R. Sylvester-Bradley

Raising yield potential of wheat. III. Optimizing partitioning to grain while maintaining lodging resistance

A substantial increase in grain yield potential is required, along with better use of water and fertilizer, to ensure food security and environmental protection in future decades. For improvements in photosynthetic capacity to result in additional wheat yield, extra assimilates must be partitioned to developing spikes and grains and/or potential grain weight increased to accommodate the extra assimilates. At the same time, improvement in dry matter partitioning to spikes should ensure that it does not increase stem or root lodging. It is therefore crucial that improvements in structural and reproductive aspects of growth accompany increases in photosynthesis to enhance the net agronomic benefits of genetic modifications. In this article, six complementary approaches are proposed, namely: (i) optimizing developmental pattern to maximize spike fertility and grain number, (ii) optimizing spike growth to maximize grain number and dry matter harvest index, (iii) improving spike fertility through desensitizing floret abortion to environmental cues, (iv) improving potential grain size and grain filling, and (v) improving lodging resistance. Since many of the traits tackled in these approaches interact strongly, an integrative modelling approach is also proposed, to (vi) identify any trade-offs between key traits, hence to define target ideotypes in quantitative terms. The potential for genetic dissection of key traits via quantitative trait loci analysis is discussed for the efficient deployment of existing variation in breeding programmes. These proposals should maximize returns in food production from investments in increased crop biomass by increasing spike fertility, grain number per unit area and harvest index whilst optimizing the trade-offs with potential grain weight and lodging resistance.

Analysing nitrogen responses of cereals to prioritize routes to the improvement of nitrogen use efficiency

The efficient use of fertilizer nitrogen (N) is crucial to sustainable human nutrition. All crops receive significant amounts of additional N in temperate environments, through fixation or fertilizer use. This paper reviews progress towards the efficient use of fertilizer N by winter wheat (Triticum aesitivum L.) and spring barley (Hordeum vulgare L.) in the UK, acknowledging that on-farm this is governed by economics. Recent multi-site N response experiments on old and modern varieties show that yield improvements since the 1980s have been accompanied by increases in economic optimum N amounts for wheat but not for spring barley. On-farm N use efficiency (NUE) has increased for barley because increased yields with optimum N were associated with compensatory decreases in grain N concentration, whereas on-farm NUE has not increased for wheat because grain N concentration has not changed and improvements in N capture were insufficient to make up for the increased yield. Genetic effects on NUE are shown to differ markedly depending on whether they are determined at a single N rate, as in variety trials, or with optimum N amounts. It is suggested that, in order to elicit faster improvement in NUE on farms, breeding and variety testing should be conducted at some sites with more than one level of applied N, and that grain N%, N harvest index, and perhaps canopy N ratio (kg N ha(-1) green area) should be measured more widely. It is also suggested that, instead of using empirical functions, N responses might be analysed more effectively using functions based on explanations of yield determination for which the parameters have some physiological meaning.

Understanding and Reducing Lodging in Cereals

Abstract Improved lodging resistance has contributed significantly to the dramatic increase in cereal yields observed in many countries during recent decades. Several advances in understanding lodging have been made since the last major review of lodging in 1973. These include: (1) a more thorough quantification of the effects of lodging on grain yield and quality, (2) collaborative studies by biologists and engineers have elucidated the mechanisms of stem and anchorage failure and the way in which cereal shoots interact with the wind and rain, (3) the development of models of the lodging process and (4) explanations for how crop husbandry decisions affect lodging. This review collates the new understanding of lodging and attempts to set out cultural and genetic-based approaches for the continued reduction of lodging risk in high-yielding cereals. The review demonstrates that the prospects for continuing to reduce lodging risk through the selection of shorter genotypes may be limited because there appears to be a minimum crop height that is compatible with high yields. There does appear to be significant scope for increasing lodging resistance by strengthening the stem and the anchorage system by exploiting the wide genetic variation in these plant characters and through crop management decisions.

Evidence for differences between winter wheat cultivars in acquisition of soil mineral nitrogen and uptake and utilization of applied fertilizer nitrogen

The response of cultivars to applied nitrogen was examined in 11 seasons, 1982-92, in two experiments per year, normally testing seven cultivars at seven rates of fertilizer nitrogen. In all, 27 cultivars were tested in 22 experiments throughout Nottinghamshire, Lincolnshire, Northampton-shire and Suffolk. Cultivars ranged in their date of introduction from Maris Huntsman (1969) to Hereward (1988). For each cultivar in each experiment, the economic optimum yield (Y opt ), the amount of fertilizer N needed to produce it (N opt ), the grain % N at N opt , the offtake of N in the grain at nil N (N off(NO) ) and N opt (N off(opt) ) and the estimated recovery of fertilizer in the grain at N opt (AFR opt ) were estimated by fitting linear plus exponential curves to data for grain yield and two-straight-line models to data for grain N offtake. From cross-site analysis, normalized cultivar means were calculated for each variate. Over the 20-year period relating to the cultivars in the trial, the contribution of new genotypes to grain yield improvement was 1.92 t/ha, Y opt increasing by 96 kg/ha per year. There was no change in grain % N at N opt . The effect of changes through breeding from 1969 to 1988 was to increase N off(opt) by 42 kg/ha (2.1 kg/ha per year), that was associated with a decrease in N off(NO) (equivalent of soil N offtake) of 15 kg/ha (0.77 kg/ha per year). Part of the increased requirement for fertilizer N was fulfilled by an increase in AFR opt of 18 % over the 20-year period. The net effect was for N opt itself to increase by 56 kg/ha (2.8 kg/ha per year). Since survey evidence indicates no general increase in N use on wheat by farmers since the mid-1980s, it appears that current fertilizer use by farmers may be underestimating the requirement for N now. Alternatively in previous years N requirements may have been overestimated. The change in N available for loss to the environment, from the balance of grain N off(opt) and N opt , was from 11 kg N/ha in 1969 compared to 25 kg N/ha in 1988. It seems possible that the potential increase in nitrate levels in groundwater associated with plant type may not have been realised because farmers have conserved the amount of N they use.

Genetic progress in yield potential in wheat: recent advances and future prospects

Knowledge of the changes in physiological traits associated with genetic gains in yield potential is essential to improve understanding of yield-limiting factors and to inform future breeding strategies. Recent advances in genetic yield potential and associated physiological changes in wheat (Triticum aestivum L.) are reviewed. Genetic gains in yield potential worldwide have been both positively correlated with harvest index (HI) and above-ground dry matter (AGDM), with more frequent reports of yield progress associated with biomass since about 1990. It is concluded that an important aim of future breeding will be the increase of biomass production while maintaining the present values of HI. In winter wheat recent biomass progress has been positively associated with pre-anthesis radiation-use efficiency (RUE) and water-soluble carbohydrate (WSC) content of stems at anthesis. Present results in two doubled-haploid (DH) populations show a positive linear relationship between stem WSC and grain yield in the UK environment. Results from various investigations worldwide in recent years have demonstrated that biomass increases have been associated with particular introductions of alien genes into wheat germplasm, e.g. the 1BL.1RS wheat-ryejtranslocation and the 7DL.7Ag wheat-Agropyron elongatum translocation. Present results confirm a positive effect of 1 BL. 1 RS on harvest biomass in two DH populations in the UK. The future prospects for identifying physiological traits to raise yield potential are considered with particular reference to winter wheat grown in northwestern Europe. It is proposed that optimized rooting traits, an extended stem-elongation phase, greater RUE, greater stem WSC storage and optimized ear morphology will be important for breeding progress in yield potential in future years.

The ability of wheat cultivars to withstand drought in UK conditions: formation of grain yield

Experiments in three dry years, 1993/94, 1994/95 and 1995/96, on a medium sand at ADAS Gleadthorpe, England, tested responses of six winter wheat cultivars to irrigation of dry-matter growth, partitioning of dry matter to leaf, stem and ear throughout the season, and to grain at final harvest. Cultivars (Haven, Maris Huntsman, Mercia, Rialto, Riband and Soissons) were selected for contrasts in flowering date and stem soluble carbohydrate. Maximum soil moisture deficit (SMD) exceeded 140 mm in all years, with large deficits ( > 75 mm) from early June in 1994 and from May in 1995 and 1996. The main effects of drought on partitioning of biomass were for a decrease in the proportion of the crop as lamina in the pre-flowering period, and then earlier retranslocation of stem reserves to grains during the first half of grain filling. Restricted water availability decreased grain yield by 1.83 t/ha in 1994 (P < 0.05), and with more prolonged droughts, by 3.06 t/ha in 1995 (P < 0.001) and by 4.55 t/ha in 1996 (P < 0.001). Averaged over the three years, grain yield responses of the six cultivars differed significantly (P < 0.05). Rialto and Mercia lost only 2.8 t/ha compared with Riband and Haven which lost 3.5 t/ha. Losses for Soissons and Maris Huntsman were intermediate. In the two years with prolonged drought, the biomass depression was on average greater for Haven (6.0 t/ha) than for Maris Huntsman (4.2 t/ha) (P < 0.05). Thus, the grain yield sensitivity of Haven to drought derived, in part, from a sensitivity of biomass growth to drought. Harvest index (HI; ratio of grain to above-ground dry matter at harvest) responses of the six cultivars to irrigation also differed (P < 0.05) and contributed to the yield responses. The smallest decrease in HI of the six cultivars with restricted water availability was shown by Rialto (-0.033); this partially explained the drought resistance for this cultivar. The largest decrease was for Maris Huntsman (-0.072). The cultivars differed in flowering dates by up to 9 days but these were poorly correlated with grain yield responses to irrigation. Stem soluble carbohydrate at flowering varied amongst cultivars from 220 to 300 g/m 2 in the unirrigated crop; greater accumulation appeared to be associated with better maintenance of HI under drought. It is concluded that high stem-soluble carbohydrate reserves could be used to improve drought resistance in the UKs temperate climate, but that early flowering seems less likely to be useful.

Feed the Crop Not the Soil: Rethinking Phosphorus Management in the Food Chain

Society relies heavily on inorganic phosphorus (P) compounds throughout its food chain. This dependency is not only very inefficient and increasingly costly but is depleting finite global reserves of rock phosphate. It has also left a legacy of P accumulation in soils, sediments and wastes that is leaking into our surface waters and contributing to widespread eutrophication. We argue for a new, more precise but more challenging paradigm in P fertilizer management that seeks to develop more sustainable food chains that maintain P availability to crops and livestock but with reduced amounts of imported mineral P and improved soil function. This new strategy requires greater public awareness of the environmental consequences of dietary choice, better understanding of soil-plant-animal P dynamics, increased recovery of both used P and unutilized legacy soil P, and new innovative technologies to improve fertilizer P recovery. In combination, they are expected to deliver significant economic, environmental, and resource-protection gains, and contribute to future global P stewardship.

The ability of wheat cultivars to withstand drought in UK conditions: resource capture

Experiments in three dry seasons (1993/94, 1994/95, 1995/96) on a loamy medium sand at ADAS Gleadthorpe, Nottinghamshire, England tested responses of green area, radiation interception, water uptake and above-ground dry matter growth of six wheat cultivars to irrigation. Cultivars differing in date of introduction (Haven 1987, Maris Huntsman 1969, Mercia 1983, Rialto 1991, Riband 1985 and Soissons 1989) were chosen for contrast in flowering time and maximum green area. Maximum soil moisture deficit (SMD) exceeded 140 mm in all years, with large deficits (> 75 mm) from early June in 1994 and more prolonged large deficits in 1995 and 1996. Restricted water availability first affected canopy expansion at a SMD of 74 mm (50% available water capacity; AW), and canopy senescence at 95 mm (64% AW). Decreases in biomass at harvest were larger with prolonged droughts in 1995 and 1996 (424-562 g/m 2 ) than in 1994 (222 g/m 2 ). Compared in the two driest years, 1995 and 1996, the decrease in biomass was greater in Haven (599 g/m 2 ) than in Maris Huntsman (420 g/m 2 ) (P < 0.05). The better drought resistance of Maris Huntsman appeared to arise through its greater water use efficiency (WUE); 6.2 cf. 5.5-5.7 g/m 2 /mm. The susceptibility of Haven to drought was possibly due to restricted water uptake, depression of radiation use efficiency (RUE) and accelerated senescence. The cultivars differed in flowering by up to 9 days but date of flowering correlated poorly with biomass response to irrigation. Early flowering was, however, correlated with favourable distribution of seasonal water use with respect to the grain filling period. The maximum green area index (GAI) of cultivars varied from 4.4 to 5.3 (P < 0.01), but differences were countered by shifts in the extinction coefficient (K), such that season-long radiation interception varied little. Green area index maxima did not therefore relate to the pattern of growth or water use, and they were correlated poorly with the biomass responses. Cultivars did not differ in their maximum depth of water extraction; they all extracted water to 1.65 m depth in each year. It is concluded that early flowering and high WUE might offer scope for improving drought resistance in the UKs temperate climate, but that small maximum GAI seems less useful.

Effect of nitrogen fertilizer applied to winter oilseed rape (Brassica napus) on soil mineral nitrogen after harvest and on the response of a succeeding crop of winter wheat to nitrogen fertilizer

Soil mineral nitrogen (N min ) was measured to 90 cm at a total of 12 sites in the UK in the autumn after an oilseed rape experiment, which measured responses to fertilizer N. On average, N min increased by 15 kg/ha per 100 kg/ha fertilizer nitrogen (N) applied to the rape, up to the economic optimum amount of N (N opt ). There were larger increases in N min where fertilizer applications exceeded N opt , thus super-optimal fertilizer applications disproportionately increased the amount of nitrate likely to leach over-winter. The small effects of sub-optimal N on N min were associated with large increases in N offtake by the oilseed rape, whereas the larger effects of super-optimal N on N min were associated with only small increases in N offtake. Over 70 % of the variation in autumn N min was explained by the previous rapes N fertilizer rate and the topsoil organic matter content. Nitrogen applied to the rape increased grain yields of the succeeding wheat crops when no further fertilizer N was applied to the wheat. It was concluded that N applied to oilseed rape significantly affected N min after harvest, and these effects were not completely nullified by leaching over-winter, so soil N supply to the succeeding wheat crop was significantly increased. Responses in grain yield indicated that each 100 kg/ha N applied to the rape provided N equivalent to c. 30 kg/ha for the following cereal. Each 1% of soil organic matter further contributed N to the wheat, equivalent to 25 kg/ha. It is important to ensure that oilseed rape receives no more than the optimum amount of fertilizer N if subsequent leaching is to be minimized. Reductions below optimum amounts will have only a small effect on leaching. Substantial changes in the economic optimum N for rape production should be accompanied by adjustment in fertilizer N application to following wheat crops. Fertilizer recommendation systems for wheat should take account of the fertilizer N applied to the preceding oilseed rape and the topsoil organic matter content.

A study of wheat development in the field : Analysis by phases

Abstract The development of several winter and spring wheat varieties was investigated at two sites with up to eight sowing dates from 1989 to 1992. The life cycle from sowing to ear emergence was considered in phases from sowing to double ridge stage, double ridge to terminal spikelet stage and terminal spikelet stage to ear emergence, and explanations for the durations of each phase were sought in terms of thermal time, effective vernalization days and photoperiod. The life-cycle duration in thermal time from sowing until ear emergence declined as the sowing date became later. Assessed using a model function, vernalization was a major factor affecting the duration of the phase from sowing to double ridge stage for winter varieties. Some spring varieties showed small responses to vernalization. Photoperiod affected the duration of both the double ridge to terminal spikelet and the terminal spikelet to ear emergence phases. Some differences among extreme types were detected. The duration of the double ridge to ear emergence phases responded similarly to mean photoperiod in two successive years. This finding should prove useful in predicting ear emergence dates. However, the duration of double ridge to terminal spikelet and terminal spikelet to ear emergence phases at comparable photoperiods differed between years. Such differences may arise because the duration of later phases may also be affected by the environment during earlier phases, particularly the sowing to double ridge phases. Also, earlier phases affected the conditions when subsequent stages were attained; for example, the photoperiod during the double ridge to terminal spikelet phase was substantially modified by duration of the sowing to double ridge phases. In view of the importance of early stages of development in crop management techniques, the characterization of differences in response to vernalization among varieties is a priority for future research. However, progress depends on a fuller understanding of the fundamental mechanisms of response to low temperature.