Kernel Growth Rate Research Articles

Heterosis for Embryo Size and Source and Sink Components of Maize

Assimilate source and sink components controlled by genotype and environment determine grain yield of maize (Zea mays L.). Field experiments were conducted in 1986 and 1987 at Gainesville, FL (29° 38′ N) on a Lakeland sand (hyperthermicoated Thermic Quartzipsamment) to assess heterosis for vegetative and yield components. In order to equalize leaf area index and light interception, parent inbred lines B73, MO17, MS71, and VA22 were planted at normal plant populations density (PPD) of 55 000 and also at 110 000 plants ha‐1, while three F1 hybrids from these inbreds were planted at 55000 and 27 500 PPD. In hybrid seeds, embryo size was associated with seed size. At common seed size (8 ram), lengths embryo and embryo axis of hybrids were greater than those of midparent and inbred line means, manifesting heterosis for embryo size. At common PPD, all biomass and grain yield source and sink components were significantly greater for hybrids than for inbreds. At common LAI and light interception achieved by doubling inbred PPD, hybrids produced significantly more biomass and kernel yield, kernel growth rate and weight, and grain‐filling duration, but kernel number for hybrids and inbreds was similar at common LAI. We conclude that hybrids as compared with their parent inbreds express heterosis in source components, including embryo size, as well as sink components, all contributing to higher biomass and grain yields of maize.

Growth and Yield of Winter Wheat as Influenced by Chlormequat Chloride and Ethephon

AbstractLodging can be a constraint in wheat (Triticum aestivum L.) production, therefore interest exists for use of plant growth regulators (PGRs) such as Chlormequat chloride or CCC (2‐Chloroethyl‐N, N, N‐trimethylammonium chloride) and ethephon [(2‐Chloroethyl) phosphonic acid] for lodging control. Field experiments were conducted in New York in 1985 and 1986 on a Honeoye silt loam (fine‐loamy, mixed mesic Glossoboric Hapludalfs) to examine the influence of CCC (1.37 kg ha−1 a.i. at Zadoks Growth Stage (GS) 31) and ethephon (0.40 kg ha−1 a.i. at GS 39) on vegetative and reproductive growth of two winter wheat cultivars, Geneva and Houser. Leaf, stem, and spike phytomass were determined at 10‐d intervals from GS 25 to 95, and kernel number and weight were determined on 15 main‐culm spikes at 3 to 4‐d intervals during the grain‐filling period. The CCC application reduced culm and total phytomass by 9% at GS 45, whereas ethephon reduced spike and total phytomass by 10 and 8% at GS 55 and GS 75, respectively. Neither PGR had an effect on total or component phytomass at harvest. In 1985, a year when no lodging occurred, ethephon and CCC yielded the same compared with the yield of the check (6.12, 6.60, and 6.39 Mg ha−1, respectively). In 1986, a year of considerable lodging, ethephon reduced lodging compared with that of CCC and the check, which was reflected in a higher kernel growth rate (1.00, 0.84, and 0.81 mg kernel−1 day−1), kernel weight (43.4, 39.8, and 38.9 mg), and grain yield 5.73, 4.71, and 5.30 Mg ha−1, respectively) in the more lodging‐susceptible cultivar, Houser. The data indicate that, under the environmental conditions of this study, CCC does not benefit wheat and that ethephon should be utilized only under severe lodging conditions.

EFFECTS OF TEMPERATURE ON RATE AND DURATION OF KERNEL DRY MATTER ACCUMULATION OF MAIZE

The response of rate and duration of kernel dry matter accumulation to temperatures in the range 10–25 °C was studied for two maize (Zea mays L.) hybrids grown under controlled-environment conditions. Kernel growth rates during the period of linear kernel growth increased linearly with temperature (b = 0.3 mg kernel−1 d−1 °C−1). Kernel dry weight at physiological maturity varied little among temperature treatments because the increase in kernel growth rate with increase in temperature was associated with a decline in the duration of kernel growth proportional to the increase in kernel growth rate.Key words: Zea mays L, period of linear kernel dry matter accumulation, controlled-environment conditions, kernel growth rate

GROWTH CHARACTERISTICS, YIELD COMPONENTS AND RATE OF GRAIN DEVELOPMENT OF TWO HIGH-YIELDING WHEATS, HY320 AND DT367, COMPARED TO TWO STANDARD CULTIVARS, NEEPAWA AND WAKOOMA

A high-yielding Canada Prairie Spring (CPS cv. HY320) semi-dwarf wheat and a high-yielding durum line (DT367) were compared to standard hard red spring (cv. Neepawa) and durum (cv. Wakooma) wheats to assess the basis for the yield differences and to examine soil moisture use. The study was carried out under dryland (moisture used from seeding to harvest was 287 mm), partial irrigation (moisture used was 374 mm) and full irrigation (moisture used was 547 mm) on a loam soil at Swift Current during 3 yr. Phenological development was delayed as much as 30 d by improved moisture conditions. At ligule of last leaf visible (LLV), HY320 had one or two more green leaves on the main tiller than did Neepawa; Wakooma generally had one more leaf than DT367. However, leaf area index was generally not different among cultivars. Harvest index was greater for the higher yielding cultivars even though cultivars did not differ in total aboveground dry matter. HY320 generally had the highest grain yield, followed by DT367, Wakooma and Neepawa. Neepawa consistently had the most heads per unit area at harvest but, because of high kernel numbers per head, HY320 produced the greatest number of kernels per unit area; as well, HY320 had large kernels; thus, it had the highest yield potential under optimum growing conditions. HY320 had the highest base temperature (Tb) for kernel growth and the highest kernel growth rates immediately after anthesis; consequently, HY320 kernels completed growth with the lowest accumulated degree days above Tb. DT367 had the highest potential kernel weight and Neepawa the lowest. There were no differences among the cultivars in the amount of moisture used between seeding and harvest.Key words: Wheat (Spring), yield components, grain development, moisture use, growing degree days, harvest index

Effect of Water Deficit during Grain Filling on the Pattern of Maize Kernel Growth and Development1

There are distinct phases of grain development in maize (Zea mays L.). Little has been reported on the relative effects of water deficits during the different phases of maize kernel development. Since endosperm cell number is important in determining sink capacity or strength, a water deficit during the lag phase may affect kernel development differently than one during the linear phase dominated by starch deposition. The objective of this research was to determine the effect of short‐ and long‐term water deficit, imposed during different phases of grain filling, on the pattern of maize kernel growth. We conducted two separate experiments. In the first, a short‐term (10 day) water deficit was imposed during endosperm cell division (the lag phase) or during the period of rapid starch deposition (the linear‐filling phase). In the second, a water deficit was initiated during the same phases, but extended until maturity. Plants were grown outdoors in Rabat, Morocco, in large pots arranged to achieve a stand density of 50 000 plants/ha. Short‐term water deficit during the lag phase resulted in a significant but transient delay in the accumulation of kernel dry mass. However, recovery from this delay was such that overall rate and duration of kernel growth were not affected. Similarly, short‐term water deficit during the linear‐filling phase had no effect on the rate or duration of kernel growth. When the long‐term water deficit treatment was initiated during the lag phase, kernel growth rate was not affected for approximately 24 days. Grain filling then terminated prematurely, resulting in a significant decrease in the duration of the grain‐filling period, and a 50% decrease in final kernel weight. When long‐term stress was imposed during the linear‐filling phase, kernel growth was not affected for 20 days, but then terminated abruptly, resulting in an 8‐day decrease in the grain‐filling period. Final kernel weight was not significantly decreased, however. Our results show that kernel growth is more sensitive to water deficits during endosperm cell division than during the period of rapid starch deposition. The higher sensitivity to water deficits during the lag phase was not a consequence of either a lack of carbohydrate availability to the grain, or an inhibition of starch synthesis, but was apparently due to an inhibition of endosperm cell division and the subsequent establishment of kernel sink capacity.

Recurrent Selection for Reduced Plant Height in Lowland Tropical Maize1

Reduced plant height is considered a desirable attribute in most maize (Zea mays L.) breeding programs. This study was conducted to evaluate direct and correlated responses to selection for reduced plant height in tropical maize. Fifteen cycles of visual, full‐sib recurrent selection for reduced plant height were conducted in one lowland tropical maize population, ‘Tuxpeño Crema I’. Cycles 0, 6, 9,12, and 15 were evaluated for correlated responses in plant morphology, grain yield, dry matter production and distribution, leaf area index, and duration and rate of kernel growth at three plant densities at two or three locations in Mexico over a period of 2 yrs. Selection resulted in a linear reduction in plant height, from 282 to 179 cm (2.4% per cycle). The estimate of broad‐sense heritability for plant height was 0.84. Optimum plant density for yield increased from 48 000 to 64 000 plants ha−1, an increase of 2.1% per cycle. As height was reduced, yield, when determined at the optimum plant density for yield for each generation, increased by 4.4% per cycle. Later cycles showed reduced total lodging, reduced barrenness, earlier flowering, and fewer leaves per plant. When grown at or near their optimum densities for grain yield, leaf area index, and total dry matter per unit area at harvest for selected cycles were similar. Yield improvement was associated with a linear increase in harvest index from 0.30 to 0.45 from the original to the 15th cycle. At 50% silking, total aboveground dry weight and its proportion as stem were reduced; kernel number per unit area at harvest increased from 1592 to 2667 kernels m −2 when grown at or near optimum density, and there was an increase in crop dry weight production from anthesis to black layer formation. Selection for reduced plant height had little effect on the effective filling period or kernel weight.

Nitrate assimilation and partitioning in detached culms of wheat

Detached wheat culms ( Triticum aestivum L. cv. Doublecrop) collected from field-grown plants 16 days after anthesis were cultured for 5 days with nutrient solution containing glutamine (7.5 mM), K 15NO 3 (15 mM), or glutamine (7.5 mM) plus K 15NO 3 (15 mM). Kernel growth rate of cultured culms equaled or exceeded that of intact field-grown plants (1.46 mg kernel −1 day −1). Net mobilization of reduced N from the peduncle occured, but this accounted for less than 23% of the net increase in grain N after 5th day of culture, regardless of N source. Glutamine had no effect on the assimilation of [ 15N]nitrate or the partitioning of reduced [ 15N]-labelled products. Peduncles contained less than 4–8% of total reduced[ 15N] excess recovered in all plant parts. Grain accumulation of reduced [ 15N] was linear,as was solution depletion, and accounted for an increasing proportion of the total reduced[ 15N] excess in all culm parts. These results provide direct evidence for nitrate assimilation by spike components.

Effect of Increased Temperature in Apical Regions of Maize Ears on Starch-Synthesis Enzymes and Accumulation of Sugars and Starch

Apical florets of maize (Zea mays L.) ears differentiate later than basal florets and form kernels which have lower dry matter accumulation rates. The purpose of this study was to determine whether increasing the temperature of apical kernels during the dry matter accumulation period would alter the difference in growth rate between apical and basal kernels. Apical regions of field-grown maize (cultivar Cornell 175) ears were heated to 25 +/- 3 degrees C from 7 days after pollination to maturity (tip-heated ears) and compared with unheated ears (control). In controls, apical-kernel endosperm had 24% smaller dry weight at maturity, lower concentration of sucrose, and lower activity of ADP-Glc starch synthase than basal-kernel endosperm, whereas ADP-Glc-pyrophosphorylase (ADPG-PPase) activities were similar. In tip-heated ears apical-kernel endosperm had the same growth rate and final weight as basal-kernel endosperm and apical kernels had higher sucrose concentrations, higher ADP-Glc starch synthase activity, and similar ADPG-PPase activity. Total grain weight per ear was not increased by tip-heating because the increase in size of apical kernels was partially offset by a slight decrease in size of the basal- and middle-position kernels. Tip-heating hastened some of the developmental events in apical kernels. ADPG-PPase and ADP-Glc starch synthase activities reached peak levels and starch concentration began rising earlier in apical kernels. However, tip-heating did not shorten the period of starch accumulation in apical kernels. The results indicate that the lower growth rate and smaller size of apical kernels are not solely determined by differences in prepollination floret development.

Effect of Temperature on In Vitro Kernel Growth of Flint and Dent Maize Hybrids1

Most maize (Zea mays L.) hybrids grown in short‐season areas are flint‐dent endosperm types. The so‐called flint hybrids are believed by some to have faster growth rates than dent hybrids at low temperatures. This study was conducted to determine the feasibility of using the in vitro kernel growth technique to investigate the effects of temperature on growth of northern flint and dent maize hybrids. Kernels of three hybrids in one experiment and five hybrids and three inbreds in a second experiment were cultured in vitro on a defined medium at 15, 20, 25, or 30°C. The kernels were removed after 2 to 7 weeks in culture and dry weights were determined. The dent endosperm hybrid ‘King KI108’ had the highest rate of kernel growth at all temperatures, whereas the European flint‐dent hybrid ‘Limagrain LG1’ had the slowest rate of growth. The Canadian flint‐dent hybrid ‘Pioneer 3995’ had an intermediate rate of growth. In one experiment, the differences were more pronounced at low temperatures than at high temperatures. Kernels of inbreds were difficult to culture in vitro, and the growth rates of their kernels were never greater than those of the F1 progenies. The comparison of flint and dent hybrids suggested that a more extensive study of the major groups of maize would result in detection of differences in growth rates of kernels at different temperatures.

Variation in Kernel Growth Characters Among Soft Red Winter Wheats1

One of the components of grain yield in wheat is kernel size. Kernel size is determined by rate and duration of kernel growth. Genetic variation and genotype ✕ environment interaction for these traits has not been assessed in a large sample of soft red winter wheats. In this study, variation in kernel growth characters was evaluated in 61 elite soft red winter wheat (Triticum aestivum L.) breeding lines and cultivars. Kernel growth rate (KGR) and effective filling period (EFP) were estimated from samples of 30 spikes in replicated plots at two locations in Kentucky. Significant genotypic variation for KGR and EFP was observed at both locations. Significant genotype ✕ environment interaction was also observed for these two traits. The KGR and EFP varied from 0.78 to 1.79 mg kernel ‒1 day ‒1 and 17.3 to 32.9 days respectively. Kernel size and grain yield were significantly correlated with KGR, while EFP was not significantly correlated with either character. The data suggest that date of physiological maturity may not always be correlated with date of anthesis. Early flowering genotypes with long EFP's reached physiological maturity later than genotypes which flowered later and had shorter EFP's. Selection for high KGR may be appropriate in developing early maturing wheat cultivars suitable for double cropping.

Sugar Efflux from Maize (Zea mays L.) Pedicel Tissue

Sugar release from the pedicel tissue of maize (Zea mays L.) kernels was studied by removing the distal portion of the kernel and the lower endosperm, followed by replacement of the endosperm with an agar solute trap. Sugars were unloaded into the apoplast of the pedicel and accumulated in the agar trap while the ear remained attached to the maize plant. The kinetics of (14)C-assimilate movement into treated versus intact kernels were comparable. The rate of unloading declined with time, but sugar efflux from the pedicel continued for at least 6 hours and in most experiments the unloading rates approximated those necessary to support normal kernel growth rates. The unloading process was challenged with a variety of buffers, inhibitors, and solutes in order to characterize sugar unloading from this tissue.Unloading was not affected by apoplastic pH or a variety of metabolic inhibitors. Although p-chloromercuribenzene sulfonic acid (PCMBS), a nonpenetrating sulfhydryl group reagent, did not affect sugar unloading, it effectively inhibited extracellular acid invertase. When the pedicel cups were pretreated with PCMBS, at least 60% of sugars unloaded from the pedicel could be identified as sucrose. Unloading was inhibited up to 70% by 10 millimolar CaCl(2). Unloading was stimulated by 15 millimolar ethyleneglycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid which partially reversed the inhibitory effects of Ca(2+). Based on these results, we suggest that passive efflux of sucrose occurs from the maize pedicel symplast followed by extracellular hydrolysis to hexoses.

Thermal Environment During Endosperm Cell Division and Grain Filling in Maize: Effects on Kernel Growth and Development in Vitro1

Relative effects of temperature during endosperm cell division (lag phase) and the effective grain‐fill period (EGFP) thephysiology of kernel development and final kernel mass in maize (Zea mays L.) were studied in vitro. Kernels were cultured at 15, 30, and 35°C during most of the lag phase. Before the start of the EGFP some kernels were transferred to each of the other temperature regimes while others remained at the original temperatures. When 30°C was maintained through the lag phase and EGFP, kernel mass was 77% of that observed for kernels from field‐grown ears. Temperature extremes (15 or 35°C) during either period of kernel development reduced final kernel mass, however, an unfavorable thermal environment during the lag phase caused the most reduction. Detrimental effects sustained in the lag phase prevailed when kernels were transferred to a more conducive (30°C) thermal environment during EGFP. Regardless of temperature during the EGFP, final kernel mass was 3 to 10 times higher when the lag phase temperature was 30°C. The data also showed that duration of EGFP was mediated primarily by prevailing temperature during EGFP while kernel growth rate during the EGFP was mediated in part by thermal history during the lag phase. High levels of soluble sugars were found in the endosperm during both stages of kernel development under adverse thermal environments (15 or 35°C). This suggested that low substrate level was not the cause of the reduced kernel growth rate and lower kernel weight observed at these temperatures. High temperature during the lag phase appeared to have the greatest negative effect on endosperm starch deposition during EGFP even when a favorable EGFP temperature (30°C) was provided. These data show that temperature extremes during the lag phase may mediate final kernel size by reducing endosperm sink capacity established during the lag phase or by reducing starch deposition in endosperm cells during the subsequent EGFP.

Accumulation and Conversion of Sugars by Developing Wheat Grains

The volume and composition of the endosperm apoplast of the developing wheat grain, comprising endosperm cavity and intercellular free-space, was examined in relation to kernel growth rate and size. Samples of the cavity sap were collected by centrifugation of kernels during the linear phase of grain growth. The cavity sap contained 10–50 mM sucrose, a small amount of hexoses but a high concentration of oligosaccharides (up to 9 times that of sucrose). In comparing cvs Yandilla King and Cleveland, high growth rate was associated with high cavity sap sucrose concentration but with low K+ concentration. K+ concentration in the endosperm cells (124 mM) was about 5 times higher than in the cavity sap (10–40 mM). Cavity sap pH was 6.3–6.6. The uptake of sucrose by endosperm cells was partly inhibited by PCMBS, an inhibitor of membrane-bound carriers. Several necessary conditions for proton cotransport during sucrose uptake by endosperm cells were met. The volume of the intercellular free-space, estimated by membrane permeating (14C-mannitol, 14C-sucrose) or non-permeating (3H-PEG900) markers averaged 2.2 μl or 5–7% of the water in grains of cvs Yandilla King, Cleveland and SUN 9E. The cavity volume was highly variable but tended to be larger in larger grains. Pulse labelling of 14CO2 to flag leaves showed that 14C-sucrose was the principal 14C-assimilate in the cavity sap and was converted to insoluble compounds in the endosperm while the cavity sap oligosaccharides acquired negligible label in 6 h.

EFFECT OF TEMPERATURE DURING GRAIN FILLING ON WHOLE PLANT AND GRAIN YIELD IN MAIZE (Zea mays L.)

In a 2-yr study, plants of an adapted, short-season single cross maize (Zea mays L.) hybrid were grown outdoors until 18 days post-silking. At that stage, the plants were transferred to controlled-environment growth cabinets where temperature effects on leaf senescence, grain and whole plant dry matter (DM) production and DM distribution were studied. The day/night temperature regimes were 25/15 °C, 25/25 °C, 35/15 °C and 35/25 °C. Higher temperatures reduced whole plant DM accumulation during grain filling. The reduction in DM accumulation was primarily related to a reduction in the period of time from 18 days post-silking until 100% leaf senescence and, to a limited extent, to a lower rate of whole plant DM production. Grain yield per plant was also lower under higher temperatures. The decreases in grain yield were almost entirely determined by a shorter duration of grain filling, while no temperature effect was observed on kernel growth rates or on kernel number per ear. During rapid grain filling, the increase in kernel DM results from utilization of a combination of assimilates temporarily stored in the vegetative plant parts and assimilates produced through current photosynthesis. Under the highest temperature regime, assimilates remobilized from other plant parts accounted for a greater proportion of kernel weight gain. In addition, there was an indication that higher night temperatures resulted in an increased proportion of gain in kernel weight resulting from remobilization of stored DM.Key words: Corn, temperature, grain-filling period, grain growth, yield components, leaf senescence

Effect of Altered Source‐Sink Ratio on Growth of Maize Kernels1

Grain yield in maize (Zea mays L.) is a function of the relationship between assimilate supply to the grain and inherent potential of the grain to accommodate this assimilate. In this study we sought to ascertain the effects of altering assimilate supply per kernel during early and late kernel development on kernel growth rate and final weight. We also sought to elucidate the effect of such alterations on the plant's compensatory responses, specifically kernel abortion and carbohydrate/ N redistribution. The single‐cross hybrid M14 ✕ W64A, selected because of intermediate weight per kernel was grown in field plots in 1979, 1980, and 1981. Assimilate enhancement per kernel was achieved by ear tip removal, which resulted in a 20 to 40% reduction in kernels per ear. Total defoliation was the assimilate reduction treatment. Both treatments were applied 12 days (early) and 24 days (late) after mid‐silking. Early defoliation greatly reduced both kernel growth rate and filling period duration, resulting in lower weight per kernel. These defoliation effects occurred despite large compensatory reductions in number of kernels per ear on treated plants. Soluble carbohydrate content of the internode above the ear of defoliated plants declined following defoliation, probably in conjunction with remobilization of stem reserves. This decline was more rapid for the later defoliated plants. In contrast, N concentration of this stem internode of defoliated plants was higher than in control plants and did not decline after treatment. Soluble carbohydrate and N concentration in the internode above the ear of plants in which ear size was reduced was higher than in control plants. Nitrogen concentration of the grain was also increased. However kernel growth rate, final kernel weight, and soluble carbohydrate concentration at maturity were not affected by this treatment. These results show that the remobilization patterns for stems in response to changes in assimilate supply, differs for N as compared to soluble carbohydrates. In addition, the data suggest that kernel growth rates and final seed size may be already approaching the upper limits of their potential in this hybrid. Undetermined factors other than assimilate supply appear to be restraining higher kernel growth rates and final weights.

I. Effects of translocation and kernel growth

A review of the literature shows that when cereal crop plants are subjected to post-anthesis stress of the photosynthetic system (such as drought and heat), kernel growth is supported to a larger extent by the mobilization of plant reserves, relative to transient photosynthesis, and that genetic variation exists in this respect. It was hypothesized that chemical destruction of the plant's photosynthetic source after anthesis may reveal genotypes that sustain translocation-based kernel growth in the absence of photosynthesis, and can then be used as an indirect screen for tolerance to post-anthesis stress. Four spring wheat cultivars (Lakhish, Cajeme, V591-51 and V582-58) were grown in the field under non-stress conditions and were sprayed once with a chemical desiccant (magnesium chlorate) 14 days after anthesis; they were compared with a non-treated control. The chemical desiccant killed awns, glumes, leaf laminae and parts of the spike-peduncle and leaf sheaths. The treatment induced earlier stem dry matter loss in two cultivars (Lakhish and V591-51, group I), and not in the two others (Cajeme and V582-58, group II). The pattern of stem dry matter loss in the group II cultivars did not differ between treatments. Kernel growth rate was reduced by the treatment, to a relatively lesser extent in group I than in group II. Cultivars of group I were therefore better than cultivars of group II in sustaining kernel growth in the absence of photosynthesis and this difference was associated with a respective difference in stem reserves mobilization.

Growth of Spring Wheat Kernels as Influenced by Reduced Kernel Number per Spike and Defoliation1

Field studies with the spring wheat (Triticum aestivum L.) cultivars ‘Era’ and ‘OlaF’ were conducted to assess the effects of reductions in kernel number per spike and defoliation of individual culms on kernel growth characteristics. The importance of stage of kernel growth at which treatments were applied was also examined by treating both at anthesis and anthesis + 14 days, the later date being after the beginning of the kernels linear dry matter accumulation period.The effects of the treatments on final weight per kernel depended on the time of treatment and on the relative weights of kernels from the untreated (control) culms. Reduction in kernel number per spike increased final kernel weight, particularly when control weights were low, whereas defoliation always reduced final weight. Treatment at anthesis usually had a larger effect.Growth rates of kernels during the linear dry matter accumulation period were enhanced by kernel removal and depressed by defoliation. Within each genotype, growth rates were highly correlated with final kernel weight. This association, however, differed between years because of variation in length of the grain filling period. No consistent effect of the treatments on grain filling duration was found.The response to early kernel number reduction, as well as the response in additional experiments to spikelet “trimming” suggested that upper limits, or potentials, for both final weight and growth rate were approached. These potential kernel weights and growth rates appeared to depend on genotype, kernel position in a spikelet, and possibly environment.

Temperature Effects on in vitro Kernel Development of Maize1

This study was conducted to ascertain the effects of temperature extremes on kernel characteristics, respiration, starch synthesis, soluble sugar, and protein content during grain filling in maize (Zea mays L.). Kernels cultured in vitro on a defined media at 15, 30, and 35 C were compared to kernels from ears developed in the field. Kernels cultured for 7 days at 35 C had higher dry weights than kernels from other treatments; however, their growth ceased by the 14th day. High soluble sugar content of aborted kernels suggests that inhibition of starch synthesis may have caused kernel abortion at 35 C. Respiration was higher for kernels from field‐grown ears. During the linear phase of dry matter accumulation, kernel growth rates were 0.65, 6.26, and 5.26 mg kernel‐1 day ‐1 for 15 C, 30 C, and field control treatments, respectively. The higher rate of kernel growth at 30 C was negated by a pronounced shortening of the duration of the grain filling period. Relative to the 30 C treatment, a three‐fold increase in duration of grain filling at 15 C was not adequate to compensate for the 90% reduction in kernel growth rate that occurred. Cool temperatures that occurred in the field or that were maintained in vitro caused soluble sugar levels to remain high and deposition of starch in the endosperm to lag several days behind that observed at 30 C.Cool temperatures did not affect the synthesis of proteins in general or cause differential synthesis of an individual or specific class of protein in the zein fraction. These data show that kernel abortion and shortening of the grain filling period at higher than optimum temperatures and severe reduction in kernel growth rates at lower than optimum temperatures are possible mechanisms by which temperature may mediate kernel growth and development and ultimately grain yield of maize.

Dry Matter Accumulation in Kernels of Maize1

Selection in a breeding program for physiological traits should not be initiated without knowledge of critical growth stages for the crop. The granin yield of maize (Zea mays L.) is the integral of kernel dry matter accumulation over time. The rate and duration of kernel fill have been suggested as factors which might be improved through selection and result in yield improvement. The purpose of these experiments was to follow kernel development at three positions on the ear when treatments were imposed which altered assimilate supply to the ear.Two maize hybrids, Pioneer brand '3388' and '3369A,' which differed in the yield components, kernel rows ear‐1, and kernel weight, were grown in field plots. Treatments were applied at 50% stand density, shoot bagging ears to allow complete pollination on a single day, and 50% leaf area removal. Kernel growth rates were determined for kernels at the base, middle, and tip regions of the ears. Hybrid 3369A had more rapid rates of kernel dry matter accumulation than did 3388 in both years.Treatments which affected assimilate supply, i.e., thinning and leaf area removal, affected kernel number (ear row)‐1through variations in the cessation of kernel development at the tip of the ears. Rates of kernel dry matter accumulation were similar for kernels from basal and middle regions of the ears, regardless of assimilate supply. Tip kernels may have filled at slower rates than did kernels from the other two regions. Altered duration of the linear phase of kernel dry matter accumulation due to treatments affecting assimilate supply was suggested as one cause of variations in kernel weight on an ear as well as between ears of the same hybrid. A critical stage in the development of maize that is highly dependent on assimilate supply is the 2‐ to 3‐week period after 50% silking; the period when final kernel number is determined.

Rate and Duration of Growth of Kernels Formed at Specific Florets in Spikelets of Spring Wheat1

In an earlier study, large differences in individual kernel weight were observed within spikelets of spring wheat genotypes (Triticum aestivum L. em Thell). To account for these differences, growth rates and effective filling periods were measured in field experiments for kernels formed at each floret position in central spikelets. The genotypes ‘Era’, ‘8037’, and ‘72316’, which differed in kernel weight and kernel number per spikelet, were used. Kernels formed in the fourth and fifth florets (numbered from the spikelet base) displayed significantly lower growth rates than proximal kernels during the period of linear kernel dry matter accumulation. Kernel growth rates during this period over all years, genotypes, and floret positions were significantly correlated with final individual kernel weights.Another factor that influenced kernel weight within a spikelet was the earlier occurrence of anthesis in the proximal florets compared to distal ones. Kernels formed in proximal florets already had a considerable weight advantage even before the period of linear dry matter accumulation. Effective filling period durations of proximal kernels of Era and 72316 were longer, a probable consequence of the earlier anthesis in these florets. Cessation of dry matter accumulation (and the associated sharp decline in kernel water content) were judged to occur at approximately the same time for all kernels in a spikelet. Additional research oriented toward understanding those factors responsible for establishing kernel growth rate during the linear dry matter accumulation period is advocated.