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Progressive N assimilation by maize kernels may constrain dry matter (DM) accumulation and final kernel weights (KW). We sought to better understand whole-plant and kernel N mechanisms associated with incremental DM and N accumulation patterns in kernels during grain fill. Maize was grown with multiple fertilizer N rates and N timings or plant densities to achieve a wide N availability gradient. Whole-plant DM and N sampling enabled determination of apparent N nutrition sufficiency at flowering (NNIR1) and when linear-fill began (NNIR3). Linear-plateau, mixed-effects models were fitted to kernel DM and N accumulation data collected weekly from early R3. Higher N supply, regardless of application timing or plant density, increased grain-fill duration (GFD) and, more inconsistently, effective grain-filling rate (EGFR). Kernels accumulated DM and N for similar durations. Both final KW and kernel N content increased consistently with N availability mostly because of higher kernel N accumulation rates (KNAR) and duration (KNAD). Both NNIR1 and NNIR3 were positively associated with KNAD and KNAR, and less strongly with EGFR. These results confirm the direct role of kernel N accumulation, in addition to prior NNI, in limiting KW gain rates and duration during grain filling.

Summary More than half of the world's food is provided by cereals, as humans obtain >60% of daily calories from grains. Producing more carbohydrates is always the final target of crop cultivation. The carbohydrate partitioning pathway directly affects grain yield, but the molecular mechanisms and biological functions are poorly understood, including rice (Oryza sativa L.), one of the most important food sources. Here, we reported a prolonged grain filling duration mutant 1 (gfd1), exhibiting a long grain‐filling duration, less grain number per panicle and bigger grain size without changing grain weight. Map‐based cloning and molecular biological analyses revealed that GFD1 encoded a MATE transporter and expressed high in vascular tissues of the stem, spikelet hulls and rachilla, but low in the leaf, controlling carbohydrate partitioning in the stem and grain but not in the leaf. GFD1 protein was partially localized on the plasma membrane and in the Golgi apparatus, and was finally verified to interact with two sugar transporters, OsSWEET4 and OsSUT2. Genetic analyses showed that GFD1 might control grain‐filling duration through OsSWEET4, adjust grain size with OsSUT2 and synergistically modulate grain number per panicle with both OsSUT2 and OsSWEET4. Together, our work proved that the three transporters, which are all initially classified in the major facilitator superfamily family, could control starch storage in both the primary sink (grain) and temporary sink (stem), and affect carbohydrate partitioning in the whole plant through physical interaction, giving a new vision of sugar transporter interactome and providing a tool for rice yield improvement.

Sorghum bicolor is a C4 grass widely cultivated for grain, forage, sugar, and biomass. The sorghum Dry Stalk (D) locus controls a qualitative difference between juicy green (dd) and dry white (D‐) stalks and midribs, and co‐localizes with a quantitative trait locus for sugar yield. Here, we apply fine‐mapping and genome‐wide association study (GWAS) to identify a candidate gene underlying D, and use nearly isogenic lines (NILs) to characterize the transcriptional, compositional, and agronomic effects of variation at the D locus. The D locus was fine‐mapped to a 36 kb interval containing four genes. One of these genes is a NAC transcription factor that contains a stop codon in the NAC domain in the recessive (dd) parent. Allelic variation at D affects grain yield, sugar yield, and biomass composition in NILs. Green midrib (dd) NILs show reductions in lignin in stalk tissue and produce higher sugar and grain yields under well‐watered field conditions. Increased yield potential in dd NILs is associated with increased stalk mass and moisture, higher biomass digestibility, and an extended period of grain filling. Transcriptome profiling of midrib tissue at the 4–6 leaf stages, when NILs first become phenotypically distinct, reveals that dd NILs have increased expression of a miniature zinc finger (MIF) gene. MIF genes dimerize with and suppress zinc finger homeodomain (ZF‐HD) transcription factors, and a ZF‐HD gene is associated with midrib color variation in a GWAS analysis across 1,694 diverse sorghum inbreds. A premature stop codon in a NAC gene is the most likely candidate polymorphism underlying the sorghum D locus. More detailed understanding of the sorghum D locus could help improve agronomic potential in cereals.

Grain yields of maize in the lowland tropics are generally limited by short days and high temperatures that minimize durations of incident light. Little has been published on the effects of this limiting environment on the genetics of grain filling rate (GFR), and grain filling period (GFP) in tropical maize germplasm. This study sought to address these limitations. A set of 8 elite maize inbreds of tropical origin and their 28 diallel hybrids were grown in three seasons at Waimanalo, Hawaii, USA. Seasonal differences included > 100% differences in values of photosynthetic active radiation (PAR) during grain formation. Information was sought on the performance variations and the genotype by season interactions for GFR, GFP, days to mid-silk (DTS), kernel weight, with estimates of general (GCA) and specific combining ability (SCA) and their interactions with seasons. Significant differences occurred for inbreds, hybrids, and genotype by season interactions, GCA and SCA effects and their interactions with seasons, which could be attributed primarily to the differences in PAR values among seasons in Hawaii during grain filling. Additive genetic effects predominated for GFR and GFP. Breeding approaches that take advantage of additive gene effects including hybrid breeding with evaluations in multiple Hawaii seasons may be used to alter GFR and GFP.

Knowledge about the comparative development and grain filling of winter cereals under different environmental conditions is important for stable and high yielding crop production. The objective of this work was to compare patterns of grain filling in bread wheat, barley and triticale grown in the Pannonian region, as well as to investigate relationships among grain filling parameters, time to anthesis and grain yield. The trials with 12 winter cereal genotypes were carried out in four successive seasons at the location Novi Sad, Serbia. Results of this study showed that all studied grain filling parameters were significantly influenced by species, cultivar, growing season, and species by growing season interaction. Longer duration of grain filling period and period to maximum grain filling were observed in triticale and wheat cultivars compared with six and two-rowed barley. Two-rowed barley cultivars had a higher grain filling rate than other cultivars. Furthermore, a negative association between time to anthesis and grain yield indicates that cultivars with the long pre-anthesis period are not recommended for the agro-ecological conditions of the Pannonian plain. Generally, medium early cultivars of small grain cereals had the highest grain weight within species and spike type, suggesting that medium early cultivars have a balanced ratio of pre-anthesis and grain filling period. High values of final grain weight in different growing seasons indicate that weather conditions in the Pannonian plain are mainly suitable for grain growth.

Individual grain weight is a major yield component in wheat. To provide a comprehensive understanding of grain weight determination, the carpel size at anthesis, grain dry matter accumulation, grain water uptake and loss, grain morphological expansion, and final grain weight at different positions within spikelets were investigated in a recombinant inbred line mapping population of bread wheat (Triticum aestivum L.)×spelt (Triticum spelta L.). Carpel size, grain dry matter and water accumulation, and grain dimensions interacted strongly with each other. Furthermore, larger carpels, a faster grain filling rate, earlier and longer grain filling, more grain water, faster grain water absorption and loss rates, and larger grain dimensions were associated with higher grain weight. Frequent quantitative trait locus (QTL) coincidences between these traits were observed, particularly those on chromosomes 2A, 3B, 4A, 5A, 5DL, and 7B, each of which harboured 16−49 QTLs associated with >12 traits. Analysis of the allelic effects of coincident QTLs confirmed their physiological relationships, indicating that the complex but orderly grain filling processes result mainly from pleiotropy or the tight linkages of functionally related genes. After grain filling, distal grains within spikelets were smaller than basal grains, primarily due to later grain filling and a slower initial grain filling rate, followed by synchronous maturation among different grains. Distal grain weight was improved by increased assimilate availability from anthesis. These findings provide deeper insight into grain weight determination in wheat, and the high level of QTL coincidences allows simultaneous improvement of multiple grain filling traits in breeding.

Summary Grain size and filling are two key determinants of grain thousand‐kernel weight (TKW) and crop yield, therefore they have undergone strong selection since cereal was domesticated. Genetic dissection of the two traits will improve yield potential in crops. A quantitative trait locus significantly associated with wheat grain TKW was detected on chromosome 7AS flanked by a simple sequence repeat marker of Wmc17 in Chinese wheat 262 mini‐core collection by genome‐wide association study. Combined with the bulked segregant RNA‐sequencing (BSR‐seq) analysis of an F2 genetic segregation population with extremely different TKW traits, a candidate trehalose‐6‐phosphate phosphatase gene located at 135.0 Mb (CS V1.0), designated as TaTPP‐7A, was identified. This gene was specifically expressed in developing grains and strongly influenced grain filling and size. Overexpression (OE) of TaTPP‐7A in wheat enhanced grain TKW and wheat yield greatly. Detailed analysis revealed that OE of TaTPP‐7A significantly increased the expression levels of starch synthesis‐ and senescence‐related genes involved in abscisic acid (ABA) and ethylene pathways. Moreover, most of the sucrose metabolism and starch regulation‐related genes were potentially regulated by SnRK1. In addition, TaTPP‐7A is a crucial domestication‐ and breeding‐targeted gene and it feedback regulates sucrose lysis, flux, and utilization in the grain endosperm mainly through the T6P‐SnRK1 pathway and sugar–ABA interaction. Thus, we confirmed the T6P signalling pathway as the central regulatory system for sucrose allocation and source–sink interactions in wheat grains and propose that the trehalose pathway components have great potential to increase yields in cereal crops.

When wheat is subjected to heat stress during grain filling, metabolic adaptation reallocates resources to deposition of reserves and prioritizes a stable filling rate. Abstract Wheat (Triticum aestivum) is particularly vulnerable to heat stress during the grain filling stage, and this can adversely affect the final yield. However, the underlying physiological and molecular mechanisms are largely unknown. In this study, the effects of heat stress on grain filling were investigated using wheat varieties with different levels of thermotolerance. Decreased grain weights and filling durations, increased protein contents, and stable filling rates across diverse varieties under different heat regimes suggested a general mechanism for heat adaptation. Proteomic analysis identified 309 heat-responsive proteins (HRPs), and revealed a general decrease in protein synthesis components and metabolic proteins, but a significant increase in stress-response proteins and storage proteins. Metabolomic analysis identified 98 metabolites specifically changed by heat stress, and suggested a global decrease in the content of carbohydrate metabolites, an increased content of amino acids, and stable levels of starch synthesis precursors. The energy-consuming HRPs suggested that less energy was channelled into metabolism and protein synthesis, whereas more energy was allocated to the stress response under elevated heat conditions. Collectively, the data demonstrated a widely distributed mechanism for heat adaptation of metabolism, in which the assimilation and energy required for metabolism and protein synthesis are reallocated to heat protection and deposition of reserves, resulting in increased storage protein accumulation and a stable filling rate.

Rice grain yield and quality are predicted to be highly vulnerable to global warming. Five genotypes including heat-tolerant and susceptible checks, a heat-tolerant near-isogenic line and two hybrids were exposed to control (31 °C/23 °C, day/night), high night-time temperature (HNT; 31 °C/30 °C), high day-time temperature (HDT; 38 °C/23 °C) and high day- and night-time temperature (HNDT; 38 °C/30 °C) treatments for 20 consecutive days during the grain-filling stage. Grain-filling dynamics, starch metabolism enzymes, temporal starch accumulation patterns and the process of chalk formation were quantified. Compensation between the rate and duration of grain filling minimized the impact of HNT, but irreversible impacts on seed-set, grain filling and ultimately grain weight were recorded with HDT and HNDT. Scanning electron microscopy demonstrated irregular and smaller starch granule formation affecting amyloplast build-up with HDT and HNDT, while a quicker but normal amylopast build-up was recorded with HNT. Our findings revealed temporal variation in the starch metabolism enzymes in all three stress treatments. Changes in the enzymatic activity did not derail starch accumulation under HNT when assimilates were sufficiently available, while both sucrose supply and the conversion of sucrose into starch were affected by HDT and HNDT. The findings indicate differential mechanisms leading to high day and high night temperature stress-induced loss in yield and quality. Additional genetic improvement is needed to sustain rice productivity and quality under future climates.

Anatomical and histochemical studies of ovary and caryopsis of sorghum reveal the importance of the chalazal complex in transporting nutrients from maternal sources to the filial diploid embryo and triploid endosperm. The presence of starch, protein, lipid, Ca, K, Mg, and Fe in various tissues at different stages of development can be revealed by a variety of histochemical techniques. Vascular supply ends at the base of the ovary and transport occurs through vascular parenchyma, pigment strand and nucellar projection where symplastic continuity is broken. Nutrients unloaded into an apoplastic placental sac then enter the endosperm and embryo through the aleurone transfer cells. The later possess characteristic wall ingrowth. The single layer of aleurone surrounding the endosperm may also help in transport during later stages of grain-filling. Grain-filling in C4 sorghum is compared with other C4 and C3 grasses showing the variety of strategies evolved to transport nutrients into filial tissues. Standardization of terminologies to describe the tissues of the crease region will help in further research and communication.