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Cassava (Manihot esculenta Crantz) roots spoil 2 to 3 d after harvest because of postharvest physiological deterioration (PPD), which has remained an unsolved problem. Roots from different sources of germplasm were evaluated 5, 10, 20, and 40 d after harvest and some were found to be tolerant to PPD. Three genotypes showed zero levels of PPD even 40 d after harvest. Tolerance to PPD in roots with high carotenoid levels may be explained by their anti-oxidant properties. Irradiation of seeds in mutagenized populations may have silenced one of the genes involved in the expression of PPD. The tolerance to PPD found in other sources cannot be properly explained. The identification of several sources of resistance (and at least two different modes of action) suggests that now there are alternatives available for solving this problem, benefiting millions of resource-limited farmers worldwide. doi: 10.2135/cropsci2009.11.0666

Inbreeding offers numerous advantages but also presents some problems for the genetic improvement of cassava (Manihot esculenta Crantz). One of the concerns is that inbreeding depression (ID) may be too drastic for (even partially) inbred materials to survive. A large experiment was conducted to quantify ID in cassava. [S.sub.1] families from eight elite cultivars, each represented mostly by over 90 cloned genotypes, were produced and evaluated in a replicated trial at one location. The results from this evaluation allowed measurement of ID levels for key traits such as fresh root yield (63.9%), fresh foliage yield (37.9%), harvest index (26.5%), plant height (10.1%), and dry matter content in the roots (5.3%). Results also indicated (as expected) that ID, for a given trait, was not uniform among the eight families evaluated. For reasons provided in the article, the ID levels measured in this study may have been overestimated. Compared with the first experiences introducing inbreeding in temperate and tropical maize (Zea mays L.), we concluded that the use of homozygous progenitors in future cassava genetic enhancement is feasible. The average ID and frequency distributions within [S.sub.1] families for different traits correlated well with the relative importance of nonadditive genetic effects controlling their inheritance as reported in the literature.

Little is known about the inheritance of agronomic traits in cassava (Manihot esculenta Crantz). The vegetative multiplication of cassava allows cloning of individual genotypes, and separates environmental from genetic variation in the within-family variation. Knowing the magnitude of between- and within-family variation is important for defining breeding strategies and for measuring different components of genetic variances, particularly the seldom-estimated epistasis. A group of nine genotypes, adapted to midaltitude environments, was used for a diallel study. Thirty clones were obtained from every [F.sub.1] cross (each clone was represented by six plants), and planted in three replications at two locations. Statistical differences among crosses were found for fresh-root yield, harvest index, root dry-matter content, and reaction to mites (Mononychellus tanajoa Bondar) and to whiteflies (Aleurotrachelus socialis Bondar). General combining ability (GCA) was significant for all traits except fresh-root yield and dry-matter content, and specific combining ability (SCA) was significant for all traits except whitefly damage score. Fresh-root yield was the only trait with significant epistatic effects, which, combined with a large dominance variance, suggested a prevalence of nonadditive effects. The introduction of inbreeding would be one approach for the efficient exploitation of these nonadditive effects found for fresh-root yield. For the remaining traits, epistasis was negligible and current schemes exploiting additive effects may suffice. Abbreviations: GCA, general combining ability; SCA, specific combining ability., CASSAVA, ALONG WITH MAIZE (Zea mays L.), sugarcane (Saccharum spp.), and rice (Oryza sativa L.), constitutes the most important source of energy in the diet of most tropical countries of [...]

Cassava (Manihot esculenta Crantz) is an important commodity for industrial processes in tropical countries as one of the few alternatives to compete with imported maize (Zea mays L.). To maintain this competitiveness, cassava breeding needs to be as efficient as possible. This study provides one of the first attempts to produce quantitative genetic data to aid breeding efficiency, through the analysis of a diallel set among nine parental clones adapted to the midaltitude valleys environment. Thirty clones represented each Fl cross (with three exceptions). Evaluations were conducted in two contrasting environments with three replications in each location. The specific combining ability (SCA) effects were relatively more important than general combining ability (GCA) effects for root yield. In the case of harvest index, dry matter content (DMC) and plant type architecture GCA effects were about twice as large as those from SCA effects. Reaction to mites (Mononychellus tanajoa Bondar) and white flies (Aleurotrachelus socialis Bondar) (based on single-location data) showed the strongest influence of GCA effects on the expression of a given trait. Yield data demonstrates the excellent potential of this crop for the tropical production of starch and source of energy in animal feed that could compete with the international markets., CASSAVA, ALONG WITH maize, sugarcane (Saccharum officinarum L.), and rice (Oryza sativa L.), constitute the most important sources of energy in the diet of most tropical countries of the world. [...]

Acid soils cover approximately 30% of the global ice-free land area. Nearly 26 million hectares of maize (Zea mays L.) is already planted on acid soils. Maize yields are low on acidic soils because of toxicities of Al, Mn, and Fe, and deficiencies of P, Ca, Mg, and K. Acid-tolerant maize cultivars would be an environmentally friendly and relatively inexpensive method for improving maize production on acidic soils. Little information is available in the literature on the magnitude of reciprocal effects on yield and other agronomic traits when maize is grown on acidic soils. A diallel study, involving eight segregating populations and their 56 reciprocal crosses, was conducted in five acidic-soil environments to determine relative importance of nuclear and cytoplasmic factors for yield, days to silk, ear height, ears per plant, and ear rot. Average (h) and specific ([s.sub.ij]) heterosis accounted for 65 and 31% of the total sum of squares for heterosis for yield. Population heterosis effects ([h.sub.j]) for yield were not significant, suggesting [h.sub.j] effects would be of little value in selecting populations for developing superior hybrids. Specific heterosis effects were negative and significant (P [is less than] 0.05) for yield and ears per plant only for the cross of CMS 36 X ETO Blanco, suggesting a minor role for nonadditive gene effects in determining yield of specific cross combinations. The absence of reciprocal differences for all traits indicated that tolerance to soil acidity was controlled by nuclear genes., Acid soils cover approximately 30% of the total ice-free land area or about 3950 million hectares of the earth's surface. Of the total acid-soil area, 40.9% occurs in the Americas, [...]

Maize (Zea mays L.) is grown on approximately eight million hectares of acidic soils, where yields are low because of the toxicity of Al and Mn and deficiency in Ca, Mg, and P. Maize cultivars tolerant to soil acidity would increase yields on such soils. A diallel study was conducted involving six soil-acidity tolerant and two susceptible segregating populations to identify superior germplasm to develop cultivars for acidic soils. The eight populations and their 28 crosses were evaluated in seven acidic soil environments. Tolerant populations averaged higher in yield (2.19 vs. 1.58 Mg [ha.sup.-1]; P < 0.01), ears per plant (0.79 vs. 0.64; P < 0.05), and ear height (61.6 vs. 51.4 cm; P < 0.01), and fewer in days to silk (68.8 vs. 69.7 d; P < 0.05) than the susceptible populations. Mean squares of parents vs. crosses were highly significant for yield, ear height, and ears per plant, and significant for days to silk, indicating heterosis for these traits. Crosses between tolerant populations tended to yield higher (3.00 Mg [ha.sup.-1]) than those between tolerant and susceptible populations (2.40 Mg [ha.sup.-1]) and between susceptible populations (2.01 Mg [ha.sup.-1]). General combining ability (GCA) was highly significant for all traits, but specific combining ability (SCA) was significant only for ears per plant. Reciprocal recurrent selection would be effective in developing superior cultivars for acidic soils and should include populations 90SA-3 and 90SA-4 or CMS-36 for yellow endosperm cultivars and 90SA-6 and 90SA-7 for white endosperm cultivars.

Soil acidity reduces maize (Zea mays L.) yields on approximately equal to 8 million hectares of tropical soils. Maize cultivars with tolerance to soil acidity can alleviate this problem in developing countries. We used a North Carolina Design I mating system to study the inheritance of tolerance to soil acidity in a tropical maize population, SA-3, in order to efficiently develop maize cultivars tolerant to acidic soils. We studied 256 full-sib progenies, involving 64 males and four females within each male, that were grouped in eight sets and field planted using two replications under one normal and three acidic soil environments during 1990-1991 in Colombia. Across acidic soils, additive genetic variance (V(sub A)) was similar to dominance variance (V(sub D)) for grain yield (0.15 vs. 0.13) greater than V(sub D) for plant height (47.97 vs. 6.83) and ears per plant (0.01 vs. 0.00), and less than V(sub D) for days to silk (1.26 vs. 2.98). Heritability, estimated using half-sib family means, averaged 36.3 + or - 19.9% for yield, 30.9 + or - 19.8% for days to silk, 51.5 + or - 19.3% for plant height, and 40.5 + or - 19.8% for ears per plant. Additive X environment interaction was the more important component of genetic variance for all traits studied. Although grain yield showed a positive additive genetic correlation (r(sub A) = 0.84) with ears per plant, direct selection for yield was more effective for improving yield under acidic soils. The magnitudes of additive and additive X environmental variances and of r(sub A)s among the environments suggest that recurrent selection, based on multilocation testing, would be effective in improving grain yield, days to silk, plant height, and ears per plant under acidic soils.

Soil acidity reduces maize (Zea mays L.) yields on about eight million hectares in developing countries. We report on response to selection for tolerance to soil acidity, using an altered version of modified ear-to-row (MER) and full-sib (FS) selection. In the MER selection, 120 half-sib (HS) families were evaluated under 45 and 80% Al saturation. One to three ears from each of the best 30% of the families were selected each cycle. After 16 cycles of MER selection, FS selection was initiated. Two hundred and fifty FS families were evaluated at five to six acid (ASEs) and one normal soil environments (NSE) and the best 25% were selected each cycle. Cycles 2, 4, 6, 8, 10, 12, and 14 of MER and 0, 1, and 2 of FS selection were evaluated in three to nine replications at six ASEs and five NSEs during 1990-1991. Across the 11 environments, gain from selection in yield averaged 40 kg ha to the -1 cycle to the -1 (1.49%) with MER and 250 kg ha to the -1 cycle to the -1 (8.10%) with FS selection. Across the six ASEs, yield improvements of 40 kg ha to the -1 cycle to the -1 (1.99%) with MER and 310 kg ha to the -1 cycle to the -1 (13.96%) with FS selection were obtained. Yield also improved across the five NSEs by 50 kg ha to the -1 cycle to the -1 (1.10%) with MER and 150 kg ha to the -1 cycle to the -1 (3.31%) with FS selection. Results indicate that tolerance to soil acidity can be improved with recurrent selection and that the progress will be higher with a system more efficient at reducing the experimental error and genotype x environment interaction.

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Knowledge on the reproductive biology of cassava, relevant to breeders and molecular geneticists, is still limited. Therefore, different studies were carried out to determine the duration of stigma receptivity and the rate of pollen tube growth. Inflorescences were covered for up to 3 days after the first opening of the bracts (e.g. anthesis day) to prevent open pollination. Results indicate that fruit and seed set are drastically reduced when flowers were covered for 2 or 3 days. However, fruits and seeds were obtained even from flowers that had been covered for 3 days after anthesis, although at low frequency. The rate of pollen tube growth was assessed in many combinations of female and male progenitors crossed through controlled pollinations and collecting the pistils at varying hours after pollination (HAP). Pollen tube growth is fast during the first 6 HAP reaching the tip of the nucellar beak. The growth slows down thereafter, taking 10 additional hours to reach the end of the beak. The growth of pollen tubes slows down even further until they enter the embryo sac. Only 10% of samples showed pollen tubes entering the embryo sac between 48 and 66 HAP. Although several tubes may reach the nucellar beak, only one was observed entering the embryo sac. Results, across the different experiments, were highly variable suggesting that the timeline of fertilization is influenced both by genotypic and environmental factors as well as the manual manipulation of inflorescences and cyathia. [ABSTRACT FROM AUTHOR]