Increased Harvest Index Research Articles

Analysis of improvements in nitrogen use efficiency associated with 75 years of spring barley breeding

A retrospective analysis of the effects of nearly 75 years of breeding on the nitrogen use efficiency (NUE) of spring barley (Hordeum vulgare L.) was undertaken to identify physiological mechanisms governing NUE and targets for future improvement. Fifteen varieties, selected to be as genetically dissimilar as possible and to represent the breeding period from 1931 to 2005, were grown at three site-year combinations in the NE of Scotland. Varieties were grown with zero or 110kgNha−1 supplied as ammonium nitrate. Averaged across site-years, breeding was shown to increase yield and NUE (grainyieldNsupply−1) by 1 and 1.2% per year respectively in the presence of fertilizer. Mean grain weight was larger in modern compared to old varieties grown both with and without N fertilizer. The greater response of yield to fertilizer in modern varieties was associated with the production of more grainsm−2. Significant variation was found between genotypes in efficiencies of N uptake (NupE; N offtake per unit N supply) and N utilization (NutEg, grain yield per unit N offtake). Differences in NutEg contributed 60% to the variation in NUE, whilst NupE contributed 40%. The improvement in NutEg was mostly the result of increases in harvest index (HI) and was stable across environments. NupE was positively correlated with post-anthesis, but not pre-anthesis, dry matter accumulation and N uptake, which suggests that greater NupE of some varieties was the result of an increased N demand from a large grain sink. Across all varieties NupE was not related to NutEg suggesting that improvements in each may be selected independently. The results also indicated possible opportunities for improving the efficiency of pre-anthesis N uptake.

Allocation patterns and phenology in wild and selected accessions of annual and perennial Physaria (Lesquerella, Brassicaceae)

Two processes that determine a good performance of plants in arid environments are phenology and resource allocation patterns. With a longer growing season and larger allocation to root, perennials achieve better access to resources and are more resilient to stresses than annuals. In traditional agricultural systems selection for optimal soil nutrient uptake has been a secondary breeding objective, because crops receive subsidies of water and fertilizers. However in arid lands, caution is required during domestication, to avoid changes in structural traits which may be the basis for sustainable production. Due to inherent differences in hierarchy among annual and perennial species, we propose that the changes in phenology and allocation brought about by selection will depend on the life cycle. We performed field studies comparing wild and selected accessions of annual and perennial species of Physaria. Life cycle determined the functional basis of seed yield. In annuals, selection resulted in early anthesis (1 week earlier), a lower allocation to roots and leaves (twofold lower), and an increase in harvest index (an increase of 62%). Selected perennials had higher biomass at maturity (45% higher), linked to a longer reproductive period (3 weeks longer) than their wild relatives. The vegetative allocation found in wild perennials remained unchanged after selection. While annuals selected for seed yield could compromise the capacity for acquisition of resources, selection in perennials did not modify the allocation strategy responsible for their positive adjustment to low resource environments. We found a trade off between seed yield potential and yield stability that resulted in lower performance of selected accessions in low quality environments in relation to their wild relatives.

Microsatellite mapping of genes for semi-dwarfism and branched spike in Triticum durum Desf. var. ramosoobscurum Jakubz. “Vetvistokoloskaya”

A combination of increased harvest index and increased seed number per plant may improve wheat yield. We found an accession Triticum durum Desf. var. ramosoobscurum Jakubz. “Vetvistokoloskaya” R-107, whose plant height was short and spike was branched. To characterize R-107, we mapped the genes for semi-dwarfism and branched spike. Semi-dwarf gene Rht_R107 (Reduced height R107) from R-107 was allelic to Rht11 and Rht-B1b on chromosome 4BS. Rht_R107, as well as Rht11, was recessive and insensitive to gibberellic acid. Microsatellite mapping indicated that Rht_R107 was linked to Xwmc48 (1.2 cM) on chromosome 4BS, and Rht11 was also linked to Xgwm495 marker (19.4 cM) on chromosome 4BS. The locus responsible for branched spike in R-107 was mapped by genotyping three F2 populations using microsatellite markers. Phenotypic analysis revealed that branched spike was under control of a recessive allele at a single locus. The bh (branched head) locus was located on chromosome 2AS and the marker Xgwm425 flanked the bh gene proximally.

Genetic gains in grain yield, net photosynthesis and stomatal conductance achieved in Henan Province of China between 1981 and 2008

Knowledge of the changes in agronomic and photosynthetic traits associated with genetic gains in grain yield potential is essential for an improved understanding of yield-limiting factors and for determining future breeding strategies. The objectives of this study were to identify agronomic and photosynthetic traits associated with genetic gains in grain yield of facultative wheat ( Triticum aestivum L.) between 1981 and 2008 in Henan Province, the most important wheat producing area in China. During the 2006–2007 and 2007–2008 crop seasons, a yield potential trial comprising 18 leading and new cultivars released between 1981 and 2008 was conducted at two locations, using a completely randomised block design of three replicates. Results showed that average annual genetic gain in grain yield was 0.60% or 51.30 kg ha −1 yr −1, and the significant genetic improvement in grain yield was directly attributed to increased thousand grain weight which also contributed to the significant increase in harvest index. The genetic gains in rates of net photosynthesis at 10, 20 and 30 days after anthesis were 1.10% ( R 2 = 0.46, P < 0.01), 0.68% ( R 2 = 0.31, P < 0.05) and 6.77% ( R 2 = 0.34, P < 0.05), respectively. The rates of net photosynthesis at 10 ( r = 0.58, P < 0.05), 20 ( r = 0.59, P < 0.05) and 30 ( r = 0.65, P < 0.01) days after anthesis were closely and positively correlated with grain yield. A slight decrease in leaf temperature and an increase in stomatal conductance after anthesis were also observed. Grain yield was closely and positively associated with stomatal conductance ( r = 0.69, P < 0.01) and transpiration rate ( r = 0.63, P < 0.01) at 30 days after anthesis. Therefore, improvement of those traits was the likely basis of increasing grain yield in Henan Province between 1981 and 2008. The genetic improvement in yield was primarily attributed to the utilization of two elite parents Yumai 2 and Zhou 8425B. The future challenge of wheat breeding in this region is to maintain the genetic gain in grain yield and to improve grain quality, without increasing inputs for the wheat–maize double cropping system.

Changes in evapotranspiration over irrigated winter wheat and maize in North China Plain over three decades

Abstract Evapotranspiration (ET) is an important component of the water cycle at field, regional and global scales. This study used measured data from a 30-year irrigation experiment (1979–2009) in the North China Plain (NCP) on winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) to analyze the impacts of climatic factors and crop yield on ET. The results showed that grass reference evapotranspiration (ETo, calculated by FAO Penmen–Monteith method) was relatively constant from 1979 to 2009. However, the actual seasonal ET of winter wheat and maize under well-watered condition gradually increased from the 1980s to the 2000s. The mean seasonal ET was 401.4 mm, 417.3 mm and 458.6 mm for winter wheat, and 375.7 mm, 381.1 mm and 396.2 mm for maize in 1980s, 1990s and 2000s, respectively. The crop coefficient (Kc) was not constant and changed with the yield of the crops. The seasonal average Kc of winter wheat was 0.75 in the 1980s, 0.81 in the 1990s and 0.85 in the 2000s, and the corresponding average grain yield (GY) was 4790 kg ha−1, 5501 kg ha−1 and 6685 kg ha−1. The average Kc of maize was 0.88 in the 1980s, 0.88 in the 1990s and 0.94 in the 2000s, with a GY of 5054 kg ha−1, 7041 kg ha−1 and 7874 kg ha−1, respectively, for the three decades. The increase in ET was not in proportion to the increase in GY, resulting improved water use efficiency (WUE). The increase in ET was possibly related to the increase in leaf stomatal conductance with renewing in cultivars. The less increase in water use with more increase in grain production could be partly attributed to the significant increase in harvest index. The results showed that with new cultivars and improved management practices it was possible to further increase grain production without much increase in water use.

Physiological attributes associated with yield and stability in selected lines of a durum wheat population

Further increasing yield potential remains one of the main objectives of wheat breeding, even in stressful environments. In general, past genetic gains were associated with increases in harvest index, and future gains should be related to greater biomass. Identifying genetic sources for such improvement may be relevant. Researchers of TRITIMED identified DH lines of durum wheat apparently possessing not only high yield potential but also good yield stability. We aimed to determine physiological attributes responsible for yield and stability among a set of genotypes derived from two parents (Cham 1 and Lahn) and four of the most promising lines of the DH population (2401, 2408, 2410, 2517). Seven field trials were carried out within the Mediterranean agricultural region of the Ebro Valley, under a wide range of conditions (ca 2–10 mg ha−1). In four of these experiments, sub-plots were included with source-sink manipulations imposed after anthesis. Cham 1, a cultivar known for high yields in semi-arid conditions, showed the highest yield potential. Although it showed less yield stability than Lahn, even under the lowest yielding conditions its yield was not significantly lower than that of Lahn. RILs 2408, 2410, 2004 and 2517 slightly outyielded Lahn in high-yielding conditions, but under poorer environments they tended to yield less. Interestingly, yield differences were closely related to their biomass rather than harvest index. Thus yield differences relating to the number of grains per m2 were due to differences in spike dry matter at anthesis, reflecting in part genotypic differences in crop growth from jointing to anthesis. In general grain weight did not respond to spike trimming after anthesis, although in two experiments the grain weight of Cham 1 did so. Thus, even the highest-yielding cultivar possessed grains that overall seemed more limited by its constitutive capacity to grow than by the availability of resources to reach this capacity (though occasionally they may be co-limited). Overall, the most interesting feature was the empirical evidence that improvement of biomass within elite material is a worthwhile objective.

Genetic gain in yield and associated changes in phenotype, trait plasticity and competitive ability of South Australian wheat varieties released between 1958 and 2007

To quantify the genetic gain in yield and associated changes in phenotype, we compared 13 South Australian wheat varieties released between 1958 and 2007. Crops were grown in three environments with a range of yield between 4.1 and 6.1 t/ha. Yield increased linearly with year of cultivar release at a rate of 25 ± 3.4 kg/ha per year. Yield improvement was associated with a linear increase in harvest index over the whole period 1958–2007 and increased shoot biomass for varieties released after the early 1980s. A non-linear model with an inflection point at 1982 ± 1.6 emphasised two phases in the time trend of grain size: it decreased between 1957 and 1982 and increased afterwards. The plasticity of grain size increased 2-fold after 1982. Grain number increased until the early 1980s and stabilised afterwards. Grain number was associated with crop growth rate between stem elongation and anthesis, and grain size was associated with crop growth rate per grain. Crop growth rate between stem elongation and anthesis increased after the early 1980s in parallel with increased radiation-use efficiency and independently of changes in capture of radiation. Candidate traits to explain the improvement in radiation-use efficiency include increased stomatal conductance and greener leaves. The concentration of water-soluble carbohydrates in shoots at anthesis increased with year of cultivar release at 0.12 ± 0.018% per year. In two out of three environments, yield response to competition declined with year of cultivar release, which was consistent with the communal plant ideotype.

The effect of alternate furrow irrigation under different nutritional element supplies on some agronomic traits and seed qualitative parameters in corn (Zea mays L.)

A new irrigation method under different nutritional element supplies for maize production was designed and tested for some agronomic traits and seed qualitative parameters with a split-plot field experiment. Irrigation was applied through furrows in three ways as the main plots: alternate furrow irrigation (AFI), fixed furrow irrigation (FFI), and conventional furrow irrigation (CFI). AFI means that one of the two neighboring furrows was alternately irrigated during consecutive watering. FFI means that irrigation was fixed to one of the two neighboring furrows. CFI was the conventional way where every furrow was irrigated during each watering. Each irrigation method was further divided into five sub-treatments with different fertilizer combinations: (1) P+N (control) (2) P+N+K (3) P+N+K+Zn (4) P+N+K+Zn+B (5) P+N+K+Zn+B+Fe. The results indicate that water stress effects caused by furrow irrigation on yield may be alleviated by more frequent irrigation intervals. We concluded that AFI is a way to save water in arid areas where maize production relies heavily on repeated irrigation. Fertilized combinations influenced dry matter partitioning to seed filling. Thus, sufficient both macro and micro nutritional elements increased harvest index which was mostly due to more number of seeds per row than higher individual grain weight. Complete fertilizer combination increased total above ground biomass through more radiation use efficiency and by increasing leaf area. In order to utilize the water sources efficiently and increase corn production under limited water supply, we propose the use of circular irrigation care along with instance, K, Zn, B and Fe fertilizer. Key words: Alternate furrow irrigation, corn (Zea mays L.), fertilizer combination,water stress.

Raising yield potential of wheat. II. Increasing photosynthetic capacity and efficiency

Past increases in yield potential of wheat have largely resulted from improvements in harvest index rather than increased biomass. Further large increases in harvest index are unlikely, but an opportunity exists for increasing productive biomass and harvestable grain. Photosynthetic capacity and efficiency are bottlenecks to raising productivity and there is strong evidence that increasing photosynthesis will increase crop yields provided that other constraints do not become limiting. Even small increases in the rate of net photosynthesis can translate into large increases in biomass and hence yield, since carbon assimilation is integrated over the entire growing season and crop canopy. This review discusses the strategies to increase photosynthesis that are being proposed by the wheat yield consortium in order to increase wheat yields. These include: selection for photosynthetic capacity and efficiency, increasing ear photosynthesis, optimizing canopy photosynthesis, introducing chloroplast CO(2) pumps, increasing RuBP regeneration, improving the thermal stability of Rubisco activase, and replacing wheat Rubisco with that from other species with different kinetic properties.

Response of seed yield and its components of red gram (Cajanus cajan L. Millsp.) to elevated CO&lt;sub&gt;2&lt;/sub&gt;

Pigeon pea (Cajanus cajan L. Millsp.) is an important grain legume crop of the semi arid tropics and is a major dietary protein source. The extra short duration cultivar of pigeon pea ICPL 88039 was evaluated at ambient&lt;br /&gt;(370 &amp;mu;mol/mol) and twice the ambient (700 &amp;mu;mol/mol) concentrations of CO&lt;sub&gt;2&lt;/sub&gt; in open top chambers (OTCs). The results showed that the crop recorded a significant positive enhanced response for total biomass, fodder yield, grain yield, number of pods and seeds per plant, test weight and HI at elevated CO&lt;sub&gt;2&lt;/sub&gt;. The ANOVA revealed significant differences in response of the characteristics to CO&lt;sub&gt;2&lt;/sub&gt; concentrations. Under elevated CO&lt;sub&gt;2&lt;/sub&gt; the total biomass recorded an improvement of 91.3%, grain yield 150.1%, fodder yield 67.1%. The major contributing components for improved grain yield under elevated CO&lt;sub&gt;2&lt;/sub&gt; were number of pods, number of seeds and test weight which recorded an increase of 97.9%, 119.5% and 7.2%, respectively. The crop maintained a significant positive increase of harvest index (HI) at elevated CO&lt;sub&gt;2&lt;/sub&gt; with an increment of 30.7% over ambient values. This increase in HI was due to its improved pod set and seed yield under enhanced CO&lt;sub&gt;2 &lt;/sub&gt;concentration thereby emphasizes this crop for sustained food with nutritional security under climate change scenario. &amp;nbsp;

Maize DELLA Proteins dwarf plant8 and dwarf plant9 as Modulators of Plant Development

DELLA proteins are nuclear-localized negative regulators of gibberellin signaling found ubiquitously throughout higher plants. Dominant dwarfing mutations of DELLA proteins have been primarily responsible for the dramatic increases in harvest index of the 'green revolution'. Maize contains two genetic loci encoding DELLA proteins, dwarf plant8 (d8) and dwarf plant 9 (d9). The d8 gene and three of its dominant dwarfing alleles have been previously characterized at the molecular level. Almost 20 years after the initial description of the mutant, this investigation represents the first molecular characterization of d9 and its gibberellin-insensitive mutant, D9-1. We have molecularly, subcellularly and phenotypically characterized the gene products of five maize DELLA alleles in transgenic Arabidopsis. In dissecting the molecular differences in D9-1, a critical residue for normal DELLA function has been uncovered, corresponding to E600 of the D9 protein. The gibberellin-insensitive D9-1 was found to produce dwarfing and, notably, earlier flowering in Arabidopsis. Conversely, overexpression of the D9-1 allele delayed flowering in transgenic maize, while overexpression of the d9 allele led to earlier flowering. These results corroborate findings that DELLA proteins are at the crux of many plant developmental pathways and suggest differing mechanisms of flowering time control by DELLAs in maize and Arabidopsis.

Breeding and Cereal Yield Progress

ABSTRACTThis paper reviews recent progress in wheat (Triticum aestivum L.), rice (Oryza sativa L.), and maize (Zea mays L.) yields resulting from substantial breeding efforts in mostly favorable environments and examines its physiological basis. Breeding and improved agronomy lift potential yield (PY), namely yield with the best variety and management in the absence of manageable abiotic and biotic stresses, and PY increase is a key component of progress in farm yield (FY), the other component being closure of the PY to FY gap. Changes in PY and FY are reviewed for several key production regions, namely the United Kingdom and the Yaqui Valley of Mexico for wheat, Japan and Central Luzon in the Philippines for rice, and Iowa and briefly sub‐Saharan Africa for maize. The PY growth rates have fallen and are currently generally no more than 1% per annum and usually much less. The trajectory of FY with time often closely parallels PY, but, especially in developing countries, there remain large yield gaps. In at least one instance (maize in Iowa) the gap between PY and FY appears to be closing rapidly. Current genetic progress is linked to increased biomass accumulation, and this will remain the way forward in the future given the limits to increased harvest index (HI). There is evidence that recent progress is related to increased photosynthesis (e.g., greater radiation use efficiency (RUE) at the canopy level and/or maximum photosynthetic rate Pmax at saturating irradiance at the leaf level) before and around anthesis. There is no theoretical reason why this trend cannot continue, especially given the vast genetic resources already found within each crop species. However, it will not be easily or cheaply accomplished, so prospects for higher rates of potential yield growth appear to be limited, notwithstanding new molecular tools and claims to the contrary. Closing the yield gap, therefore, becomes more important. Many factors are involved, but breeding can also help farmers achieve this through, for example, improved host plant resistance.

Breeding Maize for a Bioeconomy: A Literature Survey Examining Harvest Index and Stover Yield and Their Relationship to Grain Yield

ABSTRACTThe use of maize (Zea mays L.) stover as a feedstock for cellulosic biofuels production will create demand for maize hybrids with greater stover yield. It is expected that grain yield will remain the most critical trait and continue to drive hybrid sales, requiring that any increases in stover yield be made without sacrificing grain yield potential. The objective of this review was to determine the relationship between grain yield, harvest index, and stover yield to illuminate the potential for increasing both grain and stover yield through breeding. In contrast to what has been observed in other crops, gains in maize grain yield over time in the U.S. Corn Belt have been accompanied by increases in stover yield. Where recurrent selection on grain yield has been most successful, stover yield also increased while harvest index has been relatively stable. The opposite situation has been observed in tropical germplasm, where gains in grain yield have been associated with increasing harvest index and relatively constant biomass yield. We expect that stover yield of Corn Belt hybrids will continue to increase along with grain yield, resulting in future hybrids capable of producing both more food and biofeedstock for energy production. If maize breeders pursue selection for increased stover yield, we found no evidence to suggest that breeding for stover yield will necessarily reduce rate of gain in grain yield.

Plant growth regulators: a success story in perennial ryegrass seed crops

The introduction of the plant growth regulator trinexapac-ethyl (Moddus) has increased seed yields in perennial ryegrass (Lolium perenne) by up to 50% in New Zealand. Application rates from ten trials have confirmed that approximately 1.6 l/ha are optimum. Yield increases are a result of increased harvest index and better dry matter partitioning to the harvested parts. The resulting changes in harvest components include increasing seed head density and seeds per spikelet (better seed retention) associated with delayed lodging. Keywords: Lolium perenne, lodging, Moddus, seeds/spikelet, trinexapac-ethyl

Agronomic and physiological contributions to the yield improvement of soybean cultivars released from 1950 to 2006 in Northeast China

Increasing yield is a high priority in most breeding programs. Approximately 600 soybean cultivars had been released by the end of the last century in Northeast China. Understanding the agronomic and physiological changes is essential for planning further plant breeding strategies in soybean. In this study, 45 representative soybean cultivars, from maturity groups 00 and 0, released from 1950 to 2006 in Northeast China were compared in field conditions for 3 consecutive years. A positive correlation between seed yield and year of cultivar release was indicated with a 0.58% average annual increase. Seed number per plant was the most important contributor to yield gain, with a 0.41% increase per year. Pod number per plant and seed size varied slightly with the year of cultivar release. Although variation in protein was from 37.0% to 45.5%, and oil concentration was from 16.7% to 22.0%, their concentrations were not consistently related to year of cultivar release. A 33% increase in the photosynthetic rate, 10.6% increase in plant dry weight and 19.0% increase in harvest index (HI) were found, while leaf area index (LAI) decreased by 17.3%. Modern cultivars have higher photosynthetic rates than their predecessors. The reduced plant height gave increased resistance to lodging, with the lodging score dropping from 3.2 in 1951 to 1.0 in 2006. Seed resistances to disease and pest infestation were also improved. Yield stability was enhanced over years, which could be attributed to the stable pod production across different environments. A flow diagram to explain the contributors to genetic improvement of soybeans in Northeast China was developed.

Molecular detection of genomic regions associated with grain yield and yield-related components in an elite bread wheat cross evaluated under irrigated and rainfed conditions

Grain yield and grain weight of wheat are often decreased by water-limitation in the north-eastern cropping belt of Australia. Based on knowledge that CIMMYT lines are well-adapted in this region, a recombinant inbred line (RIL) population between two elite CIMMYT bread wheats (Seri M82 and Babax) was evaluated under water-limited environments. Fourteen productivity traits were evaluated in 192 progeny in up to eight trials. For three aggregations of the environments (all, high yield or low yield), multiple quantitative trait loci (QTL) were detected, each explaining <15% of variation. Co-location of multiple trait QTL was greatest on linkage groups 1B-a, 1D-b, 4A-a, 4D-a, 6A-a, 6B-a, 7A-a and an unassigned linkage group. Two putative QTL (LOD > 3) from Seri (6D-b and UA-d) increased grain yield and co-located with a suggestive (2 < LOD < 3) and a putative QTL for increased stem carbohydrate content (WSC), respectively; the latter QTL also co-located with a putative anthesis QTL for earlier flowering. Both QTL were detected only in high yield (>4t ha(-1)) environments. A third increased grain yield QTL (7A-a) from Babax co-located with QTL for increased grain number. Six putative QTL increased grain weight and co-located with QTL for harvest index, grains per spike and spike number. Three putative QTL for increased grains per spike co-located with strong QTL for earlier flowering, increased grain weight and fewer spikes. A group of progeny that exceeded the mean grain yield and grain weight of commercial checks had an increased frequency of QTL for high WSC, large grain size, increased harvest index and greater height, but fewer stems, when compared to low yielding (20% less), low grain weight progeny. These findings were consistent with agronomic analyses of the germplasm and demonstrate that there should be opportunities to independently manipulate grain number and grain size which is typically difficult due to strong negative correlations.

Responses of legumes to rhizobia and arbuscular mycorrhizal fungi: A meta-analysis of potential photosynthate limitation of symbioses

Legumes are prized for their seed protein and lipid mass fractions. Since legumes spend up to 4–16% of photosynthesis on each of the rhizobial and arbuscular mycorrhizal (AM) fungal symbioses, it might be expected that positive responses in yield due to rhizobial and AM symbioses are accompanied by decreases in seed protein and lipid mass fractions due to a photosynthate (C) limitation. We performed a meta-analysis of 348 data points from published studies with 12 legume species to test whether yield, harvest index, and seed protein and lipid mass fractions are affected by symbioses. There was a significant increase in yield due to rhizobial inoculation (16% in the field; 59% in pot experiments). There were no responses of yield to AM fungi and rhizobial + AM fungi inoculations in the field (presumably because an AM fungi-free control cannot be ensured), but significant responses in pots (45% with AM fungi; 44% with rhizobial + AM fungi). Rhizobial inoculation improved seed protein mass fraction by 7% in the field; AM fungi increased this parameter by 14% in pots. There were no discernable effects of symbioses on seed lipid mass fraction. Rhizobial symbioses in the field increased harvest index (+5%), but AM fungi did not affect harvest index. In conclusion, increases in yield due to symbioses also resulted in increases in seed protein and constant lipid mass fractions, indicating that legumes are not C-limited under symbiotic conditions.

Root size, distribution and soil water depletion as affected by cultivars and environmental factors

Field experiments were carried out in two parts at the Luancheng Experimental Station in the North China Plain. Part I, which was a continuous experiment that ran from 1990 to 2008, investigated the change of grain yield, evapotranspiration (ET), root size and soil water utilization of winter wheat ( Triticum aestivum L.) under three water regimes (rain-fed, two and four irrigation applications). Part II used 10 cultivars released from 1970 to 2000 that were grown under the same condition for two seasons (2005/2006 and 2006/2007) to compare root size, root:shoot ratio and soil water depletion (SWD). The results of testing in Part I showed that the yield and ET of winter wheat gradually increased from 1990 to the present. There was no consistent change in total root length (TRL) over time. The difference in root size among seasons and irrigation treatments mainly occurred in the upper soil profile, where the root length density (RLD) was greater. No direct relationship was found between root size and soil water use. Thus, TRL was not a factor that indicated the water extracting capacity of crops. The results from Part II revealed that the seasonal ET of earlier released cultivars (ERC) was similar to that of recently released cultivars (RRC) under well-watered conditions. However, ET was slightly increased from ERC to RRC under water deficit conditions, indicating high soil water depletion by RRC. TRL decreased from ERC to RRC and was significantly correlated with plant height. The breeding of winter wheat that reduced plant height not only increased harvest index, but also reduced root size, resulting in a smaller root:shoot ratio. The reduction in TRL from ERC to RRC mainly occurred in the top soil profile. The results from both tests indicate that TRL is not a factor that determines soil water use; rather, the distribution of RLD along the soil profile plays more important role in soil water utilization. Smaller root size in the upper soil layer did not affect soil water uptake, and might be more economical in terms of production efficiency.

GRAIN YIELD AND MORPHO-PHYSIOLOGICAL CHANGES FROM 60 YEARS OF GENETIC IMPROVEMENT OF WHEAT IN IRAN

SUMMARYIn order to understanding the changes in morpho-physiological traits of wheat cultivars released from 1940 to 2000 in Iran and to determine the association of these traits with grain yield, an experiment was conducted with 15 wheat cultivars released over the past 60 years using an randomized complete block design with four replications. The experiment was conducted on an experimental farm of Arsanjan Islamic Azad University in 2005/6 and 2006/7. Results indicated that cultivar grain yield was significantly correlated with year of release over the 60 years (r= 0.878,p&lt; 0.01). Harvest index and biological yield increased significantly with increasing grain yield. The change in the photosynthesis rate was not significant during this period but transpiration rate and stomatal conductance and chlorophyll index increased significantly. Among yield components, grain number per ear increased significantly but increase in ear number was not significant. Plant height decreased significantly in new cultivars. In conclusion the results showed that wheat yield improvement in Iran is associated with increasing harvest index, grain number per ear, transpiration rate, stomatal conductance and decreasing plant height.

Genetic loci responding to two cycles of divergent selection for grain yield under drought stress in a rice breeding population

Molecular marker loci responding to selection under drought stress were monitored in a rice breeding population obtained by crossing a tolerant parent (Apo) to a susceptible parent (IR64). The 40 highest-yielding lines under stress and non-stress conditions obtained after two cycles of divergent selection under drought stress and non-stress conditions, respectively were genotyped using 72 polymorphic and widely distributed SSR markers. Ten loci (RM572, RM6703, RM71, RM3387, RM5686, RM520, RM510, RM256, RM269 and RM511) showing highly significant allele frequency differences between the two sets were identified. Favorable alleles at eight of these loci came from the tolerant parent, and at two (RM3387 and RM510) from the susceptible parent (IR64). Effects of these loci on grain yield were tested in five independent experiments covering a range in soil moisture levels. Results showed that at six loci (RM572, RM6703, RM520, RM256, RM269, and RM511), Apo alleles had highly significant effects on grain yield in at least three of the four stress trials but only two of these loci (RM572 and RM511) also affected grain yield under non-stress conditions. In all these cases, the effects of loci generally increased with stress level. Apo alleles at these loci seem to enhance yield under stress mainly by increasing harvest index and reducing flowering delay. Large-effect quantitative trait loci (QTLs) affecting grain yield under upland drought stress have already been found previously in other populations near RM6703, RM520, and RM511. Thus, these regions appear to be important in explaining genetic variation for upland drought tolerance in rice.