Hybrid performance of sorghum and its relationship to morphological and physiological traits under variable drought stress in Kenya

Sorghum, Sorghum bicolor L. Moench, is grown mostly in semi-arid climates where unpredictable drought stress constitutes a major production constraint. To investigate hybrid performance at different levels of drought stress, 12 single-cross hybrids of grain sorghum and their 24 parent lines were grown in eight site-season combinations in a semi-arid area of Kenya. In addition, a subset of 20 genotypes was evaluated at the seedling stage under polyethylene glycol (PEG)-induced drought stress. Environmental means for grain yield ranged from 47 to 584 g/m²reflecting the following situations: two non-stress, one moderate pre-flowering, four moderate terminal and one extreme drought stress. Mean hybrid superiority over mid-parent values was 54% for grain yield and 35% for above-ground biomass. Across environments, hybrids out-yielded two local varieties by 12%. Differences in yield potential contributed to grain yield differences in all stress environments. Early anthesis was most important for specific adaptation to extreme drought. Field performance was not related to growth reduction and osmotic adjustment under PEG-induced drought stress. In conclusion, exploitation of hybrid vigour could improve the productivity of sorghum in semi-arid areas.     

Fluorimetric Determination of the Genetic Variability Existing for Chilling Tolerance in Sweet Sorghum and Sudan Grass

Chilling temperatures drastically inhibited the photochemical quenching of chlorophyll fluorescence (q_Q) measured in intact leaves photosynthesizing under steady-state conditions. This effect appeared, however, to be characteristic of chilling-susceptible plant species and was not observed in plants which are known to be chilling-tolerant, indicating that the measurement of q_Q can serve in practice to estimate rapidly the relative chilling tolerance of crop plants. A large number (28) of sweet sorghum (Sorghum bicolor) and Sudan grass (Sorghum sudanense) genotypes were screened for chilling tolerance using this rapid q_Q method. Although sweet sorghum and sudan grass obviously behaved as chilling-sensitive plants, a considerable variation for chilling susceptibility was observed among the different genotypes tested. Some sweet sorghum varieties, such as ‘Dale’ and ‘Keller’, and most of the sudan grass varieties appeared to possess a certain degree of resistance towards low temperature stress, indicating the existence of useful germplasms in Sorghum for improving stress tolerance.     

Quantitative-genetic parameters of sorghum growth under striga infestation in Mali and Kenya

To estimate quantitative‐genetic parameters of sorghum for resistance to the hemi‐parasitic weed striga [Striga hermonthica (Del.) Benth.] and for agronomic traits, 36 diallel F₂ populations and their nine parental lines were evaluated under severe striga infestation at two locations each in Mali and Kenya. Location means for grain yield ranged from 132 to 254 g/m². F₂ populations outyielded lines on average by 18%. For striga emergence traits, F₂ heterosis values ranged from ‐36% to 232% among populations. Genetic and genotype x environment interaction variances of lines and F₂s were highly significant for all traits. Broad‐sense heritabilities for areas under striga severity progress curves and grain yield were 0.83 and 0.90 in lines, and 0.81 and 0.89 in F₂s, respectively. General and specific combining ability, and their interaction effects with locations were significant for most traits. F₂ superiority for grain yield under striga infestation demonstrates the potential merit of heterozygous cultivars in the target areas. Significant genotype x environment interaction entails multilocational testing to identify stable resistance. A combination of resistance with striga tolerance is recommended to breeders.     

Mapping quantitative trait loci for flag leaf senescence as a yield determinant in winter wheat under optimal and drought-stressed environments

The timing of flag leaf senescence (FLS) is an important determinant of yield under stress and optimal environments. A doubled haploid population derived from crossing the photo period-sensitive variety Beaver,with the photo period-insensitive variety Soissons, varied significantly for this trait, measured as the percent green flag leaf area remaining at 14 days and 35 days after anthesis. This trait also showed a significantly positive correlation with yield under variable environmental regimes. QTL analysis based on a genetic map derived from 48 doubled haploid lines using amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers, revealed the genetic control of this trait. The coincidence of QTL for senescence on chromosomes 2B and 2D under drought-stressed and optimal environments, respectively, indicate a complex genetic mechanism of this trait involving the re-mobilisation of resources from the source to the sink during senescence. This revised version was published online in July 2006 with corrections to the Cover Date.     

The variation of relative water content,SPAD chlorophyll meter reading,stomatal conductance,leaf area,and specific leaf area of Jerusalem artichoke genotypes under different durations of terminal drought in tropical region

This work assessed the responses of Jerusalem artichoke (JA) genotypes experienced to different durations of terminal drought for relative water content (RWC), SPAD chlorophyll meter reading (SCMR), stomatal conductance (SC), leaf area (LA) and specific leaf area (SLA), investigated the significant contributor of physiological traits to yield and yield reduction and diversity of physiological traits in terminal drought tolerance genotypes. Field research was conducted during the late-rainy season for 2 years using a split-plot design with four replications. The three irrigation treatments involved no-drought (SD0), short (SD1) and long (SD2) durations of terminal drought were arranged as main plots, and sub-plots were six genotypes of JA. The current study revealed that RWC, SC, LA, and SLA, were drastically reduced whilst SCMR values slightly increased under SD1 and SD2 compared to SD0. According to multiple linear regression indicated that SLA (SD0), and RWC (SD1 and SD2) had high contributions to tuber fresh weight (TFW), whereas SC had a high contribution to TFW reduction under terminal drought. Principal component analysis also confirmed that the diversity of LA, RWC, and SLA had maximized contributing traits, followed by HI, SCMR and SC in these breeding materials of JA under three irrigation treatments. Our finding emphasized that JA125 (low SLA), HEL256 and JA37 (high SLA) lead to the high performance of TFW under SD0, and they exhibited high TFW under SD1 by performing medium RWC. Likewise, JA4 and JA37 had high TFW under SD2 caused by medium RWC. HEL253 and JA60 were identified as terminal drought-tolerant genotypes because of performed medium SC, which contributed to a low reduction in TFW. Our promising results are the basis for further studies, SLA, RWC and SC are an important keys for screening drought tolerance (low economic yield reduction) and these genotypes studied could be used for parental lines to improved drought tolerance progenies through breeding programs under different durations of terminal drought-prone areas.     

QTL mapping for heading date,leaf area and chlorophyll content under cold and drought stress in two related recombinant inbred line populations (Japonica rice) and meta‐analysis

Rice is highly susceptible to drought and cold. The goal of this study was to identify the QTLs affecting the rice heading date (HD), leaf area (LA) and chlorophyll content (CC) under cold and drought stress. A total of twenty‐nine and thirty‐eight additive QTLs were detected from two crosses of ‘Dongnong422’/‘Kongyu131’ and ‘Xiaobaijingzi’/‘Kongyu131’, respectively. qHD1‐7‐1, qHD1‐7‐2, qHD1‐2‐1, qLA1‐7‐1, qLA1‐6‐3 and qCC1‐7‐1 show adaptive effects under cold treatment, while qHD2‐2‐3, qHD2‐2‐2, qLA2‐7‐3 and qCC2‐5‐1 were important for explaining the genetic mechanism during drought tolerance. Additionally, nine and five additive × environment interaction QTLs were detected for two RILs, respectively. RIL26 and RIL73 were two lines that are associated with cold and drought for HD. 339 QTLs related to HD, CC and LA of rice from database and our research were projected onto the genetic map. Nine cloned genes and nineteen homologous candidate genes related to HD, CC, cold tolerance and drought tolerance were mapped by meta‐analysis. These results lay the foundation for the fine mapping of QTLs related to HD, LA and CC for marker‐assisted selection.     

Cold tolerance during juvenile development in sorghum: a comparative analysis by genomewide association and linkage mapping

Improved cold tolerance during the juvenile phase is a major breeding goal to develop new sorghum cultivars suitable as an alternative energy crop in temperate regions. The objectives of this study were to identify marker‐trait associations for cold tolerance in a sorghum diversity panel fingerprinted with 2620 single nucleotide polymorphism (SNP) markers and to detect quantitative trait loci (QTL) in two F_2:3 populations. Traits of interest were dry matter growth rate (DMGR), leaf appearance rate (LAR), chlorophyll content (SPAD) and chlorophyll fluorescence (F_v′/F_m′ and Ф_PSII) in relation to temperature. The association panel comprised 194 genotypes, while the F_2:3 populations consisted of 80 and 92 genotypes. All populations were tested under controlled conditions in a minimum of four temperature regimes ranging from 9.4°C to 20.8°C. QTL were identified for means across environments and regression parameters describing temperature effects. Several marker‐trait associations for mean (m) DMGR, base temperature (T_b) of SPAD and Ф_PSII and temperature effect on LAR were validated by QTL detected in population 1 or 2. Promising QTL regions were found on chromosomes SBI‐01, SBI‐02, SBI‐03, SBI‐04, SBI‐06 and SBI‐09, among them genomic regions where candidate genes involved in the C‐repeat binding pathway or encoding cold‐shock proteins are located.     

Unravelling the relationship between a seasonal environment and the dynamics of forage growth in grazed swards

It is well reported in the scientific literature that pastures can have similar net forage accumulation when managed with contrasting structures. However, we hypothesized that the dynamics of forage accumulation in grazed swards is linked to seasonal-environmental conditions. Marandu palisadegrass (Brachiaria brizantha [Hochst. ex A. Rich.] was used as the forage species model. The experimental treatments were four grazing heights (10, 20, 30 and 40 cm) allocated to experimental units according to a randomized complete block design with four replicates and evaluated throughout four contrasting environmental seasons (summer, autumn, winter–early spring and late spring). Under rainy and warm periods, greater net forage accumulation was observed in pastures maintained taller; on the contrary, during the mild and dry periods, net forage accumulation rate reduced as grazing height increased. Such patterns of responses were related to compensations between tiller population density and tissues flows during summer and late spring and the reduced capacity of taller canopies to compensate lower population with greater growth rates during autumn and winter–early spring. Grazed swards changed their patterns of forage growth as they transitioned from favourable to more abiotic stressful conditions, suggesting that seasonal adjustments in grazing intensities are necessary in order to maximize forage production.     

The G locus in Petunia hybrida Vilm: negative relationship to endogenous chlorophyll,starch and sugar

Suumary Three sets of Petunia hybrida Vilm. breeding lines with each set comprised of three genotypes, multiflora (gg), grandiflora (GG), and the heterozygote (Gg) were used in biochemical analyses to determine the possible mode of action of alleles at the G locus and their pleiotropic effects. Chlorophyll extractions did not correlate with visual observations that seedlings of the gg genotype have darker green foliage than seedlings of the GG genotypes. Likewise, chlorophyll, starch and sugar content were not related to greater seedling vigor, an apparent pleiotorpic effect to the g allele. Inconsistent differences in total soluble sugars and starch content were found with respect to the three genotypes. A qualitative difference in leaf sugar profiles as revealed by thin-layer and gasliquid chromatography separation was found. The Gg and gg genotypes consistently exhibited two extra bands at R_f 0.01 and at 0.52 to 0.56 which were absent in extracts from the GG genotype. The unknown compounds did not co-chromatograph with several sugar standards.     

Stay green trait: variation, inheritance and its association with spot blotch resistance in spring wheat (Triticum aestivum L.)

One thousand four hundred and seven spring wheat germplasm lines belonging to Indian and CIMMYT wheat programs were evaluated for stay green (SG) trait and resistance to spot blotch caused by Bipolaris sorokiniana during three consecutive crop seasons, 1999–2000, 2000–2001 and 2001–2002. Disease severity was recorded at six different growth stages beginning from tillering to late milk stage. SG trait was measured by following two approaches: difference for 0–9 scoring of green coloration (chlorophyll) of flag leaf and spike at the late dough stage (GS 87) and a new approach of leaf area under greenness (LAUG). Germplasm lines showed a wide range (7–89) for LAUG and were grouped into four viz., SG, moderately stay green, moderately non-stay green and non-stay green (NSG). However, very few (2.2%) lines showed high expression of SG trait, i.e., LAUG >60. LAUG appeared to be a better measure of SG trait than a 0–9 scale. Mean spot blotch ratings of SG genotypes were significantly lower than those of NSG genotypes at all growth stages. Two spot blotch resistant genotypes (Chirya 3 and Chirya 7) having strong expressions of SG trait were crossed with NSG, spot blotch susceptible cv. Sonalika. Individually threshed F₂ plants were used to advance the generations. SG trait and spot blotch severity were recorded in the parents and F₁, F₃, F₄, F₅, F₆ and F_6–7 generations under disease-protected and inoculated conditions. SG trait in the F₁ generation was intermediate and showed absence of dominance. Evaluation of progenies (202–207) in the segregating generations revealed that SG trait was under the control of around four additive genes. Lines homozygous for SG trait in F₄, F₅, F₆ and F_6–7 generations showed significantly lower mean area under disease progress curve (AUDPC) for spot blotch than those with NSG expression. A positive correlation (0.73) between SG trait and AUDPC further indicated a positive influence of SG on severity of spot blotch. The study established that variation for SG trait exists in spring wheat; around four additive genes control its inheritance in the crosses studied and there is positive association between SG trait and resistance to spot blotch.     

Expression of the limited‐transpiration trait under high vapour pressure deficit in peanut populations: Runner and virginia types

Drought can be a critical limitation on peanut yield. A physiological trait that may help to ameliorate drought is limited transpiration (TR_lim), defined as a limitation on further increases in transpiration rate (TR) under high vapour pressure deficit (VPD) conditions. The advantage of the TR_lim trait is that it allows plant water conservation to increase soil water availability for use during late‐season drought. While this trait has been identified in peanut, there is no information of how readily the trait may be transfer to progeny lines. The objective of this study was to obtain preliminary information on the expression of the TR_lim trait in two peanut progeny populations. One population was runner type of 88 RILs derived from the mating of Tifrunner × NC 3033. The second set was selected for the virginia‐type phenotype of large pods obtained from mating of PI 585005 (ICGV 86015) and N0808olJCT, both of which expressed the TR_lim trait. A two‐tier screen was applied to both populations. The initial screen was based on exposure of de‐rooted shoots to silver ions. Fifteen runner type and 12 virginia type were selected for direct measures of transpiration response to varying VPD. The results from each of the two populations showed that an effective expression of the TR_lim trait occurred in about 30% of the progeny in each population. While these results do not offer a definitive index of inheritance, they do indicate that there appears to be a strong possibility of transferring the TR_lim trait to progeny genotypes.     

Relationships between cold- and salt-tolerance during seed germination in tomato: Germplasm evaluation

Thirty tomato accessions representing six Lycopersicon species were evaluated for the rate of seed germination under no stress, cold-stress and salt-stress conditions. Most accessions responded similarly to both cold- and salt-stress conditions (i.e. they were equally sensitive or tolerant to both stresses), however, a few accessions exhibited more sensitivity (or tolerance) to one stress than the other. In addition, some accessions that germinated relatively rapidly under non-stress conditions exhibited great sensitivity to both cold stress and salt stress. Across accessions, significant (P < 0.01) positive phenotypic correlations were observed between germination rate under control and cold stress (r_P= 0.89), control and salt stress (r_P= 0.63) and cold stress and salt stress (r_P= 0.77). The results indicate that the rate of tomato seed germination under non-stress, cold- and salt-stress conditions may be controlled by the same genes (or physiological mechanisms), but additional components may be involved which affect germination rate under specific stress conditions.     

The relationship between the regeneration system and genetic variability in the cucumber (Cucumis sativus L.)

Durum wheat has been subjected to intense breeding in Italy due to its local economic importance. Four groups of Italian cultivars representative of different breeding eras were compared in northern Syria for yield potential and morphophysiological features at a moderately favourable site, and drought tolerance at a stressful site. Group 1 included indigenous landraces; Group 2 comprised genotypes selected from exotic landraces (released in 1920's–1930's); Group 3 included genotypes selected from crosses or mutagenesis involving Group 2 materials (1950's–1960's); Group 4 comprised genotypes selected from crosses between CIMMYT and Group 2 materials (from 1970's). Under moderately favourable conditions, a yield increase of 1.03 t ha–¹ was observed from Group 1 through Group 4, corresponding to a genetic gain of about 0.017 t ha–¹ per year. Such increase was only partly accounted for by a parallel increase in spike fertility and seed weight. Plant stature decreased dramatically from Group 1 to Group 4; a remarkable reduction of height was already attained in Group 3, before the introduction of dwarfing genes from bread wheat. Earliness of heading and grain filling duration increased consistently across breeding phases, the length of crop cycle remaining almost unaltered. No significant improvement of drought tolerance resulted from the breeding activity, suggesting the need to put more emphasis on selection for real stress tolerance rather than escape. Overall variation for morphophysiological traits, assessed by a principal components analysis, highlighted the great diversity among the Group 1 cultivars, while variation within Groups 3 and 4 was lower. Indigenous landraces, little used in the breeding history, as well as novel, unexploited exotic germplasm sources could contribute to broaden the crop genetic base in the region.     

Yield and grain protein concentration in bread wheat: how to use the negative relationship between the two characters to identify favourable genotypes?

A study of the relationship between grain yield (GY) and grain protein concentration (GPC) in bread wheat was carried out on a 11-year series of trials conducted by the Groupement d’Etude des Variétés Et Semences (GEVES) for the registration of new cultivars on the French National List. Values for GY and GPC came from 458 individual trials, grouped together in 21 series of bi-annual and multi-site data. The correlations between the two characters, calculated environment by environment, appeared highly variable due to high “genotype × environment” interactions for GY and GPC. The use of mean values, calculated on the 21 series of GEVES trials, enabled a better assessment of the relationship between the two characters, and an algorithm was proposed to avoid bias due to potential outliers. Using the well-assessed relationship obtained, grain protein deviations (GPDs) were defined as the standardized residuals of the regression of GPC on GY. These deviations appeared to have a partly genetic basis, as the lines with high deviations were about the same in the two independent datasets constituted by the two consecutive years of GEVES experiments. Some lines used as standards in GEVES trials obtained significant GPD for different series of bi-annual and multi-site trials, confirming the genetic origin of high-GPD. Simulations made to determine the minimum experimental design, showed that at least five sites per year for two consecutive years, were necessary to have a good assessment of the GY–GPC relationship, and hence reliable estimates of GPD.     

The relationship between grain weight and alpha-amylase in winter wheat: varietal comparison from UK field experiments

The relationship between Hagberg falling number and grain size was investigated in field-grown winter wheat varieties. Data was obtained for all varieties included in the UK Recommended List from 1997 to 2007. Overall, a negative association was found between thousand grain weight and Hagberg falling number. The negative association was detected between varieties in all years and within varieties in the majority of cases. Detailed Recommended List data from 2001 was used to examine variation in the relationship across 19 trial sites. Again, a negative association overall with both within and between variety associations was detected. However, there were also many cases where the association was not found. The association appears to be driven by both genetic and environmental factors; however, the association is not consistent enough to indicate a direct mechanistic link across all varieties.     

Drought Tolerance and Water Use of Cereal Crops: A Focus on Sorghum as a Food Security Crop in Sub‐Saharan Africa

Sub‐Saharan Africa (SSA) faces twin challenges of water stress and food insecurity – challenges that are already pressing and are projected to grow. Sub‐Saharan Africa comprises 43 % arid and semi‐arid area, which is projected to increase due to climate change. Small‐scale, rainfed agriculture is the main livelihood source in arid and semi‐arid areas of SSA. Because rainfed agriculture constitutes more than 95 % of agricultural land use, water scarcity is a major limitation to production. Crop production, specifically staple cereal crop production, will have to adapt to water scarcity and improved water productivity (output per water input) to meet food requirements. We propose inclusion and promotion of drought‐tolerant cereal crops in arid and semi‐arid agro‐ecological zones of SSA where water scarcity is a major limitation to cereal production. Sorghum uniquely fits production in such regions, due to high and stable water‐use efficiency, drought and heat tolerance, high germplasm variability, comparative nutritional value and existing food value chain in SSA. However, sorghum is socio‐economically and geographically underutilized in parts of SSA. Sorghum inclusion and/or promotion in arid and semi‐arid areas of SSA, especially among subsistence farmers, will improve water productivity and food security.     

Breeding for yield stability in unpredictable environments: single traits,interaction between traits,and architecture of genotypes

Summary Attempts to identify individual traits to use as an indirect measure for grain yield have shown some degree of success in environments where crop yields are affected by no stress or only by predictable stresses. However, analytical breeding has been largely unsuccessful in the case of a) stressful environments characterized by low yields due to high variability in the frequency, timing, duration and severity of a number of climatic stresses, and b) breeding programs where the major objective is greater yield stability defined as a reduction in the frequency of crop failures. Experimental evidence suggests that, when environmental variability is high due to unpredictable differences in frequency, timing and severity of various climatic stresses, each time different combinations of several traits are likely to confer drought resistance. Interaction among traits in determining overall crop response to variable stresses is expected to occasionally enhance the importance of a specific trait in a specific stress situation. In this context it becomes difficult to consider drought resistance as a character with its own identity in terms of inheritance.Assessment and verification of traits based on the use of isogenic lines tends to oversimplify the interactions between traits, as the approach provides information on the effect of a specific trait only in a specific genetic background.At the population level of organisation, the dilemma between selection for individual traits and specific combinations of traits can be translated into the dilemma between selection for individual genotypes and specific combinations of genotypes. The evidence that natural selection under stress conditions has not been able to identify either a single trait or a single genotype with a given adapted architecture of traits, is discussed in relation to germplasm development philosophies to stabilize yield in unpredictably stressed environments.