Effective Grain-filling Period Research Articles

Water replenishment to maize under heat stress improves canopy temperature and grain filling traits during the reproductive stage

The occurrence of heat often exacerbates drought stress by increasing atmospheric vapor pressure deficit and decreasing transpiration efficiency. Water replenishment to increase soil water content (SWC) could be an effective measure for improving crop resilience in heat. However, field observational evidence and the underlying mechanism for the mitigation were still limited. For this, two widely adopted maize hybrids (ZD958 and XY335) were planted in 2019 and 2020 in North China Plain with three growth environments during the reproductive stage: Normal field conditions (NC), High-temperature episodes (HT) and High-temperature together with elevating SWC (HT-W). Averagely, high temperature reduced grain yield by 21.6-65.1% in HT treatment compared with NC treatment. In HT-W treatment, grain yield was improved by 11.8-13.5% while SWC was elevated by 27.6–51.5% in 0–80 cm soil profile compared with HT treatment. In HT-W treatment, water replenishment to maize under heat stress improved canopy temperature and grain filling traits. The daily maximum canopy temperature was reduced by 1.0–3.1 °C. The effective grain-filling period (Pf) was extended by 1.0–1.6 days and the mean grain-filling rate (Gmean) was increased by 8.2%-10.5%. The improvement of Gmean and Pf for ZD958 was greater than XY335. Furthermore, the significant correlations between SWC in 0–20 cm soil profile and Gmean, and between SWC in 40–80 cm soil profile and Pf for both hybrids were observed. As a result, the fresh weight, volume, and water content of the grain were significantly improved in HT-W treatment compared with HT treatment. Overall, per unit increase in SWC (0–80 cm) decreased the daily maximum canopy temperature by 0.3 °C, increased dry matter accumulation by 1.5%, and grain yield by 2.1%. The information obtained from this study will be helpful to develop efficient management to protect against heat stress in the field.

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.

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.

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.

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.

SOURCE-SINK MANIPULATION EFFECTS ON MAIZE KERNEL QUALITY

Evaluation of source or sink limitations on maize (Zea mays L.) yield and yield components is important for the rational design of agricultural practices as well as breeding strategies. There is little information on how the source or sink limitations during the effective grain-filling period affect final kernel quality. The objective of this study was to evaluate the effect of source/sink manipulation in different times after mid-silking stage on kernel weight (KW), kernel number per square meter (KN m−2), kernel protein and oil of different maize hybrids. Field experiments were conducted at the experimental field of College of Agriculture, Shiraz University, Shiraz, Iran, located at Bajgah (52° 35′ N and 39° 4′ E and 1810 m above sea level, asl) during the 2008 and 2009 growing seasons. The treatments included three hybrids and defoliation that were arranged in the main, and subplots, respectively. Leaves removal treatments were imposed when plants were at the mid-silking stage. Silking was taken as the time when 50% of the plants in a row presented visible silks. Defoliations included control, 50% of defoliation at 25 (early defoliation), and 35 (late defoliation) days after silking (defoliation treatments were applied to all plants of each plot). The experiments were conducted in randomized complete block (RCB) design with three replications and the treatments in a split-plot arrangement. Early defoliation greatly reduced kernel growth rate and grain filling period, resulting in lower mean kernel weight. Defoliation treatments modified KN m−2 and kernel number per ear (KNPE) and variations in these components affected protein and oil content. Decreased source size by defoliation decreased kernel protein content dramatically with no significant change in oil content. Differences among treatments appeared to be related only to the post-flowering source/sink ratio. Therefore, to improve protein yield in maize, hybrids and agronomic practices should aim to increase the post-flowering source/sink ratio.

Yield components of maize as affected by short shading periods and thinning

Maize (Zea mays) grain yield has been described to be particularly susceptible to environmental conditions around silking; however, a better temporal description of the effect of resource deprivation during this period is needed. Additionally, yield progress and the subsequent increase in the demand of assimilates may result in source limitation during the grain-filling period in current hybrids. This work assessed the effect of (i) short (~5 days) and intense shading stresses imposed at different times, and (ii) thinning during the effective grain-filling period, on yield components of an Argentinean, widespread hybrid. Grain yield was affected by resource availability during an extended period from ~300 growing degree-days (GDD) before silking to ~780 GDD after silking (base temperature = 8°C). Kernel number (KN) was reduced by shading treatments imposed within a relatively extended period of ~700 GDD centred on silking. Within this period, we establish a critical period of ~30 days around silking (i.e. –200 to 250 GDD after silking), in which KN susceptibility was maximal. The variation in KN during this period of 450 GDD was mainly accounted for by resource availability and not by timing of treatment imposition within this window. A direct relationship between KN and weight per kernel (KW) for shading treatments imposed from 0 to 200 GDD after silking indicated that compensation of KN reduction by KW increase might not be expected when stress occurred immediately after silking. Kernel number and KW presented an inverse relationship when shading took place after 200 GDD after silking. In addition, thinning after the onset of the effective grain-filling period increased KW. The results indicate that, even in the undisturbed crop, KW was limited by source capacity during grain filling. It is suggested that there is a need to reconsider current agronomic practices and breeding strategies, focusing on the source capacity during the grain-filling period.

Heat stress in temperate and tropical maize hybrids: Differences in crop growth, biomass partitioning and reserves use

Maize (Zea mays L.) hybrids with tropical genetic background are a promising source of heat stress tolerance, but their performance in high yielding environments remains poorly understood. Our objective was to assess (i) genotypic differences in the ecophysiological determinants of grain yield; i.e., fraction of light intercepted by crop (fIPAR), radiation use efficiency for biomass production (RUE), and harvest index (HI), and (ii) the responses of mentioned traits to brief episodes of high temperature. The contribution of stored reserves to grain yield was also analyzed. Field experiments included three contrasting maize hybrids (Te: temperate; Tr: tropical; TeTr: Te×Tr) grown under two temperature regimes (control and heated) during daytime hours. We tested heating (ca. 33–40°C at ear level) along three 15-d periods (GS1: pre-silking; GS2: from silking onwards: GS3: active grain filling). Heat stress had no effect on leaf area and fIPAR, but heating during grain filling affected light capture through reduced cycle duration, especially for the Te hybrid (average of −16.5 d). Heating caused a large reduction in RUE, but this trait had a rapid recovery after heat removal and final shoot biomass was not much affected (between −3% and −33%). HI was markedly reduced by heating and its variation was associated with changes in reserves use (r2=0.61). Grain yield in heated plots was better explained (r2≥0.92) by the variation in HI than by the variation in final shoot biomass (r2≥0.59). Heat effects on grain yield were larger (i) when they occurred around flowering (−527gm−2 for GS1 and −545gm−2 for GS2) than during grain filling (−352gm−2 for GS3), and (ii) for the Te hybrid (−599gm−2) than for the TeTr (−440gm−2) and the Tr hybrids (−384gm−2). Heating around silking (GS1 and GS2) caused apparent accumulation of reserves during the effective grain-filling period. The opposite trend was detected among plots heated during active grain formation (GS3). The tropical genetic background did not penalize yield potential and conferred an enhanced capacity for enduring heat effects.

Inter-plant variation of grain yield components and kernel composition of maize crops grown under contrasting nitrogen supply

High intra-specific competition pressure, which is common at most maize ( Zea mays L.) cropping conditions, promotes inter-plant variation and the appearance of extreme plant hierarchies with different ability to capture scarce resources (i.e., dominant and dominated plants) within a stand. The objectives of the current work were to analyze (i) inter-plant variation of grain yield per plant (GYP), GYP components (KNP: kernel number per plant; KW: kernel weight), and kernel composition, together with those of their physiological determinants, i.e., plant growth (PG) rate around silking (PGR S), PGR S per kernel (PGR S KNP −1) and PG during the effective grain-filling period per kernel (PG GF KNP −1), under contrasting N supply and (ii) the contribution of dominant and dominated plants to changes in inter-plant variation and mean values of the studied traits. For these purposes two maize hybrids previously characterized by their contrasting inter-plant variation under N stress (low: AX820 and high: AX877) were cultivated at high stand densities (9 and 12 pl m −2) at two N supplies (N 0: control and N 200: 200 kg N ha −1) without water restrictions. For AX820, PGR S data set at both N levels explored a similar range (1–7.4 and 1.2–7.4 g pl −1 d −1 for N 0 and N 200, respectively) with a positive skewness in N 0, and an almost normal distribution of data in N 200. In contrast, for AX877, inter-plant variation of PGR S exhibited a normal distribution in both N levels, and N fertilization only produced a displacement of data to higher PGR S values (0–4.3 and 0.7–5.7 g pl −1 d −1 for N 0 and N 200, respectively). The effect of inter-plant variation of PGR S on the coefficient of variation (CV) of KNP was of a greater magnitude in AX877 than in AX820 due to the more linear KNP response to PGR S of the former. For both hybrids, mean values of KW increased and the CVs decreased in response to high N supply. Differences among plants and N levels in KW were related to the duration of the effective grain-filling period. Inter-plant variation of protein and starch concentrations was higher in N 0 than in N 200, but that of oil concentration was not affected by N supply. The analysis of plant hierarchies resulted useful to understand changes in mean values and frequency distributions of several agronomic traits.

Kernel weight dependence upon plant growth at different grain-filling stages in maize and sorghum

In the present study we tested how assimilate availability per kernel at different grain-filling stages may affect maize (Zea mays L.) and sorghum (Sorghum bicolor L. Moench) individual kernel weight (KW). These two species have shown a contrasting KW response to increased assimilate availability at similar seed developmental stages. Plant growth rate (PGR) per kernel was used to estimate the assimilate availability per kernel at two stages: around the early grain-filling period when kernel number per plant is also being established, and around the effective grain-filling period. We tested 3 commercial genotypes from each species, and modified the PGR by thinning or shading the stand at different developmental stages. In both species, each genotype showed a particular relationship between PGR around flowering and kernel number, which gave a range of responses in the PGR per kernel set around flowering. Final KW always increased whenever PGR per kernel around flowering was enhanced. Only sorghum showed a consistent KW increase when PGR per kernel during the effective grain-filling period was enhanced. Results confirmed that increasing assimilate availability per kernel will affect maize kernel size only if the potential set early in development is altered. Most important, we showed that linking specific KW sensibility across species at different seed developmental stages using a simple estimate of assimilate availability per seed (i.e. PGR per kernel) at each grain-filling stage helped explain most of the explored genotypic and environmental variability in final kernel size.

Source–sink relations and kernel weight differences in maize temperate hybrids

Maize ( Zea mays L.) kernel weight (KW) response to changes in assimilate availability per kernel during grain filling suggests that plants establish an early kernel sink potential that place them to grow close to a saturating assimilate availability condition during late grain-filling, meaning source limitations are common only early in kernel development. As maize reproductive efficiency in kernel set is not constant across different plant growth rates (PGR) around flowering, we used PGR per kernel during this period as an indicator of source availability per kernel. We tested whether PGR per kernel during flowering or during the effective grain-filling period were correlated to genotypic and environmental differences in final KW. Plant growth rate during both periods, KW, kernel growth rate during the effective grain-filling period, total duration of grain filling and kernel number per plant were measured in 12 commercial genotypes differing in KW sown at two sites under full irrigation. As expected from the curvilinear response relating kernel number per plant and PGR around flowering, increased PGRs resulted in higher PGR per kernel around this period ( r 2 = 0.86; p < 0.001). Differences in final KW due to genotypes or environments were significantly explained by the PGR per kernel around flowering ( r 2 = 0.40; p < 0.001), and not by the PGR per kernel during the effective grain-filling period. Genotypes differed in kernel growth rate ( p < 0.001) and grain-filling duration ( p < 0.001). The former was well explained by PGR per kernel around flowering ( r 2 = 0.66; p < 0.001), but showed no relationship with the PGR per kernel during the effective grain-filling period. Grain-filling duration was partially explained ( r 2 = 0.27; p < 0.01) by the ratio between PGR per kernel during the effective grain-filling period and kernel growth rate, but differences in duration were negligible compared to those observed in the ratio (∼41% versus ∼130%, respectively). Together, these results support the importance of source availability per kernel during early grain filling on the determination of maize potential sink capacity and final KW. Early resource availability per kernel was accurately estimated as PGR per kernel around the period of kernel number determination, which helped explain genotypic and environmental differences in maize final KW as well as in kernel growth rate.

Grain yield components in maize: II. Postsilking growth and kernel weight

Maize kernel weight (KW) results from kernel growth during two stages of grain filling, the lag phase (formative period) and the effective grain-filling phase. Environmental conditions may affect kernel biomass accumulation in each phase. This work analyzed: (1) changes in duration and rate of kernel growth on a thermal time (°C day) basis; and (2) KW response to postsilking biomass production kernel −1 (source:sink ratio). Sowing date, plant population, and nitrogen fertilization experiments were conducted in France and Argentina to induce changes in assimilate availability per kernel. Hybrids of different KW were tested. Hybrids differed in the duration of the lag phase, which determined kernel growth rate during the effective grain-filling period for hybrids with similar grain-filling duration (ca. 745°C day). Environments with low air temperature (<19°C) and less incident solar radiation led to a smaller final KW due to reductions in photoassimilate production and its partition to the grains. A value of 240 to 270 mg kernel −1 during grain filling was determined as a threshold to have mobilization or storage of reserves. Small-kernel hybrids (KW<300 mg), with large kernel number (3500 to 5500 kernels m −2), depended more on reserve mobilization than large-kernel hybrids (KW>300 mg) with reduced kernel number (2800 to 4000 kernels m −2). For the former, grain yield increments should not be based on increased kernel number but on increased biomass production.

Ear moisture during kernel development as influenced by field and laboratory selection

Grain quality, timeliness of harvest, and profitability can be increased by improving field drying characteristics of maize (Zea mays L.) hybrids. To better understand hows genes control ear drying, I compared maize strains developed by divergently selecting three cycles for (1) high HM or low LM moisture content at 45 d post pollination in the field or (2) fast FD vs. slow ear drying SD In laboratory. A field study across five locations compared HM, LM, FD, and SD strains from each of five synthetics for grain yield, ear moisture at harvest, test weight, lodging, and other agronomic traits. I studied ear moisture during grain filling for two subsets of divergently selected strains from one and three synthetics for 2 yr. In a third 2-yr field study, I measured mature kernel weight, lag period duration (LPD), effective grain-filling period (EFPD), and rate of dry matter accumulation (RDMA) for LM and HM strains developed from each of four synthetics. When averaged across the five synthetics, both SD and LM selections produced equivalent yields but lower ear moisture at harvest than the corresponding divergent strains. The LM strains had higher test weights than HM strains. When averaged across three synthetics and 2 yr, the HM strains produced higher moisture than LM strains at 15, 30, 45, and 60 d after silking. However, environments also influenced moisture content of the kernels during grain filling. In three of the four synthetics studied, HM strains had heavier kernels than corresponding LM strains. The heavier kernels seem to be due to increased RDMA. When averaged across four synthetics, LM strains had shorter LPD than HM strains. These correlated selection responses suggest that a genetic association exists among moisture content during grain filling, moisture content at physiological maturity, moisture content at harvest, LPD, and test weight. Breeding for LM or SD should improve field-drying characteristics of maize without increasing stalk breakage or decreasing yields. Key words:Zea mays L., grain filling, dry-down rates, mass selection, breeding methods

Grain filling of R-nj color-selected maize strains divergently selected for kernel weight

Grain-fill characteristics in maize (Zea mays L.) affect yield by changing kernel weight. The objective of this study was to learn how divergent tandem selection for R-nj color expression and kernel weight affected rate of dry matter accumulation (RDMA), effective grain-filling period (EFPD), and lag phase duration (LAGP). We studied development of apical, mid-ear, and basal kernels in two genetic backgrounds. We derived 12 maize strains by tandem selection within each of two early-maturing synthetics. Mass selection of synthetics NDSF and NDSD for four cycles for high (HC), random (RC), and low (LC) R-nj color expression produced six substrains. Then, four cycles of divergent mass selection for kernel weight within each color-derived substrain produced 12 substrains for study. Using the 12 strains, we conducted field experiments using a completely random experimental design within each of 2 yr at Fargo, ND. Sequential kernel samples of individual ears within each strain provided data to estimate RDMA, EFPD, LAGP, and five-kernel weight (KWT). We sampled at 3- to 4-d intervals during the linear phase of grain-filling and at maturity. Selection for HC increased RDMA but tended to decrease EFPD compared to LC strains in both NDSF and NDSD. Selecting heavier kernels increased KWT of basal and mid-ear kernels by increasing RDMA. Direct and correlated responses to R-nj color selection were evident after four subsequent cycles of divergent tandem selection for kernel weight. Therefore, R-nj expression was not a temporary maternal effect. Kernel weight selection responses differed among the color strains and synthetics. Kernel weight seemed mainly determined by RDMA that was affected by selection for R-nj color expression and for kernel mass. Key words:Zea mays L., aleurone color, mass selection, correlation, yield components

Cross-pollination effects on maize (Zea mays L.) hybrid yields

Recent studies have shown that pollen from a long-season maize (Zea mays L.) hybrid increased yield of a short-season hybrid by lengthening the effective grain-filling period, while the reciprocal cross did not alter this period or yield. This effect (metaxenia) was evaluated further in the studies reported here with hybrids of more diverse maturity and under both high and low N fertility. In the first year of this study (1989), sib- and cross-pollinations were made among B73Ht × Mo17 (B × 7) and two early-silking hybrids, LH59 × LH146 (L × 6) and Pioneer 3732 (3732) under N-sufficient (275 kg ha−1) and two lower N regimes (17 and 67 kg ha−1). Only a few significant effects were observed and these were noted at high N with one exception. With 3732 pollen, grain yield of B × 7 was decreased at 275 kg N ha−1, and physiological maturity occurred 3 d earlier. Yield of 3732 was increased by L × 6 pollen in comparison with B × 7 pollen. Kernel number and average kernel weight were not altered by pollen source. Pollen type did not affect yields under low N fertility, except for a reduction when B × 7 was pollinated by L × 6 at the 67-kg N ha−1 rate. In 1990, under N-sufficient fertility, B73Ht × LH156 (B × 6), a late-silking hybrid, and LH146 × LH82 (L × 2), an earlier hybrid, were sib- and cross-pollinated with B × 7 and 3732. The only significant effect observed was that L × 2 pollen increased B × 6 yield. Thus with the hybrids used, yields of early-season types were not altered by cross-pollination with long-season types. Previous results showing increased yields when 3732 was pollinated by B × 7 were not duplicated in either year, suggesting metaxenia effects are highly dependent upon environment.Key words: Metaxenia, xenia, cross-pollination, maize, yield, N levels

Effect of high temperature on grain-growth in field-grown maize

Little information is available on the effect of high temperature on grain-growth and on final yield in field-grown maize ( Zea mays L.). To redress this situation, a study was run on maize sown at five dates and grown in field plots in a tropical environment under high water and nutrient supply. For each sowing, mean temperature during grain-filling was relatively stable, but across sowings ranged from 25 to 32°C. The effects of these temperature regimes on the linear rate of grain-growth, the duration of the effective grain-filling period, the rate of development of milk-line and black-layer as indicators of maturity, the rate of increase in harvest index during grain-filling, and grain-yield were examined. The rate of grain-growth increased and the duration of grain-filling was shorter as temperature increased. However, contrary to other reports for maize grown in controlled environments, grain-yield was not responsive to temperature over the range from 25 to 32°C during grain-filling. This stability in grain-yield was associated with relatively similar incident-radiation receipt in all sowings. Whilst the rate of both milk-line and black-layer development increased with temperature, the development of milk-line was less variable and proved to be the better indicator of the end of effective grain-filling. Harvest index increased linearly during grain-filling in a similar manner to the rate of grain growth. The rate of harvest-index increase was relatively stable across sowings, with a mean value of 0.015 day −1. An important implication of the linear increase in harvest index is that grain-yield accumulation can be estimated from the crop biomass at any stage of grain-growth without knowledge of grain number nor the rate of grain-growth.

Effect of high temperature on the rate and duration of grain growth in field-grown Sorghum bicolor (L.) Moench

Sorghum was sown at six dates in a tropical environment and grown under high water and nutrient supply. For each sowing, mean temperature during grain-filling was relatively stable, but across sowings it ranged from 24.6 to 30.2�C. The effect of this variation on the following was examined: the linear rate of grain growth, the duration of the effective grain-filling period, the rate of increase in harvest index during grain-filling, the rate of development of black layer (as an indicator of maturity) and final grain yield. Contrary to previous findings from controlled environment experiments, the rate of grain growth increased linearly with temperature to 30�C. However, the duration of grain-filling and the rate of development of black layer were not closely related to mean temperature. Final grain yield was not necessarily adversely affected by high temperature during grain-filling. Also, high grain yield was not necessarily associated with long duration of grain-filling. Similar to the rate of grain growth, harvest index increased linearly during grain-filling. The rate of increase in harvest index was not significantly related to temperature, with a mean value across sowings of 0.0185 day-1. An important implication of the linear increase in harvest index is that grain yield accumulation can be estimated from the crop biomass at any stage of grain growth without knowledge of grain number or the rate of grain growth.

Effect of nitrogen supply on the comparative productivity of maize and sorghum in a semi-arid tropical environment III. Grain yield and nitrogen accumulation

Differences in grain yield and grain nitrogen content due to variable N supply in maize and sorghum, grown under irrigation in the semi-arid tropics, were associated with differences in their rates of accumulation and in the extent of mobilization of pre-anthesis dry matter and N. Increases in harvest index, nitrogen harvest index, and grain size during grain-filling were essentially linear, and the rates were constant under variable N supply. Differences in harvest index at maturity at different levels of applied N were small relative to differences in biomass and grain yield, and differences in grain size were small relative to differences in grain number. In effect, variation in harvest index and grain size at maturity was associated with variation in the length of the effective grain-filling period. Nitrogen supply had a greater effect on the effective duration of grain-filling in maize than in sorghum. Maximum grain yields at high levels of N uptake were greater in maize (10.0 t ha −1 at 15.5% moisture) than in sorghum (7.2 t ha −1 at 14% moisture). These differences were related to differences in biomass production, rate of increase in harvest index and the effective duration of grain-filling. The rate of increase in harvest index was greater, but biomass production was less and the duration of grain-filling was shorter, in sorghum than in maize.

Effects of nitrogen on crop development and grain growth of winter wheat in relation to assimilation and utilization of assimilates and nutrients.

Grain growth and yield components of winter wheat cv. Lely were studied in a field experiment in 1976 with 4 rates of N (50, 100, 100 + 50 or 100 + 100 kg N/ha). Growing conditions were characterized by a high level of solar radiation, warmth, ample nutrient supply and no damage by diseases. N raised grain number/m2 from 16 700 to 20 600 and grain yield from 640 to 821 g dry wt./m2. Grain growth duration was short, due to warmth, but the rate of the grain filling was very high (from 24.0 to 29.2 g/m2 day during the effective grain-filling period). A high grain yield was associated with a high grain N content which resulted in a grain protein yield ranging from 63.8 to 107.1 g/m2 with increased N rate from 50 to 200 kg/ha. The carbohydrate demand of the grains was provided by current photosynthesis and relocation of stem reserves. The latter was reflected in a decline of the stem wt. after the mid-kernel filling stage. N and P demands of the grains were supplied by withdrawal from the vegetative organs (leaves, stem, chaff) and to a large extent by post-floral uptake and assimilation. Under the prevailing growing conditions the grains turned out to be very strong sinks for carbohydrate, N and P as shown by the harvest indices. Additional N dressings increased the harvest indices of DN, N and P from 0.40 to 0.48, from 0.75 to 0.81 and from 0.91 to 0.93 resp. It was suggested that a more restricted vegetative crop development at high N levels and a longer duration of root activity, photosynthesis and grain growth after anthesis would considerably favour grain yield. (Abstract retrieved from CAB Abstracts by CABI’s permission)