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This paper reviews the results of published and unpublished experiments over 30 years looking at the effect on yield of planting density and row spacing, and of raised bed planting, of modern irrigated short wheat cultivars under the favourable climatic conditions of northwest Mexico. With optimum planting dates (mid Nov. to mid Dec.), regardless of row spacing, wheat yield was remarkably insensitive to planting density within the range normally studied (80–400 plants/m2). However yield was sensitive to row spacing: the most sensitive cultivars (erect dwarf cultivars) lost yield at spacing 30 cm and greater, while the least sensitive (some taller vigorous semidwarf cultivars) tolerated spacing up to at least 50cm without yield loss. Yield sensitivity to wide spacing was unaffected by density but varied between years, being less in favourable cooler years, and was markedly greater with later plantings (Jan.). The response to yield on narrow raised beds (2–3 rows per bed, 75–90 cm between bed centres), which have many agronomic advantages, could be explained by the aforementioned yield responses to spacing, in this case the furrow gap or distance between rows on adjacent beds being critical. Yield testing with a furrow gap of 44–50 cm appeared to select cultivars more suited to this system, but it wasn’t clear whether such cultivars represented the best option under normal 20 cm row spacing in the absence of raised beds. When testing was extended to very low plant densities, yield was remarkably insensitive (maximum yield often achieved with only 16/m2), provided the planting date was normal and the planting arrangement did not deviate greatly from a square grid, with the honeycomb design giving an additional small yield advantage. These yield responses reflect the great plasticity in wheat imparted especially by tillering, and appear to be reasonably well explained by the notion that maximum yield requires close to full light interception no later than just before flag leaf emergence, being the onset of dry weight accumulation in growing spikes. Greater leaf greenness and probably higher radiation use efficiency with very low densities (and wide gaps), and associated delays in 50% flowering, may also assist in compensation for gaps. In contrast to much conventional wisdom, it is suggested that a worthwhile breeding objective would be selection for high per spaced-plant yield, with a view to achieving normal (or better) commercial yields at very low plant densities (

This position paper honours agricultural scientist and colleague, Professor Bob Loomis, by discussing the urgent global challenge of food security and the related impacts on the environment facing agricultural science and society in the next critical 20 years. It uses the concepts of potential and actual (farm) crop yields and the yield gap between them to assess current and future opportunities for food supply to satisfy increasing demand. The cropping world is seen in two parts. The first part predominantly comprises low-input farming with very large yield gaps and a faster growing demand that can only be met with increasing imports. For these regions, a well-established strategy is outlined for crop intensification through yield-gap closure that is essential for reducing rural malnutrition and poverty, and curtailing the likelihood of high food prices. For success, it must be complemented with strategies to remove the serious institutional and infrastructural barriers faced by farmers. The second part has more or less intensified, and yield gaps are generally small to moderate: it will fairly comfortably meet the demand from population growth. For these regions, some further yield gap closure is still possible but more importantly greater potential yields are required although the chances of accelerating this are discussed and seen to be limited. For all regions, sustainable intensification of cropping, predominantly on existing arable lands, is the best way forward. Combining sustainability with intensification is not a contradiction and is, in fact, essential; sustainability requires the efficient use of all inputs in cropping, and husbandry of the soil and agricultural biodiversity needed to continue to raise productivity. Off-farm environmental impacts are inevitable, but not insurmountable, hurdles. All aspects of sustainability require boosted RD&E and sound rural policies. Greater management skills for farmers and all others involved in crop production are also essential. Contestation based on biophysical aspects of food production and its impacts can be resolved through effective research and development with farmers, while that based on Northern cultural and normative views must not be allowed to obscure the goal of affordable food for all, and reward for farmers comparable with the rest of their societies.

In order to study the effect of duration of the vegetative period (sowing to floral initiation) on potential yield, sister spring wheat semi-dwarf cultivars, Yecora and Cajeme, the latter having an extra vernalization sensitive allele (Vrn-A1v) and a photoperiod sensitive one (Ppd-D1b), were grown at 4–5 sowing dates over 5 years under irrigation and high fertility in northwest Mexico (latitude 27°N). In the earliest sowings (late Oct-early Nov) Cajeme had a 20 day longer vegetative period; this delay decreased steadily to 8 days in the latest sowings (mid-late January); anthesis date for Cajeme was, respectively, 17 and 6 days later. Relationships to minimum temperature levels in the vegetative phase strongly suggest that this sowing date by cultivar interaction arises largely because of the difference in vernalization alleles in an environment where there is limited vernalizing cold. Cajeme produced a greater maximum number of shoots, more spikelet nodes per spike, greater green area index, and greater above ground dry matter at anthesis and at physiological maturity. However Cajeme yielded no more than Yecora, even when yield was plotted against anthesis date, tending to have fewer grains/m 2 . A similar conclusion was reached when grains/m 2 were related to preanthesis photothermal quotient, and grain weight to grain filling mean temperature: both cultivars responded similarly although Cajeme had slightly fewer grains/m 2 and heavier kernels, thus weather around flowering dominated determination of these yield components. Some other yield components were also slightly, but significantly, affected by cultivar in a manner independent of flowering date and weather. Thus Cajeme had a significantly higher spike dry matter at anthesis and a significantly lower fruiting efficiency of Cajeme (73.8 versus 84.2 grains/g spike dry matter). It is suggested that the latter was a consequence of the longer vegetative period leading to greater tillering, poorer tiller survival and a more competitive preanthesis canopy, causing poorer floret survival in grain-bearing spikes. The excessive tillering may have been exaggerated by supplying all nitrogen fertilizer at sowing.

The paper refers to spring wheat breeding for potential yield, yield in the absence of manageable stresses, under irrigated low latitude conditions, representing a major environment for wheat production. Initially it discusses two propositions both over 50 years old. Donald argued that for greater yield, early generation selection should target uncompetitive plant traits or “communal” plants. In contrast, Fasoulas proposed the opposite, namely selection of plants with high yield under nil-competition, aided by the use of a honeycomb design to control for soil variation. Despite some promising results, the absence of definitive proof of the relevance to yield breeding of either concept, wheat breeders have generally opted for an intermediate approach with focus on measuring yield in plots as early as possible. They have been successful but yield progress is slowing. The advent of seeding machinery permitting regular planting patterns through seed singulation, and the reporting of optimal densities for maximum yield per hectare as low as 20 plants/m2 under irrigation and excellent management in northwest Mexico ( Fischer et al., 2019 ), suggest that Fasoulas’s method needs reinvestigating at least for wheat. Yield crossovers (i.e. genotype rank changes) with increasing plant density from the very low density for nil-competition (around 1.4/m2) are expected to be less likely if the target crop density is as low as say 20/m2. There are enough unknowns in the physiology of the wheat crop response to density to suggest that this is quite possible. The approach opens up new possibly more efficient strategies for early generation (e.g. F2) selection for potential yield and could give greater yield progress per unit cost. It would be especially suited for hybrid wheat breeding and production systems. Finally the paper discusses, in the light of the advances in precision agriculture, the opportunities for the agronomy of crops and breeding-nurseries based on low-density honeycomb planting. The major challenge, namely weed control, is manageable.

Given the importance of crop yield and yield progress, this review endeavours to clearly define the different representations of yield, discuss their measurement, and elucidate some controlling factors in yield change. For a field, farm, district or region, average farm or actual yield (FY) is central, but potential (and water-limited potential) yield (PY, PYw) is also an important yardstick. PY is defined here as the measured yield of the best cultivar, grown with optimal agronomy and without manageable biotic and abiotic stresses, under natural resource and cropping system conditions representative of the target area. Economic yield, governed by considerations of profit and risk, and record and theoretical yield, complete the picture. Yield gap is defined as the difference between PY and FY under the same environment. Across most crop-region combinations in the last 2 to 3 decades, FY progress has been associated with both PY progress and yield gap closing, and a simple model, based on linear regression against time, is proposed for understanding this. PY advance is the result of plant breeding and new agronomy (and their interaction, usually positive), while yield gap closing arises with the adoption by farmers of known innovations faster than new ones are invented. Unravelling the true technological component in apparent progress in PY, and especially in FY, is not necessarily simple, and confounding factors are listed and discussed.

This paper is a response to Sinclair and Jamieson [Sinclair, T.R., Jamieson, P.D., 2006. Grain number, wheat yield, and bottling beer: an analysis. Field Crops Res. 98, 60–67] who propose that bulk carbon and nitrogen accumulation are fundamental to grain yield determination in wheat (Triticum aestivum L.), while challenging the common approach to yield through the separate processes of grain number determination, followed by grain filling, as governed by source–sink balance then. The response focuses on yield determination under potential conditions for which genetic and agronomic progress is clearly associated with increased grain number, herein abbreviated to KNO (kernels m−2). It argues that grain yield in modern cultivars is still limited by post-anthesis sink (KNO) and that understanding KNO determination is therefore useful for predicting physiological routes to higher yield. KNO determination appears to be strongly related to dry matter accumulation in spikes at anthesis (g m−2), governed by events in the last 20–30 days before anthesis, while some modern cultivars show higher grain number per unit spike weight. Post-anthesis photosynthesis and crop dry weight accumulation have increased as KNO has increased with breeding. There is no evidence for effects of N on KNO apart from those operating via dry matter accumulation, or for grain N demand limiting post-anthesis photosynthesis. Beyond this simple model, several other linkages that might exist between the pre- and post-anthesis periods are explored. Such linkages could help maintain the balance between the post-anthesis sink and the source required to fill the sink, and constitute common underlying processes which to some extent reconcile the model of Sinclair and Jamieson (2006) with current mainstream thinking about grain yield in wheat. Reports of an increasing amount of pre-anthesis carbohydrate reserves in the crop with breeding progress is a good example, but overall it is concluded that at least under potential conditions, the commonly accepted approach to grain yield determination is not invalidated by Sinclair and Jamieson (2006).

Six greenhouse and three field experiments were conducted at the International Center for Improvement of Maize and Wheat (CIM-MYT), Mexico, on bread wheat (Triticum aestivum L.) genotypes to ascertain the role of osmotic adjustment (OA) in sustaining grain yield and its stability under water deficit conditions. Under simulated water deficit conditions of the greenhouse and field, the genotypes differed considerably regarding OA, ranging from 0.31 to 0.86 MPa and from 0.60 to 0.99 MPa, respectively. When the mean values of OA across the six greenhouse experiments were regressed against those of grain yield obtained at different moisture levels of the line source, the correlation coefficient value increased with the increase in moisture stress, turning positively significant (P < 0.001) at the highest water deficit level. Similarly, osmotic potential at full turgor (OP100) and turgor potential (TP), measured in the greenhouse, were positively correlated (P < 0.05) with grain yield at the highest line source water deficit level. Osmotic adjustment and grain yield, both measured in the simulated water deficit condition in the field, were also positively correlated (P < 0.05). Besides, OA maintained yield stability through maintenance of turgor under water deficit during reproductive period of crop growth. The heritability of OA, OP100, and TP, computed by pooled analysis of variance across the six greenhouse experiments, was 0.74, 0.73, and 0.79, respectively. The results indicated that OA, as well as OP100 and TP, could be used as screening tools for drought-resistant bread wheat genotypes in the greenhouse. This study also demonstrated the appropriate greenhouse screening methodology in this regard.

Cropping systems in the sub-humid tropical highlands are characterised by continuous cultivated cereal monoculture, leading to serious erosion and fertility decline. There is a lack of information on improved agronomy, in particular crop rotation, the use of legumes, reduced tillage and crop residue retention, designed to counter these problems. Over the period 1991–1995 an experiment was carried out in the central highlands of Mexico, at El Batan (latitude 19°31′N, longitude 98°50′W, altitude 2250 m) to test the effect of improved agronomy. Under rainfed conditions (mean annual rainfall 603 mm, one crop per year), binary rotations of maize–wheat, maize–vetch, wheat–vetch and wheat–medic pasture were compared with continuous wheat and maize. Most rotations were tested under the four combinations of tillage (zero versus cultivated) and crop residue (retained versus harvested). All plots were split for nitrogen fertilizer, and appropriate herbicides were used for weed control. Maize after wheat outyielded continuous maize under all conditions of tillage, residue and nitrogen fertilization. Within the maize–wheat rotation, zero tillage with residue retention was clearly superior (average yield across N levels of 5025 kg/ha at 10% moisture) to the other tillage-residue combinations (average 4249 kg/ha), and during dry periods, showed less wilting. With continuous maize, yield was especially poor with zero tillage regardless of residue (average 3113 kg/ha), and this was associated with poor early growth and variable stunting even under wet conditions and for which there is no clear explanation. Maize after vetch yielded well with zero tillage (4372 kg/ha), but poorly with cultivation (3128 kg/ha), possibly due to less soil water at sowing in the latter. The main treatment yield variation was associated with either wilting score or radiation interception or both, all measured before tasseling. Observations of ponding and runoff during rain events indicated that runoff was negligible where crop residue was retained on the surface with zero tillage, but significant in all the other tillage-residue combinations. Without nitrogen, maize after vetch outyielded the other rotations, but these latter yielded more with N, having a much greater response to fertilizer nitrogen (18.4 kg/kg versus 4.5 kg/kg). The average net economic benefit, calculated for each cropping system by partial budgeting, was best for farmer practice with fertilizer, although vetch–maize with zero tillage and residue removal, and wheat–maize with cultivation and residue removal, both with nitrogen fertilizer, were close behind this treatment. Residue retention was disadvantaged by the high value of residues as fodder, and wheat rotations by the lower yield relative to maize, for no grain price advantage in the Mexican market.

There are large agricultural areas in the world where wheat yields are limited by low phosphorus (P) availability. Breeding for P uptake and P utilization efficiency may reduce this problem. This study was conducted to determine the contribution of P uptake and utilization efficiency to grain yield of selected spring wheat genotypes in different environments. Thirty-eight semidwarf spring bread wheat (Triticum aestivum) genotypes were grown in two experiments in Mexico, each on an acid Andisol under rainfed conditions and on a calcareous Aridisol with irrigation, without (−P) and with 35 kg P per ha fertilized (+P). Without P fertilization, grain yield ranged from 0.8 to 4.6 t ha−1 in the acid soil and from 2.4 to 5.2 t ha−1 in the calcareous soil. With P fertilization, this range was even larger. Under conditions of P deficiency, i.e. in the acid soil at −P and +P (high P adsorption) and calcareous soil at −P (P-depleted soil), P uptake explained 71–100% of the variation in grain yield, and was highly correlated with grain yield (r=0.79–0.95). In contrast, at +P in the calcareous soil, P utilization efficiency explained 60–63% of the variation in grain yield. Here, low grain P concentration was related to high grain yield (r=−0.40 to −0.59). In the calcareous soil, the harvest index was correlated with grain yield, irrespective of the P level. In the acid soil, post-anthesis P accumulation was important. It was positively correlated with grain yield, whereas in the calcareous soil, no post-anthesis-P accumulation occurred. Here, grain P accumulation at maturity was completely determined by translocation of pre-anthesis shoot P. We conclude that the combination of improved P uptake and P utilization efficiency in the same genotypes requires selection under both high and low-P conditions.

Meeting the demands of the current tera-scale visualization community requires the use of large- and small-scale tiled displays. The Lawrence Livermore National Laboratory (LLNL) efforts in this area have led to the creation of one of the largest interactive tiled displays built to date, and a number of new efforts for "personal" tiled displays. Visualization hardware and software now being built will display up to 15 times the number of pixels in a typical desktop display. We outline the system implemented at LLNL for the creation and support of large, tiled, multi-pipe graphics displays. The system builds on a simple, portable, parallel API for tiling OpenGL commands in parallel to multiple contexts on a set of X servers. This API has allowed us to retrofit existing visualization and analysis codes to support these displays with very little effort. While this work represents a first step toward the ultimate goal of scalable visualization in individual office spaces, the result has been the creation of useful new visualization workspaces and tools for our users.

This study examines the utility of measurements of canopy temperature depression (TD) below air temperature as an indirect selection criterion for grain yield in breeding of wheat (Triticum aestivum L.). TD differences can be rapidly assessed using a hand-held infra-red thermometer and may reflect differences in stomatal conductance. Replicated small plot experiments were conducted with 23–24 spring wheat cultivars planted at two dates in the winter cropping cycles of 1991–92 and 1992–93 at a hot low latitude location in Mexico. Across a two fold range in grain yield at each planting, TD was usually highly significantly correlated with grain yield. Averaging the results for each year, the phenotypic correlation TD vs. yield was 0.84** and 0.89** for 1991–92 and 1992–93, respectively. Correlations were strongest when TD was measured between noon and 4 pm, and were unaffected by crop stage of development (pre-heading, heading, grain fill) or timing relative to irrigation. TD was positively correlated with stomatal conductance. We discuss possible causes of these high correlations, and their implications for wheat breeding.

Genetic and agronomic contributions to yield gains for wheat (Triticum aestivum L.) in the Yaqui Valley of northwest Mexico were estimated for the period from 1968–1990. Five problems associated with estimating sources of yield gains were considered, namely: (1) adjusting yield gains for variation in weather over the study period; (2) considering annual cultivar-by-weather interactions, which are potentially problematic when relative yields of cultivars are generated over only part of the study period; (3) overestimating relative yields of cultivars if the check cultivar(s) become susceptible to disease; (4) ensuring that yields as estimated from research station trials represent cultivar performance under farmers' conditions; and (5) allowing for cultivar-by-management interactions. With these factors considered, 28% of the weather-adjusted yield gain of 103 kg ha−1 y−1 was attributed to genetic gain (i.e., cultivar improvement). If the yield gains had not been adjusted for annual weather variation, genetic gain would have been overestimated at 50%. By contrast, 48% of the gain was attributed to increased use of N fertilizer, driven at least in part by a decline in N prices; no N-by-cultivar interactions were apparent. The remainder of the yield gap (24%) could not be attributed to specific factors, although P application rates increased over time, and negative deviations from linearity were associated with years having greater than normal rainfall in November and December (a phenomenon which can delay sowing and/or reduce crop stand). In addition to identifying factors contributing to yield gains, technologies that have contributed benefits not measured by yield are discussed.

Differences in response to vernalization in wheat ( Triticum aestivum L.) were quantified through controlled environment experiments with 26 lines with diverse geographical origins. Vernalization treatments of 0 to 56 d were applied to plants at their first leaf stage. All plants headed irrespective of duration of vernalization treatment. Vernalization response was assessed through the change of final leaf number (FLN) on the main stem at heading. Five lines did not respond to vernalization. FLN for vernalization-sensitive lines generally decreased to a minimum as days of vernalization treatment increased. Plants at and after the stage where additional vernalization did not reduce FLN were vernalization insensitive. The quantitative features of this vernalization response, up to the point of insensitivity, were characterized with a linear regression: ( F i − 6) = α − βT v , where F i is FLN observed for a particular vernalization treatment, T v is the time in days of that vernalization treatment, and α and β are the Y-intercept and the slope of the regression, respectively. This model fitted the experimental results well. The parameters α and β varied among lines, and are useful for quantifying vernalization response in wheat. The implication of each parameter can be interpreted biologically: α is the “changeable number of leaves”, i.e., how many leaves can be potentially decreased by vernalization treatment, and β represents the “exchange rate” between leaf numbers and vernalization days, i.e., how many leaves can be reduced by one day of vernalization treatment.

Vernalization treatments of 0 to 70 d initiated when 0 to 8 leaf tips were visible were applied to plants of the winter wheat ( Triticum aestivum L.) cultivars Pioneer 2548 and Augusta. All plants headed irrespective of duration of vernalization. Unvernalized plants of Pioneer 2548 and Augusta had mean final leaf numbers (FLN) of 20.8 ± 1.3 and 21.7 ± 1.0, respectively. Increased duration of vernalization generally reduced FLN within an age treatment until an age-dependent point of vernalization insensitivity was reached. Estimates of the minimum days of vernalization required to reach vernalization insensitivity decreased in a linear fashion as plant age at the onset of vernalization treatment increased. The number of leaves appearing after the onset of vernalization insensitivity averaged 6.3 ± 0.5. FLN minus six appears to be a valid estimate in our experimental conditions for the onset of vernalization insensitivity, at least for plants that had six or more leaves appearing after the end of vernalization treatment. Linear regressions of FLN minus six against days of vernalization were significant for both cultivars (for treatments with six or more leaves emerging after vernalization). The Y-intercepts of the fitted regressions were close to values obtained by subtracting six from FLN of unvernalized plants. Both intercept and slope were controlled genetically. Accumulated plant age, expressed as leaf stage, enables attainment of vernalization insensitivity, independent of, or in combination with vernalization treatment.

Date of sowing experiments were established under irrigation in Ludhiana, in the Indian state of Punjab, from 1985 to 1992, using a total of 66 spring wheat ( Triticum aestivum L.) cultivars and 10 sowing dates. To study interactions among cultivars, sowing dates and years and to estimate losses with delayed sowing for grains m −2 and grain yield, an orthogonal subset of four years, three cultivars (PBW 34, PBW 154 and PBW 226) and seven sowing dates was used. To study the relationship between temperature and radiation, and yield and yield components, the same three cultivars were used with all the available years and sowing dates for each. The optimum sowing dates for yield were 5 November for PBW 34 (long-season) and 15 November for cultivar PBW 154 (medium-season) and PBW 226 (short-season). The combined effect of pre-heading radiation and temperature, referred to as the photothermal quotient (PTQ), was a better predictor of grains m −2 than was either variable alone. The sole effect of post-heading temperature was the best preditor of individual kernel weight. Yield and grains m −2 were maximized for all cultivars when exposed to the highest PTQ between 20 days before to 10 days after heading. Hence, the optimum heading date was the same for all three cultivars and, for the years studied, was in mid-February. After the optimum sowing date, there was a 0.8, 0.7 and 0.7% day − yield reduction for PBW 34, PBW 154 and PBW 226, respectively. The optimum heading date is earlier than observed for several other important areas of irrigated spring wheat. This may be explained by the rapid increase in temperature from February onwards at Ludhiana. We discuss the use of PTQ and temperature to determine optimum heading dates and, therefore, sowing dates for different cultivars.

Field experiments were conducted with modern semidwarf spring wheat cultivars representing a range of yield potential in order to compare their adaptation to levels of interplant competition for resources. Levels of competition were altered, in both leaf canopy and underground, by manipulating rows of plants in eight-row yield plots. Competition for light was decreased by bending adjacent rows away from the central two rows thus permitting greater penetration of light to lower leaves of the central rows (light treatment). Reduced competition underground was realized, simultaneously with reduced competition for light, by uprooting the plants adjacent to the central two rows (root/light treatment). These treatments were implemented at the developmental stage of flag-leaf ligule emergence (FLLE), when interplant competition was assumed to become maximal. While the average yield responses were 25%, and 40% to the light and root/light treatments respectively, there was a significant interaction with genetic material. Low-yield-potential (LYP) lines responded more to reduced competition than high-yield-potential (HYP) lines suggesting that the greater efficiency of the HYP lines, reflected in their higher yield, is related to their better adaptation to interplant competition. Physiological bases for improved performance are examined and discussed in relation to ideas about crop ideotypes and possible implications to breeding. These data appear to support the idea that genes conferming yield potential through improved adaptation to the crop environment are associated with a less competitive phenotype.

A methodology for estimating yield gain using predictive models was tested in the Yaqui Valley of northwestern Mexico. The irrigated valley is representative of regions producing 40% of the developing world's wheat ( Triticum aestivum L.) and has been the subject of intensive wheat research investment during the last 30 years. Over the 1968–1990 (year of planting) study period, the linear increase in yield (± standard error) was 57.2 (± 12.8) kg ha −1 y −1 ( r 2 = 0.49) or 1.27 (± 0.26) % y −1 ( r 2 = 0.51). However, considerable variation in both yield and average growing season temperature were observed over the period. In an attempt to allow for yearly weather variation, two yield models (one simulation-based and one regression-based) assuming no change in cultivar or management were used to predict the weather-based potential yield of each year. These predicted yields showed a significant linear decline over the study period apparently due to a small increase in temperature. When average Valley yields were adjusted against simulated yields to take into account the variation in weather, the overall linear rate of yield gain increased to 103 (± 14.8) kg ha −1 y −1 ( r 2 = 0.69) or 1.91 (± 0.22) % y −1 ( r 2 = 0.78). Consequently, the allowance for the temporal temperature change showed that the returns to improved crop management and breeding have been superior to those suggested by the increase in actual Valley yields.

In order to understand the mechanism by which the timing and amount of N fertilizer affect grain yield of wheat, a large experiment comprising 32 N treatments (from nil to 320 kg N ha−1, applications from seeding to booting) was conducted using a high-yielding semidwarf spring cultivar grown under low natural fertility at Griffith, New South Wales (34°S). The weather was close to normal and supplemental spring irrigations eliminated the possibility of water stress. Crop management was otherwise optimal and there was no disease or lodging. Grain yield ranged from 170 to 750 g m−2 across N fertilizer treatments the response to N was not reduced with applications as late as the onset of stem elongation. Grain yield was very closely correlated with kernels m−2 and with total biomass production. Kernel number was in turn closely correlated with spike dry weight (g m−2) at anthesis and with crop dry weight accumulation during the spike growth phase commencing 1 week before fag leaf emergence. Kernels per unit spike weight was unaffected by N but dry matter partitioning to spikes as increased by early N stress. Numerical components of kernel number did not give a useful quantitative explanation of these N responses. Delay in N application up until the onset of stem elongation did not limit N response because N uptake and leaf area responded quickly enough for close to full radiation interception to be reached before the onset of spike growth. Crop dry weight accumulation, generally directly related to absorbed photosynthetically-active radiation (PARA), was also assisted by small increases in radiation-use-efficiency (RUE, g top dry matter MJ−1 PARA) with higher N content in foliage. Peak RUE values (mean 2.86±0.03 g MJ−1) were close to other published values for wheat. The apparent recovery of N fertilizer was little affected by N amount and timing (average 63%). N uptake continued throughout grain filling in almost all treatments but especially those with late applications. N-utilization efficiency (grain yield/N uptake), however, declined with large N doses and with applications after the onset of stem elongation: values ranged from 30 to 60 g g−1. Utilization efficiency declines were largely associated with increases in grain N concentration and, to a lesser extent, reinforcing decreases in N harvest index.

To improve N-use efficiency in wheat (Triticum aestivum L.), this study determined the latest stage of development at which supplemental N fertilizer would elicit a full grain yield response. Three experiments were carried out in 1986 with early-June-sown irrigated semidwarf spring wheats in a low fertility field at Griffith in New South Wales (34°S). N was supplied as NH4NO3 top dressings and crop management was otherwise optimal. The grain yield response to N given at seeding was very high (in the main experiment the initial slope was 38 kg grain/kg N, with 180 kg N/ha increasing yield from 1812 kg/ha to 7113 kg/ha). Single N dressings given in this and an adjacent second experiment indicated no significant decrease in yield response until dressings were delayed beyond the onset of stem elongation. There was no evidence of synergism between early and late split N dressings. The apparent N recovery in the crop at maturity ranged from 44–77% (mean 63%) and did not decline strongly until all N was given after stem elongation began. 15N recovery in crop from labelled fertilizer applied to microplots ranged from 40 to 56% and increased slightly with later N dressings, while losses decreased. With delay of single N dressings, grain protein content only increased with dressings after the onset of stem elongation. Excluding those treatments where all N was given after this stage, protein responses were quantitatively related inversely to grain yield responses (r2=0.49), with negative responses accompanying the large yield responses (>35 kg grain/kgN) generated by initial increments of N. Examination in a third experiment of possible predictors of the optimum amount of supplemental N to be given at the onset of stem elongation suggest that stem base NO3-N concentration and culm number m−2 could serve this role.

Five different homoleptic gallium complexes with malonic diester anions [Ga(ROCOCHOCOR)3] [R =Me (1), Et (2), iPr (3), tBu (4) and SiMe3 (5)] have been synthesised and characterised by 1H and 13C NMR, IR spectroscopy, electron ionisation mass spectrometry (EI-MS), and single-crystal X-ray diffraction. The thermal properties of the obtained compounds were evaluated by thermogravimetric studies to assess their suitability as precursors for the metal-organic chemical vapour deposition (MOCVD) of Ga2O3 thin films. MOCVD of Ga2O3 thin films was carried out starting from compound 2 in light of the promising features of this precursor. The as-deposited layers are amorphous and can be transformed into the monoclinic ²-Ga2O3 phase upon annealing at 1000 °C ex situ. The film morphology was studied by scanning electron microscopy (SEM), and its composition was investigated by energy-dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS). Almost stoichiometric Ga2O3 thin films with low levels of carbon incorporation were obtained.

In southern New South Wales, wheat is sown in the autumn (April–June), usually after a period of fallow to conserve soil water. In order to predict the effect of fallow and of new tillage systems, in particular no-fallow followed by direct drilling, on total available soil water (ASW) at sowing and on moisture-based sowing day probabilities, a simulation model combined with historical daily weather data was used. The model comprised a fallow subroutine with no plants, a no-fallow component involving weed germination, growth and water use, both followed by a wheat crop phase for continuous simulation. The model was calibrated and satisfactorily validated using data from several tillage experiments conducted in the region over the period 1979–1984. It was then used to compare long fallow (LF, 1 September start), short fallow (SF, post-wheat-maturity start, November–December), late short fallow (SFl, 15 February start) and grazed no-fallow (NoFg) over 41 years at Yass (684 mm rainfall), Wagga Wagga (566 mm) and Griffith (420 mm). On average, ASW on 1 May was 88 mm (LF), 61 mm (SF), 52 mm (SFl) and 33 mm (NoFg), with greater fallowing gains the wetter the site. Sowing probability in May averaged 77%/day for all fallow treatments and 54%/day for grazed no-fallow. In a number of years, especially following no-fallow, there were few sowing days in May. The average green herbage (weeds) consumption by sheep (at 10 animals/ha) on NoFg from harvest to 1 May was only 33 g/m2. Results were insensitive to the assumed starting weed leaf area index (range 0·001–0·025) after weed-germinating rain, and to stocking rate (range 0–40 per hectare), but were moderately sensitive to the upper and lower limits of seed-bed water content for sowing. Model assumptions are discussed and it was concluded that quantification of the likely cost of not fallowing in terms of reduced ASW and reduced number of sowing days was possible.

This paper examines annual variation in the effects of fallow on available soil water at sowing and on sowing day probabilities, derived from simulation using the soil water/weed growth model described in Part I (Fischer et al., 1990). The simulation assumed a typical red-brown earth soil profile and was continuous for the period 1943–1983 at Wagga Wagga (mean annual rainfall 566 mm) and at Griffith (420 mm), sites in the wheatbelt of southern New South Wales. The extra water stored on 1 May by long fallow (1 September start) compared to short fallow (November–December start), and short fallow compared to no-fallow, was moderately closely related (r2 = 0·44 to 0·53) to rain during the early part of the respective fallow periods. Effects on sowing time were estimated assuming wheat sowing required 15 days commencing on 2 May. On average, the median sowing date was 10–14 days later for no-fallow compared to fallow, being considerably later in some years due to topsoil dryness and somewhat earlier in fewer years due to better trafficability in the absence of fallow. Considering only the cost of tillage and the benefit of extra stored water and more timely sowing, increased duration of fallow did not show a positive net benefit in all years. Tactics whereby fallowing was only initiated according to criteria which might increase the chance of a positive outcome were investigated. The best tactics tested were based on fallow initiation if available soil water exceeded 40 mm. However, the improvement in cumulative net benefit never exceeded 30–50% of the difference in net benefit given by a fixed fallowing strategy (fallow in all years on a given date) and that which would have been given by perfect foresight (fallow only in those years giving positive net benefit).

A full design approach for developing very highspeed transimpedance amplifier (TIA) monolithic microwave integrated circuits (MMICs) to be economically produced in large quantities is described. As an application example, the letter reports design, experimental performances, yield, and size data for a 5-Gb/s MMIC TIA using a low-cost 0.5-/spl mu/m GaAs field-effect transistor (FET) technology, showing outstanding experimental performances and optimized for a large-volume industrial production.

The advances in computer applications and technology are providing more and more data about the distribution system that can be used for analysis. One new technology is automated meter reading (AMR) systems. AMR systems provide access to the consumption and status of individual customers. This summary outlines research work to incorporate AMR information in outage management The first technique combines AMR data with SCADA and trouble call data to identify an outage and then uses AMR polling to verify the outage and its level. The second technique provides a way to use AMR to confirm restoration of all customers below an outaged device.

A new full design method for developing high bit rate TIA MMICs to be economically produced in large quantities is described in detail. As an example of the method application, the design is reported of a MMIC TIA for 5 Gbit/s optical receivers, having a new circuit topology which allows outstanding improvements of the receiver bandwidth, noise and dynamic range to be obtained. Finally, the great potential of this topology for future developments of high performance and much faster circuits is briefly outlined.

The paper presents a GaAs MMIC transimpedance amplifier for a high bit rate (2.5 Gb/s) optical communication system. A GaAs integrated circuit with a transimpedance gain of 63 dBohm, a bandwidth of 1.8 GHz with high capacitance photodiode operation and low power consumption is demonstrated. A complete set of simulated and measured results are presented to confirm the effectiveness of the design approach. The circuit design was carried out at R&D Center of TELEBRAS together with University of Rome through the ITU/UNDP project and the prototypes were fabricated at GEC-Marconi.

The paper presents a new technological solution for fabricating microwave integrated circuits (MICs). This innovative approach is based on a deposited-multichip modules (MCM-D) technology developed at the R&D Centre of TELEBRAS. A description of the processing is presented and some results of MICs fabricated on alumina 96% are shown. The main features of this packaging technology are its low cost (

This paper reviews recent progress in wheat (Triticum aestivum L.), rice (Oryza sativa L.), and maize (Zea mays L.) yields resulting from substantial breeding efforts in mostly favorable environments and examines its physiological basis. Breeding and improved agronomy lift potential yield (PY), namely yield with the best variety and management in the absence of manageable abiotic and biotic stresses, and PY increase is a key component of progress in farm yield (FY), the other component being closure of the PY to FY gap. Changes in PY and FY are reviewed for several key production regions, namely the United Kingdom and the Yaqui Valley of Mexico for wheat, Japan and Central Luzon in the Philippines for rice, and Iowa and briefl y sub-Saharan Africa for maize. The PY growth rates have fallen and are currently generally no more than 1% per annum and usually much less. The trajectory of FY with time often closely parallels PY, but, especially in developing countries, there remain large yield gaps. In at least one instance (maize in Iowa) the gap between PY and FY appears to be closing rapidly. Current genetic progress is linked to increased biomass accumulation, and this will remain the way forward in the future given the limits to increased harvest index (HI). There is evidence that recent progress is related to increased photosynthesis (e.g., greater radiation use effi ciency (RUE) at the canopy level and/or maximum photosynthetic rate P max at saturating irradiance at the leaf level) before and around anthesis. There is no theoretical reason why this trend cannot continue, especially given the vast genetic resources already found within each crop species. However, it will not be easily or cheaply accomplished, so prospects for higher rates of potential yield growth appear to be limited, notwithstanding new molecular tools and claims to the contrary. Closing the yield gap, therefore, becomes more important. Many factors are involved, but breeding can also help farmers achieve this through, for example, improved host plant resistance.

Restoration of distribution systems is a complicated process, especially after storms, when a large number of outages can occur. Many utilities are implementing automated meter reading (AMR) systems that can aid in the restoration process. This paper presents work done to utilize the capabilities and information provided by a wireless AMR system, including the on-demand read feature, to develop a polling procedure to identify system conditions. It takes advantage of the connection information provided by a utility and the performance of the wireless meter communication systems.

Theory (Craig and Gordon 1965; Dongmann et al. 1974; Sternberg et al. 1986; Farquhar and Lloyd 1993) suggests that the oxygen isotope ratio (d 18 O) of plant material should reflect the evaporative conditions under which the material was formed, so that differences in stomatal conductance should show up in plant d 18 O. To test this theory we measured the oxygen isotope ratio of organic matter from flag leaves at anthesis and grain at harvest from eight cultivars of spring wheat (Triticum aestivum L.) grown under irrigation in each of three seasons in Mexico. The cultivars ranged widely in stomatal conductance and in average grain yield, with which conductance was positively correlated. Supporting theory, the oxygen isotope ratio of flag leaves (d 18 Ol) was found to correlate negatively with stomatal conductance for two of the three seasons. The significant correlations are consistent with high conductance cultivars having lower leaf temperatures and kinetic fractionation factors, and higher vapour pressure fractionation factors and Péclet numbers, all of which combine to result in less enriched d 18 Ol. Yield (grain weight per square meter) was also found to be significantly negatively correlated with d 18Ol in two of the three seasons. d 18 Ol was as good a predictor of yield as stomatal conductance, and significantly better than carbon isotope discrimination. Correlations between grain d 18 O (d 18 Og) and physiological parameters were less clear. Significant negative correlations between d 18 Og and stomatal conductance, leaf temperature and yield were found only during the first season. By measuring the oxygen isotope ratio of cellulose extracted from leaf samples, the difference in fractionation factors (ecp) for cellulose and whole leaf tissue was assessed. ecp was found to be variable, and more negative when d 18 Oc and d 18 Ol were lower. Cultivar means for d 13 C and d 18 O of whole leaf material were found to be significantly positively related, and the factors required to produce such a relationship are discussed.

A new circuit topology suitable for designing high performance and very wideband monolithic transimpedance amplifiers to be economically produced in large quantities is described. Some outstanding experimental results are reported, which were obtained from a 5 GBit/s MMIC TIA using this topology, fabricated with low cost 0.5 /spl mu/m GaAs FET technology.

Sixteen spring wheat genotypes were grown under hot, irrigated, low latitude conditions, during the 1990-1991 and 1991-1992 winter cropping cycles in Mexico, Egypt, India and the Sudan, and in the 1990 and 1991 winter cycles in Brazil. The genotypes were chosen to represent a range of genetlc diversity within modem wheat varieties. In addition to grain yield, in Mexico a number of morphological and physiological traits were measured throughout the growing season on two sowing dates (December and February), to evaluate their correlation with yield in the other countries. The morphological traits - above-ground biomass at maturity, grains m-2, days to anthesis and days to maturity - measured on both sowing dates in Mexico showed generally significant correlations with yields measured at the other sites. Moreover, ground-cover estimates early in establishment, and at anthesis, also showed some association with performance at these sites, but only when measured on the hotter, February sowing date in Mexico. Membrane thermostability (MT), measured on field-acclimated flag leaves showed generally significant correlations with performance at all sites, and with MT measurements made on heat- acclimated seedlings of the same genotypes grown in growth chambers. Flag-leaf photosynthesis measured at booting, anthesis and during grain filling on both sowing dates, was generally significantly correlated with grain yield at all sites, as was rate of loss of leaf chlorophyll content during grain filling. Stomatal conductance was also significantly correlated with performance at all three stages. Canopy temperature depression (CTD) measured with an infrared thermometer was significantly positively correlated with performance at the international sites when measured between 1200 and 1600 hours, after full canopy establishment. The correlation of CTD with yield was not affected by the irrigation status of the crop under well-watered conditions. The possible use of these traits in selection for yield under hot conditions is discussed.

Dryland wheat was fertilized with ammonium nitrate or liquid urea-ammonium nitrate at the time of sowing or about 3 months later (generally at the terminal-spikelet stage) on a well-drained site near Harden on the south-west slopes of New South Wales. The experiments continued from the second to the fifth year (1981-1984) of the cropping phase of a crop-pasture rotation. The maximum agronomic efficiencies for yield in the four consecutive years were 19, 4, 23 and 25 kg grain per kg of applied nitrogen (N). The three large responses were obtained in wetter than average seasons and the small response was obtained during drought. In the last three years of the study the yield response to nitrogen at the terminal-spikelet stage was found to be close to but slightly less than that for N applied at sowing. In those years the agronomic efficiencies for the late-applied N were 0, 22 and 22. The apparent recovery of fertilizer N in the above-ground parts of the crop at maturity was up to 70% of the fertilizer applied in the year of sowing, and, after the drought during which there was little uptake of fertilizer N, up to 62% by the subsequent crop. The fertilizer efficiencies in the non-drought years were higher than generally reported in south-eastern Australia, and indicate potential for profitable delayed application of N fertilizer to wheat. Grain-protein responses were variable from year to year and are discussed against a simple theoretical background of the amount of N applied and grain-yield response.

Sowing date, sowing rate and row spacing effects were studied on high input crops at Griffith, N.S.W., between 1983 and 1985 using 25 bread wheats (Triticum aestivum L.) and 3 triticales (X Triticosecale Wittmack). The aim was to identify improved management practices and genotypes through a better understanding of development and growth of irrigated wheat grown under high-yielding conditions. The genotypes were chosen to represent a wide range in genetic background, maturity and stature. Growing period durations were between 208 days and 100 days for early April and mid-August sowings, respectively, with differences in anthesis dates within sowing dates of up to 45 days. Genotypes were classified into six major maturity groups. There was no maturity type that could flower close to 1 October from a wide range of sowing dates since anthesis was delayed by 0.3 to 0.5 days per 1-day delay in sowing. Increased daylength sensitivity tended to delay anthesis relative to the timing of floral initiation and terminal spikelet formation. The end of tillering was generally associated with the attainment of 50-60% light interception rather than a given development stage of the inflorescence. Spike density was not closely related to maximum tiller number but depended on genotype, environment and plant density. Leaf appearance rate was influenced by environment and genotype, but was independent of spike development. For a given final leaf number, internode elongation started at a later leaf number for later sowing dates, resulting in reductions in both node number and height. Crop height decreased by up to 5 cm per 1-week delay in anthesis date. The period of full light interception decreased from 133 days to 43 days between April and August sowings, respectively. The timing of reproductive development determined the green area duration, but the initial development and size of the canopy was less affected by it, because of adjustments in number and type of tillers, and size and thickness of leaves. The development and maintenance of an adequate canopy was not restricted by earliness, shortness or low sowing rates (50 kg seed/ha) for April-July sowing dates.

Sowing date, sowing rate and row spacing effects were studied on irrigated wheat crops at Griffith, N.S.W. during 1983-85 using genotypes differing in maturity, stature and genetic background. The aim was to identify better management practices and genotypes through a better understanding of development and growth of wheat grown under high-yielding conditions. Maximum yield was up to 891 g/m2. The average yield reduction was 50 g/m2 or 6% per 1-week delay in anthesis after 1 October, but varied between 2 and 23%, depending on the season. Lodging was a significant problem in all three years, with less lodging for later sowing dates, earlier maturity types or shorter stature. Plant spacing, through variations in row spacing (17-45 cm) or sowing rate (50-200 kg/ha) did not significantly affect grain yields, but lodging was reduced by increased row spacing and reduced sowing rate. Dry weight at anthesis (600-1 500 g/m2) explained 65% of the variation in lodging, with severe lodging risks for weights over 900 g/m2. Harvest index improved with later sowing or earlier maturity and was, among genotypes within a sowing, negatively correlated with anthesis date, height, lodging score and final leaf number on the main stem. Nitrogen uptake usually ceased before anthesis. Genotypic differences in grain protein concentrations of more than 2% were found. Some genotypes combined high yield with high grain protein concentration (e.g. 717 g/m2, 14.1% protein). Significant genotype effects on spike density, kernel weight, kernel growth rate, and number of kernels per m2, per spike and per g chaff weight were identified, but none seemed to restrict yield. There was much compensation between traits. For example, high kernel numbers (per g chaff, spike or m2) were associated with low kernel weights and vice versa, both within and between genotypes. It was concluded that short-stature, early-maturing, low spike-bearing cultivars are most suited to high-yielding conditions from any sowing date, provided flowering occurs after late September, as such crops have a reduced lodging risk and use assimilates and N most efficiently. Genotypes were highly adaptable and many morphogenetic traits differed widely between genotypes, but were usually similar among dwarf or semidwarf, and among early or late maturing genotypes.

Experiments were undertaken at Griffith, N.S.W., using a range of genotypes, sowing dates and plant spacing to identify management strategies and genotypes that would increase irrigated wheat yields and minimize lodging risk. Results are used in this paper in an analysis of potential yield and optimum anthesis date, as influenced by temperature, irradiance, sowing date and genotype. Lodging duration was used to predict potential yields in absence of lodging from the lodging-affected yields in the study. Lodging duration between 7 days after mid-anthesis and maturity was found to best explain early and late lodging effects on yield. Yield reductions due to lodging were up to 45%. Predicted potential yields (Yp) were 800-950 g/m2 and the end of the optimum anthesis period varied from year to year. Average temperature (T, �C) and total irradiance (+R, MJ/m2) for a preanthesis period of 500�C days (>3�C) or a maximum of 60 days explained 61% of the variation in Yp: Yp=981 - 53.4T+ 0.51 +R (g/m2). Using historical weather data and frost risk restrictions indicated an optimum anthesis period between 22 September and 10 October when average predicted yields were reasonably stable. Flowering after mid-October caused reductions in average predicted yield of 70 g/m2 or 11 % per 1-week delay in anthesis. Kernel weights decreased by 5% per 1�C above 14�C, but this decrease was also associated with increased kernel numbers. High-yields under irrigation can only be achieved consistently and efficiently with lodging resistant (short, stiff stems) or avoiding (early maturing) genotypes. Very early maturing types for late sowing dates are currently not commercially available. Adjusted management practices (e.g. relatively late sowing) and lower target yields are recommended for current lodging susceptible varieties.

In order to investigate the adaptation of wheat to hot tropical environments, the phasic development and spike size of a wide range of wheat genotypes was studied during two winter seasons at four irrigated tropical sites in Mexico of similar latitude (18–20°N) but varying altitude (60–2640 m). The responsiveness of genotypes to photoperiod and vernalization was quantified in a growth cabinet experiment. The development of most genotypes was much faster and leaf and spikelet numbers in the field were reduced at the hotter (= lower) compared to the cooler sites. However, genotypes with marked photoperiodic sensitivity or responsiveness to vernalization (especially the latter) flowered almost as late at the lowest hottest site (mean minimum temperatures > 14°C) as at cooler sites. Delay due to lack of cold in the vernalization-responsive cultivars, confirmed by testing seed-vernalized material in the field, was effected up to initiation of the terminal spikelet and resulted in more leaves, spikelets and tillers than other cultivars at the hotter sites. Delay by photoperiodic sensitivity extended beyond terminal spikelet initiation. Duration of grain growth was uniformly shorter in all cultivars under the hotter conditions, irrespective of daylength or vernalization sensitivities. It is concluded that while photoperiod and vernalization sensitivity delayed the rate of development up until flowering, neither mechanism is likely to give improved adaptation to warm tropical conditions because the delay was not matched by increases in the number of grains per spikelet or per spike.

The effect of lodging caused by overturning in saturated soil was studied in high-yielding crops of irrigated semidwarf wheat ( Triticum aestivum L. and T. turgidum. L. var durum ) during two years in northwest Mexico and one in southeast Australia. Plots lodged artificially or lodged naturally were compared to plots which never lodged, usually because of protection provided by supportive mesh. Culm lodging to an almost horizontal position caused grain yield to be reduced by 7–35%, with the greatest effect when lodging occurred in the first 20 days after anthesis. In one crop the yield reduction was clearly much less than in others with the same cultivar. Lodging just before anthesis was less deleterious, perhaps because the crop was able to right itself quickly by node bending. Kernel number per unit area tended to be reduced by early lodging, and kernel weight by later lodging accompanied by small increases in grain nitrogen percentage. Lodging during rainy ripening conditions increased grain sprouting. Lodging after anthesis reduced crop growth rate, and the adverse effect of lodging on grain yield is ascribed to this reduction in photoassimilate supply. The crop in which lodging had least effect on grain yield was characterized by a reduced degree of source limitation during grain filling.

In order to understand the effect of temperature on yield and the genetic adaptation of wheat to hot low latitude environments, replicated plot experiments with 14 or more diverse spring wheat genotypes were conducted over two winter seasons (1973–74 and 1974–75) and several sowing dates in central Mexico (latitude 19–20°N). There were two hot low-altitude sites (60 and 940 m) and two cool highland sites (2249 and 2640 m); mean January temperatures were 21, 20, 11 and 7°C, respectively. Cloudiness reduced solar radiation and diurnal temperature range at the lowest site. Crops were grown under optimal agronomy and irrigation, with disease control, although it was not possible to prevent serious foliar disease ( Helminthosporium saticum subsp. tritici ) during grain filling at the lowest site. Environment (site and sowing date) had a major effect on grain yield, which ranged from 74 to 587 g m −2 across the eleven sowings made. Yield was clearly poorest at the lowest site, increasing markedly at the higher hot site and still further at the cooler sites. At the hot sites November-December sowing yielded more than January sowing. The variation in yield was closely associated with variation in kernel number m −2 ( r = 0.98), which in turn was associated with total dry weight m −2 at anthesis ( r = 0.97) and photothermal quotient (mean solar radiation divided by mean temperature) over the 30 days preceding anthesis ( r = 0.88). Poorest growth, kernel number and hence yield at the lowest site was probably due to higher temperatures accelerating development, to lower solar radiation reducing assimilation rates, and possibly to reduced diurnal temperature range; specific leaf area was however greatest at this site. Genotypic effects were small relative to environmental ones, and genotype x site interaction for grain yield was not significant. Within each site yield showed stronger phenotypic correlations with kernal number m −2 and with harvest index, than with kernel weight and biological yield, respectively. Genotypes which were relatively insensitive to photoperiod and vernalization, and hence early, were best throughout, and especially at the hot lower sites. Late genotypes, sensitive to vernalization or very sensitive to photoperiod, gave a poor relative performance at the lowland site, where yield and kernel number m −2 showed negative phenotypic correlations with days to anthesis. This occurred despite more tillering and more initiated spikelets per spike, and could be only partly attributed to higher temperatures encountered by these genotypes during their later stages of development.

The supply of soil nitrogen to successive wheat crops was compared for direct-drilled and conventional tillage systems on a red earth in south-eastern Australia. There was no significant effect of tillage on the total plant uptake of nitrogen, nor on the decline of soil nitrogen during a 5-year cropping cycle. During the growth of the last two crops in the cycle, the rates of net mineralization, nitrate leaching from the topsoil and crop-nitrogen uptake were measured, for both tillage systems, by an in situ sequential sampling method. For the first of these crops the rates of net mineralization were unaffected by tillage, but for the second crop the rates were greater on the direct-drilled soil. Variation in the rates of mineralization within both seasons could largely be explained by changes in temperature and soil water content. Leaching of nitrate from the topsoil was consistently greater for the direct-drilled soil, where seedlings initially accumulated less nitrogen than those growing on conventionally-cultivated soil. However, the differences in uptake did not persist, and at maturity similar amounts of nitrogen, representing less than 50% of the seasonal net mineralization, were accumulated in the above-ground parts of crops grown under the two systems.

Summary Double labelling experiments employing 42K and 36Cl (using 10 mM KCl and 0.1 mM CaCl2 solutions) have shown that guard cells in floating epidermal strips of Vicia faba accumulate approximately 3 K ions for every Cl ion during stomatal opening. Light or CO2-free air stimulated stomatal opening and the uptake of 42K and 36Cl by the guard cells. These results suggest organic acid anions (approximately 100 mM if divalent) acting as counter ions for K. DCMU and FCCP inhibited stomatal opening, and 42K and 36Cl accumulation in similar proportions so that the K content of guard cells almost always reflected the extent of stomatal opening. The effects of other inhibitors are also discussed.

A coupled two-layer model of energy partitioning between soil and foliage for application to sparse crops was used to calculate the mean stomatal resistance from infrared measurements of foliage temperature. The model was tested against diurnal measurements of stomatal resistance, made with a diffusion porometer on two contrasting plots of wheat. One plot was kept well watered (W) and the other plot was left unwatered (D) for 40 days. Soil water deficit conditions had developed on the D plot by the time of measurement. To further test the model, a sensitivity analysis of the calculated stomatal resistance for a wide range of hypothetical leaf areas, foliage temperatures and sensible heat fluxes from the soil was conducted. At 0800 hours the foliage temperature of the W plot was 0.2°C cooler than the D plot. When measurements ceased at 1600 hours the W plot was 3.7°C cooler. As a result of these temperature differences the calculated mean leaf stomatal resistance deviated little from 90 s m −1 on the W plot but increased during the day from 130 to 250 s m −1 on the D plot. These calculated values accounted for 76% of the variance measured by porometer and were close to the 1:1 relationship. The sensitivity analysis indicated the importance of using a coupled two-layer model when calculating stomatal resistance from sparse, water stressed crops. Without such a model the effects of leaf area index on bulk boundary layer resistance and sensible heat flux from soil are ignored and the calculated stomatal resistance is overestimated. This results because drying of the soil can elevate soil surface temperature causing an increase in foliage temperature without any stomatal change.

Atmospherically-induced plant water deficit and/or high temperature effects have been postulated as possible limitations to grain yield in irrigated sub-humid environments. An experiment was conducted in northwest Mexico to test this hypothesis, using a high yielding dwarf variety of wheat ( Triticum aestivum cv Yecora 70) grown under intensive irrigation. Misting during grain filling was automatically activated at ear temperatures above 25°C. Measurements of xylem water potential (ψx) and leaf relative water content (LRWC) on the control and misted areas, respectively, showed mean daily (13h30–15h30) values of 14.5 and 9.5 bars for ψx and 85.5 and 90.5% for LRWC. Stomatal resistance measurements indicated no difference between the two treatments throughout a diurnal cycle. Grain growth rate was slower in the misted treatment, apparently as a result of lower ear temperature. Treatments had no effect on final grain size and grain yield. Atmospherically-induced stress did not appear to limit grain yield under the meteorological conditions experienced during this experiment.

Several theoretical stochastic models for the competition between crop plants, supposed sited at the points of a rectangular lattice within a large horizontal field, and weeds randomly located in the same field, are investigated. All plants crowd horizontally for space until every point of the field is occupied by either crop or weed. The most sophisticated model allows for different radial expansion rates and starting times. The resulting formulae strongly suggest that, for maximal weed suppression, the crop plants should be sited in a square lattice (other things being equal).

The specific heats, C , of the high- T c superconductors YBa 2 Cu 3 O 7 , La 1.85 Ca 0.15 CuO 4 and La 1.85 Sr 0.15 CuO 4 have been measured from 0.4 K to above the superconducting transition temperature in magnetic fields, H , from 0 to 7 T. From measured changes in C with H , values of γ( H ) = [ C ( H )/ T ] T =0 and Δ C / T c = [( C s − C n )/ T ] T = T c have been evaluated. These results have been used to estimated the normal state γ, the fraction of the sample that is superconducting in zero field, | s , and the value of the upper critical field, [ H c2 ] T =0 .

This paper describes associations between yield performance under drought and morpho-physiological traits, determined both under drought and non-drought conditions, for a large set of diverse cultivars of bread wheat (Triticum aestivum L.), durum wheat (T. turgidum L.), triticale (X Tritosecale Wittmack) and barley (Hordeum vulgare L). Performance under drought was considered in terms of grain yield and drought susceptibility, the latter being proportional to the decrease in yield relative to yield without drought. Species effects are considered, but greatest attention is paid to associations amongst bread wheat cultivars (n = 34), based on phenotypic and genotypic correlations, multiple linear regression and principal components analysis. Under drought, which reduced yield on average 60%, greater yield was most closely associated with greater total dry weight at maturity. Variation in traits associated with plant water relations had only a minor influence. The best prediction of yield under drought, from traits measured in the absence of drought (non-drought traits), was given by a linear model containing total dry weight, kernel weight and leaf waxiness, all with positive coefficients. Drought susceptibility, as defined, was unrelated to plant water relations under drought, but was related to various non-drought traits. It increased with increased non-drought yield, harvest index, kernels per sq metre, kernels per spike and leaf water potential, and with decreased plant height and waxiness. One part of these associations with drought susceptibility appeared to be related to variation in height, probably arising from the action of the Norin 10 dwarfing genes; part, however, was independent of height. The relationships suggested that direct selection for increased yield in the absence of drought, or selection via most of the non-drought traits, increased drought susceptibility and, depending on drought level, may increase or decrease yield under drought. No trait had clearly desirable effects on yield both in the absence of drought and drought susceptibility; total dry weight appeared to have the least undesirable effect on susceptibility. ______________________ *Part 11, Aust. J. Agric. Res. 30: 801 (1979).

The relative increase in final kernel weight (SR, %) with an artificial reduction of about 80% in the number of grains per spike was assumed to estimate the degree of post-anthesis assimilate or source limitation to grain weight in wheat. This assumption was supported by comparisons with other treatments designed to alter the ratio of source to grain number. SR was determined in crops grown under irrigation and high fertility in north-west Mexico over a 5 year period, in order to examine the effect of environment (year, sowing date) and cultivar on source limitation. Some old tall cultivars and many modern short cultivars of wheat (Triticum aestivum L., T. turgidum L.) and triticale (X Tritosecale Wittmack) were studied. For modern cultivars, kernel weight always appeared to be limited by source, SR averaging 21% over all years for eight such cultivars. For a given cultivar, environmental effects were considerable, being partly explained by a positive relationship between SR and mean temperature after anthesis. Each year SR was significantly affected by cultivar and, despite cultivar by year interaction, showed some consistency between years. Old bread wheats and triticales usually showed low source limitation (SR