General Combining Ability Mean Squares Research Articles

Assessment of grain quality traits in rice under normal and water deficit condition

The main objective of this study was to assess the combining ability for rice grain quality to recognize the most desirable genotypes for rice breeding programs. A half diallel cross was made among eight rice genotypes. The obtained 28 F1 and their parents were evaluated in two separate irrigation experiments; the first experiment was irrigated every four days (normal condition). The second experiment was irrigated every ten days (a stress condition). The two experiments were designed in randomized complete block design with three replications. The results indicated that water deficit treatment significantly reduced all the studied traits except for amylose content % was increased under stressed treatments. General combining ability mean squares were highly significant for all the studied features except gelatinization temperature under drought irrigation. Specific combining ability mean squares were highly significant for grain elongation % under both conditions and amylose content % under water deficit; this indicates the importance of additive and non-additive genetic effects in the inheritance of these traits. Two crosses, Sakha106 × Sakha107 and Sakha106 × Sakha108 had the highest desirable mean values for hulling (%), milling (%), head rice, and gelatinization temperature, also lowest amylose content under both conditions. Three crosses, Giza 179 X Sakha 107, Giza 179 X Sakha 108, and Sakha 106 X Sakha 108, had the highest mean values of grain elongation % under both conditions. The cross Sakha107 × sakha108 showed highly significant positive desirable SCA effects for hulling (%) milling (%) gelatinization temperature and grain elongation. The varieties Sakha 107 and Sakha108 were the best general combiners and could be used in breeding programs for improving all the studied traits under both conditions.

Grain Fe and Zn content, heterosis, combining ability and its association with grain yield in irrigated and aerobic rice

Genetic improvement of rice for grain micronutrients, viz., iron (Fe) and zinc (Zn) content is one of the important breeding objectives, in addition to yield improvement under the irrigated and aerobic ecosystems. In view of developing genetic resources for aerobic conditions, line (L) × tester (T) analysis was conducted with four restorers, four CMS lines and 16 hybrids. Both hybrids and parental lines were evaluated in irrigated and aerobic field conditions for grain yield, grain Fe and Zn content. General Combining Ability (GCA) effects of parents and Specific Combining Ability (SCA) effects of hybrids were observed to be contrasting for the micronutrient content in both the growing environments. The grain Fe and Zn content for parental lines were negatively correlated with grain yield in both the contrasting growing conditions. However, hybrids exhibited positive correlation for grain Fe and Zn with grain yield under limited water conditions. The magnitude of SCA mean squares was much higher than GCA mean squares implying preponderance of dominance gene action and also role of complementary non-allelic gene(s) interaction of parents and suitability of hybrids to the aerobic system. The testers HHZ12-SAL8-Y1-SAL1 (T1) and HHZ17-Y16-Y3-Y2 (T2) were identified as good combiners for grain Zn content under irrigated and aerobic conditions respectively.

Genetic analysis of Agronomic Traits of Wheat under Terminal Drought Stress

Seven wheat genotypes (Sirvan, Zarin, Pishgam, Karim, Baran, Heidary, and Rizhav) were crossed in diallel mating to develop 42 F1's hybrids. The resultant hybrids along with parents were evaluated during cropping season 2015-16 at agriculture and natural resources Pardis of Tehran University, Iran, using RCB design with 3 replications under drought stress and normal field conditions from the booting stage till physiological maturity. General combining ability (GCA) and specific combining ability (SCA) for agronomic traits were determined. The main objective of the research was the identification and proper selection of best-performing wheat parental genotypes and best F1's hybrids, based on GCA and SCA estimates. Significant differences were observed among the wheat genotypes for all the reported traits in this article. Results for combining ability analysis indicated that mean squares of GCA and SCA effects were significantly high for most of the traits. The estimates of σ2gca and σ2sca and its ratio (σ2gca/σ2sca) indicated that non-additive genetic expression was dominant for most of the traits studied, then the heterosis breading is a useful program, however selection on superior hybrids should be postponed to the next generation for these traits in recombination program. It is concluded that Zarin, and Baran are recommended as the best general combiners for a future wheat breeding program. The maximum values of SCA observed in flag leaf length in normal and peduncle length in drought condition related to crosses of Rizhav with other parents. So, it is suggested these series of crosses can be down to improve these traits at the mentioned condition

Heterosis, Combining Ability and Gene Action for Yield in Bottle Gourd

The experiment was conducted at the experimental field of Olericulture Division, Horticulture Research Centre (HRC), Bangladesh Agricultural Research Institute (BARI), Gazipur, Bangladesh during the winter season of 2018-2019 to study the genetic architecture of yield in a seven parent half diallel cross of bottle gourd. The values of mean square for GCA (general combining ability) and SCA (specific combining ability) were highly significant which suggested the presence of both additive and non-additive genetic variance in the population. But the higher magnitude of GCA compared to SCA indicated predominance of additive genetic variance. In most of the cases, the cross between poor and poor parents showed positive SCA effect for fruit yield, which indicated the higher yield. The estimates of significant positive better parent heterosis ranged from 6.27 to 49.72 percent. Analysis of genetic components of variation suggested that additive components were more important in the inheritance of fruit yield. This character was observed being controlled by two to three pairs of genes or groups of genes. Narrow sense heritability was 23 percent indicating probability of selection in generations. The graphical analysis also indicated wide genetic diversity among the parents.

Genetic Architecture of Yield in Bottle Gourd (<i>Lagenaria siceraria</i> (Mol.) Standl.)

The genetic architecture of yield was studied in a five parent half diallel cross of bottle gourd at the experimental field of Olericulture Division, Horticulture Research Centre (HRC), Bangladesh Agricultural Research Institute (BARI), Gazipur during the winter season of 2016-17. The values of mean square for GCA (general combining ability) and SCA (specific combining ability) were highly significant which suggested the presence of both additive and non-additive genetic variance in the population. But the higher magnitude of GCA compared to SCA indicated predominance of additive genetic variance. In most of the cases, the cross between poor and poor parents showed positive SCA effect for fruit yield, which indicated the higher yield. The estimates of mid parent heterosis ranged from −19.0 to 31.8 percent and the better parent heterosis ranged from −28.4 to 20.5 percent. Analysis for genetic components of variation suggested that additive components were more important in the inheritance of fruit yield. This character was observed being controlled by two to three pairs of genes or groups of genes. Narrow sense heritability was 55 percent indicating probability of selection in generations. The graphical analysis also indicated wide genetic diversity among the parents.

Genetic studies of agronomic and kernel quality traits in white maize (<i>Zea mays</i> L.) inbred lines

Field experiments were conducted at National Horticultural Research Institute, Bagauda (11°33´N; 8°23´E) in the Sudan Savannah and Institute for Agricultural Research, Samaru (11011´N; 07038’E) in the Northern Guinea Savanna ecological zones of Nigeria. The experiments were conducted between July-October, 2014 rainy season to estimates mean squares for general combining ability (GCA), specific combining ability( SCA) and interaction between general combining ability and location (GCA x location) and specific combining ability and location Interactions ( SCA x location) for agronomic traits and kernel quality traits. The study comprised of 15 hybrid lines, 6 parental lines along with 4 checks which were laid out in partially balanced lattice design with three replications. Analysis of variance revealed significant (P≤ 0.01) GCA for days to maturity (27.74), ear length (8.85cm) , ear diameter (22.25cm), plant height (1501.60cm), field weight (1.70kg), moisture content at harvest (16.48%) and yield (10947553.16 kgha-1) indicating the additive gene action contributing the expression of these traits. The mean squares due to SCA were highly significant (P≤ 0.01) for days to maturity (7.49), ear length (10.06cm), ear diameter (26.96cm) and moisture content at harvest (6.78%) indicating the preponderance of non-additive gene action in the expression of these traits. The mean squares for GCA× location and SCA× location were not significant for all traits except percent whole kernel which shows highly significant GCA × location interaction (2077.97%) and this indicated that, different parental genotypes behaved differently with respect to this trait under different location.Keywords: Agronomic traits, Genetic, General Combining Ability, Inbred lines, Kernel Quality and Specific Combining Ability

Heterotic Patterns of IITA and CIMMYT Early‐Maturing Yellow Maize Inbreds under Contrasting Environments

Drought and low soil nitrogen (low‐N) constitute major constraints to maize (Zea maysL.) production in West Africa (WA). The International Institute of Tropical Agriculture (IITA) and International Center for Maize and Wheat Improvement (CIMMYT) have developed drought and low‐N tolerant inbreds for Africa. Combinations between selected IITA and CIMMYT inbreds could produce outstanding hybrids. Diallel crosses of 12 IITA and five CIMMYT early yellow inbreds plus four checks were evaluated under drought, low‐N and optimal conditions at four locations in Nigeria for 2 yr. The objectives were to examine the combining ability of the inbreds for grain yield and other traits, classify the lines into heterotic groups, identify best testers under the contrasting environments, assess genotype × environment interactions and evaluate the yield performance and stability of single‐cross hybrids. Grouping of the inbred lines was done with heterotic group's specific combining ability (SCA) and general combining ability (GCA) (HSGCA), single nucleotide polymorphism based genetic distance (SNP‐GD) and GCA effects of multiple traits (HGCAMT) methods. The GCA mean squares for all traits were greater than SCA mean squares in all environments. The inbreds were classified into four heterotic groups each across environments by HSGCA and SNP‐GD while HGCAMT method placed them into three groups. HGCAMT was the most efficient followed by HSGCA and then SNP‐GD method. The HGCAMT identified inbreds ENT 17, ENT 15, and ENT 8 as best testers for heterotic Groups 1, 2, and 3. Hybrids TZEI 17 × ENT 15, and TZEI 149 × ENT 15 were outstanding across environments.Core IdeasSeventeen inbreds from CIMMYT and IITA were classified into four heterotic groups each across environments by HSGCA and SNP‐GD while HGCAMT method placed them into three groups.The HGCAMT is the most efficient heterotic grouping method and offers a great opportunity for grouping IITA inbreds that are yet to be grouped.The HGCAMT identified inbreds ENT 17, ENT 15, and ENT 8 as best testers for heterotic Groups 1, 2, and 3, across environments.ENT13, TZEI 17, and TZEI 16 may provide genes for drought and low‐N tolerance for broadening and diversifying genetic base of germplasm and to develop hybrids adapted to sub‐Saharan Africa.Planned crosses involving IITA and CIMMYT inbreds could produce outstanding hybrids for commercialization in SSA.

Combining Ability Analysis of Storage Root Yield and Related Traits in Cassava at the Seedling Evaluation Stage of Breeding

Selection of genotypes at the seedling evaluation stage of cassava breeding for advancement is largely based on high heritability traits. Moreover, combining ability and family data at this stage are rarely analyzed, leading to a loss of potential genetic information. This study was conducted to estimate the general combining ability (GCA) of nine cassava parents and specific combining ability (SCA) of their crosses for storage root yield (SRY) and related traits, as well as to determine the gene action controlling these traits. Thirty-six full-sib cassava families were developed from a 9 x 9 half-diallel mating design and evaluated in a 6 x 6 triple lattice design. The family mean squares (MS) were significantly different for all traits assessed, suggesting significant differences among families for all traits. The GCA MS were significant for all traits, whereas SCA MS were significant for only storage root number (SRN). Percentage sum of squares attributable to GCA accounted for >50.0% of variability expressed by families in five of the seven traits studied, indicating predominance of additive gene effects in controlling expression of most traits. Parent CT4 was the best general combiner for SRY, SRN, and resistance to cassava brown streak and mosaic diseases, while crosses NASE3 x CT2, CT5 x CT3, and NASE3 x CT4 had desirable SCA effects for SRY. Correlation analysis indicated a possibility of selecting for SRY and quality traits simultaneously. Simultaneous selection of these traits at the seedling generational stage of cassava breeding could reduce the cassava breeding cycle.

Rendimento de carcaça de codornas de corte em cruzamentos dialélicos

Objetivou-se com este trabalho avaliar o peso e rendimento de carcaça e cortes de codornas por meio da metodologia de cruzamentos dialélicos. Foram utilizadas quatro linhagens de codornas de corte, denominadas L1, L2, L3 e L4. O sistema de cruzamento proporcionou 16 grupos de progênies, sendo quatro parentais (puros), seis mestiços F1 e seis mestiços F1 recíprocos, alojadas em bateria experimental em três repetições. Aos 42 dias de idade, cinco machos de cada unidade experimental foram escolhidos ao acaso e destinados ao abate, para avaliação do peso aos 42 dias de idade, peso e rendimento de carcaça, do peito, das pernas e das asas. A análise dialélica foi desenvolvida de forma univariada, considerando-se a metodologia de dialelos completos, incluindo as p² combinações das linhagens, a partir das médias das combinações genotípicas. As análises de capacidade combinatória mostraram significância para capacidade geral de combinação (CGC) para peso corporal, da carcaça, peito, pernas e asas, enquanto a capacidade específica de combinação (CEC) foi significativa apenas para o peso e rendimento de pernas. Os quadrados médios da CGG apresentaram, de forma geral, valores maiores que da CEC, revelando a predominância de efeitos aditivos na expressão dessas características, o que indica que melhorias nestas características podem ser conseguidas por meio de processos de seleção intrapopulacional. As linhas L1, L2 e L4 apresentaram valores positivos para CGC em praticamente todas as características avaliadas, e podem ser consideradas superiores à média das linhagens envolvidas no dialelo, sendo, portanto, favoráveis para proporcionarem aumento nas características avaliadas. Foi observado maior efeito dos genes aditivos, indicando que a seleção das linhas puras pode trazer ganhos satisfatórios, sendo a linha L2 a mais promissora, com base em sua capacidade geral de combinação. Para peso de pernas, o cruzamento L2xL4 seria o mais indicado e, para o rendimento de pernas, o cruzamento L1xL4, com base na capacidade específica de combinação.

Genetic analysis of grain yield and other traits of extra-early yellow maize inbreds and hybrid performance under contrasting environments

Maize (Zea mays L.) is a major staple crop in West and Central Africa (WCA) but production is constrained by Striga hermonthica parasitism and recurrent drought in the savannas where maize yield potential is highest. Two studies were conducted for 2 years at 5 locations in Nigeria. The objective was to determine the combining ability of seven extra-early yellow-endosperm inbreds, place them into heterotic groups, identify the best testers and superior single-cross hybrids under drought, Striga infestation, optimal conditions and across environments. General combining ability (GCA) and specific combining ability (SCA) mean squares were significant for grain yield and most other traits across test environments. Mean squares for GCA were larger than those of SCA in all environments, indicating that additive gene action was more important in the inheritance of traits. Three testers (TZEEI 79, TZEEI 76 and TZEEI 63) and opposing heterotic groups (TZEEI 95, TZEEI 78, TZEEI 79), (TZEEI76, TZEEI 82) and (TZEEI 63, TZEEI 58) were identified across environments. TZEEI 79 had the highest positive GCA effects for grain yield across environments. Furthermore, TZEEI 95×TZEEI 79 was identified as the potential single-cross tester for the development of three-way and double-cross hybrids. Results of repeatability analysis showed that the managed drought test site, IKDT, was the most repeatable even though it was not very discriminating. In contrast, ZAWW had high representativeness, discriminating ability and repeatability, indicating that it was the ideal test location. The best extra-early hybrid TZEEI 79×TZEEI 63 under drought stress and TZEEI 82×TZEEI 79 under Striga infestation were comparable in yield to the best open-pollinated early check. GGE biplot analysis revealed TZEEI 79×TZEEI 76 as the highest yielding and most stable hybrid across environments. The available extra-early maize inbred lines are not only drought escaping but also possess genes for drought tolerance.

Combining Ability of Early‐maturing White Maize Inbreds under Stress and Nonstress Environments

Maize (Zea mays L.) hybrids resistant to Striga hermonthica (Del) Benth and tolerant to drought are vital to the sustainability of productivity of the maize‐based farming systems of West and Central Africa (WCA). Thirty‐six diallel crosses derived from nine early maturing white maize inbreds were evaluated under drought, well‐watered, striga‐infested, and striga‐free conditions at five locations in Nigeria between 2007 and 2009. The objective was to examine the combining ability, performance and stability of the inbreds, and to identify the heterotic groups. Additive and nonadditive gene actions were important in the control of the inheritance of drought tolerance and striga resistance in the inbreds. General combining ability (GCA) mean squares of grain yield and other traits were larger than those of specific combining ability (SCA), indicating that additive gene action was more important in the inheritance of drought tolerance. The GCA mean squares for striga damage rating at 8 and 10 weeks after planting (WAP) were about six and five times greater than those of the SCA, indicating that additive gene action played a major role in the inheritance of the striga traits. The GGE biplot analysis revealed that TZEI 4 and TZEI 5 were the most promising inbreds in yield performance and stability across the test environments. Two heterotic groups were identified; [TZEI 7, TZEI 19, TZEI 2, TZEI 4] and [TZEI 5, TZEI 3]. Tester TZEI 3 was the closest to the ideal tester. Entry TZEI 7 had the highest GCA effects and TZEI 2 the lowest.

General and Specific Combining Ability for Quantitative Characters in Sunflower

Seven sunflower (Helianthus annuus L.) inbred lines were crossed in 7´7 half diallel to obtain 21 F1 hybrids which further used to estimate general combining ability (GCA) and specific combining ability (SCA) effects for yield, head diameter, 1,000-seed weight, plant height and oil content of sunflower. The 7 inbred lines and 21 hybrids were planted in a randomized complete block design with three replications at Nakhon Ratchasima, Thailand during 2008-2009. General combining ability and specific combining ability were estimated for seed yield, head diameter, 1,000-seed weight, plant height and oil content. The results revealed that mean squares for GCA were highly significant for head diameter and significant for yield and oil content. Mean squares for SCA were highly significant for 1,000-seed weight and plant height, while those of yield, head diameter and oil content were non-significant. Components of variance showed that the GCA variance was higher than the SCA variance for yield, head diameter and oil content. These results indicated that additive gene action was more important than non-additive gene action for these traits. Inbred line 5A exhibited the highest GCA effects for yield and oil content, followed by the line 2A. Among all the crosses, 2A´5A showed the greatest positive SCA effects for 1,000-seed weight and oil content. Thus, the two inbred lines (2A and 5A) revealed good potential to be used as parents for hybrid.

Heterotic patterns in rapeseed (Brassica napus L.): I. Crosses between spring and Chinese semi-winter lines

Chinese semi-winter rapeseed is genetically diverse from Canadian and European spring rapeseed. This study was conducted to evaluate the potential of semi-winter rapeseed for spring rapeseed hybrid breeding, to assess the genetic effects involved, and to estimate the correlation of parental genetic distance (GD) with hybrid performance, heterosis, general combining ability (GCA) and specific combining ability (SCA) in crosses between spring and semi-winter rapeseed lines. Four spring male sterile lines from Germany and Canada as testers were crossed with 13 Chinese semi-winter rapeseed lines to develop 52 hybrids, which were evaluated together with their parents and commercial hybrids for seed yield and oil content in three sets of field trials with 8 environments in Canada and Europe. The Chinese parental lines were not adapted to local environmental conditions as demonstrated by poor seed yields per se. However, the hybrids between the Chinese parents and the adapted spring rapeseed lines exhibited high heterosis for seed yield. The average mid-parent heterosis was 15% and ca. 50% of the hybrids were superior to the respective hybrid control across three sets of field trials. Additive gene effects mainly contributed to hybrid performance since the mean squares of GCA were higher as compared to SCA. The correlation between parental GD and hybrid performance and heterosis was found to be low whereas the correlation between GCA((f + m)) and hybrid performance was high and significant in each set of field trials, with an average of r = 0.87 for seed yield and r = 0.89 for oil content, indicating that hybrid performance can be predicted by GCA((f + m)). These results demonstrate that Chinese semi-winter rapeseed germplasm has a great potential to increase seed yield in spring rapeseed hybrid breeding programs in Canada and Europe.

Genetic Responses of Single Crosses of Maize to Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze

Striga hermonthica (Del.) Benth. and Striga asiatica (L.) Kuntze infest millions of hectares of land under cereals in sub Saharan Africa, threatening food security. One of the major crops threatened is maize (Zea mays L.), a staple food for many Kenyans. A diallel cross involving 16 inbred lines was used to study the genetics of resistance to S. hermonthica and S. asiatica. Three inbred lines each from TZL comp 1, Zea diploperennis Iltis, Doebley & Guzman x Zea mays (Diplo pool), and a low emergence stimulant pool (LE Pool) classified as resistant were crossed to adapted but susceptible inbred lines CML-204, CML-373, and CML-395 from CIMMYT and MUG-1, MUG-2, MUG-3, and MUG-4 from Kenya. The hybrids were evaluated for two seasons at Matuga for S. asiatica and at Kibos and Alupe for S. hermonthica. General combining ability mean squares were higher than specific combining ability mean squares for all traits for the two species, indicating additive genetic effects were more important. Inbreds producing crosses with better resistance than the local checks were identified for both Striga species. Diplo pool and LE Pool were better sources of resistance to S. hermonthica and LE Pool and TZL pools were better sources of resistance or tolerance to S. asiatica.

Using scab resistant donors in apple breeding

Apple cultivars were crossed according to the half-diallel and topcross mating design. Apple scab resistance of seedlings was estimated by using a 0–5 scale: 0 – no infection on leaves, 5 – more than 75% of leaf area infected. Percentage of scab resistant seedlings in crosses involving cultivars Prima varied from 35.8% to 67.6%, involving cultivars Florina and Arbat – 51.9–59.5% and 53.5–72.5%, respectively. Significant differences for resistance to scab were found among the crosses. Both general combining ability (GCA) and specific combining ability (SCA) were highly significant. The significance of GCA for scab resistance shows that genetic variability exists for this trait and selection should result in genetic progress, because the mean square for GCA was larger than that calculated for SCA. Therefore, selection of parents on its phenotype should be effective for development of resistant apple cultivars. The significant SCA estimates indicate that a large number of resistant seedlings from particular crosses should be selected.

Combining ability effects of clonal rootstocks and scions in rubber trees (Hevea)

Rootstocks and scions combinations of five rubber tree [Hevea brasiliensis (Willd. ex Adr. de Juss.) MuellArg.] clones were evaluated for rubber yield and growth vigor to detect the rootstock with the best general combining ability (GCA) and the best specific combining ability (SCA), using the method 1, suggested by Griffing. The clones (scions) involved were: IAN 873, RRIM 600, RRIM 701, PB 235 and GT 1 grafted in five different rootstocks from illegitimate seeds of the same clones. The ANOVA mean squares for GCA and SCA reciprocal effects were highly significant for both characters. The general GCA: SCA ratios were 24:1 and 4:1 for rubber yield and growth vigour, respectively. The rootstock of the RRIM 600 clone was the best general combiner for rubber yield (g i = 8.25), followed by the PB 235 (g i = 5.20).Although negative, the rootstock of the RRIM 600 clone showed the best GCA (g i = -0.0354) for growth vigour, followed by rootstock of the GT 1 clone which presented a g i = -0.5194 value. The maximum SCA effects for rubber yield (s ij = 5.55) was obtained by IAN 873 rootstocks combined with the RRIM 600 clone as scion, followed by the PB 235 (rootstock) combined with the RRIM 600 scions (s ij = 3.06). For growth vigour, the best specific combining ability effect of s ij = -1.64 was obtained by the IAN 873 clone in combination with the RRIM 701 clone, with their respective rootstock and scion. As for rubber yield, the RRIM 600 rootstocks were classified as the best general combination. In addition, the IAN 873 rootstocks combined with the RRIM 600 scion showed the best specific combining ability.

Combining Ability of Callus Induction and Plant Regeneration in Sorghum Anther Culture

Fifteen F1 hybrids derived from a six-parent diallel mating set (without reciprocal) of sorghum genotypes were used to investigate the combining ability for callus induction and plant regeneration. Anthers at a young stage were collected from the parents and hybrids, which were grown in a greenhouse, and cultured on a modified Murashige and Skoog medium containing 1.0 mg L−1 IAA, 2.5 mg L−1 2,4-D and 2.0 mg L−1 kinetin. Calli of 2 to 5 mm in diameter were transferred to a regeneration medium containing 2.5 mg L−1 kinetin and 3.0 mg L−1 IAA for plant regeneration. The general combining ability (GCA) accounted for a larger proportion than the specific combining ability (SCA) for callus induction (179.83, mean square for GCA vs. 31.75, mean square for SCA) and for plant regeneration (1225.73, for GCA vs. 410.37, for SCA). Some parents and hybrids showed high callus induction ability accompanied with high green-plant regeneration ability. Thus, improvement of the rate of green-plant regeneration in anther culture through crossing and selection may be possible.

Heterosis and combining ability in a half diallel cross of kenaf ( Hibiscus cannabinus L.) in south Italy

The occurrence of heterosis has been evaluated in 45 F 1 combinations derived from ten parental genotypes of kenaf ( Hibiscus cannabinus L.) according to a diallelic scheme. In addition studies have been conducted to assess the combining ability for yield components, using method 2, model I, of the Griffing diallel analysis. The characteristics analyzed were: plant height, basal diameter, fresh and dry weight of bark and core of the useable stalk portion. Dry bark/dry woody core weight was evaluated also. The highly significant F-test values demonstrated the existence of variability for general combining ability (GCA) and specific combining ability (SCA), and for all the characters analysed. Mean squares of GCA were always (except for basal diameter) higher than those of SCA. However, when compared in terms of the average squared effects (components), the effects of SCA were greater than those of GCA, indicating the importance of non-additive genetic effects in determining the expression of yield components. Lines HC 625, HC 622 and HC 602 were the best general combiners for the most important characters. Heterosis for dry bark weight was highest in HC 602×HC 726.

A Comparison of Pseudomonas solanacearum- resistant Tomato Cultivars as Hybrid Parents

Seven tomato Lycopersicon esculentum lines, `Venus', `Saturn', `Rodade', `Rotam 4', `Hawaii 7998', `UC-82B', and `Stevens', and their 21 crosses were evaluated for their ability to resist infection by seven virulent strains of Pseudomonas solanacearum E.F. Sm. representing race 1 biovars 1 and 3. The Gardner and Eberhart model III analysis was used to evaluate the response of lines in crosses to P. solunaceurum infection. General combining ability mean squares were significant for all strains and in 5 of 7 instances specific combining ability mean squares were significant. In four instances the parent vs. cross contrast was significant. `Hawaii 7998' was resistant to all seven strains of P. solanacearum, whereas `Rotam 4' and `Rodade' were resistant to biovar 3 and two races of biovar 1. `Venus' and `Saturn' were resistant to two other biovar 1 strains. `Hawaii 7998' transmitted greater resistance than the other resistant parents, but its small fruit size and indeterminate growth habit make it a poor choice for a hybrid parent.

Diallel Analysis of Relative Growth Rates in Maize Synthetics

Previous studies indicated that relative growth rates (RGR) based on easily obtained measurements [(leaf number ✕ leaf length ✕ leaf width ✕ O.5)/days to silking] hold promise as a selection tool to increase grain yields in early‐maturing maize (Zea mays L.). The objective of this field research was to evaluate the interrelationships among RGR, grain yield, yield components, and components of RGR. Three independent diallel sets among eight parental synthetics were evaluated in up to nine environments during 3 yr. General combining ability (GCA) and specific combining ability (SCA) effects were estimated using Griffing's Method 4, Model 1. Correlations among GCA effects (rA) for all traits were obtained. Mean squares for GCA were highly significant for all traits in one or more of the diallels, while SCA effects were significant in 50% of the F‐tests. The relative magnitudes of ratios ef the GCA component to GCA pros SCA components (mean squares) suggested that it should be easier to select parents of high‐performing progenies for kernel row number, kernel weight, RGR, leaf length, and silking date than for kernels per row or grain yield. Correlations among GCA effects estimated across environments for three diallels among eight synthetics indicated that GCA effects for RGR were highly correlated (rA = 0.82) with GCA effects for grain yield. Highly significant correlations among GCA effects for RGR, ears per plant, and kernels per row (primary yield components), as well as highly significant correlations among GCA effects for yield and leaf number, leaf length, and days to silking (RGR components), indicated that these traits are not independent. Results confirmed the suggestion that selection for increased RGR would increase grain yields in early maize.