Physiological processes associated with genetic progress in yield potential of wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) is the most widely grown of any crop and provides one-fifth of the total calories of the world's population. Since the 1960s, increases in productivity have been achieved as a result of wide-scale adoption of Green Revolution technologies. However, in spite of growing demand, the challenges of increasing production to feed an estimated world population of 9 billion in 2050 are still considerable. Due to the increased demand, it is estimated that food production must be increased by about 50% by the year of 2050. Improving wheat productivity through developing cultivars with high yield potential and with high adaptability to specific environments is the key objective in the wheat breeding programs worldwide to fill the gap between the production and the demand. The overall aims of the present study were to: (i) investigate the physiological basis of yield potential progress from 1966 to 2009 in spring bread wheats released at the International Center for Maize and Wheat Improvement (CIMMYT) in the irrigated high potential environment of NW Mexico, (ii) investigate the physiological basis of effects of the tiller inhibition Tin1A gene on ear-fertility traits and yield potential and interactions with plant density in NW Mexico and UK environments in lines of a doubled-haploid (DH) population segregating for Tin1A/non-Tin1A alleles and (iii) identify breeding targets for new cultivars with higher yield potential. Four experiments were conducted in NW Mexico at the CIMMYT research station at Ciudad Obregon. Two of these experiments studied a set of 12 historic CIMMYT spring wheat cultivars released from 1966 to 2009 in 2008/9 and 2009/10. The other two experiments examined selected lines from a doubled-haploid (DH) population derived from a cross between CIMMYT spring wheat L14 and UK winter wheat Rialto contrasting for the presence/absence of the TinlA allele for tiller inhibition and their interaction with seed rate in 2008/9 and 2009/10. In addition, two other field experiments were conducted in the UK, one in 2008/09 at KWS UK Ltd in Thriplow, Hertfordshire and one in 2009110 at the University of Nottingham Farm, Sutton Bonington campus, Leicestershire. The plant material for both of these experiments was selected lines from the CIMMYT spring wheat advanced line Ll4 (+Tin1A allele) x UK winter wheat Rialto (-TinlA allele) DH population and the Rialto parent. In the experiment at Thriplow in 2008/09 the DH lines were examined at one seed rate and in the experiment at Sutton Bonington in 2009/10 at two seed rates. At the CIMMYT site in 2008/9 and 2009/10, a randomized complete block design was implemented with four replications for the experiments examining the CIMMYT wheat historic releases and a split-plot randomised complete block design with three replications was implemented for the experiments examining the +/- Tin1A DH lines, with three seed densities (50, 150 and 450 seeds per square metre); seed rates were randomized on main plots and eight genotypes randomized on sub-plots. At the UK site, in the KWS experiment, 24 DH lines (12+Tin1A allele) and (12-TinlA allele) from the L14 x Rialto population were used. There was only one seed rate (300 seeds m-2) and a completely randomised design in three replicates was implemented. The same 24 DH lines were examined in the experiment at the SB site, at two seed rates (40 and 320 seeds m-2) in a split plot randomised complete block design in three replicates. Seed rate was randomized on main plots and DH lines were randomized on sub-plots. In all experiments examining the DH lines of the Ll4 x Rialto population, lines were selected in pairs so that the two groups of +Tin1A and -Tin lA lines were approximately balanced for flowering time and plant height, i.e. every +TinlA line has a non-Tin1A pair with similar height and flowering date. Plots were sampled for destructive measurements of dry weight and DM partitioning and ear-fertility traits at four stages in the historic experiments at (GS3l, GS39, GS61+7d and at maturity) and at two stages in the DH population experiments (GS61+7d and at maturity). The water soluble carbohydrate (WSC) content of the stems plus attached leaf sheaths was also measured at GS61+7d and at maturity. In the historic experiments, at GS 61+14 days, a degraining treatment was implemented by removing all spikelets from one side of the ear (i.e. ca. 50% of the spikletes) in the histories experiment. Non-destructive measurements were taken for stomatal conductance, canopy temperature, fractional photosynthetically active radiation (PAR) interception and normalized difference vegetative index at various dates both pre- and post-anthesis in the historic experiments. In the experiments examining the set of 12 historic CIMMYT spring wheat releases, results showed that from 1966 to 2009 the linear rate of genetic gain in yield potential was 32 ha-1 yr-1 (0.59 % yr-1) (r = 0.76. P = 0.01). Yield progress was primarily associated with harvest index (percentage above-ground DM as grain DM) in the period from 1966 until about 1990 increasing from 43% to 49%, but deceased with year of release thereafter. A non-linear genetic gain in AGDM was evident over the 43-yr period with AGDM increasing from about 1990 from which point it increased rapidly to 2009. There was no association between genetic progress in grain yield and grain number per m2 in this set of 1 cultivars; a small increase in ears per m2 was counteracted by a decrease in grains per ear. However, grain weight tracked the improvement in yield potential over the 43-year period with a linear increase of 0.23 mg yr-1. No change was found in rachis length with plant breeding; however, number of fertile spikelets per ear decreased since about 1990 and was associated with the decrease in grains per ear. There were statistically significant differences in above-ground DM production at all growth stages and a tendency to produce more biomass during the GS31 to GS61+7d phase with year of release. No differences amongst cultivars were found in the amount of radiation intercepted by the whole canopy from GS3l to GS6l+7 days. Although not conclusive, since Bacanora was an exception to the trend and radiation-use efficiency (above-ground biomass per unit PAR interception; RUE), there was a tendency for RUE to increase with year of release which was consistent with a positive association with crop growth rate (above-ground DM per m2 per day; CGR) and the trend for an increase in biomass accumulation during the stem-elongation phase with plant breeding. Although there was a trend for an increase in biomass accumulation from GS31 GS61+7d this was counteracted by a decrease in ear DM partitioning so that ear DM per m2 at GS61+7d and grains per m2 did not change with plant breeding. Results showed that the improvement in the individual grain weight from 1966 to 2009 in this set of cultivars was associated with improvements in the grain filling rate from 1966 to ca. 1990 and in the duration of grain filling from ca. 1992 to 2009. Averaging across years, there was a significant positive association between post-flowering canopy-temperature depression and grain yield. Fractional PAR interception by the canopy layers of the ear, flag leaf and the penultimate leaf was increased with year of release since about 1990. This increase in the fractional interception of PAR correlated significantly with the grain weight and grain yield amongst the 12 cultivars. Grain growth of the cultivars in this historic set was generally sink limited rather than source limited. There was no change in source-sink balance as indicted by grain growth responses to the degraining treatment with year of release. The percentage increase in grain weight in the manipulated ears ranged amongst cultivars from 0.5 to 13.2%, but differences between cultivars in the response to degraining were not statistically significant and the responses were not correlated with year of release. The results also indicated that potential grain weight has increased with plant breeding over the 43-year period, since the final grain weight of the grains in the degrained ears increased linearly with year of release. Overall the contribution of stem WSC to grain DM growth in the current study was relatively low (4 - 18%) which is consistent with the hypothesis that under the high radiation, modern cultivars are more likely to be sink than source limited. Results of TinlA experiments showed that under the UK environment, TinlA lines produced more grain yield than non TinlA lines under high seed densities. The main yield component explaining this was grains per m2. TinlA lines had a longer rachis, a wider ear, and more total and fertile spikelets per ear than non-TinlA lines. Non-TinlA lines produced more ears per plant than TinlA lines under low seed density; however, both groups of lines had similar values of ears per plant under high seed density. Non-TinlA lines produced heavier grains than TinlA lines either under high or low seed density, and individual grain weight was not affected by the increase of seed density. Under the Mexican environment, there was a slight increase in the yield of non-TinlA compared to TinlA lines, but only at low seed density. This increase was attributed mainly to heavier individual grains in non-TinlA than TinlA lines. Although Tinl A lines produced fewer ears m-2 than non-Tin 1A lines (-13.4, -12.5 and -11.3%, under seed densities of 50,150 and 450 seed m2, respectively). TinlA lines produced more grains m-2 at all seed densities and this resulted from a longer rachis, more total and fertile spike lets per ear and thus more grains per ear. There was a reduction in the grain yield in non-Tin1A lines with increasing seed density. However. TinlA lines continued to increase grain yield with increasing seed density, so that the two groups of lines yielded similarly at high plant density. This result therefore supported the hypothesis that Tin1A lines yield relatively better under high seed density than non Tin1A lines and may have a higher economic optimum seed density. Overall, in contrast to in the UK experiments, it seemed that under the high temperature and radiation environment in NW Mexico, possessing the Tin1A allele may not be an advantage compared to the non-TinlA allele even under high seed rates. Overall, several target traits were identified for future improvement of yield potential in CIMMYT plant breeding and worldwide in the present study. Genetic progress in grain yield potential in the CIMMYT spring wheat program from 1966 to 2009 of 37 kg ha-2 yr-1 was positively associated with above ground biomass and grain weight. The amount of radiation intercepted during the stem-elongation phase did not change with breeding; however, there was an apparent tendency for RUE to be increased with year of release. Potential grain weight also tracked increases in final grain weight. Therefore target traits as selection criteria in wheat breeding programs for yield potential should include a combination of traits favouring enhanced RUE during the stem elongation and potential grain weight. In addition, in winter wheat in the UK restricted tillering with the introduction of the TinlA allele offers scope to increase grains m-2 and grain yield, associated with more fertile spikelets per ear and grains per ear, in wheat crops with established plant densities in the range ca. 150 - 200 plant m-2

Physiological processes associated with genetic progress in yield potential of wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) is the most widely grown of any crop and provides one-fifth of the total calories of the world's population. Since the 1960s, increases in productivity have been achieved as a result of wide-scale adoption of Green Revolution technologies. However, in spite of growing demand, the challenges of increasing production to feed an estimated world population of 9 billion in 2050 are still considerable. Due to the increased demand, it is estimated that food production must be increased by about 50% by the year of 2050. Improving wheat productivity through developing cultivars with high yield potential and with high adaptability to specific environments is the key objective in the wheat breeding programs worldwide to fill the gap between the production and the demand. The overall aims of the present study were to: (i) investigate the physiological basis of yield potential progress from 1966 to 2009 in spring bread wheats released at the International Center for Maize and Wheat Improvement (CIMMYT) in the irrigated high potential environment of NW Mexico, (ii) investigate the physiological basis of effects of the tiller inhibition Tin1A gene on ear-fertility traits and yield potential and interactions with plant density in NW Mexico and UK environments in lines of a doubled-haploid (DH) population segregating for Tin1A/non-Tin1A alleles and (iii) identify breeding targets for new cultivars with higher yield potential. Four experiments were conducted in NW Mexico at the CIMMYT research station at Ciudad Obregon. Two of these experiments studied a set of 12 historic CIMMYT spring wheat cultivars released from 1966 to 2009 in 2008/9 and 2009/10. The other two experiments examined selected lines from a doubled-haploid (DH) population derived from a cross between CIMMYT spring wheat L14 and UK winter wheat Rialto contrasting for the presence/absence of the TinlA allele for tiller inhibition and their interaction with seed rate in 2008/9 and 2009/10. In addition, two other field experiments were conducted in the UK, one in 2008/09 at KWS UK Ltd in Thriplow, Hertfordshire and one in 2009110 at the University of Nottingham Farm, Sutton Bonington campus, Leicestershire. The plant material for both of these experiments was selected lines from the CIMMYT spring wheat advanced line Ll4 (+Tin1A allele) x UK winter wheat Rialto (-TinlA allele) DH population and the Rialto parent. In the experiment at Thriplow in 2008/09 the DH lines were examined at one seed rate and in the experiment at Sutton Bonington in 2009/10 at two seed rates. At the CIMMYT site in 2008/9 and 2009/10, a randomized complete block design was implemented with four replications for the experiments examining the CIMMYT wheat historic releases and a split-plot randomised complete block design with three replications was implemented for the experiments examining the +/- Tin1A DH lines, with three seed densities (50, 150 and 450 seeds per square metre); seed rates were randomized on main plots and eight genotypes randomized on sub-plots. At the UK site, in the KWS experiment, 24 DH lines (12+Tin1A allele) and (12-TinlA allele) from the L14 x Rialto population were used. There was only one seed rate (300 seeds m-2) and a completely randomised design in three replicates was implemented. The same 24 DH lines were examined in the experiment at the SB site, at two seed rates (40 and 320 seeds m-2) in a split plot randomised complete block design in three replicates. Seed rate was randomized on main plots and DH lines were randomized on sub-plots. In all experiments examining the DH lines of the Ll4 x Rialto population, lines were selected in pairs so that the two groups of +Tin1A and -Tin lA lines were approximately balanced for flowering time and plant height, i.e. every +TinlA line has a non-Tin1A pair with similar height and flowering date. Plots were sampled for destructive measurements of dry weight and DM partitioning and ear-fertility traits at four stages in the historic experiments at (GS3l, GS39, GS61+7d and at maturity) and at two stages in the DH population experiments (GS61+7d and at maturity). The water soluble carbohydrate (WSC) content of the stems plus attached leaf sheaths was also measured at GS61+7d and at maturity. In the historic experiments, at GS 61+14 days, a degraining treatment was implemented by removing all spikelets from one side of the ear (i.e. ca. 50% of the spikletes) in the histories experiment. Non-destructive measurements were taken for stomatal conductance, canopy temperature, fractional photosynthetically active radiation (PAR) interception and normalized difference vegetative index at various dates both pre- and post-anthesis in the historic experiments. In the experiments examining the set of 12 historic CIMMYT spring wheat releases, results showed that from 1966 to 2009 the linear rate of genetic gain in yield potential was 32 ha-1 yr-1 (0.59 % yr-1) (r = 0.76. P = 0.01). Yield progress was primarily associated with harvest index (percentage above-ground DM as grain DM) in the period from 1966 until about 1990 increasing from 43% to 49%, but deceased with year of release thereafter. A non-linear genetic gain in AGDM was evident over the 43-yr period with AGDM increasing from about 1990 from which point it increased rapidly to 2009. There was no association between genetic progress in grain yield and grain number per m2 in this set of 1 cultivars; a small increase in ears per m2 was counteracted by a decrease in grains per ear. However, grain weight tracked the improvement in yield potential over the 43-year period with a linear increase of 0.23 mg yr-1. No change was found in rachis length with plant breeding; however, number of fertile spikelets per ear decreased since about 1990 and was associated with the decrease in grains per ear. There were statistically significant differences in above-ground DM production at all growth stages and a tendency to produce more biomass during the GS31 to GS61+7d phase with year of release. No differences amongst cultivars were found in the amount of radiation intercepted by the whole canopy from GS3l to GS6l+7 days. Although not conclusive, since Bacanora was an exception to the trend and radiation-use efficiency (above-ground biomass per unit PAR interception; RUE), there was a tendency for RUE to increase with year of release which was consistent with a positive association with crop growth rate (above-ground DM per m2 per day; CGR) and the trend for an increase in biomass accumulation during the stem-elongation phase with plant breeding. Although there was a trend for an increase in biomass accumulation from GS31 GS61+7d this was counteracted by a decrease in ear DM partitioning so that ear DM per m2 at GS61+7d and grains per m2 did not change with plant breeding. Results showed that the improvement in the individual grain weight from 1966 to 2009 in this set of cultivars was associated with improvements in the grain filling rate from 1966 to ca. 1990 and in the duration of grain filling from ca. 1992 to 2009. Averaging across years, there was a significant positive association between post-flowering canopy-temperature depression and grain yield. Fractional PAR interception by the canopy layers of the ear, flag leaf and the penultimate leaf was increased with year of release since about 1990. This increase in the fractional interception of PAR correlated significantly with the grain weight and grain yield amongst the 12 cultivars. Grain growth of the cultivars in this historic set was generally sink limited rather than source limited. There was no change in source-sink balance as indicted by grain growth responses to the degraining treatment with year of release. The percentage increase in grain weight in the manipulated ears ranged amongst cultivars from 0.5 to 13.2%, but differences between cultivars in the response to degraining were not statistically significant and the responses were not correlated with year of release. The results also indicated that potential grain weight has increased with plant breeding over the 43-year period, since the final grain weight of the grains in the degrained ears increased linearly with year of release. Overall the contribution of stem WSC to grain DM growth in the current study was relatively low (4 - 18%) which is consistent with the hypothesis that under the high radiation, modern cultivars are more likely to be sink than source limited. Results of TinlA experiments showed that under the UK environment, TinlA lines produced more grain yield than non TinlA lines under high seed densities. The main yield component explaining this was grains per m2. TinlA lines had a longer rachis, a wider ear, and more total and fertile spikelets per ear than non-TinlA lines. Non-TinlA lines produced more ears per plant than TinlA lines under low seed density; however, both groups of lines had similar values of ears per plant under high seed density. Non-TinlA lines produced heavier grains than TinlA lines either under high or low seed density, and individual grain weight was not affected by the increase of seed density. Under the Mexican environment, there was a slight increase in the yield of non-TinlA compared to TinlA lines, but only at low seed density. This increase was attributed mainly to heavier individual grains in non-TinlA than TinlA lines. Although Tinl A lines produced fewer ears m-2 than non-Tin 1A lines (-13.4, -12.5 and -11.3%, under seed densities of 50,150 and 450 seed m2, respectively). TinlA lines produced more grains m-2 at all seed densities and this resulted from a longer rachis, more total and fertile spike lets per ear and thus more grains per ear. There was a reduction in the grain yield in non-Tin1A lines with increasing seed density. However. TinlA lines continued to increase grain yield with increasing seed density, so that the two groups of lines yielded similarly at high plant density. This result therefore supported the hypothesis that Tin1A lines yield relatively better under high seed density than non Tin1A lines and may have a higher economic optimum seed density. Overall, in contrast to in the UK experiments, it seemed that under the high temperature and radiation environment in NW Mexico, possessing the Tin1A allele may not be an advantage compared to the non-TinlA allele even under high seed rates. Overall, several target traits were identified for future improvement of yield potential in CIMMYT plant breeding and worldwide in the present study. Genetic progress in grain yield potential in the CIMMYT spring wheat program from 1966 to 2009 of 37 kg ha-2 yr-1 was positively associated with above ground biomass and grain weight. The amount of radiation intercepted during the stem-elongation phase did not change with breeding; however, there was an apparent tendency for RUE to be increased with year of release. Potential grain weight also tracked increases in final grain weight. Therefore target traits as selection criteria in wheat breeding programs for yield potential should include a combination of traits favouring enhanced RUE during the stem elongation and potential grain weight. In addition, in winter wheat in the UK restricted tillering with the introduction of the TinlA allele offers scope to increase grains m-2 and grain yield, associated with more fertile spikelets per ear and grains per ear, in wheat crops with established plant densities in the range ca. 150 - 200 plant m-2

Food security through translational biology between wheat and rice

Wheat and rice are the most important food crops in agriculture providing around 50% of all calories consumed in the human diet. While both are C3 species, the evolution and domestication of wheat and rice occurred in very different environments, resulting in diverse anatomical and metabolic adaptation. This review focuses on the current understanding of their adaptation in an agronomic context. The similarities and differences between wheat and rice are discussed, focusing on traits related to phenology, photosynthesis, assimilate partitioning, and lodging resistance, these being the main abiotic drivers of yield expression in most agro‐ecosystems. Currently, there are significant knowledge gaps in the major biological processes that account not only for differential adaption among cultivars within each species, but even between the two species. By addressing what is known as well as where gaps exist in a comparative context, this review aims to highlight translational research approaches that could provide insights into the genetic improvement of both crops

Avoiding lodging in irrigated spring wheat. II. Genetic variation of stem and root structural properties

Lodging-related traits were evaluated on the CIMMYT Core spring wheat Germplasm Panel (CIMCOG) in the Yaqui Valley of North-West Mexico during three seasons (2010–2013). Genetic variation was significant for all the lodging-related traits in the cross-year analysis, however, significant G × E interaction due to rank changes or changes in the absolute differences between cultivars were identified. The inconsistences on cultivar performances across seasons particularly reduced the heritability of key characters related to root lodging resistance (anchorage strength). Target characters related to stem lodging resistance (stem strength) showed good heritability values equal or above 0.70. Positive correlations between stem strength and stem diameter and between root plate spread and root strength were found. Selecting for greater stem diameter and wall width, greater root plate spread and shorter plant height could enable breeders to increase lodging resistance by increasing stem strength, root strength and decreasing plant leverage, respectively. Achieving a lodging-proof crop will depend on finding a wider root plate spread and implementing new management strategies. Genetic linkages between lodging traits will not constrain the combination of the key lodging-trait dimensions to achieve a lodging-proof ideotype. However, strong association between stem strength and stem wall width will increase the total biomass cost needed for lodging resistance

Avoiding lodging in irrigated spring wheat. I. Stem and root structural requirements

A model of the lodging process has been successfully adapted for use on spring wheat grown in North-West Mexico (NWM). The lodging model was used to estimate the lodging-associated traits required to enable spring wheat grown in NWM with a typical yield of 6 t ha−1 and plant height of 0.7 m to achieve a lodging return period of 25 years. Target traits included a root plate spread of 51 mm and stem strength of the bottom internode of 268 N mm. These target traits increased to 54.5 mm and 325 N mm, respectively, for a crop yielding 10 t ha−1. Analysis of multiple genotypes across three growing seasons enabled relationships between both stem strength and root plate spread with structural dry matter to be quantified. A NWM lodging resistant ideotype yielding 6 t ha−1 would require 3.93 t ha−1 of structural stem biomass and 1.10 t ha−1 of root biomass in the top 10 cm of soil, which would result in a harvest index (HI) of 0.46 after accounting for chaff and leaf biomass. A crop yielding 10 t ha−1 would achieve a HI of 0.54 for 0.7 m tall plants or 0.41 for more typical 1.0 m tall plants. This study indicates that for plant breeders to achieve both high yields and lodging-proofness they must either breed for greater total biomass or develop high yielding germplasm from shorter crops

Leaf photosynthesis and associations with grain yield, biomass and nitrogen-use efficiency in landraces, synthetic-derived lines and cultivars in wheat

Future genetic progress in wheat grain yield will depend on increasing above-ground biomass and this must be achieved without commensurate increases in N fertilizer inputs to minimise environmental impacts. Our objective was to quantify variation in grain yield, above-ground biomass and N-use efficiency (NUE) and associated traits in a panel of diverse hexaploid wheat germplasm comprising: (i) landraces from the AE Watkins collection, (ii) synthetic-derived hexaploid lines in a cv. Paragon spring wheat background and (iii) UK modern cultivars including cv. Paragon under low N and high N conditions. A field experiment was carried out in two seasons examining 15 genotypes (five landraces, five synthetic-derived (SD) hexaploid lines and five UK modern cultivars) under low N and high N conditions at Nottingham University farm, UK. Machine-harvested grain yield, above-ground biomass and NUE were measured. Physiological traits were assessed including flag-leaf light-saturated photosynthetic rate (Amax) and relative chlorophyll content (SPAD) under HN conditions; and flag-leaf senescence duration and rate and Normalized Difference Vegetative Index (NDVI) under LN and HN conditions. Under HN conditions, the modern cultivars overall produced higher grain yield than the SD lines (+9.7%) and the landraces (+60.4%); and the modern cultivars and SD lines also produced higher biomass than the landraces (30.3% and 28.4%, respectively). Under LN conditions, reduction in grain yield and biomass compared to HN conditions was least for the landraces (−1% and −8.6%, respectively), intermediate for the SD lines (−7.4 and −10.2%, respectively) and highest for the modern cultivars (−9.3 and −24.6%, respectively). As a result, the SD lines had higher biomass (+17%) than the modern cultivars under LN conditions. Under HN conditions the synthetic derivatives (23.8 μmol m−2 s−1) and modern cultivars (241.1 μmol m−2 s−1) had higher pre-anthesis Amax than the landraces (19.7 μmol m−2 s−1) (P < 0.001). Pre-anthesis Amax was strongly positively linearly associated with above-ground biomass (R2 = 0.63, P < 0.001) and grain yield (R2 = 0.75, P < 0.001) amongst the 15 genotypes. Flag-leaf Amax was also positively linearly associated with flag-leaf relative chlorophyll content at anthesis (R2 = 0.74; P < 0.001). Comparing the SD lines to the recurrent parent Paragon, under HN conditions one line (SD 22) had higher pre-anthesis flag-leaf Amax than Paragon (P < 0.05). Under LN conditions one line (SD 24, +27%) had higher yield than Paragon (P < 0.05) and two lines (SD 24 and SD 38, +32% and +31%, respectively) had more biomass than Paragon (P < 0.05). Our results indicated that introgressing traits from synthetic-derived wheat and landraces into UK modern wheat germplasm offers scope to raise above-ground biomass and grain yield in moderate-to-low N availability environments

Genetic dissection of the relationships between grain yield components by genome-wide association mapping in a collection of tetraploid wheats

Increasing grain yield potential in wheat has been a major target of most breeding programs. Genetic advance has been frequently hindered by negative correlations among yield components that have been often observed in segregant populations and germplasm collections. A tetraploid wheat collection was evaluated in seven environments and genotyped with a 90K SNP assay to identify major and stable quantitative trait loci (QTL) for grain yield per spike (GYS), kernel number per spike (KNS) and thousand-kernel weight (TKW), and to analyse the genetic relationships between the yield components at QTL level. The genome-wide association analysis detected eight, eleven and ten QTL for KNS, TKW and GYS, respectively, significant in at least three environments or two environments and the mean across environments. Most of the QTL for TKW and KNS were found located in different marker intervals, indicating that they are genetically controlled independently by each other. Out of eight KNS QTL, three were associated to significant increases of GYS, while the increased grain number of five additional QTL was completely or partially compensated by decreases in grain weight, thus producing no or reduced effects on GYS. Similarly, four consistent and five suggestive TKW QTL resulted in visible increase of GYS, while seven additional QTL were associated to reduced effects in grain number and no effects on GYS. Our results showed that QTL analysis for detecting TKW or KNS alleles useful for improving grain yield potential should consider the pleiotropic effects of the QTL or the association to other QTLs

Physiological processes associated with genetic progress in yield potential of wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L.) is the most widely grown of any crop and provides one-fifth of the total calories of the world's population. Since the 1960s, increases in productivity have been achieved as a result of wide-scale adoption of Green Revolution technologies. However, in spite of growing demand, the challenges of increasing production to feed an estimated world population of 9 billion in 2050 are still considerable. Due to the increased demand, it is estimated that food production must be increased by about 50% by the year of 2050. Improving wheat productivity through developing cultivars with high yield potential and with high adaptability to specific environments is the key objective in the wheat breeding programs worldwide to fill the gap between the production and the demand. The overall aims of the present study were to: (i) investigate the physiological basis of yield potential progress from 1966 to 2009 in spring bread wheats released at the International Center for Maize and Wheat Improvement (CIMMYT) in the irrigated high potential environment of NW Mexico, (ii) investigate the physiological basis of effects of the tiller inhibition Tin1A gene on ear-fertility traits and yield potential and interactions with plant density in NW Mexico and UK environments in lines of a doubled-haploid (DH) population segregating for Tin1A/non-Tin1A alleles and (iii) identify breeding targets for new cultivars with higher yield potential. Four experiments were conducted in NW Mexico at the CIMMYT research station at Ciudad Obregon. Two of these experiments studied a set of 12 historic CIMMYT spring wheat cultivars released from 1966 to 2009 in 2008/9 and 2009/10. The other two experiments examined selected lines from a doubled-haploid (DH) population derived from a cross between CIMMYT spring wheat L14 and UK winter wheat Rialto contrasting for the presence/absence of the TinlA allele for tiller inhibition and their interaction with seed rate in 2008/9 and 2009/10. In addition, two other field experiments were conducted in the UK, one in 2008/09 at KWS UK Ltd in Thriplow, Hertfordshire and one in 2009110 at the University of Nottingham Farm, Sutton Bonington campus, Leicestershire. The plant material for both of these experiments was selected lines from the CIMMYT spring wheat advanced line Ll4 (+Tin1A allele) x UK winter wheat Rialto (-TinlA allele) DH population and the Rialto parent. In the experiment at Thriplow in 2008/09 the DH lines were examined at one seed rate and in the experiment at Sutton Bonington in 2009/10 at two seed rates. At the CIMMYT site in 2008/9 and 2009/10, a randomized complete block design was implemented with four replications for the experiments examining the CIMMYT wheat historic releases and a split-plot randomised complete block design with three replications was implemented for the experiments examining the +/- Tin1A DH lines, with three seed densities (50, 150 and 450 seeds per square metre); seed rates were randomized on main plots and eight genotypes randomized on sub-plots. At the UK site, in the KWS experiment, 24 DH lines (12+Tin1A allele) and (12-TinlA allele) from the L14 x Rialto population were used. There was only one seed rate (300 seeds m-2) and a completely randomised design in three replicates was implemented. The same 24 DH lines were examined in the experiment at the SB site, at two seed rates (40 and 320 seeds m-2) in a split plot randomised complete block design in three replicates. Seed rate was randomized on main plots and DH lines were randomized on sub-plots. In all experiments examining the DH lines of the Ll4 x Rialto population, lines were selected in pairs so that the two groups of +Tin1A and -Tin lA lines were approximately balanced for flowering time and plant height, i.e. every +TinlA line has a non-Tin1A pair with similar height and flowering date. Plots were sampled for destructive measurements of dry weight and DM partitioning and ear-fertility traits at four stages in the historic experiments at (GS3l, GS39, GS61+7d and at maturity) and at two stages in the DH population experiments (GS61+7d and at maturity). The water soluble carbohydrate (WSC) content of the stems plus attached leaf sheaths was also measured at GS61+7d and at maturity. In the historic experiments, at GS 61+14 days, a degraining treatment was implemented by removing all spikelets from one side of the ear (i.e. ca. 50% of the spikletes) in the histories experiment. Non-destructive measurements were taken for stomatal conductance, canopy temperature, fractional photosynthetically active radiation (PAR) interception and normalized difference vegetative index at various dates both pre- and post-anthesis in the historic experiments. In the experiments examining the set of 12 historic CIMMYT spring wheat releases, results showed that from 1966 to 2009 the linear rate of genetic gain in yield potential was 32 ha-1 yr-1 (0.59 % yr-1) (r = 0.76. P = 0.01). Yield progress was primarily associated with harvest index (percentage above-ground DM as grain DM) in the period from 1966 until about 1990 increasing from 43% to 49%, but deceased with year of release thereafter. A non-linear genetic gain in AGDM was evident over the 43-yr period with AGDM increasing from about 1990 from which point it increased rapidly to 2009. There was no association between genetic progress in grain yield and grain number per m2 in this set of 1 cultivars; a small increase in ears per m2 was counteracted by a decrease in grains per ear. However, grain weight tracked the improvement in yield potential over the 43-year period with a linear increase of 0.23 mg yr-1. No change was found in rachis length with plant breeding; however, number of fertile spikelets per ear decreased since about 1990 and was associated with the decrease in grains per ear. There were statistically significant differences in above-ground DM production at all growth stages and a tendency to produce more biomass during the GS31 to GS61+7d phase with year of release. No differences amongst cultivars were found in the amount of radiation intercepted by the whole canopy from GS3l to GS6l+7 days. Although not conclusive, since Bacanora was an exception to the trend and radiation-use efficiency (above-ground biomass per unit PAR interception; RUE), there was a tendency for RUE to increase with year of release which was consistent with a positive association with crop growth rate (above-ground DM per m2 per day; CGR) and the trend for an increase in biomass accumulation during the stem-elongation phase with plant breeding. Although there was a trend for an increase in biomass accumulation from GS31 GS61+7d this was counteracted by a decrease in ear DM partitioning so that ear DM per m2 at GS61+7d and grains per m2 did not change with plant breeding. Results showed that the improvement in the individual grain weight from 1966 to 2009 in this set of cultivars was associated with improvements in the grain filling rate from 1966 to ca. 1990 and in the duration of grain filling from ca. 1992 to 2009. Averaging across years, there was a significant positive association between post-flowering canopy-temperature depression and grain yield. Fractional PAR interception by the canopy layers of the ear, flag leaf and the penultimate leaf was increased with year of release since about 1990. This increase in the fractional interception of PAR correlated significantly with the grain weight and grain yield amongst the 12 cultivars. Grain growth of the cultivars in this historic set was generally sink limited rather than source limited. There was no change in source-sink balance as indicted by grain growth responses to the degraining treatment with year of release.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Jughrafiyah al 'Alim al Tsalits

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