Maize Kernel Set Research Articles

Ear reproductive development components associated with kernel set in maize: Breeding effects under contrasting environments

Context or problemKernel number per ear (KNE) is the main driver of improved grain yield per ear (GYE) in maize (Zea mays L.). Under non-limiting conditions, KNE responded to variations in ear developmental traits such as the number of completely developed florets (CDFE) and exposed silks per ear (ESE), which have been positively affected by breeding. This trend needs to be assessed under common abiotic constraints that usually affect rainfed maize production. Objective or research questionThe aim of the current research was to study breeding effects on maize grain yield and its associated reproductive developmental traits as well as the sources of loss of kernel number under a wide range of growing conditions per plant. MethodsField experiments were conducted at two sites of the Central Pampas of Argentina, where hybrids of different year of release (from 1980 to 2012) were evaluated under 10 managed environments generated by the combination of (i) three growing seasons (2015–16: S1; 2017–18: S2; 2018–19: S3), (ii) two nitrogen rates (N0: no nitrogen added; N200: 200 kg N ha-1), and (iii) two stand densities in S1 and S2 (9 and 12 plants m-2). Plant biomass at R1 was used as a classifier of pre-flowering growth conditions (LOW, MID-LOW, MID-HIGH, and HIGH environments). ResultsAlthough modern hybrids always presented more GYE and KNE than old hybrids, the reproductive developmental processes behind this response were different depending on the environment. Under HIGH environments, the KNE improvement (4.53 kernels year-1) was the result of a greater CDFE (7.55 florets year-1) and more ESE (6.01 silks year-1). Under LOW environments, the genetic progress in KNE (2.41 kernels year-1) was associated with a reduced kernel abortion (−0.48 % year-1). Non-significant trend with year of release was observed in spikelet growth rate in all groups of evaluated environments, however, there was a significant decrease in the residual effect of the relationship between spikelet growth rate during the critical period and kernel abortion under LOW environments. ConclusionsTraits responsible for improved grain yield differed across environments because the relative importance of the determinant processes of kernel set vary across them. Under low-yielding environments, decreased kernel abortion is more beneficial than increased potential ear size (i.e., florets per ear) to enhance kernel number and grain yield. Implications or significanceThis finding highlights the relevance of focusing on processes conducive to kernel abortion under stress-prone environments to improve maize grain yield.

Dissecting the critical stage in the response of maize kernel set to individual and combined drought and heat stress around flowering

Increases in frequency and magnitude of drought and heat events are the prime abiotic constraints, which cause considerable adverse effects on maize kernel set and grain yield. However, there is little information about a comprehensive and systematical understanding to identify the critical stage in the response of kernel numbers per spike (KNS) to individual and combined drought and heat stress around flowering. Here we evaluated three successive phases (florets development, florets pollination and seed setting) of kernel set under drought, heat and combined drought and heat stress (DS, HS and DHS) with two maize hybrids using the field site ponds covered with a rain shelter in 2018 and 2019. As an average in two years, KNS was reduced by 18.9 % in DS treatment, 10.8 % in HS treatment and 37.2 % in DHS treatment compared with the control treatment. The most considerable adverse impacts during the three phases were all observed in DHS treatment, followed by DS and HS treatments. Among the three phases, the impact of stress on seed setting was more substantial than that in the other two phases for both hybrids. Kernel abortion was the main reason for the decrease in KNS due to the stress, which resulted from the prevention of sugar–starch conversion. The starch content in the kernel was reduced by 42 % in DS treatment, 29 % in HS treatment and 58 % in DHS treatment on the 8th day after silking. This finding would supply a reference for breeders to evolve strategies in developing stress-resilient and high-yielding hybrids under climate change.

KNMAIZE–A Model to Investigate the Dynamics of Kernel Set in Maize

Core Ideas The dynamics of kernel set in maize was modeled at the individual kernel level.Kernels survived when enough assimilate was available to meet their needs.The model captures the dynamics of ear size, plant growth rate, and kernel growth. The adjustment of kernels plant−1 to the assimilate supply is an important component of the yield production process in maize (Zea mays L.). A simulation model (KNMAIZE) was developed to better understand this process at the individual kernel level. Kernel survival was determined by comparing the assimilate supply (calculated from solar radiation) partitioned to reproductive growth during the initial stages of kernel development with the kernel assimilate requirement (based on published kernel growth curves) on a daily basis. When model parameters were optimized by maintaining the ratio of the sink (total kernel growth rate) and the source (total assimilate supply) equal to 1.0 during the linear phase of kernel growth, the model accurately mimicked the effect of plant growth rate, individual kernel growth rate, and ear size on kernels plant−1. The model also mimicked the plant density‐kernels m−2 relationship. Short‐term (3 or 5 d) variation in plant growth rate influenced kernels plant−1 11 to 15 d after the beginning of kernel growth. Modeling kernel number at the individual kernel level makes it possible to capture important aspects of the dynamics of kernel set that will lead to better predictions of kernel number and a clearer understanding of the elements of maize productivity in the field.

Silk Development and Kernel Set in Maize as Related to Nitrogen Stress

Nitrogen deficiency limits yield in maize (Zea mays L.) through reduced kernel number. A day‐by‐day investigation of silk emergence and kernel establishment has not been conducted to determine when the reduction occurs. A field study was established near Adair, IL, in 2013 and Sciota, IL, during 2014 to investigate the effect of N deficiency on silk number per ear (SPE) and kernel number per ear (KPE). Three N treatments (0 [0N], 56 [56N], and 201 kg N ha−1 [201N]) and two DuPont Pioneer hybrids (P1395HR and P1498HR) were tested. The 0N treatment reduced specific leaf N by 33%, grain biomass by 35%, and KPE by 21% in 2013 and specific leaf N by 49%, grain biomass by 58%, and KPE by 50% in 2014, compared with 201N. Reduced N status had no effect on spikelet number in 2013 but reduced spikelet number by 7% in 2014. Nitrogen stress had no influence on the rate of silk emergence in 2013 but reduced silk emergence by 142 SPE d−1 in 2014. Compared with 0N, 201N increased the rate of KPE establishment by 67 KPE d−1 in 2014. Over 2 yr and three N rates, >76% of the silks emerged within 1.6 d and 97% of the kernels were established in 1.9 d. Kernel number was reduced under N deficiency by (i) kernel establishment from emerged silks, (ii) emerged silk number, and (iii) spikelet number. Alleviation of N stress by the R1 growth stage would effectively maintain optimum SPE, KPE, and yield potential.

Roles of Carbohydrate Supply and Ethylene, Polyamines in Maize Kernel Set

Glucose appears to have an antagonistic relationship with ethylene and ethylene and polyamines appear to play antagonistic roles in the abortion of seeds and fruits. Moreover, ethylene, spermidine, and spermine share a common biosynthetic precursor. The synchronous changes of them and the relationships with kernel set are currently unclear. Here, we stimulated maize (Zea mays L.) apical kernel set and studied their changes at 4, 8, 12, and 16 d after pollination (DAP). The status of the apical kernels changed from abortion to set, showing a pattern similar to that of the middle kernels, with slow decrease in glucose and rapid decline in ethylene production, and a sharp increase in spermidine and spermine after four DAP. Synchronous changes in ethylene and spermidine were also observed. However, the ethylene production decreased slowly in the aborted apical kernels, the glucose and polyamines concentrations were lower. Ethephon application did not block the change from abortion to set for the setting apical kernels. These data indicate that the developmental change may be accompanied by an inhibition of adequate glucose to ethylene synthesis and subsequent promotion of spermidine and spermine synthesis, and adequate carbohydrate supply may play a key role in the developmental process.

Predicting Potential Kernel Set in Maize from Simple Flowering Characteristics

Accurate prediction of kernel number per plant is critical for yield simulation in maize (Zea mays L.). Model predictions of kernel number generally are based on empirical relationships between final kernel number and carbohydrate supply or plant growth at silking, without regard to flowering dynamics. The objective of this study was to develop a model for predicting kernel set from seasonal dynamics of pollen shed and silk exsertion. Hybrids were planted in isolated plots at four ratios of male fertile (MF) and male sterile (MS) plants. Dynamics of pollen shed in each plot was described by a Gauss curve and by a population index (Pind) derived from the percentage of plants at beginning, maximum, and ending pollen shed. Percentage of plants at silking followed a sigmoid curve, whereas the number of silks exposed daily per ear followed a monomolecular model. Pollen shed and silk exsertion curves were translated to potential kernel set using a published relationship. Excellent agreement was observed between predicted and measured number of florets per hectare available for pollination. The model overpredicted kernel number per hectare at high pollen shed densities. Predicted kernels per hectare were within 5% of actual kernel numbers at low pollen shed densities when delayed development of subapical ears and asynchronous pollination within ears were considered. These results indicate kernel set in maize can be predicted from simple measures of the flowering process. They confirm that kernel set is limited primarily by factors other than pollen amount when all plants in the population are MF.

Predicting Potential Kernel Set in Maize from Simple Flowering Characteristics

Accurate prediction of kernel number per plant is critical for yield simulation in maize (Zea mays L.). Model predictions of kernel number generally are based on empirical relationships between final kernel number and carbohydrate supply or plant growth at silking, without regard to flowering dynamics. The objective of this study was to develop a model for predicting kernel set from seasonal dynamics of pollen shed and silk exsertion. Hybrids were planted in isolated plots at four ratios of male fertile (MF) and male sterile (MS) plants. Dynamics of pollen shed in each plot was described by a Gauss curve and by a population index (Pind) derived from the percentage of plants at beginning, maximum, and ending pollen shed. Percentage of plants at silking followed a sigmoid curve, whereas the number of silks exposed daily per ear followed a monomolecular model. Pollen shed and silk exsertion curves were translated to potential kernel set using a published relationship. Excellent agreement was observed between predicted and measured number of florets per hectare available for pollination. The model overpredicted kernel number per hectare at high pollen shed densities. Predicted kernels per hectare were within 5% of actual kernel numbers at low pollen shed densities when delayed development of subapical ears and asynchronous pollination within ears were considered. These results indicate kernel set in maize can be predicted from simple measures of the flowering process. They confirm that kernel set is limited primarily by factors other than pollen amount when all plants in the population are MF.

Ear Temperature and Pollination Timing Effects on Maize Kernel Set

Maize (Zea mays L.) kernel set can be improved through synchronous pollination within and between ears. Reductions in kernel set could be expected because of asynchronous pollination between early‐ and late‐appearing silks. We analyzed the effect of (i) selective ear heating around the time of silking, and (ii) different time gaps between early‐ and late‐pollinated silks in an attempt to modify kernel set. Tip ear heating was expected to minimize the advantage of early silking ovaries. Lateral heating and pollination gaps were expected to exaggerate this advantage. Three pollination gaps (2, 4, and 6 d) were tested for two plant populations (3 and 9 plants m−2). Ear temperature in the heated zone averaged 4.5°C above air temperature. Temperature in the nonheated side closely followed air temperature. Treatments promoted greater differences in maize kernel number (KN) per ear (73% variation) than in the number of silks exposed 5 d after silking (6% variation). Lateral ear heating reduced KN per ear in comparison with the nonheated control, but tip ear heating did not modify KN per ear. At 9 plants m−2, synchronous pollination resulted in ∼15% increase in KN per plant. Pollination gaps of 2 and 4 d reduced KN per plant drastically (up to 51%), but the reduction was smaller for the 6‐d gap. This study (i) gives evidence of the negative impact of delayed pollination timing among silks on kernel set, which was not related to reduced silk receptivity, and (ii) defines the time gap for maximum interference of early‐ on late‐pollinated ovaries, a period shorter than 4 d.

Synchronous Pollination within and between Ears Improves Kernel Set in Maize

In maize (Zea mays L.), the later‐fertilized ovaries often abort, thereby reducing kernel set. We examined whether altering the time interval between pollination of florets within an ear or between ears could affect final kernel number per plant. Synchrony of pollination was varied by natural‐ and hand‐pollination of four hybrids, contrasting in prolificacy (ears plant−1). Plants were grown in the field at low (2.5 and 3 plants m−2) and high (7.5 and 9 plants m−2) plant populations, without water or nutrient stress. Increasing plant population generally delayed silk appearance, but most silks were exposed within 5 d after silking (DAS). Synchronous pollination of all exposed silks on apical and sub‐apical ears 5 DAS improved kernel number (KN) per plant and the floret fertility index (FFI = number of kernels/number of pollinated silks), relative to open‐pollinated plants. At low plant populations, the KN plant−1 increase resulted primarily from a large increase (39–535%, depending upon the hybrid) in kernels on sub‐apical ears. At high plant populations, only apical ears set kernels. Synchronous pollination increased KN in these ears 8 to 31%, depending on the hybrid. Thus, timing of pollination had a large impact on kernel set, and the disadvantage associated with an ontogenetic delay in silk emergence could be partially overcome by synchronous pollination. Because delayed pollination of early‐silking ovaries allowed a greater number of the late‐silking ones to set kernels, factors other than assimilate availability per fertile floret likely are involved in controlling kernel set.

Night Temperature at Silking Affects Set in Maize

Maize (Zea mays L.) grain yield is primarily associated with variation in the number of mature kernels which, in turn, depends on crop growth at silking. This study was conducted to determine whether night temperature at silking could affect kernel set in maize. Two night thermal conditions: (i) ambient and (ii) ambient plus 5 degrees C, imposed from 1 wk before to 3 wk after silking in a well-watered and fertilized maize crop, were investigated in a field experiment at Balcarce, Argentina during the 1994-1995 growing season. Development rate was hastened by heating; tassels and silks appeared 1 d earlier and the lag phase of grain filling was shortened by 5 d. Control plots required seven more days to achieve the same thermal sum than heated plots during the heating period. Plant growth per day at silking did not differ between treatments since both the crop radiation interception and radiation use efficiencies remained unchanged. Differences in respiration losses were insignificant. Spikelet morphogenesis was not affected by heating. Nevertheless, kernel abortion was increased in heated plots (36 vs. 28% for controls, P < 0.02). Final kernel number was associated with plant growth rate at silking when rate was expressed per unit of thermal time. Thus, increased night temperature at silking affected kernel set in maize mainly through increasing development rate that reduced photoassimilate production per unit of thermal time, limiting photoassimilate availability to support kernel growth at the ear tip.

Floral Asynchrony and Kernel Set in Maize Quantified by Image Analysis

AbstractA delay in the onset of silk emergence relative to pollen shed often decreases kernel set in maize (Zea mays L.). Lack of pollen, failure of silks to emerge, and loss of silk receptivity all are probable causes for kernel loss. To date, it has not been possible to distinguish between these possibilities, due to the lack of quantitative information on the intensity of pollen shed and progress of silk emergence under field conditions. Using a computer‐aided image analysis system, we measured the daily progress of silk emergence and the intensity of pollen shed in the field. Kernel set also was measured on plants whose silk emergence varied from 8 d before to 13 d after anthesis (DAA). About 200 silks emerged on the first day silks appeared and &gt;90% of the florets had silks exposed within 3d. Pollen shed followed a normal distribution with time, peaked 3 DAA, and ended 13 DAA. On ears with silks exposed to pollen for only one d, nearly all florets set kernels when pollen was shed at intensities greater than ≈ 100 grains cm−2 d−1mm; however, percent set in apical floret positions decreased when the onset of silk emergence was delayed &gt;3 DAA, despite pollen shed at a rate sufficient to ensure high kernel set. These results indicate that asynchrony between silk emergence and pollen shed decreases kernel set because silks emerge after pollen amount becomes limiting. In the current experiment, the intensity of pollen shed limited kernel set only beyond 7 DAA. Also, silks emerged prior to anthesis remained receptive to pollen for at least 5 d. This finding suggests that selection for silk emergence prior to pollen shed (protogyny) may improve kernel set in maize under conditions known to delay silk emergence.

Maize Kernel Set at Low Water Potential: II. Sensitivity to Reduced Assimilates at Pollination

Plant water deficits at pollination decrease kernel set in maize. An important component of this kernel loss is developmental failure after fertilization. This study was undertaken to determine if zygotic abortion in water‐deficient plants results primarily from a decrease in assimilate supply daring the critical period around pollination. Plants were grown in soil/sand media in a controlled environment, and photosynthesis was inhibited by water‐deficit or low‐light treatments. Plants were hand‐pollinated when silk water potential of control, low‐light, and water‐deficient plants was −0.4, −0.4, and −1.0 MPa, respectively. Corresponding leaf water potentials were −0.3, −0.3, and −1.7 MPa. The water deficit decreased ovary water potential, osmotic potential, and turgor by 0.6, 0.3, and 0.3 MPa, respectively, compared with values in well‐watered plants. Treatments were terminated 2 d after pollination, and leaf photosynthesis recovered to control values within 24 h. Kernel numbers per ear were 598 ± 17 for control, 222 ± 72 for low light, and 15 ± 6 for water‐deficit treatments. The brief inhibition of photosynthesis had little impact on C and N accumulation in vegetative sinks, but severely reduced assimilate movement to reproductive sinks. At pollination, concentrations (mg g−1 dry wt.) of fructose, glucose, and sucrose in ovaries were similar for all treatments. The high levels of these soluble sugars in both setting and aborting ovaries suggests that kernel set depends on the rate of movement of assimilates into the ovaries, rather than the concentration of soluble sugars per se. Sucrose uptake by isolated ovaries was inhibited at low water potential, suggesting a direct effect of ovary water status on kernel development. The nearly complete kernel loss observed in the water‐deficit plants indicates that low ovary water potential at pollination decreased kernel set beyond that which can be attributed to reduced assimilate supplies alone.

Maize Kernel Set at Low Water Potential: I. Sensitivity to Reduced Assimilates during Early Kernel Growth

Plant water deficits during flowering and early kernel growth reduce the yield potential of maize (Zea mays L.) by decreasing kernel number per ear. Our objective was to determine if zygotic abortion, which increases in water‐deficient plants, is caused by a decrease in assimilate supply to the ear. Plants were grown in soil/sand media in a controlled environment. Beginning at silk emergence and continuing for 3 wk after pollination, leaf photosynthesis was inhibited ≈50% (moderate) or 100% (severe) by withholding water (water‐deficit treatment) or decreasing light intensity (low‐light treatment). Silk water potential (ψw) at pollination was about −0.7 and −1.1 MPa in the moderate and severe water‐deficit treatments, compared to about −0.4 MPa in well‐watered plants. Mean kernel number per ear was 581 in control plants and was decreased 48 and 99% by the moderate and severe water‐deficit treatments, respectively. In the corresponding low‐light treatments, kernel number was decreased 40 and 100%. Thus, losses in kernel number pe ear reflected the relative inhibition of photosynthesis during early kernel growth, regardless of plant water status. Compared to controls, the water‐deficit and low‐light treatments decreased partitioning of dry matter, carbohydrates, and reduced N to the kernels. However, carbohydrates continued to accumulate immediately after pollination in the stalk and leaf tissue. This suggests that some stalk and leaf reserves were unavailable to support early kernel growth at low ψw. Collectively, these data indicate that early kernel development is highly dependent on a continued supply of carbohydrates from concurrent photosynthesis. Water deficits increase the probability of developmental failure by limiting this supply.