Final Kernel Weight Research Articles

Late-vegetative and reproductive-stage nitrogen determinants of kernel weight in maize

Differences in maize kernel weight (KW) due to soil N availability have usually been associated with the indirect role that N plays in the prevailing source-sink balance for carbon at the canopy level during reproductive development. Additionally, KW changes have also been described in response to N assimilate supply to the reproductive tissues (i.e., direct N role). Our objective was to determine the most important source factors driving KW gains with varying N and dry matter supply via the multivariate study of pertinent phenotypic relationships during late vegetative and reproductive growth. Maize responses to N rate plus N timing and(or) plant density treatments were followed in five field experiments. Principal component analyses (PCA) and partial least square (PLS) regressions explored the relationships between final KW, plant-component biomass (DM), plant-component N concentrations, N uptake at V12, R1, R3, and R6 stages, plus specific grain-filling parameters including effective grain-filling rate (EGFR), grain-filling duration (GFD), and the kernel N accumulation features of rate and duration (KNAR and KNAD, respectively). Across experimental sites, grain yields at the highest N rate were 93–196 % above those with zero N. These yield increases coincided with incremental gains in both kernel number and KW at maturity. Via PCA, final KW was more positively correlated with vegetative-tissue N concentrations (stem N % at V12 and R1, leaf N % at R1 and R3) than with biomass during the critical period. In addition, final KW was more correlated with stem biomass at R6 than at V12, R1 or R3 stages. Via PLS, the majority of KW variability was explained by critical-period N contents, either in leaf (V12 and R1) or stem (R3), while at R6 stem DM was more important in KW determination. Finally, when tested against its physiological determinants, KW at maturity was largely associated with KNAR (via PCA) and EGFR (via PLS) during the grain-filling period. Overall, these results suggest the dual importance of N in determining KW at maturity through both leaf and stem N supply during the critical period and through dry matter allocation during the effective grain filling period.

Effect of Climatic Conditions Caused by Seasons on Maize Yield, Kernel Filling and Weight in Central China

In order to evaluate the effects of climatic conditions on maize grain yield (GY), kernel weight (KW), and kernel filling and identify the optimal climatic factors for GY and KW, 2-year field experiments in three seasons, i.e., spring (SPM), summer (SUM), and autumn (AUM), on maize were conducted in Central China. The results showed that SUM had more growing degree days (GDDs) than SPM and AUM due to the higher mean temperature (MT), and also resulted in higher temperature stress (killing degree days (KDDs)) in maize growth duration. Meanwhile, after silking, SPM and SUM had more GDDs and KDDs than AUM because of the higher MT, and the accumulated solar radiation (Ra) for SUM was significantly higher than for SPM and AUM. The GY of SPM was significantly higher than that of SUM and AUM, while SUM’s GY was always the lowest, because the GDDGD, MTGD, and KDDGD played significantly negative roles on GY. The final KW for SUM was always the lowest, with GDD, MT, KDD, and Ra causing significantly negative effects, and M△T and precipitation having significant positive effects, resulting in a lower kernel filling rate during the linear kernel filling period (KFRlkf) and a lower GDD at the maximum kernel filling rate (GDDKFRmax). Maize KFRlkf has significant negative linear dependences on GDD, MT, and Ra. In summary, because of the higher MT, KDD, and GDD during maize growth and kernel filling duration negatively affecting the maize kernel filling rate, the GY and KW for SPM were the highest, and for SUM, they were the lowest; therefore, farmers should plant SPM first and then AUM in Central China.

The impact of drought and heat stress at flowering on maize kernel filling: Insights from the field and laboratory

Drought and exposure to high temperature at flowering strongly affect maize production. However, previous studies mostly focused on the effects of the stressors on kernel number. The impacts of drought and heat in isolation and combination at flowering on maize kernel filling and final kernel weight have received limited attention. Here we investigated the effects of three stressors (drought, heat, and drought plus heat) at flowering phase (from tasseling to seed setting completed) on maize kernel filling in field site plots covered with a rainout shelter and under laboratory conditions with kernel in vitro culture. In two experiments, the kernel weight of plants exposed to flowering stressors was significantly decreased relative to controls. In the field experiment, exposure to stressors decreased kernel weight by 5.0–8.3% for the basal kernel and 5.6–11.0% for the apical kernel. Accelerated leaf senescence and decreased rate of photosynthesis were observed in the field. Kernel weight was also reduced by 18.0–37.6% in the laboratory with kernel in vitro culture, suggesting that early stressors at flowering led to a sink (kernel) limit. Carbohydrate metabolism and starch biosynthesis were impaired by drought and heat individually and their combination. During the early kernel filling stage, the activities of sucrose invertase, sucrose synthase, ADP-Glc pyrophosphorylase, and starch synthase were decreased at 10 and 20 days after pollination (DAP) in the stress treatment. For both basal and apical kernels, the correlation analyses indicated the mean kernel filling rate correlated positively with zeatin riboside content at DAP 10 and indole-3-acetic acid content at DAP 20. This study provides useful information to support the development of drought- and heat-tolerant hybrids and effective management strategies to improve crop yields.

The central role of ear nitrogen uptake in maize endosperm cell and kernel weight determination during the lag period

Although kernel weight (KW) has proven to be an increasingly important driver behind grain yield (GY) variability in modern maize hybrids, nitrogen’s (N) role in the determination of individual sink capacity (i.e., potential KW) during the lag phase of reproductive development remains unclear. The research objective was to study the relationships between endosperm cell number (ECN) during the lag phase (an indicator of potential KW) and final KW within the context of changing plant N dynamics in field-grown maize during the lag phase. Field experiments tested 3–4 N rates at different plant densities or N timing applications over a 3-year period (2017–2019). Kernels were removed from ears collected at 9, 10, 13, and/or 17 days after silking (DAS) and treated for ECN determination via tissue dissection, staining, enzymatic digestion, and digital microscopy. Plant growth rate, plant N uptake rate, ear growth rate, and ear N allocation rate during the lag phase were studied, as well as plant growth rate during the critical period and its relationship with kernel number per plant. Low N treatments consistently reduced ECN, thereby limiting KW during this early post-silking stage. Following data normalization, the majority of final KW variation was consistently explained by ECN across all experiments and relative DAS sampling times. Nitrogen rate effects on potential KW were not associated with plant growth rate per kernel during the critical period bracketing silking. ECN was highly correlated with ear N allocation rate during the lag phase, but not with plant N uptake rate during that period. Overall, these results show that higher N availability, independently of N timing and plant density, increased final KW by enhancing the sink capacity of individual kernels (at a wide range of kernel numbers per plant) via gains in ECN during lag-period development.

Kernel Water Relations and Kernel Filling Traits in Maize (Zea mays L.) Are Influenced by Water-Deficit Condition in a Tropical Environment.

Water deficit is a major limiting condition for adaptation of maize in tropical environments. The aims of the current observations were to evaluate the kernel water relations for determining kernel developmental progress, rate, and duration of kernel filling, stem reserve mobilization in maize. In addition, canopy temperature, cell membrane stability, and anatomical adaptation under prolonged periods of pre- and post-anthesis water deficit in different hybrids was quantified to support observations related to kernel filling dynamics. In this context, two field experiments in two consecutive years were conducted with five levels of water regimes: control (D1), and four water deficit treatments [V10 to V13 (D2); V13 to V17 (D3); V17 to blister stage (D4); blisters to physiological maturity (D5)], on three maize hybrids (Pioneer 30B80, NK 40, and Suwan 4452) in Expt. 1. Expt. 2 had four water regimes: control (D1), three water deficit treatments [V10 to anthesis (D2); anthesis to milk stage (D3); milk to physiological maturity (D4)], and two maize hybrids (NK 40 and Suwan 4452). Water deficit imposed at different stages significantly reduced maximum kernel water content (MKWC), kernel filling duration (KFD), final kernel weight (FKW), and kernel weight ear–1 while it increased kernel water loss rate (KWLR), kernel filling rate (KFR), and stem weight depletion (SWD) across maize hybrids in both experiments. The lowest MKWC under water deficit was at D3 in both experiments, indicating that lower KFR results in lowest FKW in maize. Findings indicate that the MKWC (R2 = 0.85 and 0.41) and KFR (R2 = 0.62 and 0.37) were positively related to FKW in Expt. 1 and 2, respectively. The KFD was reduced by 5, 7, 7, and 11 days under water deficit at D3, D4 in Expt. 2 and D4, D5 in Expt. 1 as compared to control, respectively. Water deficit at D5 in Expt. 1 and D4 in Expt. 2 increased KWLR, KFR, and SWD. In Expt. 2, lower canopy temperature and electrical conductivity indicated cell membrane stability across water regimes in NK 40. Hybrid NK 40 under water deficit had significantly higher cellular adaptation by increasing the number of xylem vessel while reducing vessel diameter in leaf mid-rib and attached leaf blade. These physiological adjustments improved efficient transport of water from root to the shoot, which in addition to higher kernel water content, MKWC, KFD, KFR, and stem reserve mobilization capacity, rendered NK 40 to be better adapted to water-deficit conditions under tropical environments.

Kernel weight responses to the photothermal environment in maize dent × flint and flint × flint hybrids

AbstractMaize (Zea mays L.) grain yield is assumed to be source limited during the flowering period but sink limited during grain growth; however, environmental restrictions during active grain filling may strongly affect final kernel weight (KW). In this study, we evaluated the effect of natural changes in photothermal conditions during lag phase (LP) and effective grain‐filling period (EGFP) on KW, its physiological determinants, and the post‐flowering source–sink relationships of flint and semident germplasm. F1 hybrids of flint × flint and dent × flint background were tested during four seasons (Y1, Y2, Y3, and Y4). Across years, the highest KW (286 mg) was obtained under the maximum photothermal quotients during LP (PTQLP = 1.18 MJ m−2 °C−1) and EGFP (PTQEGFP = 1.07 MJ m−2 °C−1) of Y2, whereas the smallest KW (252 mg) and source–sink ratio during grain filling was obtained under the lowest PTQEGFP (.79 MJ m−2 °C−1) of Y3. Supra‐optimum temperatures during LP of Y3 negatively affected potential KW determination, and hence kernel growth rate (P < .001) as a result of reduced assimilate availability per kernel. Hybrids dent × flint exhibited higher grain yield, kernel number, and plant growth around flowering than flint × flint throughout evaluated seasons but had reduced source–sink relationship during grain filling (P < .05) and increased KW sensitivity (P < .001) to changes in the photothermal conditions. Results emphasized the importance of the photothermal environment during grain filling on KW determination (particularly for seasons with great photothermal imbalance between filling subphases) as well as the dependency of KW responses on the genetic background.

Endosperm cell size reduction caused by osmotic adjustment during nighttime warming in rice

High night temperature (HNT) often reduces yield in field crops. In rice, HNT during the ripening stage diminishes endosperm cell size, resulting in a considerable reduction in final kernel weight; however, little is known about the underlying mechanisms at cell level. In this study, we performed picolitre pressure-probe-electrospray-ionization mass spectrometry to directly determine metabolites in growing inner endosperm cells of intact seeds produced under HNT conditions, combining with 13C feeding and water status measurements including in situ turgor assay. Microscopic observation in the inner zone suggested that approximately 24.2% of decrease in cell expansion rate occurred under HNT at early ripening stage, leading to a reduction in cell volume. It has been shown that HNT-treated plants were subjected to mild shoot water deficit at night and endosperm cell turgor was sustained by a decline in osmotic potential. Cell metabolomics also suggests that active solute accumulation was caused by a partial inhibition of wall and starch biosynthesis under HNT conditions. Because metabolites were detected in the single cells, it is concluded that a partial arrest of cell expansion observed in the inner endosperms was caused by osmotic adjustment at mild water deficit during HNT conditions.

Different leaf water dynamics post-anthesis affects the final kernel weight in wheat

Drought stress during wheat (Triticum aestivum L.) grain filling can affect the grain yield and its quality; effects that in some regions can be increased by the event La Niña, the cold phase of the climatic phenomenon El Niño Southern Oscillation. The aim was to evaluate if the evolution of relative water content (RWC) and stomatal conductance (gs) can be related to grain yield and its quality. Four spring wheat cultivars grown in the southern cone of South America were subjected to two levels of irrigation applied after anthesis, a well-watered treatment (Control), and another with 50 % of the Control (Stress). RWC and gs were determined weekly during the stress period and agronomic traits at harvest. With a mean grain yield decrease of 14.8 % (P<0.05), two types of response to water deficit were identified for RWC and gs. Two cultivars (Biointa 1001 and LE2333) had slower RWC and gs decreases, which were related to a lower ratio of kernels to aerial biomass (P<0.05), and to a non-significant effect in kernel weight. While, in the other cultivars (LE2249 and LE2331), the water deficit caused a more rapid RWC and gs decrease related to a higher ratio of kernels to aerial biomass, and a significant kernel weight decrease (10.4 and 20.7 %, respectively). The latter cultivars, that had more rapid leaf dehydration under water stress, may also have been source-limited during the grain-filling period.

Engineering Biomimetic Calcium Phosphate Nanoparticles: A Green Synthesis of Slow-Release Multinutrient (NPK) Nanofertilizers.

Biomimetic calcium phosphate nanoparticles (CaP) have been actively used in biomedicine, due to their high biodegradability and biocompatibility. However, much less progress has been made regarding their application in precision agriculture, i.e., for the controlled delivery of active species to plants. Herein, we report a straightforward and green synthetic method to dope CaP with potassium (K) and nitrogen (N, as nitrate and urea). By modulating the synthetic conditions in terms of maturation time (at 37 °C) and doping, we prepared K- and N-doped nanoparticles in the form of either amorphous calcium phosphate (ACP) or nanocrystalline apatite (Ap) and studied the impact of the dopants on the ACP-to-Ap transformation pathways. Importantly, we found out that ACP, isolated at low maturation times, incorporates nitrogen (in the form of nitrate and urea) to a larger extent than Ap (2.6 vs 1.1 wt %, respectively). Multinutrient nanofertilizers (so-called nanoU-NPK) with the following composition (wt %) were obtained: Ca (23.3), P (10.1), K (1.5), NO3 (2.9), and urea (4.8). The nanoU-NPK provides a slow and gradual release of the most important plant macronutrients (NPK), with N in two chemical forms, and different kinetics. The concentration of nutrients supplied by 10 g L-1 of nanoU-NPK to the media after 1 week (in mg L-1) was Ca (27.0), P (6.2), K (41.0), NO3 (134.0), and urea (315.0). Preliminary tests on durum wheat have shown that the application of nanoU-NPK allows reducing the amount of nitrogen supplied to the plants by 40% with respect to a conventional treatment, without affecting the final kernel weight per plant. The application of these slow-release NPK nanofertilizers is a promising strategy toward enhancing the efficiency of the fertilization, complying with the concept of precision agriculture.

The role of the exponential and linear phases of maize (Zea mays L.) ear growth for determination of kernel number and kernel weight

Abstract Although the importance of the critical period for establishing the maize (Zea mays L.) yield components of kernel number (KN) and kernel weight (KW) is well known, there is little detailed information on how early ear development during the critical period and lag phase are influenced by hybrid improvement or N stress. Seven hybrids commercialized between 1946 and 2015 were compared under a range of N stress created by five N fertilizer treatments: no N applied or a total of 220 kg N ha−1 where N was applied either at the four-leaf stage (V4), at flowering, or a combination of these two times. Ears (cobs plus kernels) were sampled five times at weekly intervals, from approximately 14 days before until 14 days after flowering, to determine dry weight, N concentration, and N content. The dynamics of these traits were modeled using an expolinear function and parameters estimated using a non-linear mixed model. Hybrid and N treatment influences on the exponential and linear phases of ear growth were determined by comparing parameters. The greatest impact of genotype or plant N status was realized in the linear phase when modern hybrids exhibited higher linear growth rates and ears accumulated progressively more N with increasing V4 stage N rate. Neither genotype nor N stress resulted in different ear growth or N accumulation rates during the exponential phase; however, modern hybrids exhibited a longer exponential phase duration. Use of expolinear function with parameters estimated within the non-linear mixed models enabled us to gain insight on development phases modified as the result of genetic improvement or affected by N stress. During the exponential phase, which overlaps with the lag phase when cell division is a key determinant of potential kernel weight and sink strength, neither hybrid era nor N treatments substantially impacted ear growth. Instead, the most important early ear growth characteristics to differentially impact final KN and KW at maturity were the duration of the lag phase and ear growth rates during the linear phase. Future research is suggested to unravel how the duration of the exponential phase is related to cell number and kernel size determination, and how these in turn affect kernel growth rate during the linear phase.

Source-sink relations and kernel weight in maize inbred lines and hybrids: Responses to contrasting nitrogen supply levels

Maize (Zea mays, L.) kernel weight (KW) is regulated by the source-sink relationship at the early grain-filling period, when potential kernel size is defined, and at the effective grain-filling period, when final KW is established. The relative importance of each period is expected to depend upon the occurrence or not of environmental restrictions to plant growth. The objective of this study was to analyze the effect of nitrogen (N) availability on KW determination in two different groups of genotypes (inbreds and hybrids) when it is caused by changes in the source-sink relationship at mentioned periods. Six inbred lines of different genetic background and their F1 derived hybrids were tested in the field under two contrasting soil N levels (N0: low; N1: high). Almost all evaluated traits had greater mean values (P < 0.001) at N1 than at N0 level, except the plant growth rate per kernel during the critical period for kernel set (PGRCP kernel−1). However, the magnitude of the response to increased N was considerably higher for hybrids than for inbreds. Under contrasting N availability, plant growth during grain filling (PGGF) was the main determinant of increments registered in the source-sink ratio during this stage (i.e. PGGF kernel−1), and KW was chiefly associated with this ratio (r2 ≥ 0.50, P < 0.001). By contrast, the association between KW and PGRCP kernel-1 was important only at N1, and more robust for hybrids (r2 = 0.61, P < 0.001) than for inbreds (r2 = 0.34, P < 0.001). The uncoupling between both source-sink ratios observed at N0, as a result of a decreased post-flowering growth in this treatment, was responsible for differences in KW response to the PGRCP kernel−1 detected between N levels. The KW of inbreds was almost unaffected by mentioned N effects on the analyzed source-sink ratios, a trend attributed to negative inbreeding depression effects on traits controlling seed expansion. These differential responses between genotypic groups should be considered in breeding programs targeting KW increase.

Timing of Water Deficit Limits Maize Kernel Setting in Association With Changes in the Source-Flow-Sink Relationship.

The kernel setting of maize varies greatly because of the timing and intensity of water deficits. This variation can limit leaf productivity (source), the translocation of assimilated sugars (flow), and yield formation (sink). To explain the decline in kernel setting of maize under water deficits from the perspective of source-flow-sink, a 3-year experiment was conducted under a rain shelter. Five water regimes were studied. One regime included well-irrigated (CK) treatment. Four regimes involved water deficits: irrigation was withheld during the 6- to 8-leaf stage (V6−8), the 9- to 12-leaf stage (V9−12), the 13-leaf stage to tasseling stage (V13−T), and the silking stage to blister stage (R1−2). Water deficit effects on kernel setting began when the water deficit occurred at V9 and became more significant with time. Kernel weight was reduced by 12 and 11% when there were water deficits during V9−12 and V13−T, respectively. This was the result of reduced leaf area (limited source) and an altered vascular bundle in the ear peduncles (limited assimilate flow). The reduced vascular bundle number, rather than the ear peduncle cross-sectional area, significantly affected the final kernel weight when exposed to a water deficit prior to the silking stage. The water deficits prior to and close to the flowering stage significantly reduced ear kernel number; that is, 14 and 19% less during V13−T and R1−2, respectively, compared with the kernel number during the CK treatment. This reflects a smaller sink under water deficit conditions. Additionally, ovary size was reduced the most in the V13−T water deficit compared with other treatments. After rewatering, the water deficit before or during flowering stage continued to have residual effects on grain-filling in the late growth period. The grain-filling rate decreased under the V9−12 water deficit; the grain-filling duration shortened under the R1−2 water deficit; and both negative effects occurred under the V13−T water deficit. This study clearly indicated that (1) the water deficit during the vegetative organ rapid growth period both limited leaf source development and assimilate flow and slowed down kernel development, and (2) the water deficit just before and during flowering reduced kernel sink. Deficits at both times could retard grain-filling and reduce maize yield. The results of the present study might guide irrigation practices in irrigated maize or inform the management of sowing time in rainfed maize, to desynchronize the water deficit and the plant’s reactions to such deficits at different stages.

Spraying exogenous 6-benzyladenine and brassinolide at tasseling increases maize yield by enhancing source and sink capacity

The endosperm contributes approximately 80% of the final kernel weight in maize. Plant hormones that affect cell proliferation might increase endosperm cells and establish greater kernel sink capacity, which would improve grain yield. In this study, 6-benzyladenine (BA) and brassinolide (BR) were separately sprayed at tasseling to regulate kernel endosperm cell division, and EDAH (a mixture of ethephon and diethyl aminoethyl hexanoate) was foliar applied at the jointing stage to enhance lodging resistance. The results showed that the two hormones sprayed at tasseling could hamper leaf senescence, improve photosynthesis and significantly increase the number of kernel endosperm cells by 6.2-40.4%, resulting in a high grain filling rate and high kernel weight. Additionally, the tasseling treatments increased grain yield by 2.9-16.0% in both years. There were no significant differences between EDAH and control conditions in sink capacity and yield. The correlation analysis suggested that grain weight and yield had a close correlation with the number of endosperm cells and the filling rate in the lag stage. We concluded that the lag stage played a vital role in constructing sink capacity, and treating maize with BA or BR at tasseling can apparently increase the source and sink capacity to improve the grain filling rate and yield.

Maize kernel weight responses to sowing date-associated variation in weather conditions

Variation in weather conditions during grain filling has substantial effects on maize kernel weight (KW). The objective of this work was to characterize variation in KW with sowing date-associated weather conditions and examine the relationship between KW, grain filling parameters, and weather factors. Maize was sown on eight sowing dates (SD) at 15–20-day intervals from mid-March to mid-July during 2012 and 2013 in the North China Plain. With sowing date delay, KW increased initially and later declined, and the greatest KW was obtained at SD6 in both years. The increased KW at SD6 was attributed mainly to kernel growth rate (Gmean), and effective grain-filling period (P). Variations in temperature and radiation were the primary factors that influenced KW and grain-filling parameters. When the effective cumulative temperature (AT) and radiation (Ra) during grain filling were 950°C and 1005.4MJm−2, respectively, P and KW were greatest. High temperatures (daily maximum temperature [Tmax]>30.2°C) during grain filling under early sowing conditions, or low temperatures (daily minimum temperature [Tmin]<20.7°C) under late sowing conditions combined with high diurnal temperature range (Tmax-min>7.1°C) decreased kernel growth rate and ultimately final KW. When sowing was performed from May 25 through June 27, higher KW and yield of maize were obtained. We conclude that variations in environmental conditions (temperature and radiation) during grain filling markedly affect growth rate and duration of grain filling and eventually affect kernel weight and yield of maize.

Contribution of Reserves to Kernel Weight and Grain Yield Determination in Maize: Phenotypic and Genotypic Variation

Maize (Zea mays L.) kernel weight (KW) and grain yield depend on plant growth during active grain filling and reserves use. The objective of our study was to analyze the phenotypic and genotypic variation in these traits in a family of recombinant inbred lines (RIL). In two field experiments we measured plant grain yield (PGY) and its components (KW and kernel number per plant, KNP), biomass production per plant and per kernel during active grain filling, and apparent reserves use (ARU) per plant (ARUP, difference between PGY and plant biomass production during active grain filling) and per kernel (ARUK, difference between KW and plant biomass production per kernel during active grain filling). Heritability (h2) and phenotypic plasticity were computed for all traits. Large differences were always evident among genotypes, but phenotypic plasticity was (i) low for KW and plant biomass at R2 and physiological maturity; (ii) intermediate for KNP and PGY; and (iii) high for plant growth, plant growth per kernel after R2, and ARUs. Traits with highest h2 were KW (0.70), KNP (0.61), and ARUP (0.59). Final KW was related to plant growth per kernel (r2 = 0.64; P &lt; 0.001) but not to ARUK, and ARUP was driven (r2 ≥ 0.49; P &lt; 0.001) by KNP. Because of its positive relationship with KNP (main determinant of PGY), high h2 and high phenotypic plasticity, breeding must consider the increase in ARUP for improving grain yield, an objective that needs to be coupled with large reserves accumulation before silking to avoid the risk of lodging.

Genetic dissection of the maize kernel development process via conditional QTL mapping for three developing kernel-related traits in an immortalized F2 population.

Kernel development is an important dynamic trait that determines the final grain yield in maize. To dissect the genetic basis of maize kernel development process, a conditional quantitative trait locus (QTL) analysis was conducted using an immortalized F2 (IF2) population comprising 243 single crosses at two locations over 2years. Volume (KV) and density (KD) of dried developing kernels, together with kernel weight (KW) at different developmental stages, were used to describe dynamic changes during kernel development. Phenotypic analysis revealed that final KW and KD were determined at DAP22 and KV at DAP29. Unconditional QTL mapping for KW, KV and KD uncovered 97 QTLs at different kernel development stages, of which qKW6b, qKW7a, qKW7b, qKW10b, qKW10c, qKV10a, qKV10b and qKV7 were identified under multiple kernel developmental stages and environments. Among the 26 QTLs detected by conditional QTL mapping, conqKW7a, conqKV7a, conqKV10a, conqKD2, conqKD7 and conqKD8a were conserved between the two mapping methodologies. Furthermore, most of these QTLs were consistent with QTLs and genes for kernel development/grain filling reported in previous studies. These QTLs probably contain major genes associated with the kernel development process, and can be used to improve grain yield and quality through marker-assisted selection.

Changes in Kernel Filling with Selection for Grain Yield in a Maize Population

ABSTRACTGenetic improvement in kernel filling characteristics has been an important contributor to hybrid maize (Zea mays L.) yield increases. However, changes in kernel filling characteristics have not been evaluated within the context of recurrent selection programs. The objective of this study was to determine if rate and duration of kernel filling and their interaction with plant density changed with long‐term recurrent selection for agronomic performance in the Iowa Stiff Stalk Synthetic (BSSS) population. Populations selected for improved grain yield and agronomic performance were compared with the unselected base population, BSSS, at average final plant densities of 4.7 and 6.6 seeds m−2 at two locations near Ames, IA in 2009 and 2010. Treatments were replicated three times per location in a split plot design. Selected populations had a longer period of kernel filling but did not have a higher rate of kernel filling. Selected populations also had reduced final kernel weight at higher plant density, whereas unselected populations did not differ in final kernel weight. Reduction in final kernel weight at high plant density in selected populations suggests that increases in grain yield at high plant density in selected populations resulted from components of grain yield other than final kernel weight.

Heat stress in temperate and tropical maize hybrids: Kernel growth, water relations and assimilate availability for grain filling

Several studies have indicated that maize (Zea mays L.) kernel weight is severely affected by heat stress, but this response was never evaluated under field conditions. Our objective was to assess the effect of brief episodes of above-optimum temperatures on the dynamics of biomass and water accumulation in kernels of maize hybrids with contrasting tolerance to heat stress. Heat effect on assimilate supply from the plant (i.e., current biomass production and water-soluble carbohydrates in stem) to developing grains was also analyzed. Field experiments included a factorial combination of (i) three hybrids (Te: temperate; Tr: tropical; TeTr: Te×Tr), (ii) two temperature regimes, control and heated during daytime hours (ca. 33–40°C at ear level), and (ii) three 15-d periods (GS1: immediately before anthesis; GS2: from silking onwards; GS3: early phase of active grain filling). Heat effects on final kernel weight were larger (i) when they occurred during the first half of effective grain filling (−23.1% for GS3) than around flowering (−4.8% for GS1, −6.3% for GS2), and (ii) for the Te hybrid (−20.4%) than for the TeTr (−8.6%) and the Tr (−6.8%) hybrids. Heating around flowering (i) enhanced the assimilate availability per kernel during the effective grain-filling period, (ii) increased carbohydrates reserves in stem at physiological maturity, (iii) and had no significant effect on the dynamics of biomass and water accumulation in kernels. The opposite trend was detected among plots heated during GS3, which mostly exhibited the interruption of grain filling. Robust associations were established between (i) carbohydrate reserves in stem at physiological maturity and assimilate availability per kernel during effective grain filling (r2=0.49; P<0.001), and (ii) the rate of water loss from kernels and the duration of effective grain filling (r2=0.71; P<0.001). These responses underlay the enhanced sensitivity to heat stress of the hybrid with full temperate genetic background.

Genetic Analysis of Grain Filling Rate Using Conditional QTL Mapping in Maize

The grain filling rate (GFR) is an important dynamic trait that determines the final grain yield and is controlled by a network of genes and environment factors. To determine the genetic basis of the GFR, a conditional quantitative trait locus (QTL) analysis method was conducted using time-related phenotypic values of the GFR collected from a set of 243 immortalized F2 (IF2) population, which were evaluated at two locations over 2 years. The GFR gradually rose in the 0–15 days after pollination (DAP) and 16–22 DAP, reaching a maximum at 23–29 DAP, and then gradually decreasing. The variation of kernel weight (KW) was mainly decided by the GFR, and not by the grain filling duration (GFD). Thirty-three different unconditional QTLs were identified for the GFR at the six sampling stages over 2 years. Among them, QTLs qGFR7b, qGFR9 and qGFR6d were identified at the same stages at two locations over 2 years. In addition, 14 conditional QTLs for GFR were detected at five stages. The conditional QTL qGFR7c was identified at stage V|IV (37–43 DAP) at two locations over 2 years, and qGFR7b was detected at the sixth stage (44–50 DAP) in all four environments, except at Anyang location in 2009. QTLs qQTL7b and qQTL6f were identified by unconditional and conditional QTL mapping at the same stages, and might represent major QTLs for regulating the GFR in maize in the IF2 population. Moreover, most of the QTLs identified were co-located with QTLs from previous studies that were associated with GFR, enzyme activities of starch synthesis, soluble carbohydrates, and grain filling related genes. These results indicated that the GFR is regulated by many genes, which are specifically expressed at different grain filling stages, and the specific expression of the genes between 16–35 DAP might be very important for deciding the final kernel weight.

Trait dissection of maize kernel weight: Towards integrating hierarchical scales using a plant growth approach

Maize (Zea mays L.) yield is a function of the number harvested kernels per unit land area and the individual kernel weight (KW). Kernel weight and its development show a wide variability due to the genotype, the environment, the crop management, and all possible interactions. Commercial maize hybrids differ markedly in the patterns (rate and duration of kernel growth) behind differences in final KW. The same can be observed when public or elite proprietary maize inbred lines are analyzed. To progress in our understanding of KW variability, we reviewed and discussed current knowledge for analyzing kernel growth as an integrated system, modulated by processes linking different levels of organization (the different kernel tissues, the whole kernel, the plant, the canopy). Ideas on how to integrate this knowledge towards the development of a multi-hierarchical scale framework for predicting KW under different growth environments are currently needed, as they have high relevance for dissecting the genetic basis of kernel growth and maize yield definition at the canopy level. At the kernel and tissue level, we highlight the need of focusing and studying traits like assimilate movement into developing kernels, endosperm cell division, endosperm cell death, kernel internal homeostasis and kernel water relations. These specific processes need to be connected affecting the rate or duration of grain filling at the whole kernel level for studying kernel size variability. At the plant and canopy levels, the dynamic response of both kernel number and potential KW to plant growth around flowering appears critical for understanding KW variations, as maize final KW is highly associated with the potential KW determined during the first stages of grain filling. Focusing on the period around flowering in this species is important for yield improvement through both yield components, kernel number and size. At present it seems that maize potential yield can only be improved by modifying the sink capacity established at the end of the lag phase and increasing the source strength during grain filling so as to fulfill this potential.