Lateral Florets Research Articles

Phenotypic characterization and seed viability test in ex-situ conserved Ethiopian cultivated barley (Hordeum vulgare L.) landraces

BackgroundNowadays, most of the Ethiopian barley landraces had been lost from farmer’s field and exclusively found ex-situ conserved at the Ethiopian Biodiversity Institute (EBI). Those ex-situ conserved are generally believed to be representative of the original population and possess high genetic diversity and important unique genes that are useful for tackling the various biotic and abiotic stresses in the face of the current climate change. Thus, this research was aimed at testing the performance of 150 ex-situ conserved landraces that had been collected from Arsi and Bale highlands, Southeastern Ethiopia. The landraces were tested at multiple test locations over two years (2021 and 2022).ResultsAll the tested landraces showed a good germination rate regardless of their long storage duration. In addition, performance of all the qualitative traits revealed a varying frequency for each character state. For example, most of the accessions (51.3%) had six kernel row numbers (KRN). All the remaining accessions had two rows (28.7%) and irregular KRN with variable lateral florets (20%). Likewise, some of the quantitative traits considered showed a significant variation among the landraces. However, there observed a significant variation for all the interaction effects in some of the traits considered signifying the importance of considering environment effects while targeting genetic selection and improvement of ex-situ conserved germplasms. The phenotypic coefficients of variation (PCV) were considerably high to medium in most of the traits considered including seed yield per hectare (SYPH) but with no associated higher genotypic coefficients of variation (GCV). Moreover, all the traits showed a far greater phenotypic coefficient of variation (PCV) to that of genotypic coefficients of variation (GCV) once again suggesting the pronounced effect of environmental factors to the variation. This was far supported by the significantly higher absolute magnitudes in phenotypic correlation compared to their corresponding genotypic correlation in most of the traits. Low estimates of heritability and genetic advance observed in all the traits considered except seed yield per hectare indicate importance of the trait for selection in Ethiopian barley improvement programs. Clustering patterns of the accessions, in narrow sense, revealed the existence of low divergence among the samples.ConclusionEthiopian barley landraces are promising candidates for further yield improvement and conservation. However, further regular testing and screening should be conducted for the ex-situ conserved landraces because of the current erratic climate change. In addition, more robust molecular marker systems could be used to clearly reveal the extents of genetic diversity and to facilitate the breeding and conservation of Ethiopian barley landraces.

Analyses of MADS-box Genes Suggest HvMADS56 to Regulate Lateral Spikelet Development in Barley.

MADS-box transcription factors are crucial regulators of inflorescence and flower development in plants. Therefore, the recent interest in this family has received much attention in plant breeding programs due to their impact on plant development and inflorescence architecture. The aim of this study was to investigate the role of HvMADS-box genes in lateral spikelet development in barley (Hordeum vulgare L.). A set of 30 spike-contrasting barley lines were phenotypically and genotypically investigated under controlled conditions. We detected clear variations in the spike and spikelet development during the developmental stages among the tested lines. The lateral florets in the deficiens and semi-deficiens lines were more reduced than in two-rowed cultivars except cv. Kristina. Interestingly, cv. Kristina, int-h.43 and int-i.39 exhibited the same behavior as def.5, def.6, semi-def.1, semi-def.8 regarding development and showed reduced lateral florets size. In HOR1555, HOR7191 and HOR7041, the lateral florets continued their development, eventually setting seeds. In contrast, lateral florets in two-rowed barley stopped differentiating after the awn primordia stage giving rise to lateral floret sterility. At harvest, the lines tested showed large variation for all central and lateral spikelet-related traits. Phylogenetic analysis showed that more than half of the 108 MADS-box genes identified are highly conserved and are expressed in different barley tissues. Re-sequence analysis of a subset of these genes showed clear polymorphism in either SNPs or in/del. Variation in HvMADS56 correlated with altered lateral spikelet morphology. This suggests that HvMADS56 plays an important role in lateral spikelet development in barley.

LF1 regulates the lateral organs polarity development in rice.

The patterning of adaxial-abaxial tissues plays a vital role in the morphology of lateral organs, which is maintained by antagonism between the genes that specify adaxial and abaxial tissue identity. The homeo-domain leucine zipper class III (HD-ZIP III) family genes regulate adaxial identity; however, little information is known about the physical interactions or transcriptionally regulated downstream genes of HD-ZIP III. In this study, we identified a dominant rice mutant, lateral floret 1 (lf1), which has defects in lateral organ polarity. LF1 encodes the HD-ZIP III transcription factor, which expressed in the adaxial area of lateral organs. LF1 can activate directly the expression of LITTLE ZIPPER family gene OsZPR4 and HD-ZIP II family gene OsHOX1, and OsZPR4 and OsHOX1 respectively interact with LF1 to form a heterodimer to repress the transcriptional activity of LF1. LF1 influences indole-3-acetic acid (IAA) content by directly regulating the expression of OsYUCCA6. Therefore, LF1 forms negative feedback loops between OsZPR4 and OsHOX1 to affect IAA content, leading to the regulation of lateral organs polarity development. These results reveal the cross-talk among HD-ZIP III, LITTLE ZIPPER, and HD-ZIP II proteins and provide new insights into the molecular mechanisms underlying the polarity development of lateral organs.

Multifloret spikelet improves rice yield.

The typical rice (Oryza sativa) spikelet contains a single fertile floret and produces only one grain; by contrast, Brachypodium distachyon spikelets contain multiple fertile florets and produce several grains. To increase yield, rice breeders have traditionally focused on panicle morphology (branch number and length, spikelet density), but have not considered the number of florets in each spikelet. Production of rice spikelets with more florets could further increase the number of grains per panicle. Here, we describe two novel approaches - altering meristem determinacy and restoring lateral floret formation - for breeding rice cultivars with a multifloret spikelet, thereby increasing the number of grains per panicle and potentially improving yield.

Characterization and fine mapping of nonstop glumes 2 (nsg2) mutant in rice (Oryza sativa L.).

In cereal crops, the grain number per panicle and the grain yield are greatly affected by the number of florets in a spikelet. In wild-type rice, a spikelet only produces one fertile floret and beneath the floret are a pair of sterile lemmas and a pair of rudimentary glumes. This study characterized a rice spikelet mutant nonstop glumes 2 (nsg2). In the nsg2 mutant, both the sterile lemmas and rudimentary glumes were elongated, and part of sterile lemma looked like a lemma in appearance, shape and size. Detailed histological analysis and qPCR analysis revealed that the sterile lemmas in the nsg2 mutant had homeotically transformed into lemma-like organs. This phenotype indicates that NSG2 is involved in the regulation of spikelet development and supports the long-held view that sterile lemmas were derived from the lemmas of the two lateral florets. This implies that the rice spikelet has the potential to be restored to the "three florets spikelet", which may have existed in its ancestors. Genetic analysis reveals that the nsg2 trait is controlled by a single recessive gene. The NSG2 gene was finally mapped between markers R-20 and R-39 on chromosome 7 with a physical region of 180 kb. The two MYB family factors LOC_Os07g44030 and LOC_Os07g44090 might be involved in the development of the spikelet and floral organ, and they were considered as candidate genes of NSG2. These results provide a foundation for map-based cloning and function analysis of NSG2, as well as evidence to support "three-florets spikelet" breeding in rice.

Extreme Suppression of Lateral Floret Development by a Single Amino Acid Change in the VRS1 Transcription Factor.

Increasing grain yield is an endless challenge for cereal crop breeding. In barley (Hordeum vulgare), grain number is controlled mainly by Six-rowed spike 1 (Vrs1), which encodes a homeodomain leucine zipper class I transcription factor. However, little is known about the genetic basis of grain size. Here, we show that extreme suppression of lateral florets contributes to enlarged grains in deficiens barley. Through a combination of fine-mapping and resequencing of deficiens mutants, we have identified that a single amino acid substitution at a putative phosphorylation site in VRS1 is responsible for the deficiens phenotype. deficiens mutant alleles confer an increase in grain size, a reduction in plant height, and a significant increase in thousand grain weight in contemporary cultivated germplasm. Haplotype analysis revealed that barley carrying the deficiens allele (Vrs1.t1) originated from two-rowed types carrying the Vrs1.b2 allele, predominantly found in germplasm from northern Africa. In situ hybridization of histone H4, a marker for cell cycle or proliferation, showed weaker expression in the lateral spikelets compared with central spikelets in deficiens Transcriptome analysis revealed that a number of histone superfamily genes were up-regulated in the deficiens mutant, suggesting that enhanced cell proliferation in the central spikelet may contribute to larger grains. Our data suggest that grain yield can be improved by suppressing the development of specific organs that are not positively involved in sink/source relationships.

LATERAL FLORET 1 induced the three-florets spikelet in rice

The spikelet is a unique inflorescence structure in grass. The molecular mechanisms behind the development and evolution of the spikelet are far from clear. In this study, a dominant rice mutant, lateral florets 1 (lf1), was characterized. In the lf1 spikelet, lateral floral meristems were promoted unexpectedly and could generally blossom into relatively normal florets. LF1 encoded a class III homeodomain-leucine zipper (HD-ZIP III) protein, and the site of mutation in lf1 was located in a putative miRNA165/166 target sequence. Ectopic expression of both LF1 and the meristem maintenance gene OSH1 was detected in the axil of the sterile lemma primordia of the lf1 spikelet. Furthermore, the promoter of OSH1 could be bound directly by LF1 protein. Collectively, these results indicate that the mutation of LF1 induces ectopic expression of OSH1, which results in the initiation of lateral meristems to generate lateral florets in the axil of the sterile lemma. This study thus offers strong evidence in support of the "three-florets spikelet" hypothesis in rice.

Natural diversity of inflorescence architecture traces cryptic domestication genes in barley (Hordeum vulgare L.)

Many-grained mutants occurring spontaneously among their less well-endowed field mates may have appeared to early farmers as fortunate twists of fate foreboding wealth and abundance. In domesticated barley, the number of kernel rows in spike can be tripled by recessive mutant alleles at the Six-rowed spike 1 (vrs1) locus that abolish the suppression of lateral spikelet fertility. In another barley row-type, so called intermedium-spike (int), lateral floret size is often intermediate between six-and two-rowed types. Phenotypic and sequence analyses of our intermedium-spike collection revealed that other genes can increase the size of florets and even stimulate occasional grain setting in lateral spikelets. Here, we show that a complete six-rowed phenotype occurs in a diverse panel of intermedium-spike barley carrying wildtype Vrs1 in the presence of the Int-c.a allele of the intermedium spike-c (int-c) gene, previously considered only as a modifier of lateral spikelet fertility. Int-c.a-type alleles had arisen before domestication and are associated with the enlargement of lateral florets in wild barley, suggesting that natural selection/evolution acts towards reduced lateral floret size. Since Int-c.a cannot overcome the suppression of lateral florets in the genomic background of wild barleys, we infer the existence of other gene loci, at which novel alleles or allelic combinations were selected for after domestication, to increase grain number of barley independently of Vrs1.

Genetic mapping of the labile (lab) gene: a recessive locus causing irregular spikelet fertility in labile-barley (Hordeum vulgare convar. labile)

The recessive labile locus mapped on chromosome 5HL causes irregular spikelet fertility and controls floret development as well as row-type in barley. The labile-barley displays a variable number of fertile spikelets at each rachis internode (0-3 fertile spikelets/rachis internode) which is intermediate between that observed in two- or six-rowed types. Previous re-sequencing of Vrs1 in 219 labile-barley (Hordeum vulgare L. convar. labile) accessions showed that all carried a six-rowed specific allele. We therefore hypothesized that this seemingly random reduction in spikelet fertility is most likely caused by the labile (lab) locus, which we aimed to phenotypically and genetically define. Here, we report a detailed phenotypic analysis of spikelet fertility in labile-barleys in comparison to two- and six-rowed genotypes using scanning electron microscopy analysis. We found that the first visible morphological deviation occurred during the stamen primordium stage, when we regularly observed the appearance of arrested central floral primordia in labile but not in two- or six-rowed barleys. At late stamen and early awn primordium stages, lateral florets in two-rowed and only some in labile-barley showed retarded development and reduction in size compared with fully fertile lateral florets in six-rowed barley. We used two F2 mapping populations to generate whole genome genetic linkage maps and ultimately locate the lab locus as a recessive Mendelian trait to a 4.5-5.8 cM interval at approximately 80 cM on chromosome 5HL. Our results will help identifying the role of the lab gene in relation to other spikelet fertility factors in barley.

Hexastichon and intermedium mutants in barley

Homozygous intermedium mutations from two-row barley are generally characterized by their lateral florets being of increased size and fertility. Median florets develop normally. Sixty allelism-tested intermedium mutations represent nine different gene loci, each locus possessing a particular type of lateral floret development. The intermedium character is usually recessive. Two cases of dominant mutations have been recorded. The F2 generation of certain loci primarily give rise to double-recessive hexastichon plants (for instance int-c x int-d). Other locus combinations give rise, in individual progenies, to double-recessive hexastichon or irregular and deformed F2 plants (for instance int-a x int-c), and others, finally, produce double-recessive irregular and deformed F2 plants exclusively and no hexastichons (for instance int-a x int-e). Hexastichon types, consequently, may arise in two ways: (1) by direct mutation in the two-row locus of distichon types (Vv), and (2) as double-recessives in the second generation of hybrid combinations of special intermedium loci and alleles.

Progressive promotion of lateral floret development in three-and four-gene combinations of intermedium genes in barley

Lundqvist, U. and Lundqvist, A. 1990. Progressive promotion of lateral floret development in three- and four-gene combinations of intermedium genes in barley. —Hereditas 113: 237–242. Lund. Sweden. ISSK 00 18–0661. Received September 19. 1990. Accepted October 4, 1990 Gene interaction promoting laterai floret development in barley was studied in a total of 220 F3 families from prominent F2-plants selected in three- and four-gene segregations derived from double mutant constellations of intermedium genes combined in crosses. The study was based on 4 independently inherited loci, the three recessive int-a. int-c, and int-e, and the semidominant Int-d. Three-gene F2-segregations. totally 17, were based on the 7 inrermedium genes a9. a27. a62, a63, a12. a36, and a65. The only four-gene segregation included a9, c63. d12, and e65. Awn development, fertility, and seed development for lateral florets of the main spike showed a highly significant promotion in three-gene segregants compared with the corresponding double mutants. An apparent further progressive enhancement of the lateral floret charactera was obtained when a fourth intermedium gene was introduced into the segregation, particularly for fertility and seed size. This observation may have practically important implications for plant breeding in six-rowed barley, when specifically efficient constellations of alleles belonging to different intermedium loci have been tried out.

The co-operation between intermedium genes and the six-row gene hex-v in a six-row variety of barley

The interactive competence of the genes promoting lateral floret development in the successful Svalöf six-row variety ‘Agneta’ was studied. In this variety, genes for enhanced development of lateral florets occur not only at the six-row locus hex-v, but also at a recessive intermedium locus, int-c. Three int-c mutants, c5, c13, and c29, were crossed to ‘Agneta thus there was no dominant normal allele at int-c present in the crosses, whereas the hex-v locus would segregate for the normal and the six-row allelic states. F2 data for lateral floret characters (awn length, fertility, seed size) were contrasted to standards considered to provide the phenotypic spectra corresponding to the different genotypic constituents of the F2 segregations (two-row variety, VV CC; int-c mutants, Wcc; six-row mutant, vv CC; and six-rowed variety ‘Agneta’, vv cc). The analyses of observed data in relation to expectations on various hypotheses for interaction between hex-v and int-c led to the following main conclusions. The degrees of phenotypic enhancement may differ among characters of lateral floret development and among int-c mutant genes. There are no indications that the int-c allele present in ‘Agneta’ is the superior enhancer of the hex-v gene vA in this variety. Nor can we presume that vA is a superior six-row gene. Most probably, it would pay the efforts in a plant-breeding program with six-row barley to try various constellations of hex-v and int-c mutants.

Lateral floret development in barley as influenced by recessive intermedium genes in a heterozygous state

Effects of gene interaction between heterozygous recessive intermedium genes and the semidominant Int-d gene on lateral floret development in barley were studied in various F, constellations with I, 2, or 3 int genes added to a heterozygous Int-d gene. The study was based on 13 recessive intermedium mutants belonging to the three most frequently mutated int loci, int-a (5 alleles), int-c (5 alleles), and int-e (3 alleles); and 5 alleles of the semidominant locus Int-d. The different loci were represented by the alleles a9a10a14a27a30; c3, c13, c29c62c63; e20e23e65; and d12, d22 d24d25d36. Comparisons between the different constellations were made on data for lateral floret awn length, fertility, and kernel size. The recessive int genes were able to enhance the expression of Int-d. The degree of enhancement may differ among allelic states of Int-d and among characters of lateral floret development, and there is progressive enhancement when two or three recessive int genes are added as potential enhancers to the initial effector. Differences among the three int loci in their enhancing potentials are shown at higher levels of the basic effect caused by the initial effector. Locus int-c is indicated to be the superior enhancer.

Enhancing interaction of recessive intermedium genes of barley in combinations with the six-row gene hex-v

The interactive competence of a number of genes promoting lateral floret development in barley was studied with particular reference to the interaction in dihybrid and trihybrid F2 segregations after crossing involving the six-row locus hex-v and the three most frequently mutating recessive intermedium loci. int-a, int-c. and int-e. The former locus was represented by the mutation hex-v3 (derived from the two-row variety ‘Bonus’) and the allelic state hex-vA in the six-row commercial variety ‘Agneta’. The latter loci were each represented by three allcles (-a9, -a10 -a27; -c5 -c13 -c29; -e20-e23 -e65) derived from the two-row varieties ‘Bonus’ or ‘Foma’, whereas ‘Agneta’ contributed the int-c allele -cA. F2 data for lateral floret characters (awn length. fertility, seed size) were contrasted to standards considered to provide the phenotypic spectra corresponding to the different genotypic constituents of the F2 segregations. The analyses of observed data in relation to expectations on various hypotheses for interaction between hex-v and the recessive int genes led to the following main conclusions. In no case were there indications of total failure to enhance the phenotypic expression above the level of the corresponding standard categories. The degrees of phenotypic enhancement may differ among characters of lateral floret development, among int loci, and among allelic states of the latter, whereas differences between the two representatives of hex-v were not discernible.

Lateral floret development in barley as influenced by recessive int genes in a heterozygous state

Lateral floret development in barley as influenced by recessive int genes in a heterozygous state

Gene interaction of induced intermedium mutations of two-row barley

Totally 13 intermedium mutants belonging to the more or less pronouncedly dominant locus Int-d of barley have been crossed to mutant representatives of 8 recessive int loci, and the dihybrid F2 classification patterns were analysed for deviations from ratios expected for gene combinations where previous analyses have excluded linkage as a disturbing factor. Among the present 98 combinations between lnt-d alleles and the 8 int loci, 40 indicated, with statistical significance, an enhancement of lateral floret development in double heterozygotes, leading to their being classed as intermedium D. The three Int-d alleles ranking in the top in this respect, d40, d36, and d50, are those showing the most pronounced degrees of dominance, which could indicate that the stronger the basic effect produced by the heterozygote Int-d gene, the more efficiently will a recessive int gene in heterozygote state be able to exert a reinforcement of the phenotypic expression. In the constellation of a heterozygote lnt-d allele and a homozygote recessive gene, tendencies to epistatic interaction (leading to the intermedium D phenotype), or, less frequently, to complementary interaction (leading to the double mutant phenotype) were observed, with a fairly even distribution to the 13 lnt-d alleles. Among the recessive int loci, int-c turned out to be the most efficient, both as enhancer in double heterozygote constellations with Int-d, and in the complementary interaction in the Dd cc gene constellation.

An intermedium gene present in a commercial six-row variety of barley*

Lundovist, U. and Lundovist, A. 1987. An intermedium gene present in a commercial six-row variety of barley. —Hereditas 107 131–135. Lund. Sweden. ISSN 0018–0661. Received December 11, 1986 In a study of the inheritance of seven recessive intermedium loci (int-a, -b, -c, -e, -f -h, and -i) in relation to the semidominant six-row locus hex-v as contributed by the highbred commercial six-row variety Svalof s ‘Agneta’, F2 segregations unexpectedly revealed the presence, in this variety, of genes for enhanced development of lateral florets not only at the hex-v locus, but also at an intermedium locus, int-c. Further analyses demonstrated, among 18 alleles of int-c, a variable competence of conferring a six-row phenotype to cc Vv plants, with a single hex-v allele from ‘Agneta’. The int-c allele from ‘Agneta’ is not among the “strong” alleles. Nevertheless, this allele is likely to influence the phenotype of ‘Agneta’ favourably, in view of the fact that among mutants from the two-row state, at the hex-v locus, the development of lateral florets is generally variable, and that, in comparison with commercial six-row varieties, their lateral kernels are smaller, with shorter awns. An unintentional selection for enhancing int genes occurring as spontaneous mutations seems likely in commercial six-row varieties. Then, an optimal gene co-operation may have been attained in the ‘Agneta’ variety, “stronger”c alleles running the risk of going beyond this optimum. Or, we are just in the beginning of a deliberate use of the possibilities inherent in the co-operation between int genes and alleles at the hex-v locus.

The influence of fertiliser nitrogen application on development and number of reproductive primordia in field-grown two- and six-rowed barleys

A field experiment using different 2- and 6-rowed barley cultivars grown at two contrasting nitrogen supply rates (40 and 150 kg N/ha) without water restrictions was carried out to explore how the number of leaf primordia and the rate of leaf emergence interact to modify the duration of different pre-heading phenological phases, and to analyse the dynamics of initiation and mortality of different subcomponents of grain number per unit area, in response to different fertiliser nitrogen regimes.The lower nitrogen supply (40 kg N/ha) delayed heading time as a consequence of higher phyllochron, without any effect on the final number of leaves initiated in the apex. The different fertiliser nitrogen rates only altered the co-ordination between tiller appearance and leaf emergence in two of the 6-rowed cultivars, whereas for the rest of the cultivars, co-ordination between leaf emergence and primary tiller appearance was not altered by fertiliser nitrogen rate. The lower nitrogen supply resulted in less fertile florets per ear due to a reduction in floret primordia survival, without any effect on the maximum number of spikelet primordia initiated in the apex. The higher floret abortion observed in 6-rowed compared with 2-rowed cultivars could be associated with the fact that lateral floret primordia, in those apical and basal positions within the ear in 6-rowed cultivars, are poorly developed with respect to the central spikelets.

Does function follow form? Principal QTLs for Fusarium head blight (FHB) resistance are coincident with QTLs for inflorescence traits and plant height in a doubled-haploid population of barley

Fusarium head blight (FHB), an important disease of barley in many areas of the world, causes losses in grain yield and quality. Deoxynivalenol (DON) mycotoxin residues, produced by the primary pathogen Fusarium graminearum, pose potential health risks. Barley producers may not be able to profitably market FHB-infected barley, even though it has a low DON level. Three types of FHB resistance have been described in wheat: Type I (penetration), Type II (spread), and Type III (mycotoxin degradation). We describe putative measures of these three types of resistance in barley. In wheat, the three resistance mechanisms show quantitative inheritance. Accordingly, to study FHB resistance in barley, we used quantitative trait locus (QTL) mapping to determine the number, genome location, and effects of QTLs associated with Type-I and -II resistance and the concentration of DON in the grain. We also mapped QTLs for plant height, heading date, and morphological attributes of the inflorescence (seeds per inflorescence, inflorescence density, and lateral floret size). QTL analyses were based on a mapping population of F1-derived doubled-haploid (DH) lines from the cross of the two-rowed genotypes Gobernadora and CMB643, a linkage map constructed with RFLP marker loci, and field evaluations of the three types of FHB resistance performed in China, Mexico, and two environments in North Dakota, USA. Resistance QTLs were detected in six of the seven linkage groups. Alternate favorable alleles were found at the same loci when different inoculation techniques were used to measure Type-I resistance. The largest-effect resistance QTL (for Type-II resistance) was mapped in the centromeric region of chromosome 2. All but two of the resistance QTLs coincided with QTLs determining morphological attributes of the inflorescence and/or plant height. Additional experiments are needed to determine if these coincident QTLs are due to linkage or pleiotropy and to more clearly define the biological basis of the FHB resistance QTLs. Plant architecture should be considered in FHB resistance breeding efforts, particularly those directed at resistance QTL introgression and/or pyramiding.

Growth and Final Weight of Central and Lateral Barley Grains under Mediterranean Conditions as Influenced by Sink Strength

Grain weight largely determines the end‐use quality of barley (Hordeum vulgare L.). In six‐rowed types, central spikelets commonly bear heavier grains than lateral spikelets. However, the extent to which such differences in weight are influenced by source availability remains unclear. Field experiments were carried out in nine rainfed trials of northeastern Spain and showed large variability in grain weight. Three six‐rowed genotypes were used: Barberousse, Orria, and Plaisant. Sink strength of mainstems was decreased by half at anthesis by sterilizing 50% of fertile central and lateral florets. Variation among spikelets in grain growth parameters [maximum grain filling rate (GFR) and grain filling duration (GFD)] and in final grain weight was evaluated in control and sink‐reduced spikes. Central grains outweighed lateral ones in control spikes from 7.3 to 14.8 mg across trials. Absolute differences in weight increased progressively in those trials favoring larger grains, although lateral to central grain weight ratios (L/C) remained fairly constant, with an average value of 0.77. This advantage of central grains was mostly caused by higher GFR values. A 50%‐reduced sink increased grain weight of both central and lateral spikelets similarly and did not diminish the absolute differences in weight found in control spikes. This result suggests that central grains have a greater intrinsic potential weight. While grain yield was partially source limited in all environments, lateral grains exhibited on average a major degree of source limitation (24.2%) than central grains (19.1%). Sink reduction increased grain weight of both central and lateral grains by stimulating GFR in all trials, and also by lengthening GFD in poor rainfed environments.