SummaryExperiments were carried out to assess the increase in yield potential of winter wheat in the U.K. due to variety improvement since the early years of this century. The effects of other genetic changes were minimized by applying fungicide to control eyespot and foliar diseases, and by using nets to prevent lodging. The experiments were carried out in 1978 at Cambridge. One, on soil of high fertility in Camp Field, received 104 kg N/ha and the other, on soil of lower fertility in Paternoster Field, received 38 kg N/ha. Twelve genotypes were tested. Eight were varieties which formed a chronological series beginning with Little Joss, introduced in 1908. The remaining genotypes were recently developed selections from the Plant Breeding Institute and a line bred by the French breeders, Benoist.The average yield of the 12 varieties and lines tested was 3·96 t/ha in Paternoster Field and 6·40 t/ha in Camp Field. In both fields the two highest yielding entries, Hobbit and the advanced breeding line 989/10, outyielded Little Joss by close to 40%. Benoist 10483 was the only entry for which the percentage yield advantage depended on high soil fertility.The newer, high yielding, varieties were shorter and reached anthesis earlier than the older varieties. They had lower stem weights/m2 than the older varieties but similar maximum leaf area indices and leaf weights/m2. Within each experiment the total dry-matter production of the varieties was similar, the increase in grain yield due to variety improvement being associated mainly with greater harvest index (ratio of grain yield to grain + straw yield).It is argued that by a continuation of the trend towards reduced stem length, with no change in above-ground biomass, breeders may be able to increase harvest index, from the present value of about 50% to about 60%, achieving a genetic gain in yield of some 25%. As the limit to harvest index is approached, genetic gain in yield will depend on detecting and exploiting genetic variation in biomass production.

SummaryField experiments comparing 13 winter wheat varieties representing very old, old, intermediate and modern groups were carried out over three seasons. The experimental sites were on soils of high fertility and weeds, diseases and pests were controlled by applications of proprietary agrochemicals. Lodging was prevented by supporting the plants with coarse-mesh netting.The average yield of all varieties over the three harvest years 1984, 1985 and 1986 was 7·7 t/ha (at 15% moisture content). Compared with the very old varieties which were grown by farmers during the 19th century, the modern varieties gave 59% more grain, had 14% more ears/m2 and 30% more grains per ear, but a similar mean grain mass. The modern varieties yielded slightly more biomass (total above-ground dry matter at harvest), were much shorter and reached anthesis about 6 days earlier than the older ones. In 1984, when the yields were greater than in 1985 and 1986, the yield advantage of the modern varieties was more in percentage and absolute terms than in the other years. Also, in 1984, the differences between the very old and the modern varieties in biomass was the greatest.The genetic gain in yield measured in these experiments is compared with those estimated for other countries and reasons for the variation are discussed.

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In a field experiment with 47 wheat genotypes, plant samples were taken at anthesis and maturity and analysed for nitrogen. Taking means over all genotypes, the plants contained at anthesis 83 % of the total present at maturity, while at maturity 68 % of the plant nitrogen was present in the grain.There was significant genetic variation in most of the component attributes determining nitrogen uptake. At anthesis, the heaviest plants contained most nitrogen. Because the nitrogen concentration in the leaves (average 2-8%) was much greater than in the stems and ears (average O8 %) and the leaves comprised, on average, 37 % of the plant weight at anthesis, variation in leanness was the main cause of variation in the nitrogen content of plants of a given weight. During grain filling, plants which lost the most dry weight from their stems and leaves took up the least nitrogen.It is suggested that the strong positive correlation between the accumulation of dry matter and of nitrogen, both until anthesis and during grain filling, occurred because both carbon assimilation and nitrate reduction depend on energy made available from chloroplasts. In addition, assimilate is required to sustain the growth of roots which is necessary for continued nitrate uptake.The results show that it should be easier for breeders to select for high nitrogen uptake from among heavy than from among light genotypes. However, although dwarf genotypes tended to be light, none of the correlations found was strong enough to render it impossible to produce dwarf genotypes capable of giving high yields of high-protein grain.