coordination and genetic variability might affect estimates of thermal time progression under present and future climate scenarios. Two series of experiments were carried out under controlled conditions with eight constant temperatures ranging from 5°C to 40°C applied to seedlings and mature plants. Different growth and developmental processes were characterized, including shoot development and radicle, leaf and stem growth. Once normalized to a temperature of 20°C, the temperature responses of the different processes displayed no significant variability between temperate and Mediterranean genotypes. On the other hand, not all developmental processes within a genotype displayed a coordinated response to temperature. Significant departures from the response of shoot development in mature plants were observed regarding several processes, particularly at low and supra-optimal temperatures (up to ±5°C and ±2°C for minimal and maximal temperature estimates, respectively). Differences were particularly marked relative to node and stem elongation responses. Overall, when using the temperature dependencies of different processes to estimate cumulative thermal time, significant bias was observed in alfalfa when considering stem elongation by comparison with other processes. The consequences for improvements to forage crop models, and for the meta-analyses often used to calibrate them, are discussed.

Soybean Maturity Group Choices for Maximizing Radiation Interception across Planting Dates in the Midsouth United States

Cumulative interception of photosynthetically active radiation (CIPAR) is a key factor affecting soybean [Glycine max (L.) Merr.] yield. Under well‐watered conditions, differences in CIPAR across planting dates, maturity groups (MGs) and row spacing in the US Midsouth could partially explain yield differences. Irrigated experiments were conducted for 2 yr at three locations in Arkansas with four planting dates and 16 cultivars (MG 3–6) using a narrow (0.46–0.48 m) and a twin‐row spacing (0.19–0.20 m twin rows on 0.96 m beds) with a seeding density of 35 plants m−2. The fraction of radiation intercepted (FRI) from emergence to beginning seedfill (or complete canopy closure) was well defined for each row spacing as a sigmoidal relationship with cumulative thermal time. A bilinear relationship between CIPAR and relative yield explained 46% of the yield variation. Relative yield increased with CIPAR, reached a maximum at 687 MJ m−2 and then declined. Simulations estimated the FRI and CIPAR for a range of planting dates, MG choices, narrow and twin‐row spacings, and two locations with 30‐yr of historical weather data. Simulations identified scenarios where management of row spacing and MGs avoid low FRI during flowering and at the start of seedfill. Simulations also identified management scenarios that ensure sufficient CIPAR to obtain 95% of maximum yields. Late MGs provided greater CIPAR compared with earlier MGs, but yield decreased at high values of CIPAR emphasizing the importance of understanding mechanisms underlying the yield response to CIPAR across planting dates and MGs.

Predicting Emergence of 23 Summer Annual Weed Species

First- and second-year seedbank emergence of 23 summer annual weed species common to U.S. corn production systems was studied. Field experiments were conducted between 1996 and 1999 at the Iowa State University Johnson Farm in Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for most species were planted in plastic crates. Seedling emergence was counted weekly for a 2-yr period following seed burial (starting in early spring). Soil temperature at 2 cm depth was estimated using soil temperature and moisture model software (STM2). The Weibull function was fit to cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time (HTT), and day of year (DOY). To identify optimum base temperature (Tbase) and base matric potential (ψbase) for calculating TT or HTT, Tbaseand ψbasevalues ranging from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each species. The search for the optimal model for each species was based on the Akaike's Information Criterion (AIC), whereas an extra penalty cost was added to HTT models. In general, fewer seedlings emerged during the first year of the first experimental run (approximately 18% across all species) than during the second experimental run (approximately 30%). However, second-year seedbank emergence was similar for both experimental runs (approximately 6%). Environmental effects may be the cause of differences in total seedling emergence among years. Based on the AIC criterion, for 17 species, the best fit of the model occurred using Tbaseranging from 2 to 15 C with four species also responding to ψbase= −750 kPa. For six species, a simple model using DOY resulted in the best fit. Adding penalty costs to AIC calculation allowed us to compare TT and HTT when both models behaved similarly. Using a constant Tbase, species were plotted and classified as early-, middle-, and late-emerging species, resulting in a practical tool for forecasting time of emergence. The results of this research provide robust information on the prediction of the time of summer annual weed emergence, which can be used to schedule weed and crop management.

Prediction of annual weed seed emergence in garlic (Allium sativum L.) using soil thermal time

Avena fatua L. and Polygonum aviculare L. are two competitive weeds in garlic (Allium sativum L.) fields. Knowledge of the temporal pattern emergence will contribute to optimizing the timing of control measures, thus maximizing efficacy. The development of predictive models can contribute to control measures at early growth stages. The objective of this study was to develop and validate predictive empirical models of emergence for A. fatua and P. aviculare based on thermal time. Cumulative seedling emergence data were obtained during two years from a garlic field and used to develop and validate the models. The relationship between cumulative seedling emergences and cumulative thermal time (TT) under field conditions was analyzed using the Gompertz function. The models accounted for 98% and 96% of the variation observed in A. fatua and P. aviculare, respectively. Model validation performed well in predicting the seedling emergence of both species. According to this model, A. fatua emergence started at 381 TT after sowing and reached 50% and 90% of total emergence at 407 and 478 TT, respectively, with a soil base temperature of 1°C. P. aviculare started emergence at 410 TT after sowing and reached 50% and 90% of total emergence at 505 and 590 TT, respectively, with a base temperature of 0°C. Results indicate that these models could be useful as predictive tool contributing to a effective control of A. fatua and P. aviculare populations in garlic crops.

Root growth and anchorage by transplanted 'Tifgreen' (Cynodon dactylon x C. transvaalensis) turfgrass.

Field experiments quantified factors affecting root growth and anchorage by transplanted 'Tifgreen' (Cynodon dactylon (L.) Pers.×Cynodon transvaalensis Burtt Davy) sod, a globally important warm-season C4 turfgrass. Vertical force required to detach recently transplanted sod from underlying soil was the measure of root anchoring strength. In early spring, date of sod harvest and transplantation was important to root growth and anchorage measured 30 days after transplantation. Delaying sod harvest/transplantation by about a month after the end of the winter shoot dormancy period increased root anchoring strength 200% and root dry mass 640% during the 30 days after sodding. The strong effect of early-spring sodding date on root anchorage was related to cumulative thermal time before sod harvesting. Root anchoring strength was directly proportional to the number, but not mass, of roots produced by transplanted sod. In late spring, anchoring of sod to very firm traffic-compacted clay was 87% greater than to loamy sand, measured 14 days after sodding. N-P-K fertilisation did not affect late-spring sod anchorage to loamy sand soil, measured 18 days after sodding, but did enhance shoot density and colour. Sod root penetration into a silt loam soil was unaffected by an initially dry surface layer when sufficient irrigation was used. Overall, root anchorage by transplanted Tifgreen sod was similar to, or greater than, values reported for cool-season C3 turfgrasses in similar circumstances.

Radiation use efficiency and shoot:root dry matter partitioning in seedling growths and regrowth crops of lucerne ( Medicago sativa L.) after spring and autumn sowings

Storage of carbohydrates in roots is important for lucerne survival and regrowth rate, begins during initial growth. This study examined whether variation in radiation use efficiency (RUE) and shoot:root partitioning ( P shoot) occurring for various types of lucerne canopy, including seedling crops and regrowths in various seasons, was significant or not. Seedling and regrowth crops were monitored for four sowing dates in fully irrigated experiments on chalky soils in north-eastern France for three years. P shoot was calculated as the ratio between increments in aerial dry matter (ADM) and total dry matter (TDM) for successive crop measurements. Leaf Area Index (LAI) was regressed against cumulative thermal time (CTh) using a 5 °C base temperature, which provided the best fit ( R 2 = 0.98). The increase in LAI was exponential up to 0.7 m 2 of green leaves per m 2 of soil (350 CTh) and then became linear for seedling crops, but was linear for regrowths. During spring and summer, the slope of the linear phase of LAI increase was almost constant for all the seedling growths (0.011–0.014 CTh −1) except the last summer sowing, but was significantly variable (from 0.007 to 0.025 CTh −1) for regrowths. The RUE based on ADM and TDM (RUEa and RUEt, respectively) reached a linear phase for seedling growths when ADM exceeded 1.0 t DM ha −1. RUEa was equal to 1.72 g DM MJ −1 irrespective of sowing date, while RUEt was higher at 2.28 and 2.84 DM MJ −1 for canopies from spring and summer sowings, respectively. The RUE of seedling crops always exceeded those of regrowth crops present at the same period. Hence seedling crops of lucerne were slower to establish their LAI than regrowths, but used cumulative intercepted radiation as efficiently as the latter. The seedling P shoot varied slightly according to crop age and season unlike the regrowth P shoot which decreased significantly vs. CTh5 (−8.9 × 10 −4 CTh −1; R 2 = 0.66); while the ratio was also influenced by the daylength of the period. The P shoot of seedling and regrowth canopies evolved very differently with respect to CTh, the former remaining quite stable and the latter declining. This suggests a change in plant behaviour between seedling and regrowth crops, probably triggered by crop defoliation.

A Thermal Time Model to Predict Corn Poppy (Papaver rhoeas) Emergence in Cereal Fields

Corn poppy is the most abundant broad-leaved weed in winter cereals of Mediterranean climate areas and causes important yield losses in wheat. Knowledge of the temporal pattern of emergence will contribute to optimize the timing of control measures, thus maximizing efficacy. The objectives of this research were to develop an emergence model on the basis of soil thermal time and validate it in several localities across Spain. To develop the model, monitoring of seedling emergence was performed weekly during the growing season in a cereal field located in northeastern Spain, during 3 yr. Cumulative thermal time from sowing date was used as the independent variable for predicting cumulative emergence. The Gompertz model was fitted to the data set of emergences. A base temperature of 1.0 C was estimated through iteration for maximum fit. The model accounted for 91% of the variation observed. Model validation in several localities and years showed general good performance in predicting corn poppy seedling emergence ( values ranging from 0.64 to 0.99 and root-mean-square error from 4.4 to 24.3). Ninety percent emergence was accurately predicted in most localities. Results showed that the model performs with greater reliability when significant rainfall (10 mm) occurs within 10 d after crop sowing. Complemented with in-field scouting, it may be a useful tool to better timing control measures in areas that are homogeneous enough regarding climate and crop management.

Understorey microclimate and crop performance in a Grevillea robusta-based agroforestry system in semi-arid Kenya

The influence of dispersed trees on microclimatic conditions, gas exchange and productivity of maize ( Zea mays L.) in a Grevillea robusta-based agroforestry system in semi-arid Kenya was examined to test the hypothesis that the benefits of shade seen in savannah ecosystems may be outweighed by competition for below-ground resources. Meristem temperature, cumulative thermal time, intercepted radiation, spatial distribution of shade and gas exchange were determined for maize grown as sole crops, in an agroforestry system, or under shade nets providing 25 or 50% reductions in incident radiation to discriminate between effects of shade and below-ground competition. The major benefit of shade was to reduce exposure to the supra-optimal temperatures experienced in many tropical regions, and which are predicted to become increasingly common by climate change models. However, although trees decreased photosynthetic photon flux density ( PPFD) incident on understorey maize by ca. 30%, the yield reduction was much greater than in the 25% shade net treatment in four seasons providing contrasting rainfall. Maize yield was unaffected by 50% artificial shade in the driest season (168 mm) but decreased with increasing shade when rainfall was high (628 mm). Shade reduced meristem temperature and delayed flowering by 5–24 days depending on treatment and seasonal rainfall. Thermal time to flowering in the agroforestry system doubled from 600 to 1200 °C day as rainfall decreased. Photosynthetic and transpiration rates for understorey maize were similar to the 25 and 50% shade treatments when rainfall was high, but were ca. 10% of those for unshaded sole maize in dry seasons. PPFD-saturated photosynthetic rate was initially similar in all treatments but fell sharply in the agroforestry system as the season progressed. Radiation conversion coefficients did not differ between unshaded sole and understorey maize. The results suggest that the ameliorative influence of tree shade was greater in low rainfall seasons, as in savannah systems, but that potential benefits were outweighed by below-ground competition. This may be managed by root-pruning trees.

Decomposition of sugarcane harvest residue in different climatic zones

Sugarcane in Australia is increasingly grown under the green cane trash blanket system where harvest residues (trash) are retained on the soil surface instead of being burnt. This is considered a more sustainable system, but relatively little is known about its effects on soil carbon (C) and nitrogen (N). As part of a study to understand the effects of trash retention on soil C and N dynamics, we measured the composition and decomposition of sugarcane trash in terms of dry matter (DM), C, and N in 5 field experiments in contrasting climatic conditions in Queensland and New South Wales. The trash from newly harvested sugarcane contained large quantities of DM (7–12 t/ha) and C (3–5 t/ha), which could be estimated from cane yield, and significant quantities of N (28–54 kg/ha), which could not be predicted from cane yield. Trash quality was low (C : N ratio >70) and it took a year for most of the trash to decompose. Cumulative thermal time was the variable most closely associated with cumulative DM and C decomposition. Variation in the rate of trash DM and C decomposition between sampling dates was partially related to temperature and rainfall at 2 of the 3 sites, but was considered to be influenced by other factors (such as soil, trash, and management) as much as by climate. There were 2 phases of decomposition: an early phase when C : N ratios were high and variable and net N loss or gain was not related to C loss; and a late phase when C : N ratios were much lower and similar across experiments and net N loss was related to C loss. The rate of N loss from trash during the first 12 months was slow (1–5 kg/month), which would have been of little immediate significance for plant growth. The potential value of trash for soil N supply lies in cumulative effects over the medium–long term.

Model predicting dynamics of biomass, structure and digestibility of herbage in managed permanent pastures. 1. Model description

AbstractTo investigate seasonal and annual interactions between management and grassland dynamics, a simple mechanistic model of the dynamics of production, structure and digestibility in permanent pastures was constructed. The model is designed to respond to various defoliation regimes, perform multiple‐year simulations and produce simple outputs that are easy to use as inputs for a model of ruminant livestock production. Grassland communities are described using a set of average functional traits of their constituent grass groups. The sward is subdivided into four structural compartments: green leaves and sheath, dead leaves and sheath, green stems and flowers, and dead stems and flowers. Each compartment is characterized by its biomass, age and digestibility. Only above‐ground growth is modelled, using a light‐utilization efficiency approach modulated by a seasonal pattern of storage and mobilization of reserves. Ageing of plant parts is driven by cumulative thermal time from 1 January and by biomass flows. Age affects senescence, abscission and digestibility of green compartments and, therefore, the quality of green leaves and stems can increase or decrease over time in relation to net growth and defoliation dynamics. The functional traits having the greatest impact on model outputs are seasonal effects, period of reproductive growth and effects of temperature on photosynthetic efficiency.The functional traits of the grass groups were parameterized for temperate pastures of the Auvergne region in France. The other model inputs are few: proportion of functional groups, basic weather data (incident photosynthetically active radiation, mean daily temperature, precipitation and potential evapotranspiration) and site characteristics (nitrogen nutrition index, soil water‐holding capacity). In the context of a whole‐farm simulator, the model can be applied at a field scale.

Combined Effects of Elevated Temperature and Carbon Dioxide on Geometry of Maize Leaves

An accurate prediction of variations in water consumption from a field and in the crop growih affected by the climate change due to global warming is important for the continuous agricultural activities. Components of water consumption in a farrnland, i.e. transpiration from the crop surface and evaporation from the soil surface, are influenced by the structure of crop canopy. In this study, the combined effects of elevated temperature and carbon dioxide (C02) concentration on the geometry and longevity of leaves of maize were examined through an experiment with growih chambers. Three treatments, i.e. TLCL (air temperature day/night: 28/22'C, C02 concentration: 350ppm), TLCH (28/22'C, 750ppm), and THCH (32/26'C, 750 ppm), were performed in different closed chamber respectively, and sixteen of maize (Zea mays L. cv. Pioneer G-98) planted in a pot were grown in each treatment. The cumulative thermal time from emergence of plant until the expansion of the ith leaf became larger in the order, TLCH<TLCL< THCH. The longevity of the ith leaf became longer in the order, TLCL<TLCH<THCH. Area of the ith leaf

Effects of nitrogen and radiation on dry matter and nitrogen accumulation in the spike of winter wheat

In wheat, spike growth prior to anthesis is a key period influencing kernel set. Incident radiation and crop N status affect the accumulation of dry matter (DM) and nitrogen (N) in the spike, as observed at anthesis. However, the influence of these two factors on the dynamics of spike growth throughout the pre-anthesis period is not fully understood. Our objective was to investigate the effects of incident radiation and crop N nutrition on the dynamics of spike dry matter, N concentration and N content throughout the pre-anthesis period and to propose models describing spike DM and N content during spike growth, per unit area. We also investigated whether kernel number was related to spike N content at anthesis independently of spike DM, a controversial issue. Field experiments were conducted over two seasons with ‘Trémie’ winter wheat and over an additional season with ‘Arche’ and ‘Soissons’. Treatments involved the application of N fertiliser at different stages and rates to achieve various levels of N deficit and the use of shading nets at various periods during spike growth to reduce incident radiation. Shading reduced spike DM but increased spike N concentration; it reduced spike N content whenever there was a large decrease in incident photosynthetically active radiation (PAR). All these effects occurred only during the second half of the spike growth period, even if shading was applied from the onset of spike growth. Crop N deficit reduced spike DM, N concentration and N accumulation. Spike DM and N content were affected throughout the spike growth period if the deficit started before the beginning of spike growth, and was severe. Continuous and temporary N deficits induced similar responses. The interaction between shading and N fertilisation was not significant for spike DM and N content, but was significant for spike N concentration. Two models, one predicting spike DM and the other predicting spike N content at any time during spike growth were developed. They were validated with an independent data set and by cross-validation with the initial database. Both models fitted the observed data well. The input variables, cumulative PAR after sowing, nitrogen nutrition index (NNI) of the crop and cumulative thermal time after sowing are easy to measure or to simulate. Kernel number was correlated with both spike DM and spike N content at anthesis, demonstrating that N content played no role in determining kernel number independently of DM.

Salinity Effects on Seedling Growth and Yield Components of Rice

Flood irrigation practices that are commonly used in California during the early stages of rice (Oryza sativa L.) establishment may contribute to salinity damage and eventually decrease yield. Knowledge of salinity effects on rice seedling growth and yield components would improve management practices in fields and increase our understanding of salt tolerance mechanisms in rice. Salinity sensitivity of rice was studied to determine salinity effects on seedlings and yield components. Plants of rice cultivar M‐202 were grown in a greenhouse in sand and irrigated with nutrient solutions of control and treatments amended with NaCl and CaCl2 (2:1 molar concentration) at 1.9, 3.4, 4.5, 6.1, 7.9, and 11.5 dS m−1 electrical conductivity. Shoot dry weights of seedlings were measured at five harvests in the first month after seeding. Seedling growth was significantly reduced by salinity at the lowest salinity treatment, 1.9 dS m−1. At 1.9 and 3.4 dS m−1, significant reduction of seedling growth occurred at longer cumulative thermal time than at higher salt levels. Seedling survival was significantly reduced when salinity was 3.40 dS m−1 and higher. Highly significant linear responses of grain weight per plant, grain weight per panicle, spikelet number per panicle, and tiller number per plant to salinity were observed. There was a common lowest salt level for fertility and pollen germination beyond which they were significantly reduced by salinity. Harvest index was significantly decreased when salinity was at 3.40 dS m−1 and higher. Tiller number per plant and spikelet number per panicle contributed the most variation in grain weight per plant under salinity. Reductions in seedling survival, tiller number per plant, and spikelet number per panicle were the major causes of yield loss in M‐202 under salinity. The compensation between spikelets and other yield components was confounded with salinity effects, but was believed to be minor relative to the reduction of spikelets due to salinity and, therefore, not sufficient to offset yield loss even at moderate salt levels.

Straw incorporation and immobilization of spring‐applied nitrogen

Abstract. The effect of rate and timing of autumn/winter straw incorporation on the immobilization of spring‐applied N‐fertilizer are studied. The immobilization of 15N‐labelled fertilizer by straw was determined in pot trials with a sandy loam soil (Wick series) collected from two field experiments. In experiment 1, straw was incorporated at four different rates in the autumn; experiment 2 had a fixed rate of straw (7.5 t/ha) incorporated at monthly intervals from September to March. Immobilization in spring was not enhanced when only stubble was incorporated in autumn. However, autumn incorporation of 7.5 t/ha straw resulted in a significant increase in the immobilization of spring‐applied N, equivalent to 10 kg/ha; when 15 t/ha straw was incorporated, immobilization increased to 18 kg/ha. The enhancement of immobilization, immediately following fertilizer application, was dependent on the extent of straw decomposition prior to N application. Thus immobilization was related to cumulative thermal time (day °C above 0°C; Tsum between the date of straw incorporation and fertilizer application). Straw which had been incorporated for Tsum &gt; 1200 no longer caused appreciable immobilization of spring fertilizer.

Effect of Forcing Temperature on Flowering of Four Herbaceous Perennial Species

Four herbaceous perennial species, Delphinium grandiflorum `Blue Mirror', Hibiscus xhybrida `Disco Belle Mix', Salvia xsuperba `Blue Queen', and Veronica longifolia `Sunny Border Blue' were forced in a glass greenhouse at 15, 18, 21, 24, or 27°C under long days. Before being forced, all tested species except H. xhybrida were exposed to 5°C for 12 weeks. Increasing forcing temperature generally promoted visible bud and flowering. However, visible bud and flowering of D. grandiflorum `Blue Mirror' and flowering of V. longifolia `Sunny Border Blue' were delayed at 27°C. Although the tested species tended to have more flower buds, bigger flowers, and greater height at lower forcing temperatures, the effect of forcing temperature on those characteristics was species-dependent. Temperatures as low as 15°C decreased bud number and flower size of H. xhybrida `Disco Belle Mix'. The base temperature (Tb) and cumulative thermal time (CTT) necessary to complete the indicated developmental stage were calculated from a linear regression: 1/f = a + bT. Based this equation, days to flowering (or visible bud) at certain temperatures or the temperature required for flowering within a certain number of days can be predicted.