High-parent Heterosis Research Articles

Combining Ability and Heterosis for Forage Yield among Elite Alfalfa Core Collection Accessions with Different Fall Dormancy Responses

ABSTRACTUnderstanding genetic parameters of populations that are being considered for inclusion into plant‐breeding programs provides critical knowledge to enhance economic traits of interest. The objective of this study was to determine the genetic parameters influencing forage yield for nine populations selected from the USDA National Plant Germplasm System alfalfa (Medicago sativa L.) core collection that possessed high‐yield potential and differing fall dormancy response. Forage dry matter yields of 36 diallel hybrids, their 9 parents, and 6 commercial checks were harvested three times in each of 2 yr from seeded plots near Las Cruces, NM. Variation among hybrids for forage yield was primarily attributed to general combining ability effects, which were 5.6 times greater than specific combining ability effects. More than 70% of the hybrids performed similarly to the best commercial check. Fourteen hybrids exhibited significant mid‐parent heterosis (MPH), and seven hybrids exhibited significant high‐parent heterosis (HPH). The frequency of hybrids showing significant MPH was twofold greater for hybrids generated between parents with different fall dormancy response as compared with hybrids generated from parents with similar fall dormancy response. Eight of the nine top‐yielding hybrids were derived from populations that originated from Peru and Argentina. All hybrids with significant positive HPH involved a population from South America.

Evaluation of semi-hybrid perennial ryegrass populations

The yield potential of modern forage cultivars is a limit to production from pasture- based agriculture, and may influence profitability. Hybrid vigour has increased yield in a range of plant species, but is only partially captured in conventional forage breeding systems. The objective of this research was to assess the potential for harnessing hybrid vigour in a semi-hybrid breeding system for perennial ryegrass (Lolium perenne). Paired crosses among eight parental sources, including four cultivars and four ecotypes, were used to create 28 semi-hybrid populations. Parents, semi-hybrids and check cultivars were trialled in pure grass plots under rotational grazing for 2 years in the Manawatu. Dry matter yield data were used to estimate cumulative and seasonal patterns of hybrid vigour expression for cultivar x cultivar semi-hybrid combinations. The mean level of high parent heterosis was less than 2%. However, one combination exhibited significant (p

Heterosis and Combining Ability for Major Yield Traits of a New Wheat Germplasm Shannong 0095 Derived from Thinopyrum intermedium

The wheatgrass, Thinopyrum intermedium (Host) Barkworth & DR Dewey, shows many beneficial characteristics, such as big spikes and high resistance to many diseases. To transfer the beneficial genes of this species, many wheat- Thinopyrum intermedium alien chromosome lines were developed. Of them, Shannong 0095 (SN0095), a disomic substitution, has long spikes and flag-leaves, and thus may be an important genetic resource for wheat yield improvement. In order to realize its heterosis and combining ability on major yield traits, a 7 × 7 complete diallel design was made according to Griffing's Method-I. The results showed that heterosis for spike length (SPL), flag-leaf area (FLA), number of spikes per plant (NSP), number of spikelets per spike (NSL), kernels per spike (KPS), 1 000-kernel weight (TKW) and grain yield per plant (GYP) existed in all the crosses by SN0095, but heterobeltiosis occurred only for KPS, TKW, and GYP. The relative mid-parent heterosis (RMH) and relative high-parent heterosis (RHH) for GYP, which valued as high as 35.32 and 29.92% respectively, were the highest among all the traits mearsured. Though additive and non-additive gene effects and cytoplasmic effects (or cytoplasmic-nuclear interaction effects) were found in governing all the traits measured above, additive gene action played a predominant role. The results also showed that SN0095 was the best-general combiner for SPL and FLA, and high-general combiner for NSP amongst all the parents. Estimates of specific combining ability (SCA) showed that SN0095 could also make high-SCA combinations for GYP, such as SN0095 × Jimai 19 (JM19). SN0095 could be a unique and important parent in hybrid wheat breeding programs.

Agronomic Performance of Soybean Hybrids from Single, Three-Way, Four-Way, and Five-Way Crosses, and Backcross Populations

Male-sterility systems combined with insect-mediated cross-pollination in soybean [Glycine max (L.) Merr.] have been shown to produce large quantities of hybrid seed, which can be useful for studying heterosis and agronomic performance in different population types. The procedure was used to obtain large quantities of hybrid seed for replicated yield trials. The objective of our study was to evaluate agronomic performance of soybean single, three-way, four-way, and five-way crosses, and three backcross generations (BC1, BC2, and BC3) to estimate heterosis for yield and other agronomic traits. Parental genotypes were male-sterile, female-fertile lines, selected for excellent insect pollinator attraction, crossed to a group of male parents selected for their agronomic performance. In 2003, F1 seed of single crosses were evaluated in replicated experiments at two locations in Iowa and one location in Indiana. In 2004, three-way crosses and BC1 populations were evaluated at one location in Iowa. In 2005, hybrid seed of single, three-way, and four-way crosses, BC1, and BC2 populations of eight families were planted at one location in Iowa. In 2006, the five-way crosses and BC3 populations were planted at one location in Iowa. In all experiments, hybrids and parents were evaluated. Results showed a range in mid-parent heterosis values (MPH) for yield from −35% to +42%, and in high-parent heterosis (HPH) from −44% to +42%. For yield, backcross populations had higher heterosis estimates than the other population types. For seed protein content, mean MPH ranged from −1% to +7%, and mean HPH from −4% to +7%. For oil content, mean MPH ranged from −10% to +6%, and mean HPH from −13% to +3%. Negative heterosis values were observed, indicating that a large number of different parental combinations need to be evaluated to identify the superior heterotic combinations. These evaluations will determine the feasibility of hybrid soybean production for commercial purposes.

Assessment of agronomic performance and heterosis for agronomic traits in hybrid high erucic acid rapeseed (HEAR)

Twelve genetically diverse and geographically distinct high erucic acid rapeseed (HEAR) Brassica napus L. cultivars/lines were selected and crossed by hand emasculation in a topcross design to produce 45 F1 hybrid combinations. Seven of the twelve parents were HEAR cultivars/lines developed by the University of Manitoba (UM) and the remaining five parents were proprietary European (EU) HEAR cultivars/lines. Adequate seed was produced to evaluate the hybrids and parents in replicated yield trials in six environments in Manitoba during 2004 and 2005. Vigor, days to first flower, days to maturity, lodging, plant height, and seed yield were assessed. The hybrids were generally more vigorous, taller and more lodging resistant than their parents with similar days to first flower and days to maturity as their earlier parents. The best hybrids in this study displayed high parent heterosis for seed yield of up to 143% and commercial heterosis for seed yield of up to 99%. These hybrids were from the most genetically diverse UM × EU crosses. HEAR hybrids appear very promising approach to increase seed yields in this crop. Key words: Brassica napus L., HEAR, heterosis, hybrids, genetic diversity

Evaluation of Soybean [Glycine max (L.) Merr.] F1 Hybrids

Heterosis is an important factor in development of hybrid cultivars. Few heterosis studies have been done in soybean [Glycine max (L.) Merr.]. This is because manual cross-pollination is difficult and time-consuming, and not economical to produce large quantities of hybrid seed. Male-sterility systems identified in soybean, combined with insect-mediated cross-pollination, have been shown to produce large quantities of hybrid seed. This procedure was used in this study to produce hybrid seed for replicated yield trials with the objectives to: 1) evaluate the agronomic performance of soybean F1 hybrids; and 2) estimate heterosis for yield and other agronomic traits of the F1 hybrids. Parental genotypes were two male-sterile, female-fertile lines with the ms3 and ms9 mutations, and a group of six high-yielding male-parent lines. The experiment was conducted in two years at several locations. In 2005, twelve F1 hybrids were evaluated along with parent lines. Mid-parent heterosis (MPH) ranged from −29% to +32%, and high-parent heterosis (HPH) from −23% to +1%. In 2006, eleven hybrids were evaluated. MPH values ranged from −53% to −21%, and HPH from −66% to −35%. Seed size and seed protein content showed HPH for some combinations. For traits related to vegetative growth, such as height and lodging, positive MPH and HPH were observed. In general, depending on the year and parent combinations, there were hybrids that performed better than the mid-parent values, suggesting that heterosis was identified in soybean. Significant differences for yield between hybrids of the ms3 and ms9 groups also were observed.

Desempenho de híbridos top crosses de linhagens S3 de milho em três locais do Estado de São Paulo

Este trabalho teve por objetivo avaliar o desempenho e a heterobeltiose de 27 híbridos de linhagens S3 (parcialmente endogâmicas), oriundas de híbridos comerciais de milho, cruzadas em esquema top cross com o testador IA 33 (sintético-elite comercial) e a capacidade geral de combinação (CGC) das linhagens. Os 27 híbridos top crosses, duas testemunhas comerciais e o testador foram avaliados nos anos agrícolas 2005/2006 e 2006/2007, sob delineamento de blocos casualizados com três repetições, no Centro Experimental Central - IAC (Campinas), na APTA Regional do Nordeste Paulista (Mococa) e do Vale do Paranapanema (Palmital). Foram avaliadas as seguintes características: florescimento masculino (FM), altura da planta (AP), altura da espiga (AE), plantas acamadas e quebradas (AC+Q), massa de espigas (ME) e massa de grãos corrigida para 14% de umidade (MG). Identificaram-se híbridos top crosses promissores, com MG iguais ou superiores às testemunhas comerciais. Os resultados comprovam que alguns híbridos top crosses possuem elevado potencial produtivo, com a vantagem do menor custo de produção. Ressaltaram-se linhagens S3 com elevados valores de CGC.

HIGH PARENT HETEROSIS AND EVALUATION OF TOMATO (Solanum lycopersicum, L.) HYBRIDS INCORPORATING THE MUTANT RIN GENE

This investigation was conducted in the research facilities of the Horticulture Research Institute during the period from 2003 to 2006 to produce and evaluate some tomato hybrids having ripening inhibitor mutant gene rin and study effect of this gene on some quality characters, yield and fruit shelf life. Twenty four tomato cvs. were crossed as female parents with Incridible cv. as male parent. The cultivar Incridible carries rin mutant gene. Parents and F1 populations were planted in RCBD with 4 replicates in two successive summer seasons 2005 and 2006 in open field to evaluate them. It was found that the hybrids Black From Tula I Incridible and VFNT I Incridible produced the highest significant early yield (7.7 and 6.85 ton /feddan, respectively). Nine hybrids out of them exhibited significant positive high parent heterosis for early yield ranged from 36.81 to 146.79% with the hybrid Black From Tula I Incridible having the highest value. The hybrid Strain B I Incridible produced the highest total yield (17.54 ton/fed.) over all evaluated genotypes; the hybrid Packmore BI Incridible exhibited the highest positive high parent heterosis for the same trait. The cultivar Polish produced the highest significant average fruit weight over all evaluated cvs. with mean value 241.4 g. The cultivars Black From Tula, ranked second in this trait having mean value being 205.8g, the two hybrids VFNT I Incridible and Castle King I Incridible showed significant positive high parent heterosis. The cultivar Incridible significantly was the highest value among all evaluated genotypes in fruit firmness, fruit firmness during 35 days, and storage ability. All of the evaluated hybrids were not superior in fruit firmness compared to high parent where they showed significant negative high parent heterosis for these traits.

Results after 19 years of testing in a Pinus strobus x P. wallichiana F1 hybrid population

Full-sib families were generated from a factorial design between seven female trees of P. strobus and four male trees of P. wallichiana to combine the rapid growth of former species with high resistance to blisterrust (Cronartium ribicola) of the latter one. At age two the hybrid families were artificially inoculated with blister-rust and at age six they were field planted. Total height growth, diameter at 1.3 m, basal area, stem volume, stem straightness and tree survival were measured at age 19 after planting. Significant (p<0.05) and highly significant (p<0.01; p<0.001) differences among female effects were found for all tested traits including survival suggesting that nuclear additive genes controlled these traits. Male effects were significant only for stem straightness. In all but one trait the σ2GCA: σ2SCA and σ2GCA-F : σ2GCA-M ratios were in favour of additive variance suggesting that this variance should be used in the breeding strategy. Narrow-sense heritability at family level was 0.778 for volume growth rate and 0.861 for survival; such high values indicate that the breeding programme, has good chances of success. Two of the 11 tested parents exhibited significant positive g.c.a. effects for survival after heavy blister-rust inoculation. One parent exhibited highly significant positive g.c.a. effects for volume growth rate. The three good general combiners should be taken into account in the breeding programme. Estimate of the high-parent heterosis was negative for the volume but positive for survival. For volume, the hybrid mean was 5.9% lower than the mean of the eastern white pine but 85.4% greater than blue pine bulk lot for survival. Estimate of low-parent heterosis for survival was also positive; i. e. the hybrid population mean was 537.5% greater than eastern white pine. If the hybrids will be used in operational planting programs, a variable genetic gain between 6.0 and 10.5% for volume and between 6.8 and 9.6% for survival could be achieved.

Heterosis of purple blotch (Alternaria porri (Ellis) Cif.) resistance, yield and earliness in tropical onions (Allium cepa L.)

Five open pollinated onion cultivars Red Creole, Kaharda, Koumassa, Sokoto local and Ori were crossed in a complete diallel and their progenies were evaluated in replicated trials, at Sokoto and Talata Mafara, Nigeria, during the 2004/2005 and 2005/2006 seasons. The experiments were randomised as complete block design with three replications. 30 mls of 10−1 cfu of spore suspension of Alternaria porri prepared by serial dilution was poured into the centre of each plot (sunken bed) immediately after irrigation at 2 weeks after transplanting. Combined analysis across seasons and locations indicated significant (P < 0.05) differences between the populations with respect to all the characters under study. Crosses Ori × Koumassa and Koumassa × Ori recorded highly significant (P < 0.01) positive high parent heterosis for bulb diameter. Crosses Ori × Sokoto local and Ori × Koumassa recorded highly significant (P < 0.01) negative high parent heterosis for days to maturity. Cross Sokoto local × Koumassa exhibited highly significant (P < 0.01) positive high parent heterosis for number of leaves/plant. Crosses Red Creole × Kaharda and Kaharda × Red Creeole recorded highly significant (P < 0.01) positive high parent heterosis for disease incidence. Red Creole × Kaharda and Kaharda × Red Creeole also recorded highly significant (P < 0.01) positive high parent heterosis for fresh bulb yield. Cross Kaharda × Ori recorded highly significant (P < 0.01) positive high parent heterosis for bulb weight. The cross Kaharda × Sokoto local recorded significant (P < 0.05) negative high parent heterosis for days to maturity while the cross Red Creole × Kaharda recorded highly significant (P < 0.01) positive high parent heterosis for number of leaves/plant. Bulb diameter and bulb weight recorded highly significant (P < 0.01) and significant (P < 0.05) environmental correlation with days to maturity.

Heterosis for horticultural traits in Broccoli

Over the last three decades, broccoli (Brassica oleracea L., Italica Group) hybrids made by crossing two inbred lines replaced open-pollinated populations to become the predominant type of cultivar. The change to hybrids evolved with little or no understanding of heterosis or hybrid vigor in this crop. Therefore, the purpose of the present study was to determine levels of heterosis expressed by a set of hybrids derived by crossing relatively elite, modern inbreds (n = 9). An additional objective was to determine if PCR-based marker derived genetic similarities among the parents can be useful to predict heterosis in this crop. Thirty-six hybrids derived from a diallel mating design involving nine parents were evaluated for five horticultural characters including the head characteristics of head weight, head stem diameter, and maturity (e.g., days from transplant to harvest), and the plant vigor characteristics of plant height, and plant width in four environments. A total of 409 polymorphic markers were generated by 24 AFLP, 23 SRAP and 17 SSR primer combinations. Euclidean distances between parents were determined based on phenotypic traits. About half of the hybrids exhibited highparent heterosis for head weight (1-30 g) and stem diameter (0.2-3.5 cm) when averaged across environments. Almost all hybrids showed highparent heterosis for plant height (1-10 cm) and width (2-13 cm). Unlike other traits, there was negative heterosis for maturity, indicating that heterosis for this character in hybrids is expressed as earliness. Genetic similarity estimates among the nine parental lines ranged from 0.43 to 0.71 and were significantly and negatively correlated with highparent heterosis for all traits except for stem diameter and days from transplant to harvest. Euclidean distances were not correlated with heterosis. With modern broccoli inbreds, less heterosis was observed for head characteristics than for traits that measured plant vigor. In addition, genetic similarity based on molecular markers was more highly correlated with plant vigor characteristics than head traits. Unlike with molecular marker-based estimates of genetic similarity, euclidean distance determined using phenotypic trait data was not predictive of heterosis. In conclusion, this study has documented heterosis in Brassica oleracea L., and the ability to predict heterosis in this crop using molecular marker-based estimates of genetic similarity among parents used in producing the hybrid.

Insect-mediated cross-pollination in soybean [ Glycine max (L.) Merrill] : I. Agronomic performance

In soybean, manual cross-pollination to produce large quantities of F 1 hybrid seed for yield trials is difficult and time-consuming. Conversely, insect-mediated cross-pollination has been shown to produce large quantities of hybrid seed in soybean and could facilitate the identification of heterotic patterns. The objective of our study was: (1) evaluate F 1 hybrid soybean plants from single crosses for yield and agronomic traits over several environments and (2) compare hybrid performance of the single crosses to lines developed from three-way crosses and backcrosses. In 2003, F 1 seed of single-crosses and their parent lines were evaluated in replicated experiments at three locations. Also in 2003, three-way crosses, and BC 1F 1 seed were produced. In 2004, three-way crosses, BC 1F 1 crosses, and their parent lines were evaluated at one location. High-parent heterosis (HPH) in single-crosses for grain yield ranged from −41.11% to +11.19%; for protein content from −4.34% to +3.53%, and for oil content from −13.22% to −0.84%. In three-way crosses, HPH for yield ranged from −25.21% to −4.50%, for protein from −2.72% to +1.92%, and for oil from −5.87% to −1.20%. For BC 1F 1 crosses, HPH for yield ranged from −15.65% to +41.97%, for protein from −2.57% to +1.69%, and for oil from −2.47% to +2.22%. Although positive heterosis levels were observed across all populations tested to determine the economic feasibility it is imperative that more tests of more cross-combinations be evaluated in replicated environments. Extensive research in different environments must be conducted to determine what parental combinations will produce the highest heterosis levels, and to develop criteria for selecting the parents with the best combining ability. This will be important to maximize agronomic performance that can economically justify the use of hybrids in soybean production.

Heterosis and combining ability for grain yield and its components in selected maize inbred lines

Information on heterosis and combining ability of adapted maize (Zea mays L.) inbred lines is essential to maximize their use for variety development. This study was conducted to determine heterosis and combining ability of selected inbred lines for yield and yield components. Eight inbred lines were crossed in a diallel mating system. The crosses and parents were evaluated in a randomised complete block design with three replications at Bako Research Center, Ethiopia, in 2001 and 2002. There were significant differences between the entries for measured traits. Mean mid-parent heterosis (MPH) ranged from 2.9% for days to maturity to 89.2% for grain yield and high-parent heterosis from 0.65% for ear diameter to 64% for grain yield. All crosses exhibited positive MPH for ear and plant height, ear length, kernels per row and grain yield. General and specific combining ability effects were significant for most traits. Gutto LMS5, CML-202 and CML-387 were good general combiners for ear and plant height, and days to maturity. Line 143–5-i was the best general combiner for grain yield. This study identified inbred lines and hybrid combinations that had desirable expression of important traits. This will be useful for the development of high yielding hybrids.

Heterosis in root development and differential gene expression between hybrids and their parental inbreds in wheat (Triticum aestivum L.)

In spite of commercial use of heterosis in agriculture, the molecular basis of heterosis is poorly understood. In this study, heterosis was estimated for eight root traits in 20 wheat hybrids derived from a NC Design II mating scheme. Positive mid-parent heterosis was detected in 96 of 160 hybrid-trait combinations, and positive high-parent heterosis was detected in 79 of 160 hybrid-trait combinations. Improved differential display was used to analyze alterations in gene expression between hybrids and their parents in roots at the jointing stage. More than 990 fragments were repeatedly displayed, among which 27.52% were differentially expressed between hybrids and their parents. Four differential expression patterns were observed. Thirty differentially expressed cDNA fragments and three genes with open reading frames were cloned, and their expression patterns were confirmed by reverse-northern blot and semi-quantitative RT-PCR analysis, respectively. We concluded that these differentially expressed genes, though mostly with unknown function, could play important roles for hybrids to demonstrate heterosis in root system traits.

Heterosis for Horticultural Traits in Broccoli

Over the last 3 decades, broccoli (Brassica oleracea L., Italica Group) hybrids made by crossing two inbred lines replaced open-pollinated populations to become the predominant type of cultivar. The change to hybrids evolved with little or no understanding of heterosis or hybrid vigor in this crop. Therefore, the purpose of the present study was to determine levels of heterosis expressed by a set of hybrids derived by crossing relatively elite, modern inbreds (n = 9). A total of 36 hybrids formed by crossing nine parents were evaluated for horticultural characters, including head weight, head stem diameter, plant height, plant width (in a row), and maturity (e.g., days from transplant to harvest) in four environments. When averaged across all four environments, roughly half of the hybrids exhibited high parent heterosis for head weight (1 to 30 g) and stem diameter (0.2 to 3.5 cm). Almost all hybrids showed high parent heterosis for plant height (1 to 10 cm) and width (2 to 13 cm). Unlike other traits, there was negative heterosis for maturity, indicating that heterosis for this character in hybrids is expressed as earliness. With modern broccoli inbreds, heterosis for head characteristics appears less important than for traits that measure plant vigor.

Statistical Parameter “Essentiality of Heterosis” and Its Genetic Meaning

An additive dominant model of inheritance of a quantitative trait in a group of hybrids is considered. The parameter "essentiality of heterosis," associated with the frequency of high-parent heterosis, was proposed to characterize the expression of a trait in a group. An expression was obtained to relate essentiality of heterosis, mean dominance, and trait polygenity, which is fulfilled upon a random and independent distribution of advantageous alleles among the parents. A measure of polygenity of a trait taking into account inequality of quantitative effects of different loci was proposed.

Genetic analysis of rapeseed self-incompatibility lines reveals significant heterosis of different patterns for yield and oil content traits

Self-incompatibility is one of the most effective approaches to utilizing heterosis in oilseed rape around the world. To evaluate the heterosis of double low self-incompatibility, the possibility of combining seed yield and oil content, and the genetic effects of parents on their hybrid progenies, a 2-year field trial using a 3 x 22 NC II mating design was conducted during the 1999-2001 growing seasons in Wuhan, China. Significant differences in seed yield per plant and seed oil content were observed among the F 1 hybrids and between F 1 progenies and their parents. However, the heterosis for seed yield per plant was much greater than that for seed oil content. Mid-parent heterosis and high-parent heterosis of seed yield per plant ranged from 5.50 to 64.11% and from -2.81 to 46.02%, while those of seed oil content ranged from -1.55 to 7.44% and -3.61 to 6.55%, respectively. Non-additive genetic effects were a major mechanism that accounted for the yield heterosis in addition to additive effects. In contrast, seed oil content heterosis was mainly dependent on an additive genetic effect. General combining ability (GCA) determined the stability of hybrid cultivars. In hybrid breeding, parental materials might be selected by the sum of GCAs and variances of special combining abilities (SCAs) of female and male parents for traits affected by both additive and non-additive effects, and by the sum of GCAs of two parents for traits controlled mainly by additive effects. Primary branches and their siliques were the most important yield traits.

Heterosis for yield and its physiological determinants in wheat

Heterosis in hybrid wheat varieties produced using a chemical hybridising agent was assessed in field experiments. Hyno Esta and its parents were compared in factorial combinations of four-seed rates (25–300 seeds m−2) and two nitrogen fertilizer rates (0 and 200 kg N ha−1) in 2001/02 and again in 2002/03. Hyno Rista and Hyno Renta and their parents were compared at two-seed rates in 2001/02. Hyno Rista and its parents were added factorially to the Hyno Esta experiment in 2002/03, while Hyno Renta and Hybred and their parents were compared at two seed rates in 2002/03. Mid parent heterosis for grain yield was found in three hybrids and two of these showed high parent heterosis. High parent heterosis in Hyno Esta over a range of sowing densities was mostly exhibited in total biomass but also, in one of two years, in harvest index. High parent heterosis in Hyno Renta was associated more with harvest index than with biomass. The heterosis for biomass in Hyno Esta resulted from greater interception of photosynthetically active radiation (PAR) than the male parent, with better radiation use efficiency than the female parent. In both seasons Hyno Esta achieved grain numbers per ear at least as high as the high parent for this trait (Audace), and combined this with mean grain weights at least as heavy as the high parent for mean grain weight (Estica). Much of the increased biomass and grain yield in the hybrid came late in the season as high parent heterosis was expressed for both maximum grain filling rate and grain filling duration. Heterosis was higher when nitrogen was applied than when withheld; only greater at lower seed rates when expressed in proportionate terms (e.g. as a percentage of the parents), rather than in absolute terms (e.g. t ha−1); and greater in the year with the cooler and wetter summer.

Heterotic and seed rate effects on nitrogen efficiencies in wheat

Four field experiments over 2 years investigated whether wheat hybrids had higher nitrogen-use efficiency (NUE) than their parents over a range of seed rates and different N regimes. There was little heterosis for total N in the above-ground biomass (NYt), but there was high-parent heterosis for grain N yields (NYg) in two of the hybrids, Hyno Esta and Hyno Rista, associated with greater nitrogen harvest index (NHI). Overall, the hybrids did not significantly increase the total dry matter produced per unit N in the above-ground crop (NUtEt), but did increase the grain dry matter per unit N in the above ground crop (NUtEg). The improvement in NUtEg was at the partial detriment of grain N concentration. Heterosis for grain NYg in Hyno Esta was lower at zero-N, suggesting that it did not achieve higher yields through more efficient capture or utilization of N. The greater NHI in Hyno Esta appeared to be facilitated by both greater N uptake, and remobilization of N from vegetative tissues, after anthesis.The response of N efficiency and uptake to seed rate was dependent on N supply and season. Where N fertilizer was applied, N uptake over time was slower at the lower seed rates, but where N was withheld N capture at the lowest seed rate soon approached the N capture of the higher seed rates. During grain filling, the rate of accumulation of N into the grain increased with seed rate and the duration of N accumulation decreased with seed rate. With N applied, N yields increased to an asymptote with seed rate, when N was withheld there was little response of N yields to seed rate. In 2002, N utilization efficiency (NUtEt and NUtEg) also increased asymptotically with seed rate, but in 2003 seed rate had little effect on N utilization efficiency. When nitrogen fertilizer had not been applied, NHI consistently decreased with increasing seed rate. The timing of N application made little difference to NUE, NY, or NUtE.

Heterosis for Grain Yield and Other Agronomic Traits in Foxtail Millet

Foxtail millet [Setaria italica (L.) P. Beauv.] is a largely self-pollinating species that is used as a warm-season annual in the USA. Nearly all cultivars of this species grown in the USA are selections from land races. This research was undertaken to determine whether sufficient high-parent heterosis is expressed in foxtail millet for grain yield and other key traits to justify the development and use of varietal crosses. Seven diverse parents and 21 F2s and 21 F3s produced from biparental crosses were evaluated in five environments in 1996. Genotype × environment interaction was highly significant for grain yield, but the highest yielding entries were high-yielding in each environment. High-parent heterosis for grain yield was detected in 18 of 21 F2s. On the basis of the estimate of average heterosis, which was highly significant in every environment, the expected yield of the F1 generation was 68% greater than the average yield of the parental cultivars. This high level of heterosis for grain yield suggested that varietal crosses or other types of cultivars in which there exists a relatively high amount of heterozygosity would provide a significant yield benefit over nonhybrid cultivars. Although significant heterotic effects were observed for each of the other traits, additive effects were more important. Significant correlations between traits of the estimates of additive and/or variety heterosis effects suggested that at least some of the genes controlling grain yield, plant height, and spike length were either the same or in coupling phase linkage.