An area of plant science that is still uncertain is precisely what determines plants' demand for nitrogen (N), that is the amount of N they need to take up from the soil to meet their requirements for potential growth and synthesis of new tissue. A robust and unequivocal physiological basis from which to determine N demand is lacking. Yet N dominates plant nutrition. No nutrient is needed in larger quantities and, in most environments, no nutrient is in such limiting supply. Knowledge of the factors governing N demand is essential to predict the needs of crops under a wide range of field situations, so that growers can be given more reliable fertilizer recommendations (Greenwood 1982; van Keulen et al. 1989). This is important, not just for economic reasons, but because of the risks to the environment that can arise from the over-application of N fertilizers, in particular the problem of nitrate leaching (Addiscott et al. 1991).

From 1985 to 1987, 20 experiments were carried out on shallow chalk soils, in which soil N reserves were expected to be small, to assess seasonal variations in the response of winter cereals to applied fertilizer N, and to relate these responses to measurements of soil mineral N (SMN), temperature and soil moisture deficits (SMD).

Soil mineral N measured in autumn varied from 21 kg/ha (1986) to 73 kg/ha (1985), while SMN in spring ranged from 19 kg/ha (1987) to 91 kg/ha (1985), these values were typical of soils in long-term arable rotations. Estimates of apparent net N mineralization (AM) during the growing season were small at c. 26 kg/ha and suggested large seasonal variation. The small AM is considered to be due to the shallow topsoil drying out during the growing season. Whole crop N offtake without fertilizer N was only c. 40kg/ha. Crop N offtake, grain yield without fertilizer N and AFR (apparent recovery of fertilizer N) could not be reliably predicted by regression on SMN in autumn, SMN in spring or AM. Little or none of the variation in crop yield could be accounted for by regression on accumulated temperature over winter, maximum SMD in April to July or mean temperature in April to July.

Despite optimum grain yields being only moderate at 6·59 t/ha for winter wheat and 6·78 t/ha for winter barley, response to applied fertilizer N was large, between 3·77 and 5·38 t/ha. In consequence the requirement for fertilizer N (c. 240–250 kg/ha) was also large, but differed little between seasons. This large requirement is concluded to be a result of limited fertilizer recovery and mineralization of soil N during the growing season.

The hypothesis was tested that slow grain drying stimulates pre-maturity alpha-amylase activity in wheat (Triticum aestivum L.). Grain drying rate in 91 commercial crops of cultivars Avalon or Mercia grown over the years 1988–90 was estimated from the slope of linear regressions of moisture content on time. Incipient sprouting was detected in some samples from 1988 using the fluoroscein dibutyrate test, but results from a beta-limit dextrin gel assay indicated that pre-maturity alpha-amylase was probably the major source of alpha-amylase activity in these samples. Although year-to-year differences in ln alpha-amylase activity and Hagberg falling number tended to be associated with seasonal differences in drying rate, there was no evidence of a relationship between either ln alpha-amylase activity or Hagberg falling number and drying rate after the year effect was removed by an analysis of covariance. A second dataset from one crop in each of 14 years at one site showed significant positive linear relationships between cumulative potential evapotranspiration calculated over different periods during grain ripening and Hagberg falling number (excluding 3 years when incipient sprouting was thought to occur). It was concluded that pre-maturity alpha-amylase activity was stimulated by an unknown environmental factor differing between seasons and associated with seasonal differences in drying rate and cumulative potential evapotranspiration.

A restricted randomization procedure for self-conjugate 5 × 5 Latin squares is shown to be useful for trials on intercropping, particularly agroforestry, as records from border plots need no longer be discarded.

Twenty-six experiments were carried out in England and Wales from 1983 to 1985 to compare urea and ammonium nitrate (AN) as N top-dressing for cereals over the range 100–300 kg/ha N. Most of the experiments were sited on chalky or other soils of pH>7·0 where the greatest differences in effectiveness were expected.

The results showed that while relative N offtake in grain was 2·5% greater from AN, there was no difference in grain yield. Splitting the main N application increased N offtake from urea but not from AN. Overall, urea effectiveness from a single application at GS 31 increased with increasing rainfall on the day of application, but was not increased by subsequent rainfall. On chalk soils, urea effectiveness increased with increasing cumulative rainfall up to the fifth day after application. There was little effect of rainfall on urea effectiveness when the dressing was split. Although grain yield was unaffected by the type of fertilizer N applied, grain N concentration was usually less from urea.

The fertilizer N requirement for optimum yield (Nopt) wassimilar for both fertilizers. Splitting the main N application had no effect on yield or Nopt. We concludethat urea is a satisfactory source of N for cereals.

Use of an alpha-lattice design in 714 international yield trials of barley, durum wheat and bread wheat in 1990/91 to 1992/93 resulted in an average efficiency 18% higher than the randomized complete block design when average variance was used as the comparison criterion. Alpha-lattice was generally most efficient when the C.V.s of the trials were high. It was also slightly more efficient for low-yielding than for high-yielding trials, and for rainfed than for irrigated trials. Since the changeover to alpha-lattice designs requires no new major input or changes in present field layout, its use for international trials is recommended.

Year × variety means of wheat grain crude protein content (CP) and Hagberg falling number (HFN) surveyed in the UK from 1974 to 1993 were investigated with a residual maximum likelihood (REML) analysis using variety, nitrogen fertilizer application (kg N/ha), average monthly temperatures (°C), and total monthly rainfall (mm) as fixed terms, and year as a random term. Annual variety means were weighted according to the inverse of their variances. The statistical significance of fixed terms as they were added to the model was evaluated by comparing the Wald test statistic with critical values of chi-squared. CP, except for 1976 which was associated with prolonged drought conditions, was best described with a model combining effects for variety (P < 0·001), nitrogen application rate (0·0143, S.E. = 0·00312, P < 0·001), July temperature (0·184, S.E. = 0·0582, P < 0·01) and May rainfall (−0·00952, S.E. = 0·00233, P < 0·001). A model describing HFN combined effects for variety (P < 0·001), August rainfall (−0·930, S.E. = 0·2579, P < 0·001) and June temperature (24·4, S.E. = 7·37, P < 0·001). These models were evaluated for describing CP and HFN of cv. Avalon grown on sandy-loam from 1982 to 1992 at one site. Correlation coefficients of 0·51 (P = 0·1) and 0·72 (P = 0·012) were obtained between observed values at this site and those predicted from the nationally derived models for CP and HFN respectively. Variety effects suggested that the varieties commonly used in the mid 1970s would give higher CP and lower HFN than their more modern counterparts if grown at similar levels of nitrogen and in the same climatic conditions.

Observations were made in 1994 and 1995 in Hertfordshire of the flowering phenology and attractiveness to beneficial insects of two commercial mixtures of flowering plants intended for set-aside land. These were the Tübingen Mixture from Germany and Ascot Linde SN from the Netherlands. The mixtures were visited by 14 species of Hymenoptera, 14 species of syrphid Diptera and six species of Lepidoptera. Although the mixtures contained 12 and five plant species respectively, Phacelia tanacetifolia was the dominant species to establish, flower and attract insects in both mixtures. The other plants contributed little to flower density or insect diversity. These mixtures are therefore not suitable for UK needs using the present proportions of plant species.

Yield responses of some economically important crop plants due to foliar applications of low concentrations of 28-homobrassinolide (HBR) were studied in experiments conducted at research stations and in farmers' fields in India during 1989–95. Foliar sprays of different concentrations of HBR were applied at tillering and spike/panicle initiation in wheat (0·5 and 1·0 mg/l) and rice (0·25, 0·50 and 1·00 mg/l); at flowering and pegging in groundnut (0·25 and 0·50 mg/l); 30 and 45 days after sowing (DAS) in mustard (0·25 and 0·50 mg/l); 25 and 35 days after emergence in potato (0·25 and 0·50 mg/l); and 30, 50, 70 DAS in cotton (0·1 and 1·0 mg/l). The HBR treatments significantly (P<0·05 and P<0·01) increased grain yields in wheat, rice and mustard, pod yields in groundnut, tuber yields in potato and seed cotton yields, over control. The extent of yield improvement due to HBR was influenced by crop species, concentration of HBR, plant growth stage at application and frequency of application.

Four sets of near-isogenic lines carrying different combinations of the alleles Rht-B1b, Rht-D1b and Rht-B1c for gibberellin-insensitive dwarfism in hexaploid wheat (Triticum aestivum L.) were compared with tall controls in a series of yield trials in eastern England and central Germany. In all four varietal backgrounds the effects of Rht-B1b and Rht-D1b were similar (plant height ≈ 86 and 83% of tall controls respectively) and in combination reduced plant height to c. 58%. The Rht-B1c allele caused more severe dwarfism (c. 50%) and, when combined with Rht-D1b, reduced plant height still further to c. 41%.

Data from the trials were consistent with a model for height/yield relationships in which the pleiotropic effects of the Rht alleles on yield can be inferred from their primary function: insensitivity to gibberellin limits stem extension growth, decreasing assimilate demand for this organ and diverting it to the developing ear (which is not itself dwarfed). The net balance between the resulting increase in harvest index and the curvilinear relationship observed between plant height and total shoot yield results in optimum grain yields at intermediate plant heights.

Yield advantages of shorter plants over tall controls were evident over several trials with mean grain yields ranging from 200 to 760 g m−2. The optimum plant height for yield improvement in different genetic backgrounds was achieved by different Rht alleles according to the background varietal height, such that intrinsically taller genotypes required more potent Rht alleles to achieve maximum potential grain yield.

Ear yield components showed increases in grain number due to Rht pleiotropy, from which it is inferred that the number of grains per ear is limited by supply of assimilates pre-anthesis. Increases in grain number were associated with decreases in mean weight per grain which varied according to severity of dwarfism and varietal background, so that the net effect on grain yield per ear was sometimes positive, sometimes negative, and sometimes neutral in different Rht/variety combinations.

Although it has been generally recognized that the difference in yield potential amongst wheat cultivars released in different eras is related to differences in their reproductive sink strength, there have been few investigations about changes in source–sink ratios as a consequence of wheat breeding. In the present study, two field experiments, in which plots were fertilized and irrigated and lodging and diseases were prevented, were carried out with seven cultivars (including a commercial hybrid) representing different periods of plant breeding in Argentina from 1920 to 1990. The cultivars were defoliated during post-anthesis to analyse the response of grain weight at particular positions within the spike (which have intrinsic differences in potential size).

Individual grain weight was virtually unaffected by defoliation in the old cultivars, but modern cultivars exhibited a significant reduction in individual grain weight for several positions within the spike, although this reduction was small (c. 15%) and many grains were unaffected. In addition, no relationship was found between individual grain weight in the controls and its reduction due to defoliation.

We concluded that if the source–sink ratio is further reduced, the grain yield of modern wheats will be simultaneously limited by the source and the sink. Future breeding should therefore attempt to improve simultaneously both sink and source strengths.

The effect of nitrogen and a plant growth regulator regime (chlormequat chloride followed by 2-chloroethylphosphonic acid) on the crop performance of a hybrid (cv. Luchs) and a conventionally bred winter rye (cv. Sentinel) were investigated in two field experiments each year between 1993 and 1995 at Winchester, UK. Internode length and dry weight/unit length of internodes was measured in order to assess the effect of the growth regulator regime on stem structure. Grain yields were 15% higher in the hybrid Luchs than in Sentinel. With high levels of applied nitrogen, both cultivars lodged in all seasons and this was most severe in 1994 when 88% of the crop lodged in Sentinel and 52% in Luchs. Plant growth regulator treatment consistently reduced lodging but did not eliminate it. Reductions in lodging were not always associated with an increase in grain yield. In the hybrid cultivar, the growth regulator treatment reduced yield in 1993 and 1995 when 2-chloroethyl-phosphonic acid was applied at GS49 and GS39 respectively, but increased yield in 1994 when applied at GS37. Growth regulator consistently reduced stem length, and the percentage reduction in length of the individual internodes within the stem was strongly influenced by the timing of the 2-chloroethylphosphonic acid component of the PGR treatment. Growth regulator reduced internode lengths by up to 25% in Sentinel and 35% in Luchs, and this was associated with reductions in the dry weight of internodes by up to 32% in Sentinel and 38% in Luchs. Consequently, dry weight/unit length of the stem was not increased by growth regulator treatment. Yield reductions in Luchs following growth regulator treatment may have been due to reduced stem reserves which have been associated with tolerance of stress in rye. Both cultivars were highly responsive to nitrogen. Economic optima varied from season to season, but they ranged over three years, from 175–273 kg/ha nitrogen, and were greater than the currently recommended application rates. Crop lodging increased with increasing nitrogen rate even when plant growth regulator was applied and yield penalties from lodging would have been high, if weather conditions during grain maturity had been unfavourable.

15N-labelled fertilizer was applied at different rates (0, 30, 60, 90 kg N ha−1) and in different forms (urea or ammonium sulphate) to wheat grown in Syria in three seasons (1991/92, 1992/93 and 1994/95).

Recovery of 15N-labelled fertilizer in the above-ground crop at harvest was low (8–22%), with the amount of 15N-labelled fertilizer recovered in the crop increasing as the rate of application increased. Fertilizer application caused a significant increase in the amount of unlabelled soil N in the crop, suggesting that the application of N fertilizer caused a ‘real’ added nitrogen interaction. Recovery of 15N-labelled fertilizer in the crop was unaffected by the form of the fertilizer.

On average 31% (14–54%) of the 15N-labelled fertilizer remained in the soil at harvest, mostly in the 0–20 cm layer. At the lowest application rate (30 kg N ha−1) most of the residual fertilizer was as organic N, but at the higher application rates (60 and 90 kg N ha−1), a greater proportion of the 15N-labelled fertilizer was recovered as inorganic N, presumably as the result of top-dressing N in dry conditions in the spring. The amount of 15N-labelled fertilizer remaining in the soil increased as the fertilizer rate increased, but was unaffected by the form of fertilizer applied.

Losses of 15N-labelled fertilizer were large (>35%), probably caused by gaseous losses, either through volatilization of N from the calcareous soil, or through denitrification from wet soils rich in organic residues.

N fertilization strategies in the West Asia/North Africa (WANA) region should take note of the low recovery of N fertilizer by the crop in the season of application, and the resultant large quantities of residual fertilizer.

A field experiment was carried out at Lumle Agricultural Research Centre (LARC) farm, Nepal, during the winter seasons of 1992/93 and 1993/94 in order to study the profitability of intercropping wheat (Triticum aestivum L.) with tori (Brassica campestris var. toria) and pea (Pisum sativum L.). A sole crop of wheat planted at 120 kg seed ha−1 was compared with sole crops of tori and of pea planted at 8 kg ha−1 and 60 kg seed ha−1, respectively, wheat+tori mixed-intercropped at 120[ratio ]6, 120[ratio ]4 and 120[ratio ]2 kg seed ha−1and wheat+pea at 120[ratio ]45, 120[ratio ]30 and 120[ratio ]15 kg seed ha−1.

Results over the two seasons showed that the intercropping of wheat+pea was profitable in terms of overall grain yield, land advantage, monetary advantage, economic return and meeting the dietary requirements of the subsistence farmers, although the sole crop of pea gave the highest net return. Mixing pea with wheat did not reduce wheat yields in either year except when pea was sown at 45 kg seed ha−1, which reduced wheat yield significantly in the first season. For wheat+pea intercropping, sowing pea at 30–45 kg ha−1 was the most profitable. The wheat+tori intercrop did not perform as well and was not as profitable as either sole crop. Intercropping of tori had a negative effect on wheat yield at all seed rates in the first year.

Levels of ribulose bisphosphate carboxylase (Rubisco) and Rubisco activase were compared in leaves above the ear in two genetically related populations (Z0 and Z20) of maize (Zea mays L.). Z20 was obtained from Z0 after twenty agronomic selection cycles for grain yield improvement (c. 90% above Z0). Plants were cultivated in the highlands of Mexico and leaves were sampled weekly during the grain-filling period. Chlorophyll, soluble protein and Rubisco activity were measured. Chlorophyll and soluble protein content slowly decreased during this period, the former faster than the latter, with no significant differences between populations. During the first 40 days after anthesis, Rubisco activity was significantly greater in the high-yielding population (Z20), although Western blot analysis of Rubisco showed similar values for both populations within this period. However, the same analysis for Rubisco activase indicated a greater amount of this protein in the higher-yielding population (Z20) than the original one (Z0) during the early and middle part of the grain-filling period. The addition of Rubisco activase and an ATP-generating system to Z0 leaf extracts resulted in increased Rubisco activity. It was concluded that during grain-fill in maize, the level of Rubisco activase has a regulatory effect on Rubisco activity expression.

Agroclimatic conditions in the UK are not ideally suited to maize (Zea mays L.) as a grain crop. An alternative growing technique to direct-seeded maize was investigated at Reading University over three growing seasons, using four cultivars (Bastion, Ace, Anko and Mirna) of different FAO maturity ratings. In the main trials conducted in 1989 and 1990, seedlings were raised for 15–45 days in a glasshouse and planted in the field between early May and mid-June, at 10 plants/m2. On each planting date, maize kernels of the same cultivars were also sown in the field. At the end of each growing season, transplanted and direct-seeded crops were assessed for grain maturity and grain yield components. Averaged over year and cultivar, transplants matured significantly earlier than direct-seeded maize and tended to give higher grain yields. Their grain dry weights (86% DM) averaged 7·0 t/ha in a preliminary study in 1988, 9·0 t/ha in 1989 and 6·1 t/ha in 1990, whereas those of direct-seeded maize averaged 4·0, 8·0 and 3·0 t/ha, respectively. The highest grain weights resulted from May plantings using 15-day-old seedlings of the late cv. Mirna, and from June plantings using 30-day-old seedlings. Yield benefits from transplanting were not significant in 1989, suggesting that in a very warm season transplants may not necessarily outyield direct-seeded crops. Differences in grain weight between transplanted and direct-seeded maize are discussed with respect to grain moisture content, 1000-grain weight, ear length, and number of grains and grain rows per ear; and applications of maize transplanting for cropping systems are outlined. It is concluded that suitable combinations of cultivar, transplant age at planting and planting date can meet the thermal time requirement for a grain harvest in southern England.

Although there has been some recent interest in the morphology of individual white clover plants within established mixed swards under sheep grazing, there is little information available on the morphological changes taking place during the critical first 3 years of sward development. Undamaged white clover plants were sampled from an experiment at Plas Gogerddan, Ceredigion, comparing three contrasting white clover cultivars, sown with a common companion ryegrass cultivar, under continuous stocking with sheep. Turves (250 × 250 mm), from which individual plants were carefully extracted, were taken every month during 1990–91 (years 2 and 3 after sowing). Each plant was described in detail by assessing a range of morphological characters. Information was also gathered from each quadrat on the degree of flowering and seedling recruitment. There were clear seasonal variations in plant size and complexity with an increase in the number of simple, unbranched plants over the winter/spring period, which became the dominant plant type by June. During the later part of each year, the reverse was true. Characters associated with plant size also decreased over the winter period, followed by recovery during the ensuing summer. However the recovery was not complete and there was a strong general trend towards an increased proportion of less complex plants of reduced size over the 2 years. As plant size fell, plant number increased and morphological differences between cultivars diminished. By autumn of the second year (third year after sowing) the mean plant size reached a critical level with further stolon loss resulting in large scale plant death. Over the 2-month period August–October 1991, plant number fell by 60% and resulted in a reduction in stolon abundance of 76%; a true clover crash. The seedling data also suggest that under these conditions seedling recruitment can be up to one twentieth of that expected from a conventional sowing and can play a substantial role in the regeneration of the sward.

Maize seedlings (cultivars Bastion, Ace, Anko and Mirna) were raised for 15, 30 or 45 days in a glasshouse and planted in the field between early May and mid-June in 1989 and 1990, at 10 plants/m2. Samples of transplants and direct-seeded crops of the same cultivars were taken at regular intervals throughout the growing season for growth analyses. In both years, the 15-day-old transplants established more rapidly in the field than older transplants, and the patterns of leaf production of these young transplants were similar to those of direct-seeded maize. Transplants flowered earlier than direct-seeded maize from the same planting date. At most harvests, transplants showed significantly higher dry weights than their direct-seeded counterparts, and May-planted maize had significantly higher dry weights than June-planted crops, while cultivar effects were small. Final biomass yields were consistently higher in 1989 (17 t/ha) than in 1990 (11 t/ha). In both seasons, particularly in 1990, there was a trend for a lower harvest index in direct-seeded maize which reflected its immaturity at final harvest. The lowest harvest index (0·07) in 1990 was recorded for direct-seeded maize of the late cultivar, Mirna (June planting), the highest harvest index (0·65) for 30-day-old Mirna transplants (May-planted). A comparison of the experienced effective temperature sums during the vegetative, reproductive and grain-filling phases suggested that the transplant nursery period provides an advancement in thermal time which can affect grain-filling and crop maturity. Year to year variation in climate seems to be of primary importance for yield advantages of transplanted over direct-seeded maize.

Seventy-seven adaptive research trials were conducted, with the participation of local farmers, on their fields during the 1993 and 1994 wet seasons (southwest monsoon seasons) in the warm subhumid tropical zone on the west coast of India. The objective of the trials was to compare the agronomic and economic benefits of an improved management consisting of placing urea briquettes containing diammonium phosphate (UB–DAP) by hand (56 kg N/ha and 14 kg P/ha) after controlled transplanting using a modified 20×20 cm hill spacing with two other practices: (i) the current management system, consisting of random transplanting and two split applications of prilled urea (PU) and a basal application of single superphosphate (SSP) and (ii) the farmers' traditional management practices, involving random transplanting and broadcasting of fertilizers. In spite of marked variations in rainfall distribution patterns during the 1993 and 1994 seasons, the improved management using UB–DAP increased grain yields by 2·14 t/ha (89%) in 1993 and 1·23 t/ha (45%) in 1994 over traditional management practices and by 1·56 t/ha (52%) in 1993 and 0·83 t/ha (27%) in 1994 over the current management system. These additional yields were obtained at a 28–36% lower plant population density (25 hills/m2v. 35–39 hills/m2). The use of a modified 20×20 cm spacing could reduce by up to 50% the labour normally required for the conventional placement of UB by hand about 1 week after transplanting using the standard 20×20 cm spacing (8–10 workdays/ha). The estimated value: cost ratios for the improved management ranged from 4·9 (1994) to 8·6 (1993) and increased further when the savings in seed input were considered. Stochastic dominance analysis of the yield data suggests that the improved management of UB–DAP could be risk-free and therefore would be preferred over the other two managements by rice farmers and policy makers.

In two field experiments conducted in 1991 and 1992 at Akure, Nigeria, 0–12 and 0–24 ml/l of the plant growth regulator, paclobutrazol, and 0, 75 and 150 kg N/ha were applied to maize in factorial combinations. Substantial vegetative growth occurred with increasing N rates in 1991 and this accelerated 50% silking date. Paclobutrazol influenced maize growth at this stage only in 1992 when 12 and 24 ml/l depressed plant height and increased both stem girth and leaf number/plant.

Higher paclobutrazol application rates decreased plant height at maturity in both years while, in 1991, 150 kg N/ha enhanced it, and also stem girth and shoot dry matter yield. In 1991, both ear and grain dry weight yield/m2 increased with higher N rates while in 1992 4–24 ml/l paclobutrazol enhanced ear dry yield/m2 compared with untreated maize.

Both chlorophyll a and b in the ear leaf increased with higher rates of paclobutrazol and N fertilizer, except in 1992 when high leaf N tended to mask the effect of paclobutrazol. N fertilizer in 1991 enhanced the contents of N and Ca in the ear leaf while depressing P, compared with untreated maize.