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.

Plant breeders have long made the distinction between genes conferring resistance to the various stresses, Yield potential is defined as the yield of a cultivar when grown in which Frankel (1947) called "observable characters," environments to which it is adapted, with nutrients and water nonlimiting and with pests, diseases, weeds, lodging, and other stresses and those increasing yield potential, the "productivity effectively controlled. As such, it is distinguished from potential yield,

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Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO2 dm(-2) leaf surface hour(-1) under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3-15 hours, and there was a marked reduction in the outflow of (14)C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and (14)CO2 fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the "sink" organs.

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Seasonal control of flowering often involves leaf sensing of daylength coupled to time measurement and generation and transport of florigenic signals to the shoot apex. We show that transmitted signals in the grass Lolium temulentum may include gibberellins (GAs) and the FLOWERING LOCUS T (FT) gene. Within 2 h of starting a florally inductive long day (LD), expression of a 20-oxidase GA biosynthetic gene increases in the leaf; its product, GA20, then increases 5.7-fold versus short day; its substrate, GA19, decreases equivalently; and a bioactive product, GA5, increases 4-fold. A link between flowering, LD, GAs, and GA biosynthesis is shown in three ways: (1) applied GA19 became florigenic on exposure to LD; (2) expression of LtGA20ox1, an important GA biosynthetic gene, increased in a florally effective LD involving incandescent lamps, but not with noninductive fluorescent lamps; and (3) paclobutrazol, an inhibitor of an early step of GA biosynthesis, blocked flowering, but only if applied before the LD. Expression studies of a 2-oxidase catabolic gene showed no changes favoring a GA increase. Thus, the early LD increase in leaf GA5 biosynthesis, coupled with subsequent doubling in GA5 content at the shoot apex, provides a substantial trail of evidence for GA5 as a LD florigen. LD signaling may also involve transport of FT mRNA or protein because expression of LtFT and LtCONSTANS increased rapidly, substantially (>80-fold for FT), and independently of GA. However, because a LD from fluorescent lamps induced LtFT expression but not flowering, the nature of the light response of FT requires clarification.

Comprehensive studies in grasses show that gibberellins (GAs) play a role as a florigen. For Lolium temulentum, which flowers in response to a single long day (LD), GAs are a transmitted signal, their content increasing in the leaf early in the LD and then, hours later, at the shoot apex. There is a continuous trail of evidence of hormonal action of these GAs for L. temulentum and support for a similar role in the flowering of other LD-responsive temperate grasses and cereals. A characteristic of the initial flowering responses of grasses and cereals is their limited stem elongation. Interestingly, it is GAs with low effectiveness for stem elongation, GA5 and GA6, that reach the shoot apex and, structurally, are probably not degraded by 2-oxidase enzymes. By contrast, GA1 and GA4 cause stem elongation, may be inactive for floral evocation, and do not reach the vegetative shoot apex apparently because of susceptibility to degradation. However, GA4 can be florally active if protected against 2-oxidases either structurally or by using a 2-oxidase inhibitor. Later in inflorescence development, GA1 and GA4 can be detected at the shoot apex and are florally active if applied. The 2-oxidase restricting accessibility to the apex has probably declined at this time so there is a second florigenic, LD-regulated GA action. A growing body of molecular evidence supporting these actions of GA may provide a future basis for manipulating flowering of grasses and cereals.

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