A population of 96 doubled haploid lines (DHLs) was prepared from F1 plants of the hexaploid wheat cross Chinese Spring x SQ1 (a high abscisic acid-expressing breeding line) and was mapped with 567 RFLP, AFLP, SSR, morphological and biochemical markers covering all 21 chromosomes, with a total map length of 3,522 cM. Although the map lengths for each genome were very similar, the D genome had only half the markers of the other two genomes. The map was used to identify quantitative trait loci (QTLs) for yield and yield components from a combination of 24 site x treatment x year combinations, including nutrient stress, drought stress and salt stress treatments. Although yield QTLs were widely distributed around the genome, 17 clusters of yield QTLs from five or more trials were identified: two on group 1 chromosomes, one each on group 2 and group 3, five on group 4, four on group 5, one on group 6 and three on group 7. The strongest yield QTL effects were on chromosomes 7AL and 7BL, due mainly to variation in grain numbers per ear. Three of the yield QTL clusters were largely site-specific, while four clusters were largely associated with one or other of the stress treatments. Three of the yield QTL clusters were coincident with the dwarfing gene Rht-B1 on 4BS and with the vernalisation genes Vrn-A1 on 5AL and Vrn-D1 on 5DL. Yields of each DHL were calculated for trial mean yields of 6 g plant(-1) and 2 g plant(-1) (equivalent to about 8 t ha(-1) and 2.5 t ha(-1), respectively), representing optimum and moderately stressed conditions. Analyses of these yield estimates using interval mapping confirmed the group-7 effects on yield and, at 2 g plant(-1), identified two additional major yield QTLs on chromosomes 1D and 5A. Many of the yield QTL clusters corresponded with QTLs already reported in wheat and, on the basis of comparative genetics, also in rice. The implications of these results for improving wheat yield stability are discussed.

Using cereal crops as examples, we review the breeding for tolerance to the abiotic stresses of low nitrogen, drought, salinity and aluminium toxicity. All are already important abiotic stress factors that cause large and widespread yield reductions. Drought will increase in importance with climate change, the area of irrigated land that is salinized continues to increase, and the cost of inorganic N is set to rise. There is good potential for directly breeding for adaptation to low N while retaining an ability to respond to high N conditions. Breeding for drought and salinity tolerance have proven to be difficult, and the complex mechanisms of tolerance are reviewed. Marker-assisted selection for component traits of drought in rice and pearl millet and salinity tolerance in wheat has produced some positive results and the pyramiding of stable quantitative trait locuses controlling component traits may provide a solution. New genomic technologies promise to make progress for breeding tolerance to these two stresses through a more fundamental understanding of underlying processes and identification of the genes responsible. In wheat, there is a great potential of breeding genetic resistance for salinity and aluminium tolerance through the contributions of wild relatives.

The mean time for 50% germination at 20 8C of 12 Indian wheat (Triticum aestivum) cultivars was nearly halved, from 51 h to 27 h, by soaking seed in water for 8 h prior to sowing. A delay of 24 h without further soaking, intended to simulate postponement of sowing, reduced the time saved by priming to 16%. Priming had no effect on ®nal germination percentage. These results were used in the design of 275 on-farm, farmer-managed, participatory trials of seed priming in wheat during the 1997±98 and 1998±99 rabi (post-monsoon) seasons.In forty-one trials in tribal areas of Bihar and West Bengal states of India, seed priming gave a 13% grain yield advantage for farmers growing wheat in marginal areas with low levels of agricultural inputs. Mean bene®ts from seed priming of wheat in nine trials in Chitwan, Nepal were 17%. In high potential areas of Gujarat, India, 205 trials had higher rates of input use. Yield bene®t from priming in these trials averaged only 5% but constituted an extra 200 kg ha 71 grain at little or no cost. In 20 trials on marginal land, with slightly saline irrigation water in Ahmadwala, Pakistan, an average yield increase of 36% was obtained using seed primed with a 0.2% gypsum solution. Collaborating farmers reported that priming wheat seed overnight resulted in faster, more complete emergence, more vigorous early growth, better tillering, earlier owering, larger ears, earlier maturity and higher yields. In addition, many farmers also reported that foliage in primed plots was a darker shade of green than that in non-primed plots, suggesting that primed plants may have been using nitrogen more ef®ciently.Seed priming was popular with farmers, most of whom reported that they would prime wheat seed the following year. A survey in Gujarat in 1998±99 of 63 farmers who had tested priming in 1997±98 showed that, while 65% had primed some of their own seed, none had primed more than 50 kg, suggesting that there were practical dif®culties in priming larger volumes of seed.introdution Wheat (Triticum aestivum) is grown widely in South Asia in the rabi (post-monsoon season) after the harvest of kharif (rainy season) crops (Hobbs and Morris, 1996).

SUMMARYVariation for K" and Na^ accumulation at low salinities in hydroponic (water) culture were observed in shoots of different wheat species. Greater discrimination (in favour of K+ and against Na+ accumulation) was shown by hexaploid bread wheat {Triticum aestivum L.) than by tetraploid durum wheat (T. turgidum L.). Since Aegilops tauschii Cosson {A. squarrosa L.), the source of the D genome in bread wheat, also exhibited high discrimination between K"^ and Na^, it was concluded that the character resided in the D genome. Studies of aneuploid bread wheat lines and disomic substitution lines of D genome chromosomes for their A and B genome homoeologues in durum wheat cv. Langdon revealed that the trait was controlled by the long arm of chromosome 4D. Since the aneuploid and disomic substitution lines showed better relative salt tolerance than durum wheat, but had lower yield potentials, we recombined chromosome 4D with chromosome 4B in a tetraploid wheat background using a homoeologous pairing mutant. This produced families of 4D/4B recombinant lines, some of which exhibited the enhanced K'^/Na'^ discrimination trait. RFLP analysis confirmed that the trait was controlled by a single gene (Knal) which was completely linked to five markers on the distal third of the long arm of 4D. A second cycle of homoeologous recombination was employed to remove the distal 4D genetic material from the recombined Knal 4B/4D chromosome and to map Kfial in greater detail. By this strategy, Knal was mapped within a short 2 cM region. Genetic analysis of K"^: Na"^ ratios showed very high LOD scores in this region for plants grown in solution culture, but lower values for plants grown in the field.In general, recombinant lines which exhibited the enhanced K+/Na+ trait were slightly more tolerant of salinity in the field and in sand culture than recombinants lacking the trait. There was, however, considerable variation between individual lines. Ion discrimination and relative tolerance were also higher in a Knal recombinant (line no. R3) than in a knal recombinant (line no. R165) in sodic conditions. In these two lines the enhanced K+/Na+ discrimination trait did not alter responses to low potassium or calcium supply.