Identification of pearl millet [Pennisetum glaucum (L.) R. Br.] lines tolerant to soil salinity

Crop tolerance to salinity is of high importance due to the extent and the constant increase in salt-affected areas in arid and semi-arid regions. Pearl millet (Pennistum glaucum), generally considered as fairly tolerant to salinity, could be an alternative crop option for salt affected areas. To explore the genotypic variability of vegetative-stage salinity tolerance, 100 pearl millet lines from ICRISAT breeding programs were first screened in a pot culture containing Alfisol with 250 mM NaCl solution as basal application. Subsequently, 31 lines including many parents of commercial hybrids, selected from the first trial were re-tested for confirmation of the initial salinity responses. Substantial variation for salinity tolerance was found on the basis of shoot biomass ratio (shoot biomass under salinity/ non-saline control) and 22 lines with a wide range of tolerance varying from highly tolerant to sensitive entries were identified. The performance of the genotypes was largely consistent across experiments. In a separate seed germination and seedling growth study, the seed germination was found to be adversely affected (more than 70% decrease) in more than half of the genotypes with 250 mM concentration of NaCl. The root growth ratio (root growth under salinity/control) as well as shoot growth ratio was measured at 6 DAS and this did not reflect the whole plant performance at 39 DAS. In general, the whole plant salinity tolerance was associated with reduced shoot N content, increased K⁺ and Na⁺ contents. The K⁺/Na⁺ and Ca⁺⁺/Na⁺ ratios were also positively related to the tolerance but not as closely as the Na⁺ content. Therefore, it is concluded that a large scope exists for improving salt tolerance in pearl millet and that shoot Na⁺ concentration could be considered as a potential non-destructive selection criterion for vegetative-stage screening. The usefulness of this criterion for salinity response with respect to grain and stover yield remains to be investigated.     

Genetic analysis of grain mould resistance in coloured sorghum genotypes

Grain moulds are a major constraint to sorghum production and to adoption of improved cultivars in many tropical areas. Information on the inheritance of grain mould reaction is required to facilitate breeding of resistant cultivars. The genetic control of grain mould reaction was studied in 7 crosses of 2 resistant sorghum genotypes. P₁, P₂, F₁, F₂, BC₁ and BC₂ families of each cross were evaluated under sprinkler irrigation for field grade and threshed grade scores and subjected to generation mean analysis. Frequency distributions for grain mould reaction were derived and F₂ and BC₁ segregation ratios were calculated. Grain mould reaction in crosses of coloured grain sorghum was generally controlled by two or three major genes. Resistance to grain moulds was dominant. Significant additive gene effects were also found in all cross/season combinations. Significant dominance effects of similar magnitude to additive effects were also observed in five out of ten cross/season combinations. Gene interactions varied according to the parents with both resistant and susceptible parents contributing major genes. Choice of parents with complementary resistance genes and mechanisms of resistance will be critical to the success of resistance breeding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization and genetic control of germination-emergence responses of grain sorghum to salinity

Summary When grain sorghum is grown in saline soils, one cause of low yield is poor crop establishment. The objectives of this study were to assess the response of grain sorghum to salinity in the germination-emergence stages, study the inheritance of salt tolerance at this stage, and determine the relative contribution to final emergence of salt effects during imbibition, and after onset of germination. Twelve inbred lines and 18 F1 hybrids, resulting from an incomplete 6×6 factorial mating design, were tested for germination and emergence in folded paper at 10 salt concentrations, from 1.8 to 36 dSm^-1. The mean EC₅₀ (the electrical conductivity at which the variable score declines by 50%) for emerged seedlings production was 21.2 dSm^-1. Large genotypic differences were observed for salt tolerance at germination and emergence stages, which were not related to the viability of seeds, and poorly related to seed weight (considered as an estimate of intrinsic seed vigor). In the hybrids, these differences were due to SCA and female GCA for emergence, and female GCA for germination, though the male GCA was also significant for both characters. Line per se performance was significantly correlated to individual GCA estimates for emergence, but not for germination. Heterosis was only detected in three crosses for final emergence and in one cross for germination. The genetic differences in final emergence were mainly due to effects occurring after the onset of germination rather than a consequence of effects during imbibition.     

Genetic analysis of grain mold resistance in white seed sorghum genotypes

Grain molds in rainy season sorghums can cause poor grain quality resulting in economic losses. Grain molds are a major constraint to the sorghum production and for adoption of the improved cultivars. A complex of fungi causes grain mold. Information on genetics of grain mold resistance and mechanisms is required to facilitate the breeding of durable resistant cultivars. A genetic study was conducted using one white susceptible, three white resistant/tolerant sources, and one colored resistant source in the crossing programme to obtain four crosses. P₁, P₂, F₁, BC₁, and BC₂, and F₂ families of each cross were evaluated for the field grade and threshed grade scores, under sprinkler irrigation. Generation mean analyses and frequency distribution studies were carried out. The frequency distribution studies showed that grain mold resistance in the white-grained resistance sources was polygenic. The additive gene action and additive × additive gene interaction were significant in all the crosses. Simple recurrent selection or backcrossing should accumulate the genes for resistance. Epistasis gene interactions were observed in colored resistance × white resistance cross. Gene interaction was influenced by pronounced G × E. Pooled analysis showed that environment × additive gene interaction and environment × dominant gene interaction were significant. The complex genetics of mold resistance is due to the presence of different mechanisms of inheritance from various sources. Evaluation of segregating population for resistance and selection for stable derivatives in advanced generations in different environments will be effective.     

A single recessive gene for stem sweetness in sorghum

Summary The inheritance of stemsweetness in sorghum has followed a qualitative pattern, non-sweet being monogenically dominant over sweet. Hence sweetness was controlled by a single recessive gene. A maternal effect was not involved for the expression of sweetness.     

A test for obligate apomixis in grain sorghum R473

Summary Segregation patterns in progeny arrays of selfed plants, heterozygous for the Mdh 1 isozyme marker locus, were used in an attempt to confirm the presence of apomixis in the grain sorghum line R473. No evidence for obligate apomictic reproduction was obtained. However, our studies did not rule out the possibility of a low level of facultative apomixis in R473.     

Evaluation of greenhouse inoculation techniques to screen sorghum for resistance to downy mildew

Summary Six inoculation techniques were compared for the artificial promotion of downy mildew (Peronosclerospora sorghi) in sorghum. These were (1) sprouted seeds incubated between sporulating infected leaves, (2) sprouted seeds depped in conidial suspension, (3) sprouted seeds sprayed with conidial suspension, (4) seedlings at plumule stage inoculated with drops of a conidial suspension, (5) seedlings at plumule stage sprayed with a conidial suspension, and (6) seedling showered with conidia falling from infected leaves. Seedlings at the one-leaf stage sprayed with a conidial suspension (6 × 10⁵ ml^-1) showed the highest systemic infection (100%) in the susceptible lines IS 643 and IS 18433. This technique is effective, repeatable, and allows the deposition of a conidial suspension as a fine mist on the entire seedling surface. In the greenhouse, the technique was used to test the downy mildew reaction of genotypes previously reported as resistant (< 5% incidence) in 3–4 years of field screenings. Of the 61 genotypes tested, 21 were free from downy mildew, 14 had less than 5% incidence, and the rest showed variable susceptible reactions. Therefore, the technique can be reliably and effectively used in the greenhouse to detect disease escapes and to indentify resistance.     

Defining traits for selection using isopopulations of sorghum

Sorghum is grown in the subtropics in north-eastern Australia, where production is risky due to limited planting opportunities and highly variable rainfall during the crop cycle. To improve grain yields of sorghum there, plant breeders have adopted the empirical approach of selecting directly for grain yield, because traits likely to confer improved adaptation have not been clearly defined. Even for readily-observed traits such as maturity type, no clear selection goal had been identified for this highly variable environment. This paper examined the use of isopopulations as a tool for defining traits for selection in plant breeding programs, and discussed the merits of this approach relative to other alternatives. Phenology of sorghum in north-eastern Australia was used as a case study. Isopopulations differing in maturity were developed from three populations of sorghum, and were grown in five contrasting locations. For stable grain yields, the best time to flowering was consistently about 1200 day-degrees using a base temperature of 7°C (or about 60 days to flowering for midsummer plantings in Central Queensland). This result was in accord with other direct experimental evidence, but contrasted with recent simulations reported in the literature, which suggested a longer crop duration was preferred. Our conclusions were that the crop model failed to extend growth duration in response to water stress, thereby increasing the yield expectation for a later flowering hybrid in a poor season. The simulations also assumed a full profile of soil water at planting in every season, which would have provided a different outcome from simulations in which conditions prior to planting were permitted to impact antecedent soil water. The strengths and weaknesses of experimental and simulation approaches were discussed, before concluding that the comprehensive evaluation of traits may best be accomplished by a combination of approaches: analysis of variety trial data, direct comparisons using isopopulations or isolines, and appropriate use of a validated crop model.     

Potential of wild germplasm for increasing yield of grain sorghum

Summary Sorghum [Sorghum bicolor (L.) Moench] backcross populations containing 3 to 50% wild germplasm were evaluated in south central India for grain yield and nine related traits. No individual BC₀F₂- to BC₂F₂-derived lines were high transgressive segregates for grain yield. Only 1.5% of all BC₃F₂- or BC₄F₂-derived lines were transgressive segragates, with 26% higher mean grain yield than their respective recurrent parents. The ten highest-yielding BC₂F₂- to BC₄F₂-derived lines per mating having parent CK60B yielded an average of 14% more than CK60B, which was, at the 5% level, a statistically significant difference. However, the increased yield was associated with increased plant height. The highest-yilding lines from RS/R/A2725 x virgatum and RS/R/A2725 x verticilliflorum were an average of 13.5% higher-yielding than RS/R/A2725 (a significant difference) and were equal in plant height. Selection increased BC₂ mean grain yields by 6 to 27%. Population mean yield, mean yield of selected lines, and frequency of high-yielding lines were highest in the BC₄.Journal paper no. 380, ICRISAT, Patancheru, India; Journal paper no. J-11114, Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, USA.     

Pattern analysis of genotype × environment interaction for striga resistance and grain yield in African sorghum trials

The parasitic weed Striga hermonthica (Del.) Benth. seriously limits sorghum [Sorghum bicolor (L.) Moench] production in Sub-Saharan Africa. As an outbreeder, S. hermonthica is highly variable with an extraordinary capacity to adapt to different hosts and environments, thereby complicating resistance breeding. To study genotype x environment (G x E) interaction for striga resistance and grain yield, nine sorghum lines, 36 F₂ populations and five local checks were grown under striga infestation at two locations in both Mali and Kenya. Mean squares due to genotypes and G x E interaction were highly significant for both sorghum grain yield and area under striga severity progress curve(ASVPC, a measure of striga emergence and vigor throughout the season). For grain yield, the entry x location-within-country interaction explained most of the total G x E while for ASVPC, entry x country and entry x location-within-country interactions were equally important. Pattern analysis (classification and ordination techniques) was applied to the environment-standardized matrix of entry x environment means. The classification clearly distinguished Malian from Kenyan locations for ASVPC, but not for grain yield. Performance plots for different entry groups showed differing patterns of adaptation. The ordination biplot underlined the importance of entry x country interaction for ASVPC. The F₂ derived from the cross of the striga-resistant line Framida with the striga-tolerant cultivar Seredo was the superior entry for both grain yield and ASVPC, underlining the importance of combining resistance with tolerance in striga resistance breeding. The observed entry x country interaction for ASVPC may be due to the entries' different reactions to climatic conditions and putative differences in striga virulence in Mali and Kenya. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of suboptimal temperature on biomass production of potato populations of Andean and European origin

Summary Plants grown from seed derived by crossing conventional European S. tuberosum material were compared with plants grown from seed derived by crossing S. tuberosum with various, Andean frost resistant tuberbearing Solanum species. Biomass growth at optimal (20°/10°) and suboptimal (10°/4°) temperatures was studied.Differences in increase of fresh and dry matter were found between populations of Andean and European orgins at 10°/4°. At 20°/10° no such differences were found.At suboptimal temperature, Andean hybrids produced significantly more fresh and dry matter than European crosses in two harvests (64 and 178 days old plants). Statistically, Andean hybrids were found to produce the same amount of dry matter per day at both temperatures, over the complete growth period.Height increase and flower development were strongly depressed in European crosses under the suboptimal temperatures, but much less so in Andean crosses.The Andean material appears to be an under utilized resource in potato breeding for cool climates.     

Heritability estimates of variation for NaCl tolerance in Sorghum bicolor (L.) Moench seedlings

Summary Data on root lengths of two-week-old seedlings of 51 Sorghum bicolor (L.) Moench accessions germinated in 50, 100, 150, 175, and 200 mM NaCl solutions were used to estimate broad sense heritabilities of variation for salinity tolerance.Estimates of broad sense heritabilities ranged from 0.38 to 0.73, the highest being in 50 mM NaCl, suggesting that there is potential for improving salinity tolerance in the species through selection and breeding.     

Comparison of shoot dry weight, Na+ content and δ13C values of ari-e and other semi-dwarf barley mutants under salt-stress

Four breviaristatum (short awned and semi-dwarf) barley mutants; ari-e.1, ari-e.119, ari-e.156 and ari-e.228 were compared with other semi-dwarf mutants; Golden Promise, Alf, Pallas and Diamant along with their non-mutant parents; Bonus, Foma, Maythorpe, Bomi and Valticky, for response to salt stress. Plants were exposed to hydroponic salt treatments (NaCl at 25 and 175 mol m^-3) for 4 weeks, after which response was measured in terms of shoot dry weight, sodium content and δ¹³C. In general ari-e mutants and Golden Promise had significantly lower Na⁺ contents than the other mutants. They also had significantly more negative δ¹³C values than the other lines in stressed (175 mol m^-3 NaCl) conditions. There was a positive correlation (r = 0.71, p < 0.01) between shoot Na⁺ and δ¹³C values so that δ¹³C became less negative with increasing Na⁺ content. Shoot dry weights were compared to shoot Na⁺ and δ¹³C values. The ari-e and Golden Promise mutants showed less reduction in dry matter production in salt stress relative to the control treatment than all the other lines. The data suggest that ari-e mutants and Golden Promise are better adapted to salt stressed environments than the other lines examined. Tests for gibberellic acid sensitivity revealed that ari-e mutants and Golden Promise responded weakly to GA₃, while other dwarf mutants Pallas, Diamant and Alf along with their parents Bonus, Foma, Maythorpe, Valticky and Bomi were highly sensitive. Our results support previous findings that ari-e mutants and the GPert mutant are allelic. This revised version was published online in July 2006 with corrections to the Cover Date.     

In vitro screening and field evaluation of tissue-culture-regenerated sorghum (Sorghum bicolor (L.) Moench) for soil stress tolerance

Summary Sorghum bicolor (L.) Moench is generally quite sensitive to salt and acid (high aluminium) soil stresses, but quite tolerant of drought stress. As with any stress phenomenon, intra-specific variability exists within the genus. In vitro cell selection and somaclonal variation offer an alternative to traditional breeding methodology for generating improved breeding lines for hybrid development. A field selection protocol was developed for the three soil stresses and inter-stress evaluations were conducted in an effort to find multiple, stress-tolerant genotypes. The acid soil-drought stress, super-tolerant selections were located by the R₇ generation when exposed to a combined aluminium-drought stress field environment and when the regeneration population (number of regenerated lines from one callus source) was maintained at 15,000 plants or higher. A variant frequency of 0.1 to 0.2% for stress tolerance and acceptable agronomic traits among the surviving somaclones, provided an adequate number of phenotypes with desirable agronomic characteristics and a high level of soil stress tolerance. Subsequent research verified that the stress-tolerant regenerants had superior acid soil and drought stress tolerance to that of the donor parents, that their yield capabilities under stress were superior to their parents, and that their stress tolerance attributes were transferred in hybrid combinations. In vitro selection was not effective in increasing the number of field stress survivors. In fact, superior germplasms were developed from non-stressed callus or salt-stressed callus. In vitro selection reduced regeneration frequency and subsequent survival of plants under field stress. In vitro-stressed regenerants should be subjected only to non-stressed environments to maintain population numbers for field selection and thereafter should be subjected to stress environments during later (R₅₊) generations. The optimal strategy for the exploitation of somaclonal variation may be through short-term cell culture (< 12 months) with no attempt at in vitro selection.