Development of an appropriate breeding scheme for tolerance to Empoasca kraemeri in common bean

Summary Significant advances in increasing tolerance to the leafhopper Empoasca kraemeri Ross & Moore in common bean have been obtained using a new breeding scheme where yield under leafhopper attack is the principal selection criterion in the evaluation of progenies. However, to further refine this breeding scheme, a study was conducted to determine whether selection for nonprotected yield would be more effective in early versus late generations. Two selection strategies were compared. In Strategy I, early generation selections in the F₂ and F₃ were compared to Strategy II where late generation selections were made in the F₄ and F₅, with the F₂ and F₃ generations advanced using single pod descent and bulk practices, respectively. Yield trials of the F₆ lines from both selection strategies were conducted under nonprotected and insecticide protected treatments. No significant differences were detected between the two selection strategies. However, Strategy II did produce advanced lines with greater nonprotected yields than did Strategy I, with the best F₆ line, in three out of four crosses, coming out of Strategy II. Late generation selection is recommended over early generation selection. Results of the new leafhopper breeding scheme, based on yield, are compared to the old breeding scheme where selections were made using visual selection practices in early generations.     

Honeycomb selection for yield in early generations of spring wheat

Summary The effectiveness of the honeycomb selection method for yield in spring wheat (Triticum aestivum L.) was evaluated using progenies from two wheat crosses, Glenlea x NB131 and Glenlea x Era. Honeycomb selection was carried out in the F₂ and F₃ generations, grown at the University of Manitoba in the summers of 1980 and 1981, respectively. In both generations, divergent selection was made for both high and low yield. Plants selected in the F₃ generation were entered in an F₄ yield test in the summer of 1982. Results of the experiment showed that honeycomb selection for yield in the F₂ and F₃ generations was effective in identifying parents of high- and low-yielding lines. F₃ plants from highyielding F₂ selections gave higher yields than those from low-yielding F₂ selections by 11.5% and 13.0% for Glenlea x NB131 and Glenlea x Era crosses, respectively. The F₄ yield test showed that high yielding selections from both crosses significantly outyielded by 8.9% low yielding selections and by 14.4% the unselected composite lines. It is concluded that the honeycomb selection method can be used for early generation selection in spring wheat.     

Selection for seed yield in Andean intra-gene pool and Andean × Middle American inter-gene pool populations of common bean

Yields of large-seeded Andean (A) common bean (Phaseolus vulgaris L.) cultivars of Chile and Nueva Granada races are 40 to 60% lower compared to their Middle American (M) counterparts of small-seeded Mesoamerica and medium-seeded Durango races. Our objective was to use the concept of congruity backcrossing between Andean x Middle American inter-gene pool [AM 11833 = A 483 (A)///// A 686 (M) //// PVA 800A(A)/// ‘Carioca’ (M)// Carioca (M)/ G 19833 (A)] and between races within Andean gene pool [i.e., intra-gene pool Andean, AA 11834 = A483 (A)//// ‘Cardinal’(A) /// ‘Blanco Español’(A) // BlancoEspañol (A) / ‘Taylor’ (A)] to compare selection for seed yield improvement of large-seeded Andean beans. Seven hundred sixty seven F₂-derived F₃ (F_2:3) families were produced for each population. Visual appraisal for total plant performance, combined with seed yield from non-replicated plots was used for selection of 551 families in F_2:3, 182 families in F_2:4, and 91 families in F_2:5 in each population. Eight hundred twenty three F_5:6 lines were developed from the 91 F_2:5 families in each population. Visual selection, combined with seed yield in non-replicated plots was again used to select 294 lines in F_5:6 in each population. Similarly, 44 highest yielding F_5:7 lines were selected in AM 11833 and 39 F_5:7 lines in AA 11834. Thus, single plant selections were made in the F₂ and F₅, and plants within each plot were harvested in bulk in F₃, F₄, F₆, and F₇. Thirty nine F_5:8 lines from AA 11834 and 44 lines from AM 11833, parents, and checks were evaluated at Popayán and Quilichao, Colombia in 1998 and 1999. Selected lines in both populations, on average, out-yielded the mean of their large-seeded Andean parents. Mean yield of the lines selected from AM 11833 was 50% higher than AA 11834 lines. Twelve F_5:8 lines out-yielded the highest yielding Andean parents G 19833 and A 483 in AM 11833, whereas only one line yielded significantly higher (p < 0.05) than the highest yielding parent A 483 in AA 11834. However, none of selected lines out-yielded small-seeded Middle American parents used in AM 11833 (A 686 and ‘Carioca’). The mean 100 seed-weight of AA 11834 was 36 g compared to 28 G for AM 11833 F_5:8 lines. Selected lines had similar days to maturity as parents in AM 11833, and matured 3 d later in AA11834. Correlation coefficients between yield and 100 seed-weight were negative in both populations. Yield and days to maturity were positively correlated in AA 11834.     

Comparison of pedigree selection and single seed descent for oil yield in linseed (Linum usitatissimum L.)

Summary Four populations of linseed derived from five parents were advanced from the F₂ to the F₆ generation by SSD and to the F₅ by pedigree selection. These populations were used to compare the efficiency of the two methods in order to provide superior genotypes with respect to grain yield and oil content, i.e. oil yield. The results showed minimum differences between pedigree and SSD lines for grain yield, where in only one cross the SSD lines were significantly superior to the pedigree lines. Since pedigree selection was carried out for both, seed yield and oil content, a positive response to selection was expected. However, early selection for yield, a character with low heritability, was not successful. On the contrary, significant differences for oil content were detected between the two groups of lines in three of the four crosses studied. In these cases the pedigree lines were superior to the SSD lines. These results demonstrate that strict selection in early generations for oil content, a character with comparatively high heritability, is feasible and successful in linseed. However, selection for seed yield should be postponed to later inbred generations. Consequently, in breeding for maximum oil yield of linseed a two-step selection procedure is recommended.Abbreviations PS pedigree selection - SSD single seed descent - TGW thousand grain weight     

The effectiveness of early generation (F3) yield testing and the single seed descent procedures in two cowpea (Vigna unguiculata (L.) Walp.) crosses

Summary Two breeding procedures were compared in two cowpea crosses. Both procedures were started from the same selected F₂ plants in each cross. For the early generation yield testing, the F₃ lines were yield tested and a pedigree and bulk methods followed in F₄ and F₅. Each F₅ line was bulked to provide seed for a yield test in F₆. In the Single Seed Descent (SSD) procedure, a single seed was taken from each F₂ plant to produce the F₃ generation. The procedure was repeated for the F₄ and F₅. The F₆ SSD lines and the F₆ yield testing lines were compared in yield tests.The results showed that differences in yielding ability of F₃ lines persisted over generations indicating that selection was effective. This was confirmed by the high significant correlations between F₃ yields and those of later generations which ranged from r=0.51^* to 0.85^**.The grain yields of lines derived by the single seed descent procedure were as good as those derived from early generation yield testing.Significant linear correlation between visual rating of F₃ and F₆ yields with actual yields revealed that it is possible to identify promising lines of cowpea visually.     

Single plant selection for yield in lentil

Summary Individual plant selection for yield in lentil is problematic at a commercial crop density primarily because of inter-plant entanglement by tendrils. Visual plant selection for yield was compared with random selection in the F₅ at three plant densities (66, 133 and 200 seeds/m²) by an evaluation of F₇ progeny yields over two seasons in two populations of lentil. Random plant sampling was as effective as visual plant selection in isolating high-yielding F₇ lines. The plant density of the selection environment did not affect the response to selection. The correlations between the seed number of selected F₅ plants and the mean yield of their F₇ progenies were r=+–0.26 and –0.06 in two populations, indicating the lack of positive response to plant selection for seed number. The results show that 1) random sampling is the most economic of the methods tested of plant selection for yield and 2) the plant density of the environment for plant selection can be low enough to avoid inter-plant entanglement by tendrils, allowing a focus in plant selection on characters, other than yield, of importance to the breeding program and with a higher heritability than yield.     

Response to selection for grain yield and harvest index in F2, F3 and F4 derived lines of two wheat crosses

Summary Aspects of selection for yield and harvest index were investigated by simulating selection using data from random pedigreed F₂, F₃, F₄ and F₅ derived lines from two crosses grown in plots at two sites over two years.Improvement in yield through selection was obtained when the response was measured at the same site and in the same year as the selection. Selecting the best 10 per cent of F₂ to F₄ derived lines gave F₅ derived lines that outyielded random selections by 19 to 53 per cent for one cross and 5 to 23 per cent for the second cross. These lines were 41 to 50 per cent better than the mid-parent in one cross, but were less than the mid-parent in the other cross.However, the response to selection when measured in a different year was little better than random selection. The effect of different sites also reduced the effectiveness of selection.Selection of harvest index in early generations for improvement of yield was ineffective when response was measured at the same site in the same year, or in different years.Contrary to some theoretical proposals, the same improvement in yield was obtained by selecting in early or late generations. While high yielding genotypes may be lost by delaying selection, this is counteracted by the better predictive value of late generations due to their greater homozygosity and homogeneity.     

Introgressing resistance to bacterial and viral diseases from the middle American to Andean common bean

The genetic base of cultivars within market classes of common bean (Phaseolus vulgaris L.) is narrow. Moreover, small- and medium-seeded Middle American cultivars often possess higher yield and resistance to abiotic and biotic stresses than their large-seeded Andean counterparts. Thus, for broadening the genetic base and breeding for higher yielding multiple stress resistant Andean cultivars use of inter-gene pool populations is essential. Our objective was to determine the feasibility of introgressing resistance to Been common mosaic virus (BCMV, a potyvirus), and the common [caused by Xanthomonas campestris pv. phaseoli (Xcp) and X. campestris pv. phaseoli var. fuscans (Xcpf)] and halo [caused by Pseudomonas syringae pv. phaseolicola (Psp)] bacterial blights from the Middle American to Andean bean, using gamete selection. Also, we investigated the relative importance of the use of a landrace cultivar versus elite breeding line as the last parent making maximum genetic contribution in multiple-parent inter-gene pool crosses for breeding for resistance to diseases. Two multiple-parent crosses, namely ZARA I = Wilkinson 2 /// ‘ICA Tundama’ / ‘Edmund’ // VAX 3 / PVA 773 and ZARA II = ‘Moradillo’ /// ICA Tundama / Edmund // VAX 3 / PVA 773 were made. From the F₁ to F₅ single plant selection was practiced for resistance to the common and halo bacterial blights in both populations at Valladolid, Spain. The parents and F₅-derived F₆ breeding lines were evaluated separately for BCMV, and common and halo bacterial blights in the greenhouse at Filer and Kimberly, Idaho in 2001. They were also evaluated for the two bacterial blights, growth habit, seed color and 100-seed weight at Valladolid in 2002. All 20 F₁ plants of ZARA I were resistant or intermediate to common and halo bacterial blights in the greenhouse, but their F₂ and subsequent families segregated for both bacterial blights. Segregation for resistant, intermediate, and susceptible plants for common bacterial blight occurred in the F₁ of ZARA II. Simple correlation coefficient for common bacterial blight between the F₁ and F₁-derived F₂ families was positive (r = 0.54 P < 0.05) for ZARA II. From the F₂ to F₅ the number of families resistant to both bacterial blights decreased in both populations. Only four of 20 F₁ plants in ZARA I resulted in seven F₆ breeding lines, and only one of 32 F₁ plants in ZARA II resulted in one F₆ breeding line resistant to the three diseases. None of the selected breeding lines had seed size as large as the largest Andean parent. The use of elite breeding line or cultivar as the last parent making maximum genetic contribution to the multiple-parent inter-gene pool crosses, relatively large population size in the F₁, and simultaneous selection for plant type, seed traits as well as resistance to diseases would be crucial for introgression and pyramiding of favorable alleles and quantitative trait loci (QTL) of interest between the Andean and Middle American beans.     

Ear:stem ratios in breeding populations of wheat: significance for yield improvement

Earlier studies showed that the ratio of the weight of the wheat ear to stem at anthesis (ear:stem ratio) may give a better indication of potential yield than harvest index because it is determined early in the life cycle and is not affected by post anthesis stress. These studies concluded that selection for high ear:stem ratio at anthesis may lead to further improvement in grain yield of wheat. The present work was undertaken in the field to identify lines varying in ear:stem ratio in breeding populations and to study its implications for yield improvement.At anthesis stem length, ear length, tiller number, dry weight of stem and ear and ear:stem ratio were measured in 14 crosses on F₂ single plants and F₂ derived lines grown in the F₃, F₄, and F₅ at three locations in Western Australia over four seasons. In addition, biomass, grain yield and yield components were measured on selected crosses at two locations on F₂ derived lines grown in the F₄ and F₅. There was a considerable range of ear:stem ratio between and within the crosses studied. Although ear:stem ratio was strongly correlated with stem length, there was substantial variation within stem length classes. Ear:stem ratio had a high mean broad sense heritability (82%), whereas HI, grain yield and above ground biomass had lower heritabilities, 68, 55 and 35% respectively. Ear:stem ratio was strongly correlated between generations and sites indicating stability of this character. Ear:stem ratio had a significant positive correlation with grain yield, HI, grains per ear and per m². The correlation of grain yield with HI was equal or slightly higher than that of grain yield with ear:stem ratio.Ear:stem ratio offers promise as a predictor of HI and yield potential where post-anthesis moisture stress can influence HI. Ear:stem ratio measurement is unlikely to be adopted for selection purposes in routine breeding programs, as it is laborious and time consuming. However, ear:stem ratio could be used to identify superior parental genotypes and early generation selections from special crosses in terms of its ability to partition assimilate.     

Breeding the common bean (Phaseolus vulgaris L.) for resistance to bean common mosaic virus: alternatives to backcrossing

A series of field experiments was undertaken in order to determine whether resistance to bean common mosaic virus (BCMV) could be incorporated into genotypes of the common bean (Phaseolus vulgaris L.) suitable for cultivation in Zimbabwe without recourse to backcrossing. Six inbred genotypes carrying the resistance-conferring alleles at the loci I and Bc-3 were crossed with five locally-adapted inbred genotypes. The first experiment comprised F₃ progeny rows, each derived from a single unselected F₂ plant, the second, F₃ bulks selected for resistance, and the third, a comparison of selected and unselected F₂-derived F₄ lines. The number of days to flowering and to maturity, the incidence of mosaic and necrosis symptoms, seed yield and seed size were recorded. There was evidence that late flowering and maturity were associated with BCMV resistance in some crosses, though not strongly enough to present an obstacle to plant breeding. The incidence of virus symptoms and seed yield were influenced by genetic factors additional to the major resistance genes, and variation in seed yield was present not only between bulk populations of crosses, but also between single-row plots of lines within crosses. This indicates that early-generation selection for yield in the presence of BCMV, even among progeny selected for BCMV-resistace, is likely to be effective. However, the variation in yield among F₄ lines was least in the highest-yielding crosses, which may represent a limit to successful selection for yield. Seed size was partly under additive genetic control, but there was also evidence of non-allelic interactions. There was no association between large seed size, preferred by consumers, and susceptibility to BCMV in the progeny, indicating that the association between these characters in the parent lines is fortuitous and will not present an obstacle to plant breeding. It is noted that a considerable amount of useful genetic information can be obtained without recourse to elaborate crossing schemes, provided that unselected progeny are included in experiments as controls. The evidence presented indicates that resistance to BCMV can be combined with appropriate values of maturity date, yield and seed size without the need for backcrossing.     

Improvement of resistance to Fusarium head blight by recurrent selection in an intermating breeding spring wheat population using the dominant male-sterile gene ms 2

Four cycles of recurrent selection for FHB resistance were conducted in an intermating wheat breeding population using the dominant male-sterile gene ms ₂ during 1987–1991.Five cycles of phenotypic mass selection for male-sterile plants were evaluated using the soil-surface inoculation method in Experiment I. Experiment II evaluated changes in FHB scores during five cycles of progeny selection for fertile plants using the single-floret inoculation method. In Experiment I, the average level of FHB response increased to MR level in C₄, compared to MS level in C₀. The numbers of infected spikelets and diseased kernels decreased 0.32 and 2.68 per cycle, respectively. In Experiment II, the average level of FHB response increased to R level in C₄F₁. The numbers of infected spikelets and diseased kernels decreased 0.93 and 4.58 per cycle, respectively. In both experiments, the largest selection gains were realized in the first cycle. The frequencies of R and MR individuals were increased significantly. The frequencies of individuals with FHB response equal and/or superior to Sumai 3 were increased to 5–8% in C₄ and 25% in C₄F₁after the fourth cycle. Agronomic traits tended to be slightly improved in selected populations. Compared to 2% in C₀, about 34% of lines superior in both FHB resistance and agronomic traits in C₄F₁ were selected to enter the conventional breeding program for further evaluation. Sixty three semidwarf lines superior in both FHB resistance and yield potential were selected from the F₅ generations derived from C₁F₁ to C₄F₁. From them, two resistant cultivars with high-yielding potential were developed and commercialized in the Lower Yangtze Valley. Recurrent selection appears to be highly effective and feasible in shifting the average FHB response of the intermating population in the desirable direction, thereby enhancing the frequency of resistant individuals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Single seed descent as a breeding method for swedes (<Emphasis Type="Italic">Brassica napus</Emphasis> L. var. <Emphasis Type="Italic">napobrassica</Emphasis> Peterm)

The results are presented of two single seed descent (SSD) breeding programmes for swedes (Brassica napus L. var. napobrassica Peterm). The first programme produced cultivar Virtue and was done as part of a research programme on heterosis. It involved the production and trialling in 1991 of 95 F₆ families from a single cross made in 1985 between lines derived from cultivars Criffel and Marian. Six F₆ families were mass multiplied in polythene tunnels in 1992, using blowflies as pollinators, and trialled in 1993 and 1994 before Virtue was entered into National List (NL) trials in 1995. The second programme was done as a commercially funded breeding programme and involved the production and trialling in 1999 of 1,037 F₆ families from 15 crosses made in 1993. Fifty F₆ families were advanced to F₇ in a glasshouse in 2000 and assessed in 2001. Six F₇ families were mass multiplied in polythene tunnels in 2002 and trialled in 2003. Two cultivars, Gowrie and Lomond, from the cross between Airlie and Invitation, were produced from the programme and entered NL trials in 2004. The SSD was traditional in the sense that each advanced family was descended from a different F₂ plant without selection and a glasshouse was used for the selfing generations. However, it differed from the schemes that have been operated for soybean and spring cereals in that it was not possible to grow two or three generations a year because of the vernalization requirements of swedes, and it was not possible to grow a large number of plants at really high density because the inflorescences needed to be covered with Glassine bags to prevent cross pollination. In yield trials over 4 years, Gowrie had the highest dry-matter yield (12.59 t/ha) of the three new cultivars, out yielded Magres (11.28 t/ha) and other shopping swedes, but was not as high yielding as cultivar Kenmore (13.44 t/ha) which had been produced by pedigree inbreeding with selection. A modified SSD breeding scheme is recommended in which family selection is practised at F₃.     

Selecting superior spring wheat crosses in early generations

Summary Three methods were evaluated as potential aids in identifying superior crosses early in the breeding program: (1) F₁ yield, (2) F₂ yield, and (3) midparental yield. Two sets of spring wheat (Triticum aestivum L. em. Thell.) crosses and parents were evaluated in replicated single row plots in F₁ and in replicated multi-row plots in F₂ over a period of five years. On the basis of F₁ yields, two of the highest yielding and two of the lowest yielding crosses in each set were chosen for a more detailed selection study in subsequent generations.In Set 1, random F₂ head selections for each of the selected four crosses were evaluated in 3-row yield plots in F₄. In Set 2, F₂ plant selections were made on the basis of head weight and the highest yielding 10% evaluated for yield in F₄.Lines of crosses identified as high yielding in F₁ had significantly greater mean yields in F₄ than those of crosses that were low yielding in F₁. The high yielding crosses had three to four times as many lines yielding in the top 10% in F₄ than did the low yielding crosses.Correlations for yield were obtained between F₁ and F₂ in different years, F₂ and midparents in the same year, lower or no correlations between F₁ and midparents in the same year, and no correlations between F₁ and midparent and F₂ and midparent in different years. It is suggested that larger plot sizes rather than single rows be used to evaluate parents.Midparental yield, F₁ yield and F₂ yield tests are recommended as a progressive set of screening tests for a given set of crosses to effectively maintain the superior crosses in the breeding program.     

Studies on the breeding of self-pollinating cereals

Summary The limitations of the pedigree and mass methods of selection in the improvement of self pollinating cereal crops are discussed and the value of quantitative data in the early generations of selection is emphasised. Two systems of selection which provide such data have been introduced at the Cambridge Plant Breeding Institute.In one, known as the F₂ progeny method of selection, the progenies of selected F₂ plants are grown in yield trials without further selection during F₄, F₅ and F₆ after which single plant selections are made within the more promising progenies.In the other system, known as the pedigree trial method of selection, normal pedigree selection is carried out in F₂ and F₃. In the F₄ generation single plants are selected from the better families for continued pedigree selection and within each of these families the remaining plants are bulked to give grain for a yield trial in the following year. This process is repeated in F₅ and F₆, the grain for trial being in each case obtained from the progeny rows and not from the preceding trial.The use of these methods is illustrated by reference to wheat hybrids grown at Cambridge, and the data obtained is shown to be useful in comparing the value of different parental combinations as well as in assessing the relative merits of the selections within each cross.     

Genotype × environment interaction amongst spring barley lines grown at sites in the northern and southern hemispheres

Summary Measurements and scores for juvenile plant habit, time to ear emergence, plant height, leaf diseases and 1000 kernel weight were recorded on 123 lines of spring barley grown at Palmerston North, (40°S) New Zealand, and at Cambridge, (52°N) England. The material was grown at both sites from F₄ to F₇.For juvenile habit, time to ear emergence, plant height and 1000 kernel weight highly significant correlations were found between sites and seasons though few accounted for more than 50 per cent of the variation. There was no evidence of closer correlation between seasons within sites than between sites and seasons, and it was concluded that selection at either site should be equally effective.Analysis of yield performance over sites and seasons within sites revealed large effects due to genotypes and seasons within sites but small effect due to sites. Large interactions were found between genotypes and sites but the genotypes × seasons within sites effect was relatively small; better agreement was found in relative yields between seasons within sites than between sites, so that yield performance in one country was a poor indication of potential performance in the other country.The implications of these results for the use of a selection nursery in New Zealand are discussed.     

A comparison of yield and yield component selection in winter wheat

Summary Three populations of winter wheat were formed by crossing Avrora to Sage, TAM W-101, and Danne. Approximately 10% of the F₂ plants from these crosses were selected for high and low levels of number of tillers per plant, number of kernels per spike, 1000-kernel weight, and grain yield. Forty-eight solid seeded F₃ lines obtained from the selected F₂ plants were then selected for high and low expressions of yield components and grain yield. Realized heritabilities were estimated. Indirect responses of yield to yield component selection and direct response to selection for grain yield were measured. Heritabilities were low for tiller number, number of kernels per spike and kernel weights but were high or intermediate for grain yield when selection occurred in the F₂ generation. When selection was practiced in the F₃ generation, heritabilities for tiller number and yield were low, but were intermediate to high for number of kernels per spike and kernel weight and high heritabilities were found for kernel weight. Selection for kernel weight often increased grain yield; however, direct selection for grain yields was usually as effective.Journal article no. J-4488 of the Oklahoma Agri. Exp. Stn., Stillwater, Oklahoma 74074.     

Yield potential and drought tolerance of segregating populations of barley in contrasting environments

Summary Using the traditional approach (selection for grain yield) it has been found that F₃ families derived from F₂'s selected under unfavourable conditions were more vigorous in the early stages of growth, taller, earlier in heading and with larger yields than F₃ families derived from F₂'s selected under favourable conditions. A high and negative correlation coefficient was found between the drought susceptibility index and grain yield at the driest site, whereas at the wettest site the correlation coefficients were lower and in some cases positive, indicating the existence of traits which are desirable under drought and undesirable under favourable conditions, or vice versa.Expected responses to selection for grain yield using different selection criteria indicated that selection under stress conditions is expected to be more efficient than selection under favourable conditions when dry areas is the target environment.Expected responses to selection for grain yield using different selection criteria indicated that selection under stress conditions is expected to be more efficient than selection under favourable conditions when dry areas is the target environment.Part of this work was supported by Opec Fund for International Development.     

Effect of plant density on selection for seed yield in two population types of Phaseolus vulgaris L.

Summary Pedigree selection for seed yield, using early generation yield tests, was practiced from the F₂ to F₇ in two populations of common bean (Phaseolus vulgaris L.) in three plant densities: 66, 133, and 266 thousand plants ha^-1 at CIAT-Palmira, Colombia. The six highest yielding lines selected from each plant-density, along with 13 parents, were evaluated in a 7×7 lattice design with three replications at the three densities utilized for selection, and at 399,000 plants ha^-1 in 1988 and 1989.Based on the mean performance of selected lines and the mean of the parents, selection for seed yield was effective in all densities in both populations. However, none of the lines selected from the population within the race Mesoamerica (TC 4673) significantly outyielded their best parent under any plant density. The highest yielding lines selected from the interracial population (TR 4635) outyielded their best parent irrespective of the plant-density used for selection. The highest yielding line originated from the highest density used for selection. Low density was neither good for selection nor for evaluation and identification of high-yielding cultivars of common bean. There was no significant difference between the mean yield of lines selected at the intermediate and high population densities. The effects of plant density, year, and their interactions were significant for seed yield.     

Selection for multiple disease resistance reduces cucumber yield potential

Summary Experiments were conducted in Wisconsin and Michigan to determine whether selection for multiple disease resistance adversely affects yielding ability of commercial cucumber (Cucumis sativus var. sativus L.; CS) lines. Inbred F₄ and F₅ lines were developed from C. sativus var. hardwickii (R.) Alef. (CH) x CS which had been either selected or not selected for resistance to the causal organisms of scab, anthracnose, and downy mildew. The exotic CH germplasms possesses a multiple fruiting habit with high yield potential and the CS genotypes include several disease resistance. In each comparison between selected and unselected progeny, the unselected families either significantly outyielded, or were not significantly different than their selected counterparts. In no case did the selected progeny outyield the unselected progeny. Since this was observed for both fruit number per plant and total fruit weight per plant, and it occurred despite differences in fruit size, we conclude that selection for disease resistance led to a reduction in yield potential in these populations.Received for publication-. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.     

Relationship between relative water content during reproductive development and winter wheat grain yield

Summary Water is often the most limiting factor to winter wheat (Triticum aestivum L.) production in the southern Great Plains of the U.S.A., yet the lack of reliable screening criteria has precluded direct selection for drought resistance in breeding programs. Previous work showed that leaf relative water content (RWC) was highly heritable when measured under field-drought conditions, but its adoption as a screening tool for yield improvement requires further investigation of the genetic relationship between grain yield and RWC. Plants representing high and low yield potential under drought stress, and a random group of plants, were selected from an F₂ population having the pedigree, TAM W-101/Sturdy. Two sets of entries, each comprised of the two parents and 24 F₂-derived lines, were evaluated under a rainshelter in the F₃ (1986) and F₄ (1987) generations to determine differences in leaf RWC during reproductive development. One set of entries did not receive any water after the jointing stage, and the other set was grown under well-watered conditions. A positive relationship was observed between grain yield and RWC measured during anthesis and mid-grain fill, as the high-yield selections maintained a significantly higher RWC than the low-yield selections. Grain yield and RWC were also positively associated among random selections segregating for both traits. Subsequent adjustment of genotype means for differences in reproductive development at time of sampling underscored the need to consider differences in maturity when RWC is the selection criterion.