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Grain yields of maize in the lowland tropics are generally limited by short days and high temperatures that minimize durations of incident light. Little has been published on the effects of this limiting environment on the genetics of grain filling rate (GFR), and grain filling period (GFP) in tropical maize germplasm. This study sought to address these limitations. A set of 8 elite maize inbreds of tropical origin and their 28 diallel hybrids were grown in three seasons at Waimanalo, Hawaii, USA. Seasonal differences included > 100% differences in values of photosynthetic active radiation (PAR) during grain formation. Information was sought on the performance variations and the genotype by season interactions for GFR, GFP, days to mid-silk (DTS), kernel weight, with estimates of general (GCA) and specific combining ability (SCA) and their interactions with seasons. Significant differences occurred for inbreds, hybrids, and genotype by season interactions, GCA and SCA effects and their interactions with seasons, which could be attributed primarily to the differences in PAR values among seasons in Hawaii during grain filling. Additive genetic effects predominated for GFR and GFP. Breeding approaches that take advantage of additive gene effects including hybrid breeding with evaluations in multiple Hawaii seasons may be used to alter GFR and GFP.

Key message This research revealed diverse PTG rates among intraspecific pollen–pistil interactions that showed variable dependency on the stigma and mature TT. Pollen–pistil interactions regulate pollen tube growth (PTG) rates and are determinants of fertilization and seed set. This research focuses on the diversity of intraspecific PTG rates and the spatial and temporal regulation of PTG among Nicotiana tabacum genotypes. Nonrandom mating within self-compatible species has been noted, but little is known on the mechanisms involved. To begin research on nonrandom mating, we took advantage of the model reproductive system of N. tabacum and used seventeen diverse N. tabacum genotypes in a complete pollination diallel to measure the diversity of intraspecific pollen–pistil interactions. The 289 intraspecific interactions showed surprisingly large differences in PTG rates. The interaction between specific males and females resulted in 18 specific combining abilities that were significantly different, indicating the importance of the specific genotype interaction in regulating intraspecific PTG. No single female or male genotype exerted overall control of PTG rates, as determined by a general combining ability analysis. Slow and fast pollen–pistil interactions showed spatial differences in growth rates along the style. Slower interactions had a slower initial PTG rate while fast interactions had faster consistent rates of growth indicating spatial regulation of PTG in the pistil. Removal of the stigma or the mature transmitting tissue (TT) showed the tissue-specific component of PTG regulation. Stigma removal resulted in slower or no change in PTG rate depending on the pollen and pistil genotypes. Removal of the TT, which necessitated removal of the stigma, showed no change, slower or unexpectedly, increased growth rates relative to growth rates without a stigma. These data show the diverse nature of pollen–pistil interactions in N. tabacum genotypes providing a system to further investigate the regulation of PTG.

Alternaria blight ( Alternaria spp.) is an important sweetpotato disease in Uganda causing yield losses of over 50 % in susceptible genotypes. In Uganda, Alternaria bataticola and Alternaria alternata are the major species with A . bataticola the more aggressive of the two. The most effective control measure for this disease is the use of resistant genotypes. This study was conducted to determine the inheritance of resistance to Alternaria blight and the general and specific combining abilities of the available germplasm. Sixteen parental clones varying in reaction to Alternaria blight were crossed using the North Carolina II mating scheme. Due to incompatibility of some parents, two sets of compatible parents were formed. Differences among the families for Alternaria blight severity were significant while general combining ability (GCA) and specific combining ability (SCA) mean squares were highly significant (P < 0.001) for the disease with GCA sum of squares (SS) being more predominant at 67.4 % of the treatment SS for Set 1 and the SCA SS predominant at 54.0 % of the treatment SS for Set 2. This indicated that both additive and non-additive effects are important in controlling this trait. Some parents with high, negative GCA effects produced families with undesirable SCA effects and the reverse was also true. This implies that the best parents should not be chosen on GCA alone but also on SCA of their best crosses. The wide range in the area under disease progress curve for the families indicated that it was possible to select for highly resistant genotypes.

Combining ability is one of the most important information breeders use to identify superior inbred lines on the basis of their performance in hybrid combinations. The objectives of our study were (i) to quantify the importance of general combining ability (GCA) and specific combining ability (SCA) variances for seed yield, oil content and oil yield; and (ii) estimate GCA and SCA effects of seed yield, oil content and oil yield of inbred lines developed from advanced cycle pedigree breeding populations in sunflower. A total of 109 female S₃ cytoplasmic male sterile (CMS) lines from four bi-parental populations in advanced cycle pedigree breeding were crossed with two testers to form 218 testcross hybrids (TCHs). The TCHs were then evaluated in three environments. Variance component analysis results showed predominance of σ²gca over σ²sca for seed yield and oil yield indicating that superior TCHs can be identified based on positive and significant GCA effects of the female lines. For oil content σ²sca was predominant over σ²gca indicating that selecting for TCHs with high oil content would be best among line × tester combinations and not among female S₃CMS lines per se. The proportion of GCA and SCA effects in the best five TCHs in each breeding population also confirmed the predominance of GCA effects over SCA effects for seed yield and oil yield while for oil content both GCA and SCA effects appear to be important, with SCA effects having more influence than GCA. The best selection strategy would therefore be to capture the GCA in the early stages of inbreeding and then SCA for the few unique combinations when lines are almost fixed.

Breeding maize hybrids for drought tolerance would significantly reduce yield loss due to drought in southern Africa. Mode of gene action controlling yield and secondary traits was investigated by mating 27 inbred lines, in sets according to a North Carolina design II scheme. The resultant 72 experimental and eight commercial hybrids were evaluated in 8 x 10 α-lattice design with two replications, in four drought and two non-drought environments. Under drought conditions, only general combining ability (GCA) variance was significant for yield, indicating predominance of additive effects. In non-drought environments, both GCA and specific combining ability variances were significant for yield, indicating importance of additive and non-additive effects, respectively. Contributions of male (GCAm) and female GCA (GCAf) effects to hybrids varied depending on the trait and conditions. Superior GCAf to GCAm effects for yield under drought conditions, and for ASI, prolificacy and ear aspect under both drought and non-drought conditions, suggested that maternal effects might have modified these traits. Larger GCAm than GCAf for ASI and silking dates under normal conditions indicated that paternal genotypes played a greater role in determining these traits. Similar GCAm and GCAf for yield under normal conditions, silking and anthesis dates under drought indicated that both parents made similar contribution to hybrids. Possibly, complications due to modification of traits by cytoplasmic effects and cross-over G x E for yield might partly explain why only a few drought tolerant hybrids have been developed. Practical implications of these findings in breeding drought tolerance in maize are discussed.

Pre-harvest sprouting in wheat is the germination of seeds within the spikes when rains occur after or during grain ripening, which occurs commonly in the barani tract of Pakistan. Therefore, 10 cultivars and five advanced lines of spring bread wheat were evaluated for pre-harvest sprouting resistance. After natural rainfall, seeds were immediately collected from the wet spikes and tested for germinating ability. Three different germination tests were applied to hand-threshed seed: (1) spikes threshed on the day of sampling and germination tested immediately, (2) spikes threshed on the day of sampling and germination tested 1 week later, and (3) spikes threshed 1 week after sampling and germination test immediately after threshing. Seeds and spikes kept for 1 week were place on blotting paper at room temperature. Cultivars BARS-09, 09FJ17, Doukkala-12, NARC-09 and Ouassou-20 exhibited higher sprouting resistance while other genotypes were susceptible to pre-harvest sprouting in each of the three tests. A diallel crossing was conducted with six susceptible and two resistant genotypes to assess the genetic behavior of pre-harvest sprouting resistance. The combining ability (CA) demonstrated a higher proportion of additive genetic effects for sprouting resistance, because of higher variance of general and specific CA for both parameters under study. Doukkala-12 and BARS-09 showed increased pre-harvest sprouting resistance in their F1 descendants.

Evaluation of combining ability is a crucial process in hybrid breeding, and dissection of the genetic basis of combining ability will facilitate hybrid breeding. In this study, molecular markers significantly associated with general combining ability (GCA) of seven yield-related traits and the traits per se were detected in a set of maize introgression lines (ILs) under three environments. Totally 25 and 31 significant loci for GCA and the traits per se were commonly detected under multiple environments, respectively. Correlation analysis and comparison among these significant loci revealed that the genetic basis of GCA of these yield-related traits was generally different from that of the traits per se except for the trait of ear row number. In addition, GCA of the ILs was positively and significantly correlated to the total relative effects of significant GCA loci in the ILs in general, implying that the GCA loci identified in this study would be useful in molecular breeding. Correlation analysis also showed that the GCA of yield per plant was strongly correlated to the GCA of kernel number per row, ear length and 100-kernel-weight, thus these traits were more important in genetic improvement for GCA. Results in this study would provide useful information for hybrid breeding in maize.

[This corrects the article DOI: 10.3389/fpls.2024.1471041.].

Nematodes cause substantial grain yield loss in susceptible maize ( Zea mays L.) cultivars. This study was conducted to estimate general combining ability (GCA), specific combining ability (SCA) and genetic effects associated with nematode resistance in maize. The 30 F 1 hybrids generated from a 6 × 6 diallel and two local checks were evaluated in 2009 at three sites in Uganda. A split plot design was used with nematode treatments serving as whole plots and the hybrids as subplots but arranged in an 8 × 4 spatially adjusted alpha-lattice design. The experiment was replicated three times. Results showed GCA to be important for the reduction of P . zeae and Meloidogyne spp. densities and increase of root mass, with a contribution of 72 to 93% of the phenotypic variance. Inbreds MP709 and CML206 had the highest GCA for Pratylenchus zeae resistance, whereas for grain yield, it was CML444, CML312 and CML395 that were outstanding. The SCA influenced plant height and grain yield under nematode infestation, contributing 43 and 58% of the phenotypic variance, respectively. Observed reciprocal differences due to maternal effects also played a role in influencing the grain yield under nematode infestation. Overdominance genetic effects explained the non-additive variance recorded for the plant height, grain yield, number of root lesions, and P. zeae and Meloidogyne spp. densities under nematode infestation. The parents MP709, CML206, 5057, and CML444 contributed most of the dominant genes for the P. zeae resistance in all their crosses. The parent CML444 contributed most of the dominant genes for improved grain yield in all of its crosses. The high GCA effects among some parents support their utility in breeding of widely adapted nematode-resistant cultivars. The dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure.

Prediction of single-cross performance has been a major goal of plant breeders since the beginning of hybrid breeding. Recently, genomic prediction has shown to be a promising approach, but only limited studies have examined the accuracy of predicting single-cross performance. Moreover, no studies have examined the potential of predicting single crosses among random inbreds derived from a series of biparental families, which resembles the structure of germplasm comprising the initial stages of a hybrid maize breeding pipeline. The main objectives of this study were to evaluate the potential of genomic prediction for identifying superior single crosses early in the hybrid breeding pipeline and optimize its application. To accomplish these objectives, we designed and analyzed a novel population of single crosses representing the Iowa Stiff Stalk synthetic/non-Stiff Stalk heterotic pattern commonly used in the development of North American commercial maize hybrids. The performance of single crosses was predicted using parental combining ability and covariance among single crosses. Prediction accuracies were estimated using cross-validation and ranged from 0.28 to 0.77 for grain yield, 0.53 to 0.91 for plant height, and 0.49 to 0.94 for staygreen, depending on the number of tested parents of the single cross and genomic prediction method used. The genomic estimated general and specific combining abilities showed an advantage over genomic covariances among single crosses when one or both parents of the single cross were untested. Overall, our results suggest that genomic prediction of single crosses in the early stages of a hybrid breeding pipeline holds great potential to redesign hybrid breeding and increase its efficiency.