Genetic analysis and selection for wheat yield in drought-stressed and irrigated environments

Summary Wheat (Triticum aestivum L.) cultivars grown in the southern Great Plains of the U.S.A. are exposed to a wide range of moisture conditions due to large fluctuations in the amount and frequency of rainfall. Yield stability under those conditions is therefore a desirable trait for wheat breeders. Our primary objective was to quantify various genetic parameters for grain production in drought-stressed and irrigated environments. We also attempted to predict and measure yield responses when selection is practiced in either drought-stressed or irrigated environments, or both. Seventy F₂-derived lines from the cross, TAM W-101/Sturdy, were evaluated at Goodwell, OK, under irrigated and naturally drought-stressed conditions in 1987 and 1988. Genetic variance and heritability estimates were higher in the irrigated environment than in the drought-stressed environment. The genetic correlation coefficient for yields in the two environments was 0.20±0.16, indicating that selection of widely adapted genotypes requires testing in both environments. Based on the genetic variance/covariance structure of this particular population, the linear index which maximized the combined expected gain in both environments was 0.66Y₁ + 0.34Y₂, in which Y₁ and Y₂ are yields in the irrigated and drought-stressed environments. This index is not expected to apply across all populations; rather, it further supports the hypothesis that testing in either environment alone (drought stressed or irrigated) may not be most effective for increasing either mean productivity or yield under drought stress.     

Use of Triticum tauschii to improve yield of wheat in low-yielding environments

Triticum tauschii (Coss.) Schmal. is an ancestor of bread wheat (T. aestivum). This species has been widely used as a source ofsimply-inherited traits, but there are few reports of yield increases due tointrogression of genes from this species. Selections from F₂-derivedlines of backcross derivatives of synthetic hexaploid wheats (T.turgidum / T. tauschii) were evaluated for grain yield in diverseenvironments in southern Australia. Re-selections were made in theF₆ generation and evaluated for grain yield, yield componentsincluding grain weight, and grain growth characters in diverse environmentsin southern Australia and north-western Mexico. Re-selection was effectivein identifying lines which were higher yielding than the recurrent parent,except in full-irrigation environments. Grain yields of the selectedderivatives were highest relative to the recurrent parent in thelowest-yielding environments, which experienced terminal moisture deficitand heat stress during grain filling. The yield advantage of the derivativesin these environments was not due to a change in anthesis date orgrain-filling duration, but was manifest as increased rates of grain-filling andlarger grains, indicating that T. tauschii has outstanding potential forimproving wheat for low-yielding, drought-stressed environments.     

CIMMYT's approach to breeding for wide adaptation

Summary The wheat area in developing countries, including China, is around 100 million ha. To address the needs of these very diverse wheat growing areas, CIMMYT has defined 12 wheat mega-environments (ME). A ME is defined as broad, not necessarily continuous often transcontinental area with similar biotic and abiotic stresses, cropping systems and consumer preferences. The factors describing each ME are presented.CIMMYT's breeding methodology is centered around the development of widely adapted germplasm with high and stable yield across a wide range of environments. Segregating populations are alternating screened in two diverse environments in Mexico. One key requirement is that all germplasm is tested under near optimum conditions for its yield potential. The second one is multi-locational testing of advanced lines at sites that represent a given ME (key locations) and careful screening of germplasm for tolerance to abiotic and biotic stresses specific to that environment. This methodology has permitted the pyramiding of a large number of multiple resistance genes for use against a wide spectrum of diseases and tolerance to abiotic stresses within each ME. In addition, the widespread testing of lines allows the identification of traits which are beneficial in several environments. Data from international nurseries are used to further delineate environments within an ME. This approach has proven to be successful since around 70% of the spring wheat area in developing countries (excluding China) is planted to varieties derived directly or indirectly from CIMMYT germplasm. The performance of the bread wheat cultivar Pastor in international trials is given as an example for a wide adaptation.     

Selection environment and environmental sensitivity in barley

Summary Groups of 10 barley genotypes were selected for high grain yield under either high yielding (two groups) or low yielding conditions (two groups). The genotypes had a similar average grain yield across a wide range of yielding conditions, but differed in their linear response over environments (environmental sensitivity). The genotypes selected for high grain yield under low yielding conditions were less sensitive to changing environments than genotypes selected for high grain yield under high yielding conditions. The higher stability of genotypes selected under low yielding conditions was shown by both the linear regression analysis and the comparison of coefficients of variation. The use of a safety-first index showed that the probability of a crop failure of genotypes selected for high grain yield under high yielding conditions was between 1.8 and 2.7 times higher than for genotypes selected for high grain yield under low yielding conditions. The development of new cultivars for areas where a large proportion of the crop is grown by subsistence farmers should therefore be based on selection under low yielding conditions.     

Analysis of adaptation of barley,triticale, durum and bread wheat under Mediterranean conditions

Summary Yield data obtained from a comparative small grain cereals trial, grown for five consecutive growing seasons at a total of 23 environments in Cyprus, were subjected to regression analysis. Within each environment, yield trials consisted of a standard set of three cultivars or elite lines of barley, triticale, durum and bread wheat. The regression coefficient (b) of crop mean on the environmental index (I) and the mean square deviation from regression (sd²) were calculated for each crop. Each crop tended to have its own characteristic value of sd² and its magnitude was an excellent indicator of specific crop-environment interaction. The causes of large sd², for two of the four crops, were the susceptibilith of barley to lodging, when favourable conditions were encountered at high yielding environments, and triticale dependence on late season precipitation. Durum wheat and triticale had an average response to different yielding environments (b>1.19) and both were significantly different from those of bread wheat (1.08) and barley (0.54). Hence, barley, bread and durum wheat are specifically adapted to low, average and high yielding Mediterranean environments, respectively. The cultivation of triticale at the expence of durum wheat is not feasible. Furthermore, interactions between crops and environments demonstrated by the regression parameters, should constitute the basis for decision making, regarding crop adaptation in a region. The average yield in all environments should not be considered as a proper criterion for adaptation. In this study, triticale had a similar mean grain yield (3,842 kg/ha) to that of bread wheat, but was significantly higher yielding than barley or durum wheat (5 and 7%, respectively).     

CIMMYT's approach to breed for drought tolerance

Summary About 32% of the 99 million ha wheat grown in developing countries experiences varying levels of drought stress. Three major drought types have been identified: Late drought (LD) is common in the Mediterranean region, early drought (ED) is found in Latin America and wheat is produced on residual soil moisture (RM) in the Indian subcontinent and part of Australia. Until 1983, CIMMYT selected all germplasm under near optimum conditions for its yield potential and tested only advanced lines under drought. In spite of many critics, this approach proved to be successful, since in the mid 80's CIMMYT germplasm was grown on 45% of the wheat area in LC with annual rainfall from 300–500 mm and on 21% in areas with less than 300 mm. Since 1983, CIMMYT's drought breeding methodology is to alternate segregating populations between drought stressed and fully irrigated conditions (FI) and to test advanced lines under a line source irrigation system. To compare the efficiency of these approach, yield of four, mostly leading varieties, from each of the regions with LD, ED, RM, and FI and twelve recent CIMMYT cultivars selected for high yield under FI and RM conditions (ALT) were compared under four different moisture regimes (FI, LD, ED, and RM) in 89–90 and 90–91 in Yaqui Valley, Mexico. Genotypic correlation between yield and days to flowering, days to maturity, height, grains m^-2, TKW, test weight and grain fill period were calculated.Mean grain yield of the four best lines in the ALT group was highest under all moisture stress regimes, followed by the FI-group. However, the highest yielding cultivar within each moisture regime was from the FI-group under FI, from the LD-group under LD, and from the ALT-group under ED and RM conditions. Estimates for genetic advance suggest that FI is the best environment for increasing grain yield even in all three drought environments. This indicates that yield potential per se is beneficial also in drought environments. The highest yield in drought environments was realized by the CIM cultivars selected under FI and RM. Simultaneous evaluation of the germplasm under near optimum conditions, to utilize high heritabilities and identify lines with high yield potential, and under stress conditions to preserve alleles for drought tolerance seem at present the best strategy.     

Specific adaptation of barley varieties in different locations in Ethiopia

Barley is one of the most important cereal crops grown for the livelihoods of the poor farmers of Tigray region in northern Ethiopia. As many low input and marginal environments it has benefited less from the yield increases achieved by modern breeding. This has been largely attributed due to genotype × environment intraction (GEI). To investigate the causes of GEI, ten barley varieties including local checks (two farmers developed varieties, four modern varieties and three rare local varieties) were tested over 21 environments. Participatory methods were applied to sample an adequate number of environments spanning the regional diversity. The yielding ability and stability of the varieties was graphically depicted by GGE and PLSR biplot. There were two major groups of environments, the central and northern highlands, the latter with less rainfall and poorer soils. Rainfall per month and total nitrogen level were the environmental variables that differentiated these two groups. In Tigray, rainfall in June and July were negatively correlated with yield, reflecting waterlogging problems. The different varieties were either specifically or widely adapted across the two environments. The variety ‘Himblil’, originating in Tigray, was the highest yielding and also most stable in the region of origin. However, it was inferior to improved varieties (Shege and Dimtu) at high yield levels. The association of earliness with grain yield indicates that the trait can be effectively manipulated within the existing materials. We recommend breeding for drought/water logging resistance based on selection in the target environment as the best strategy to provide stable and high yielding varieties for Tigray.     

Introgression of resistance to late blight (Phytophthora infestans) from Solanum pinnatisectum into S. tuberosum using embryo rescue and double pollination

In order to investigate the agricultural potential of the genus Vicia, and identify traits associated with productivity and responsiveness to environment, 34 undomesticated Mediterranean accessions representing Section Narbonensis (V. johannis, V. narbonensis) and V. sativa were grown in five contrasting environments in northern Syria (growing season rainfall: 76–290 mm).Highly significant genotype × environment interactions were observed for all traits. For most of the components of yield, accession mean performance (productivity)was highly correlated with responsiveness across environments (r = 0.59–0.96), as defined by joint linear regressions. Thus high yielding genotypes tended to be relatively more productive than low yielding genotypes under conditions that favoured high yields. Regression analysis revealed that mean site yields were positively correlated to rainfall (r = 0.85) and its attendant effect on growing season length as measured by cumulative season temperature and phenology (r = 0.59–0.81).In order to examine yield related traits independently of taxonomy, genotypes were grouped into three categories using K-means clustering based on productivity and responsiveness of seed, hay and biological yield. Highly productive/responsive genotypes were tall with high harvest index, large seeds and low fecundity (seeds and pods per plant), whereas unproductive/unresponsive plants tended to be short, highly fecund, with small seeds and low harvest index. Principal components analysis showed that responsiveness, in terms of seed, hay and biological yields, was closely related to phenological plasticity. Thus highly productive/responsive genotypes were able to start flowering earlier than unproductive/unresponsive genotypes in early environments, but significantly later in late, higher rainfall environments. Plant growth habit was also related to yield responsiveness. In environments with little biomass production the proportion of erect plants was high in all three categories. In more favourable, high biomass environments, the proportion of erect plants in unproductive/unresponsive genotypes fell dramatically, but was unchanged among productive/responsive genotypes. We suggest that for unproductive/unresponsive genotypes competition for light is increased under optimal growth conditions. We argue that the optimal combination of fixed and responsive traits in high yielding genotypes results in a `compound interest-type' response to more favourable environments. Highly productive and responsive genotypes can capture resources more effectively than their low yielding counterparts, leading to a positive relationship between performance and responsiveness for most components of yield. Differences in productivity and responsiveness for seed, hay and biological yield reflected Vicia taxonomy, increasing in the following order from low to high: V. johannis, V. sativa, the small seeded V. narbonensis (salmonea, jordanica, affinis) V. n. var. narbonensis, and finally V. n. var.aegyptiaca. V. n. var. aegyptiaca showed the most agricultural potential, since the taxon contained all the properties of productive/responsive genotypes listed above, yielding >1 t/ha under extremely arid conditions (104 mm),and >2.5 t/ha on 290 mm rainfall, confirming its potential for dry environments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Pattern analysis on grain yield performance of Chinese and CIMMYT spring wheat cultivars sown in China and CIMMYT

Understanding the relationships among testing environments is essential for better targeting cultivars to production environments. To identify patterns of cultivar, environment, cultivar-by-environment interactions, and opportunities for indirect selection for grain yield, a set of 25 spring wheat cultivars from China and the International Maize and Wheat Improvement Center (CIMMYT) was evaluated in nine environments in China and four management environments at CIMMYT in Cd. Obregon, Mexico, during two wheat seasons. Genetic background and original environment were the main factors influencing grain yield performance of the cultivars. Baviacora M 92, Xinchun 2 and Xinchun 6 showed relatively more stable and higher grain yields, whereas highly photoperiod sensitive cultivars Xinkehan 9, Kefeng 6 and Longmai 19 proved consistently inferior across environments, except in Harbin and Keshan, the two high latitude environments. Longmai 26, also from high latitude environments in the north-eastern Heilongjiang province, was however probably not as photoperiodicly sensitive as other cultivars from that region, and produced much higher grain yield and expressed a broader adaptation. None of the environments reported major diseases. Pattern analyses revealed that photoperiod response and planting option on beds were the two main factors underlying the observed interactions for grain yield. The production environment of planting on the flat in Mexico grouped together with Huhhot and Urumqi in both wheat seasons, indicating an indirect response to selection for grain yield in this CIMMYT managed environment could benefit the two Chinese environments. Both the environment of planting on the flat with Chinese Hejin and Yongning, and the three CIMMYT environments planting on raised beds with Chinese Yongning grouped together only in one season, showing that repeatability may not be stable in this case.     

Recurrent selection in oat for adaptation to diverse environments

In order to test if selection can improve a population's adaptation to diverse environments simultaneously, three cycles of recurrent selection based on grain yield in Iowa, Idaho, and Norway were practiced in an oat (Avena sativaL.) population developed from North American, Scandinavian, and wild species (A. sterilis L.) germplasm sources. Specific objectives were to determine if selection: increased mean yields across environments and within all environments; changed the genetic correlation of yields in different environments; and changed genetic variation for yield within the population. We evaluated 100 to 210 randomly-chosen families from each cycle of selection at three Iowa locations, in Idaho, and in Norway for two years. Grain yield within each location and mean yields across locations increased significantly over cycles of selection. Mean yields across locations expressed as a percent of the original population mean increased at a rate of 2.6% per year. Several families from the third cycle population exhibited both high mean yields across locations and consistently high yields within all locations. Average genetic correlations of yield in different environments were higher in the second cycle than in the original population. A trend of reduced genetic variation and heritability was observed in Iowa only. These results suggest that we successfully improved mean population yield both within and across locations, and yield stability across environments, and in developing families with outstanding adaptation to diverse environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Durum wheat landraces from Syria

Syrian durum wheat landraces from diverse collection sites were evaluated for agronomic performance under arid conditions over two seasons at four locations, at two levels of nutrient availability.Grain yield differed considerably among locations. Within locations, significant population and fertilizer effects were demonstrated. In most cases, maximum yield was achieved by landraces, which demonstrates the breeding value of local germplasm. In a particular environment, the population effect was nonsignificant for total dry matter production, but significant for straw and grain yield. Nitrogen application was ineffective if moisture availability was the dominant growth limiting factor.Correlations between plant characteristics at evaluation and collection site characteristics were limited, and therefore, their utilization in germplasm evaluation appear limited. The negative relation between grain yield and soil nitrogen content at the collection site may be caused by the comparative disadvantage that germplasm from regions with favourable growing conditions has under low-yielding conditions.Particular environmental conditions at the collection site were no indicator for the response to changed growing conditions. Relatively high grain yields under good and adverse growing conditions excluded each other in all but two cases. Also, it was difficult to relate grain yield to regions of collection, as the response of individual landraces from a certain region to changing growing conditions differed strongly. Differentiation on the basis of landrace groups appears more promising. The Hauran landrace group provided the largest number of populations with relatively high grain yields over diverse environments.     

High yielding spring bread wheat germplasm for global irrigated and rainfed production systems

Global wheat (Triticum aestivum L.) production must increase 2% annually until 2020 to meet future demands. Breeding wheat cultivars with increased grain yield potential, enhanced water-use efficiency, heat tolerance, end-use quality, and durable resistance to important diseases and pests can contribute to meet at least half of the desired production increases. The remaining half must come through better agronomic and soil management practices and incentive policies. Analyses of the recent International Yield Trials indicate that grain yields of the best new entries were usually 10% higher than the local checks globally, as well as within a country across sites. Variation in yield across sites within a country/region underline the role of genotype × environment (GE) interaction and provides opportunities to select for stable genotypes, which is not often done. The lack of proper analysis undermines proper utilization of germplasm with high yield potential and stability in the national wheat breeding programs. Some of the best performers in irrigated areas were amongst the best in semiarid environments, reinforcing the fact that high yield potential and drought tolerance can be improved simultaneously. The best performing lines often had genotypic base of widely adapted genotypes Kauz, Attila, Baviacora, and Pastor, with genetic contributions from other parents including synthetic wheat. We recommend within country multilocation analysis of trial performance for a crop season to identify lines suiting particular or different locations within a country. The immediate feedback on GE interaction will also help in breeding lines for countries having substantial variation across locations and years.     

Land-races of wheat from the northern Negev in Israel

Summary Wheat has traditionally been grown by the Beduin population in the semi-arid (150 to 200 mm, mean total annual rainfall) northern Negev region of Israel.A collection was made in this area (the size of which is 150 km²) from small (0.1 to 0.5ha) fields of mixed wheat, resulting in 1553 collected spikes. Each spike was planted in a 1 m row at Bet Dagan, and grown under favorable conditions. Qualitative and quantitative data were collected from each row. Qualitative data were submitted to hierarchial clustering and the results were compared with published information on the identification, classification and distribution of the land-races of wheat in the Middle East. Triticum durum was represented in 84% of the collection. It was clustered into 22 populations, identified as 11 known varietas of T. durum. They were aggregated into five groups, similar to groups of old varieties recognized by Jacubziner (1932). While 38.5% of the collection consisted of T. durum groups villosa and sinaica, aboriginal to the northern Negev, it included also forms similar to several land-races found in the past in other parts of the Middle East. Each of the populations, and the durum collection as a whole, was very diverse for the quantitatively measured plant attributes. Triticum aestivum was represented in 15.6% of the collection, clustered into six populations. Most of the common wheat accessions were analogous to the old locally grown variety Hirbawi. Triticum compactum was represented in only eight accessions.The collection is now being evaluated as a potential genetic resource for durum wheat breeding.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel No. 374-E, 1982 series.     

Modelling wheat yield change under CO2 increase,heat and water stress in relation to plant available water capacity in eastern Australia

Increasing heat and water stress are important threats to wheat growth in rain-fed conditions. Using climate scenario-based projections from the Coupled Model Intercomparison Project phase 5 (CMIP5), we analysed changes in the probability of heat stress around wheat flowering and relative yield loss due to water stress at six locations in eastern Australia. As a consequence of warmer average temperatures, wheat flowering occurred earlier, but the probability of heat stress around flowering still increased by about 3.8%–6.2%. Simulated potential yield across six sites increased on average by about 2.5% regardless of the emission scenario. However, simulated water-limited yield tended to decline at wet and cool locations under future climate while increased at warm and dry locations. Soils with higher plant available water capacity (PAWC) showed a lower response of water-limited yield to rainfall changes except at very dry sites, which means soils with high PAWC were less affected by rainfall changes compared with soils with low PAWC. Our results also indicated that a drought stress index decreased with increasing PAWC and then stagnated at high PAWC. Under high emission scenario RCP8.5, drought stress was expected to decline or stay about the same due to elevated CO₂ compensation effect. Therefore, to maintain or increase yield potential in response to the projected climate change, increasing cultivar tolerance to heat stress and improving crop management to reduce impacts of water stress on lower plant available water holding soils should be a priority for the genetic improvement of wheat in eastern Australia.     

Recurrent selection for broad adaptation affects stability of oat

A recurrent selection program for adaptation to diverse environments was successful in improving mean oat (Avena sativa L.) grain yield within and across testing environments. The objectives of this research were to determine if this selection program also resulted in changes in other agronomic traits or altered yield stability. Additionally, we investigated how selection modified the response of genotypes to climatic conditions. We evaluated random samples of 100 families from the original population and each of three selection cycle populations in replicated yield trials in Idaho, Iowa, and Norway for two years. Yield stability was assessed via joint regression analysis and superiority analysis. For each cycle, genetic relationships among yields observed indifferent environments were assessed by estimating phenotypic correlations between pairs of target environments. The effect of climate variables on genotype-by-environment interaction (GEI) responses was determined with partial least squares regression. Selection resulted in a small increase in mean heading date, a decrease in mean test weight, and no change in total above-ground biomass or plant height. Genotypic regression coefficients on environmental indices and deviations from regression were larger in the last cycle population, but superiority analysis demonstrated that selection significantly improved the adaptability of the population to the target testing environments. Improved adaptation was also demonstrated by increased phenotypic correlations among the most divergent pairs of environments in the later cycles. Partial least squares regression of GEI effects on climate variables suggested that later cycle families tended to respond more favorably to cooler than average conditions than the original population. Selection resulted in improved yield stability as well as improved mean yield. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plant traits related to yield of wheat in early, late, or continuous drought conditions

Bread wheats (Triticum aestivum L.) were evaluated for plant characteristics contributing to grain yield and plant adaptation under various drought patterns. The usefulness of these traits as explicit selection criteria in developing drought tolerant wheat varieties was investigated in three experiments. Cultivars from four germplasm groups, representing the four relevant major and distinct global wheat growing environments, were grown under the respective simulated early, late, continuous and no drought conditions by manipulating irrigation in north western Mexico. Additionally, 560 advanced lines from the CIMMYT breeding program were grown under late drought conditions, and 16 randomly selected advanced genotypes were studied in more detail under late and no drought conditions. In these three studies, the association between yield in drought-stressed environments and yield in non drought-stressed environments was interpreted to reflect genotypic high yield potential, mainly by way of high biomass development. However, yield potential only partly explained the superior performance under drought. For each pattern of drought stress, particular and often different plant traits were identified that further contributed specific adaptation to the distinct drought stress conditions. Knowledge of these traits will be useful for developing CIMMYT germplasm for specific drought-stressed areas. Ultimately, these studies demonstrate that both yield potential and specific adaptation traits are useful criteria in breeding for drought environments, and should be combined to achieve optimum performance and adaptation to drought stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

The global adaptation of bread wheat at high latitudes

Spring sown bread wheat is grown at high latitudes in Europe, Asia and North America. However, it is not clear what the associations are among environments, particularly in Asia and North America, and whether or not cultivars developed in one region may adapt in another. A yield trial comprised of cultivars developed in northern Kazakhstan, western Siberia, the Canadian Prairies, northern USA, northeastern China and broadly adapted genotypes bred by CIMMYT in Mexico was planted in all the above mentioned environments in 2002–2004. In general, cultivars performed best within the regions they were developed. However, cultivars developed in northern Kazakhstan/western Siberia were the most broadly adapted at high latitudes; they were not significantly different for grain yield from the locally developed cultivars in both China and Canada. Stronger photoperiod response, greater plant height and larger seed weight appeared to be key adaptive features of these materials. At lower latitudes, the Kazakh/Siberian cultivars were significantly lower yielding than all other materials. When low latitude Mexican sites were removed from the analysis, the Chinese locations tended to associate, whereas most Canadian and Kazak/Siberian locations were negatively associated with those from China. SSR analysis of the cultivars from each region split the materials into two general groups, one based on North American cultivars and one comprised of Kazakh/Siberian and Chinese cultivars. Lines developed in Mexico were spread across these two groupings. Evidence suggests that considerable scope exists to improve bread wheat adaptation at high latitudes globally through intercrossing materials originating from Asia and North America.     

Daylength,Temperature and Solar Radiation Effects on the Phenology and Yield Formation of Spring Durum Wheat

Future food security will depend on crop adaptation to changing environments. We studied the limitations imposed by daylength, temperature and solar radiation on wheat yield in eight field experiments conducted at contrasting northern latitudes and involving 42 adapted spring durum wheat genotypes of divergent phenology, and reduced or without photoperiod sensitivity. Air temperatures averaged from sowing to anthesis (SA) increased from northern to southern sites, while daylength and minimum temperatures from anthesis to maturity (grain filling, GF) followed the opposite trend, due to differences in the latitude of sites. The site effect explained 96 % of the variation in the number of days SA, which was much smaller in southern sites. Average minimum daily temperatures above 6.9 °C before anthesis and below 10.8 °C during GF accompanied by photoperiods during GF of less than 14.2 h resulted in less than 14 000 kernels m^?2, which was the threshold below which kernel number limited yield. Radiation during GF lower than 1.8 kJ kernel^?1 day^?1 limited kernel weight, which was then a constraint to the achievement of yield potential.     

Characterization of resistance of cassava genotypes to bacterial blight by evaluation of leaf and systemic symptoms in relation to yield in different ecozones

Twenty-two improved and local cassava genotypes were evaluated for their bacterial blight symptom types in reaction to infection by Xanthomonas axonopodis pv. manihotis under field conditions in the forest, forest savanna transition and wet savanna zones of Togo. High genotype × environment interactions in development of each symptom type were observed. Combining data on environments and genotypes, spot, blight and wilt symptoms were positively correlated. Analysing genotype reactions across environments, indications for independent mechanisms of resistance on leaf and stem level, varying by genotype, were found. Genotypes Main27 with resistance to spot and blight symptoms and TMS4(2)1425 with resistance to wilt symptoms are recommended to breeders to introgress their resistance characteristics. Significant negative correlations were generally observed between blight and wilt symptom development and root yield across ecozones, with blight being more important under lower, and wilt under higher inoculum pressure. Genotypes TMS30572, CVTM4, TMS92/0429 and TMS91/02316 showed low spot, blight and wilt symptoms combined with high root yield across ecozones.