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Maize crop production depends on nitrogen (N) availability, N uptake by the crop and the efficiency with which absorbed N is used to produce biomass (NUEBIOM) or grain yield (NUEGRAIN). This framework assumes unique efficiency values for the whole stand, with no distinction among plants in spite of the inherent inter-plant variability of plant growth, especially under crowding stress. In this work we assessed the degree of contribution of different early-established groups of plants to crop responses to N fertilization of two maize hybrids (H) with different tolerance to crowding stress (high for AX820 and low for AX877) cultivated at two stand densities (9 and 12 pl m−2). Groups corresponded to the lower, mid and upper terciles (Ts) of the crop, representing dominated, intermediate and dominant plants, respectively. In most cases, lower and mid Ts had a greater participation in crop biomass and grain yield responses to N fertilization. The response of NUEBIOM and NUEGRAIN to N fertilization was higher for the lower and mid Ts than for the upper T. For each N level, crop NUEGRAIN was negatively related to inter-plant variability in plant NUEGRAIN. When no N was added, the reduction in crop NUEGRAIN of both hybrids was mainly caused by the increased inter-plant variability in plant N uptake (i.e. resource capture). Additionally, the crowding-intolerant AX877 under the most stressful condition (12 pl m−2 and no added N) had a reduced crop NUEGRAIN due to the enhanced plant-to-plant variability in grain yield (i.e. resource use). Consequently, the early-established plant-to-plant variability pattern conditioned crop NUEGRAIN; the predominant path was hybrid dependent. Fil: Rossini, Maria de Los Angeles. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires. Universidad Nacional del Noroeste de la Provincia de Buenos Aires. Centro de Investigaciones y Transferencia del Noroeste de la Provincia de Buenos Aires; Argentina Fil: Otegui, Maria Elena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires Norte. Estación Experimental Agropecuaria Pergamino; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal; Argentina Fil: Martínez, E.L.. Asociación de Cooperativas Argentinas C.L.; Argentina Fil: Maddonni, Gustavo Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal; Argentina

In maize (Zea mays, L.) genotypic variability in the relationship between total kernel number per plant (KNP) and plant growth rate (PGR) during the period bracketing silking (R1 ± 15 d), and in kernel set efficiency (KNP PGR−1, KSE) have been widely documented. Nitrogen supply (Ns) affects PGR and hence KNP (i.e. indirect effect on KNP), but reports on a direct N effect on KNP PGR−1 are still contradictories. Moreover, recent studies have documented that prolificacy (the number of ears per plant) was cancelled out in N-limited plants despite of their high PGR. Additionally, PGR can be differentially affected by environmental conditions (e.g. sowing dates) through indirect effects on plant size or directly, which could determine changes on KSE. Field experiments were carried-out in Parana (31° 48' S, 60° 32' W), Argentina, during two growing seasons (2014−15 and 2015−16). Two single cross hybrids were sown in two sowing dates (early: September and late: December), with three N fertilizer rates (0, 90, and 270 kg N ha−1) at three plant densities (5, 7 and 9 pl m−2). The effects of plant N status and plant size on KNP, KSE and prolificacy were study for a wide range of PGR and relative PGR (rPGR). Plant N status was estimated by using SPAD measurements on the ear-leaf blade at R1. Ranges of lower, mid and higher PGR (absolute and relative values) and SPAD units were established using 33rd and 66th percentiles. Curvilinear functions were fitted to kernel number at the apical ear (KNE1) vs. PGR and KNP vs. PGR and boundary functions were also fitted to calculate the unexplained variance (i.e. residuals) of these functions, indicative of direct plant N status effects on KSE. Residuals of KNE1 vs. PGR relationship decreased with increases in plant N status up to a threshold value of 46.8 SPAD units (R2 = 0.75, P

Selection for maize (Zea mays L.) grain yield in high-yielding environments at high population densities has favored a compact phenotype tolerant to crowding stress, bearing a single, well-grained ear. However, by contributing to vegetative and reproductive plasticity (i.e., multiple shoots and ears per plant, respectively), tillering may be adaptive in environments with low and variable availability of resources, chiefly water and nutrients, where crops are sown at low plant population density. In this work we present a robust, new conceptual framework for vegetative and reproductive plasticity in maize with direct agronomic applications, combining original data from new experiments and data reviewed from the literature. First, we describe production systems where tillering in maize would be relevant in terms of grain yield. Next, we discuss possible masked effects of genetic selection at high plant densities on tillering and present novel experimental results showing genotypic variation of tillering in modern maize hybrids and genotype x environment x management effects (plant density x location x sowing date) on tillering expression. We follow with a two-part framework to analyze tillering and prolificacy. In the first part (from axillary buds to tillers), we integrate the early effects of the light environment (through photomorphogenesis) and carbon balance on tillering emission, and discuss the environmental factors (temperature, photoperiod, radiation, water, nitrogen) that modulate tiller emission and tiller growth. In the second part (from tillers to kernels), we summarize the functional relationships governing kernel set on the ears of main shoot (apical and sub-apical ears) and tillers, focusing on the growth rate of shoot cohorts, rather than the whole plant. We then provide examples of the diverse patterns of contribution of multiple shoots to crop grain yield for maize husbandry in low-yielding environments. Finally, we address the effect of tillering on resource capture and use efficiency of maize crops by discussing its relationship with biomass and grain yield and provide supportive experimental data. We conclude with identification of knowledge gaps leading to testable hypotheses.

In maize (Zea mays L.) crops, nitrogen (N) status at silking (R1) has been used to predict grain yield (GY) response to N fertilization and to develop strategies to manage crop nutrition in order to match N supply with crop demand during growing season, such as late N fertilizations. Crop N status can be estimated by N nutrition index (NNI), which is based on actual and critical N concentration in crop biomass. Optical measurements of N concentration (e.g. SPAD readings) of the leaf blade subtending primary ear have also been used as a proxy of crop N status. Sowing date (SD) and N rates could affect soil N availability and hence crop N uptake at R1. Additionally, the effects of SD and its interaction with plant density (PD), N rates and hybrids (H) on N uptake (NuptP) and particularly on N partitioning in leaf-blades, stem + sheaths and ears could affect SPADs readings. We hypothesized that variations of GY by crop N status at R1 promoted by SD, PD and H, would be better predicted by NNI than by SPAD readings. In this study, two Hs (DK 70−10 VT3P and DK 73−10 VT3P) were cropped in two contrasting SD (early and late) in Parana, Argentina (31°44′ S 60°32′ W) at three PD (5, 7 and 9 pl m−2) with three N rates (0, 90 and 270 kg N ha-1) in order to evaluate the effect of treatments on: i) N availability, N uptake at the plant and crop level, N partitioning in leaf-blades, stem + sheaths and ears, SPAD readings and NNI at R1, and ii) the relationships among N availability and N uptake at the plant and crop level, NNI, SPAD, and GY. N concentration of leaf-blades was negatively affected by PD, but this reduction was attenuated by N rates, especially in late SD (N x PD x SD interaction). Hence, in early SD, some data of both Hs corresponding to 270 N yielded low SPAD values for NNI greater than 0.86. Consequently, crop N status was better reflected by NNI than by SPAD readings, because NNI considers N stored in the whole plant. NNI at R1 adequately described relative GY variations promoted by SD, PD, N rates and, Hs, i.e. NNI was a more meaningful crop status index than SPAD readings. Overall, our study contributes to understanding mechanisms that regulate crop N status affected by agronomical practices and adds insights to explore in late N fertilization of maize crops.

In maize (Zea mays L.) crop, “defensive practices” such as low plant densities and limited nitrogen (N) fertilization have begun to be progressively adopted in limited-production regions of the world. At low plant density, the expression of prolificacy (i.e., more than one fertile ear per plant) can stabilize maize production. Considering N as the main limiting resource, yield can be defined as a function of i) total N uptake and the efficiency to produce yield from total N uptake, i.e., N internal efficiency (NIE) or ii) total N uptake, N harvest index (NHI) and grain N concentration. The objective of this work was to describe changes in NIE and their related components regulated by prolificacy at both canopy- and plant- scales. Field studies were carried out in Buenos Aires, Argentina, during 2015–2016 and 2016–2017 seasons. Treatments were combinations of two N supplies (N-: 0 and N+: 200 kg N ha−1), two plant densities (4 and 8 plants m−2) and five maize hybrids released during the last four decades. High N supply positively impacted yield per unit area (ca. 45 %) by increasing kernel number (ca. 33 %), kernel weight (ca. 18 %) and harvest index (ca. 12 %). At low plant density, high N supply expressed the highest prolificacy (mean 1.65 ears plant−1). The older hybrids displayed the highest prolificacy (mean 1.30 ears plant−1) and the lowest yield per unit area (mean 792 g m−2), but showed the greatest grain N concentration (mean 15.1 g kg−1). However, recent hybrids resulted in medium prolificacy (mean 1.11 ears plant−1) with the highest yield per unit area (mean 910 g m−2), but with the lowest grain N concentration (mean 13.6 g kg−1). The different pattern of grain N concentration among hybrids coupled with similar NHI (mean 0.80) led to the lowest NIE of the most prolific hybrids (mean 55 g yield g N−1) relative to the less prolific ones (mean 60 g yield g N−1). Variations in NIE among genotypes and plant densities were negatively influenced by grain N concentration under N- and under N+ only when prolificacy = 1. Under N+ with prolificacy > 1, NIE variations were positively determined by NHI. For all genotypes, higher yield per plant (mean 30 %) and total N uptake (mean 25 %) were recorded for the prolific plants, reflected in the greater absolute NIE (mean 5 %). For the oldest hybrid differences between prolific and non-prolific plants in NIE (mean 46.8 vs 41.8 g yield g N-1) were greater than those recorded for the other genotypes due to the higher impact of prolificacy on yield per plant (mean 51 %) than on total N uptake (mean 26 %). Over time, improvements in NIE primarily occurred from greater yield of the apical ear. Overall, the expression of prolificacy appears to be an adequate strategy to stabilize maize production in response to improvements of environmental conditions, specially addressed by low plant density and high N supply.

Identifying optimum combinations of genotype (G) and agronomic management (M) i.e. crop design, to match the environment (E) i.e. site and expected seasonal conditions, is a useful concept to maximise crop yields and farmers’ profits. However, operationalising the concept requires practitioners to understand the likelihood of different E outcomes and GxM combinations that would maximise yields while managing risks. Here we propose and demonstrate an analysis framework to inform crop designs (GxM) at the time of sowing of a dryland maize crop, that combines data sets from multi-environment field experimentation and crop simulation modelling, and that accounts for risk preference. A network of replicated, G by M on-farm and on-research station trials (n = 10), conducted across New South Wales and Queensland, Australia, over three seasons (2014–2016) was collected. The trials consisted of combinations of commercial maize hybrids, sown at a range of plant densities and row configurations producing site average yields (Environment-yield) that varied between 1576 and 7914 kg ha−1. Experimental data were used to test the capacity of APSIM-Maize 7.10 to simulate the experimental results, and to in-silico create a large synthetic data set of multi-E (sites x seasons) factorial combination of crop designs. Data mining techniques were applied on the synthetic data set, to derive a probabilistic model to predict the likely Environment-yield and associated risk from variables known at sowing, and to derive simple “rules of thumb” for farmers that discriminate high and low yielding crop designs across the lower, middle and upper tercile of the predicted Environment-yields. Four risk profiles are described, a “Dynamic” (i.e. each year the farmer would adopt a crop design based on the predicted Environment-yield tercile and corresponding “rules of thumb”), “High rewards seeker” (i.e. each year the farmer would adopt the crop design that optimises yield for the higher tercile of Environment-yields), “Middle’er” (i.e. each year the farmer would adopt the crop design that optimises yield for the middle tercile of Environment-yields), and “Risk averse” (i.e. each year the farmer would adopt the crop design that optimises yield for the lower tercile of Environment-yields). The difference in yield between the lowest and highest performing crop design was ca. 50 % which translates into a ca. 2-fold change in water use efficiency, i.e. from 8 to 15 kg grain mm−1 rainfall. APSIM-Maize explained 88 % of the variability in the experimental data set. The validated model was used to extend the number of E sampled by adding additional sites within the same region and using historical climate records for the period 1950–2018. Crop available water at the time of sowing was a good predictor for the likelihood of the season falling within each of the three Environment-yield terciles. Recursive partitioning trees showed that plant density and hybrid were the main variables discriminating crop performance within the upper, middle and lower terciles of Environment-yields. The probability distribution functions for yield resulting from the alternative risk management strategies were tested in terms of changes in the mean yield, an index of yield stability, and down-side risk i.e. the likelihood of achieving a non-economic yield. We conclude that (i) for dryland maize cropping, the crop water availability at the time of sowing can be used to inform optimum crop designs, increase yields and yield stability and reduce down-side risks; and (ii) the proposed framework is useful to untangle complex GxExM interactions in field experimentation that provide a transferable platform to develop simple rules to identify optimum crop designs early in the season.

Inter-plant competition is a key trait of maize (Zea mays, L.) crops growth and grain yield. Spatial and temporal availability of resources like water, nutrients and radiation have been proved to impact on this trait. It is suspected that light-quality signals operate on plants´ growth variability of irrigated and fertilized maize crops. Since photoreceptors phytochromes B1 and B2 are involved in light-signals-mediated detecting neighbors, it has been speculated that phytochromes B i) affect plants growth variability from early stages of maize cycle, ii) are involved in the increasing inter-plant variability by increasing plant density, and iii) confer a higher competitive capacity of plants within a canopy. It is also unclear if shade avoidance responses are detrimental or beneficial for plants growth and grain yield in maize crops. To test these hypothesis, plants of maize inbred line France 2 wild type (WT) and the isogenic mutants lacking either phyB1 or phyB2 (phyB1 and phyB2), were cultivated in the field during two seasons in monocultures (WT, phyB1, phyB2; hypothesis i and ii) and polycultures (WT/phyB1, WT/phyB2, phyB1/phyB2 and WT/phyB1/phyB2; hypothesis iii) at contrasting plant densities (low and high) irrigated and fertilized. Plant biomass of ten tagged plants per plot were nondestructively estimated from seedling emergence to 15 days after flowering, and from individual samples at physiological maturity and coefficient of variation (CV) of plant biomass was calculated as a proxy of plants growth variability. Plant leaf area, stem length, and plant growth rate of tagged plans were measured around female flowering, i.e. silking (PGRs) and kernel number per plant (KNP) and grain yield were quantified at physiological maturity. At the lower density, no differences in plants growth variability were detected among genotypes. By contrast, at the higher density CV of WT was higher than those of mutant lines only when canopies were fully developed (i.e. after flowering). Taller plants with larger leaf area characterized WT phenotype at all densities and polycultures, which were generally reflected on higher PGRs. These reactions of WT plants allowed them to acquire more competitive ability and to set more KNP and grain yield than phyB1 plants. phyB2 may have offset differences in KNP with WT by greater grain weights. Hence, in maize crops, phytochromes B are key photoreceptors mediating the response of plants growth variability to crowding stress, without any detrimental effect on grain yield because the ability of plants to forage for light, sustains PGRs and kernel setting.

Genotypic differences in the response of maize kernel number per plant to ear growth rate around silking, caused by contrasting N availability, have been attributed to the effects of this element on reproductive efficiency (i.e. kernel set per unit of ear growth rate). The objective of current research was to assess if reduced reproductive efficiency of some genotypes under N stress is due to the effect of this nutrient on the number of completely developed florets per ear, the number of exposed silks per ear, and/or abortion of pollinated florets. Two field experiments were conducted with two hybrids previously characterized by their contrasting reproductive efficiency (high for AX820 and low for AX877) under N stress, two stand densities (9 and 12 pl m−2) and two levels of added N (0 and 200 kg N ha−1). We established links among plant and ear growth rates, reproductive traits and kernel number per plant. Reduced reproductive efficiency (quantified as kernel number per plant per unit of spikelet growth rate around silking) of both hybrids under N deficiency was mainly due to an enhanced abortion of pollinated florets of the most suppressed plants of the stand (dominated individuals). This response did not appear to be the result of low spikelet growth rate around silking, but a direct control of N on sink capacity of fertilized ovaries for assimilates allocation.

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 (PGRS), PGRS per kernel (PGRSKNP-1) and PG during the effective grain-filling period per kernel (PGGFKNP-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 12plm-2) at two N supplies (N0: control and N200: 200kgNha-1) without water restrictions. For AX820, PGRS data set at both N levels explored a similar range (1-7.4 and 1.2-7.4gpl-1d-1 for N0 and N200, respectively) with a positive skewness in N0, and an almost normal distribution of data in N200. In contrast, for AX877, inter-plant variation of PGRS exhibited a normal distribution in both N levels, and N fertilization only produced a displacement of data to higher PGRS values (0-4.3 and 0.7-5.7gpl-1d-1 for N0 and N200, respectively). The effect of inter-plant variation of PGRS 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 PGRS 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 N0 than in N200, 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. © 2011 Elsevier B.V. Fil: Mayer, Luis Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Rossini, Maria de Los Angeles. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Maddonni, Gustavo Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina

Increased plant population density in irrigated and fertilized maize crops enhances plant-to-plant variability since early vegetative stages, because the most suppressed individuals of the stand intercept less radiation per unit leaf area than the dominant ones (i.e. a size-asymmetric competition for light). Contrarily, a size-symmetric competition has been proposed for the acquisition of soil resources in a plant community (e.g. N capture per unit root length is similar among plants of different size). Hence, N fertilization effect on the variability of maize plants would depend on the initial plant-to-plant variability or on that promoted by a high plant population density. Two maize hybrids with contrasting tolerance to crowding (tolerant AX820 and intolerant AX877) were cultivated under different combinations of stand densities (6, 9 and 12plantsm-2) and N supplies (0 and 200kgNha-1) without water restrictions. Variability in plant growth rate among plants was computed along the cycle, especially after fertilizer was applied (i.e. the early reproductive period; PGRER) and during the critical period around silking (PGRCP). Plant-to-plant variability in biomass partitioning to the ear (partition index; PI), ear growth rate during the critical period (EGRCP) and kernel number per plant (KNP) was also established. Reduced N supply increased the coefficient of variation (CV) of PGRER, PGRCP, EGRCP and KNP (0.05 Fil: Rossini, Maria de Los Angeles. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Maddonni, Gustavo Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Otegui, Maria Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Maize canopies with a synchronous seedling emergence and a uniform plant spatial distribution exhibit early-established plant hierarchies (at the 4-leaf stage; V4). The dominant and dominated individuals of the stand differ in plant growth rate during both the pre-silking period (i.e. from V7 to V13; PGRPS) and the period around silking (i.e. a 30 d period centered in silking; PGRS), and in the ear growth rate around silking (EGRS). Based on the depleted availability of assimilates of the dominated plants, we tested the hypotheses that (i) the low PGRPS of dominated individuals affects the morphogenesis of the apical ear leading to a low number of completely developed flowers per ear, and (ii) the low EGRS of dominated individuals results in a pronounced asynchrony of flowering dynamics and uneven silk exsertion from the husks. Two hybrids with contrasting tolerance to crowding stress (DK752 and DK765 as the tolerant and the intolerant hybrid, respectively) were cropped under different intensities of interplant competition (6, 12, 12 plants m−2 thinned to 6 plants m−2 at V9 and 6 plants m−2 shaded from V9 onwards) during 2004/2005 and at 12 plants m−2 during 2005/2006 at Pergamino (34°56′S 60°34′W), Argentina. Dominant plants were the individuals of the stands with the highest PGRPS (ca. 1.72 and 2.56 g d−1 for dominated and dominant plants, respectively), PGRS (ca. 3.05 and 3.94 g d−1 for dominated and dominant plants, respectively) and EGRS (ca. 1.06 and 1.55 g d−1 for dominated and dominant plants, respectively). This plant type also exhibited the most synchronous flowering dynamics (anthesis–silking interval ca. 1.49 and 1.15 days for dominated and dominant plants, respectively) and the highest kernel set (ca. 401 and 572 kernels plant−1 for dominated and dominant plants, respectively). Apical ears of dominated plants exhibited a delayed in the rate of progress to successive floral stages, but the final number of completely developed flowers per ear did not differ between extreme plant types (ca. 967 and 803 completely developed flowers per ear for DK752 and DK765, respectively). Hence, kernel number per plant was not limited by the number of completely developed flowers per ear, but flowering dynamics were a decisive factor in kernel set of both plant types. Asynchronous silking within the ear of dominated plants determined a greater proportion of flowers per ear with non-exposed silks on silking + 5 d and a larger asynchrony in silk extrusion within the ear. These responses increased kernel abortion rate respect to figures obtained for dominant individuals.

Early interferences among plants within a maize stand determine the establishment of extreme plant types (i.e. dominant and dominated individuals). The development of these hierarchies takes place well before [from the seventh leaf stage (V 7 ) onwards] the start of the critical period for kernel set (i.e. a 30-day period centered in silking). Kernel number per plant (KNP) is significantly related to plant growth rate around silking (PGR S ) and biomass partitioning to the ear during this period. Previous evidence has demonstrated that at high stand densities, extreme plant types may exhibit similar PGR S values but set different KNP. We tested the hypothesis that early established plant hierarchies differ in biomass allocation to the ears during the period around silking. Two hybrids of contrasting tolerance to crowding (DK752 and DK765 as the tolerant and the intolerant hybrid, respectively) were cropped at different interplant competition intensities (6, 12, 12 pl m −2 thinned to 6 pl m −2 at V 9 and 6 pl m −2 shading from V 9 onwards) during 2003/2004 and 2004/2005 in Argentina. For all treatments, the coefficient of variation (CV) of plant biomass increased from V 3 (ca. 1.2%) to V 9-10 (ca. 22%). From V 7 onwards, plant growth rate of dominant individuals was higher ( P P S (ca. 4.5 g pl day −1 ) than dominated individuals (ca. 3.7 g pl day −1 ). As PGR S declined in response to increased plant population density (ca. 5.1 and 2.8 for 6 and 12 pl m −2 , respectively), biomass partitioning to the ear was reduced (ca. 0.44 and 0.33 for 6 and 12 pl m −2 , respectively). For all treatments, however, dominant plants exhibited a greater biomass partitioning to the ear (ca. 0.41) than the dominated individuals (ca. 0.36). Consequently, the former were the individuals with the highest ear growth rate (ca. 1.9 and 1.4 g per ear per day for the dominant and dominated plant, respectively) and KNP (ca. 623 and 490 kernels per plant for the dominant and dominated plant, respectively) of the stand. We identified three traits on DK765 related to the low tolerance to high-density stress of this genotype: (i) a higher plant-to-plant variability (CV ca. 26% and 19%, for DK765 and DK752, respectively), (ii) a lower biomass partitioning to the ear around silking (ca. 0.26 and 0.39 for DK765 and DK752, respectively), and (iii) a higher response rate of KNP to ear growth rate around silking (ca. 370–738 and 360–414 kernels per g, for DK765 and DK752, respectively). Hence, as stand density was increased, KNP of DK765 was sharply reduced, especially in the dominated individuals of the stand.

Maize (Zea mays L.) cropping conditions that promote high intra-specific competition pressure generate an increased plant-to-plant variability within the stand, and the appearance of individuals with different ability to capture scarce resources (i.e. dominant and dominated plants). The objectives of this paper were to analyze (i) stand density effects on plant biomass at physiological maturity (R6), grain yield per plant (GYP), GYP components (KNP: kernel number per plant; KW: kernel weight), and kernel composition (starch, oil and protein contents per kernel) of the mean plant of the stand (i.e. considering all individuals) and of the dominant (D) and dominated (d) individuals; and (ii) the contribution of these extreme plant hierarchies to GYP, GYP components and kernel composition of the mean plant of a stand. Four maize hybrids of contrasting KW (small and large KW) were cropped at a wide range of stand densities (3‐15 pl m � 2 ) during 1999/2000 and 2001/ 2002 in Argentina. The mean value of measured variables declined as plant density increased from 3 to 15 pl m � 2 , and plant-to-plant variability (CV: coefficient of variation) of the same variables increased with enhanced crowding. The magnitude of the reduction in mean plant values differed among variables: plant biomass at R6, GYP and KNP underwent a larger reduction (ca. 66%) than KW (ca. 14‐19%) or kernel contents (ca. 22% for oil and protein contents, and 13% for starch content). Similarly, the increase in CVs was larger for plant biomass at R6 (from ca. 13 to 40%) and GYP (from ca. 30to 58 and 15 to 38% for small and large KW hybrids, respectively)than for KW (ca. from 7to 20%). Only a slight increase in CVs of oil (6‐17%) and protein (9‐12%) concentrations of large KW hybrids was recorded. The CVof KNP followed a trend similar to that for GYP. Differences between plant categories increased when mean GYP and KNP of all individuals of the stand were smaller than 157 g pl � 1 and 649 kernel pl � 1 , respectively. Below these thresholds, the d/D ratio dropped from 0.76 to 0.30 (small KW hybrids) or to 0.40 (large KW hybrids) for GYP (r 2 = 0.76, P < 0.001), and from 0.75 to 0.38 (small KW hybrids) or to 0.46 (large KW hybrids) for KNP (r 2 = 0.59, P < 0.001). In contrast, the d/D ratio for KW varied always from 1 to 0.80 in response to decreased mean KW (r 2 = 0.39, P < 0.01). The concentration of kernel contents did not differ between plant types. Results indicate that grain yield of maize crops grown at high stand densities is composed by plants bearing very different kernel numbers, with slight differences in kernel size, and similar starch, oil and protein concentration. # 2005 Elsevier B.V. All rights reserved.

Reduced plant biomass and increased plant-to-plant variability are expected responses to crowding in monocultures, but the underlying processes that control the onset of interplant interference and the establishment of hierarchies among plants within a stand are poorly understood. We tested the hypothesis that early determined plant types (i.e. dominant and dominated individuals) are the cause of the large variability in final kernel number per plant (KNP) usually observed at low values of plant growth rate (PGR) around silking in maize ( Zea mays L.). Two hybrids (DK696 and Exp980) of contrasting response to crowding were cropped at different stand densities (6, 9 and 12 plants m −2 ), row spacings (0.35 and 0.70 m), and water regimes (rainfed and irrigated) during 1999/2000 and 2001/2002 in Argentina. The onset of interplant competition started very early during the cycle, and significant differences ( P 4–6 (DK696) and V 6–7 (Exp980). Plant population and row spacing treatments did not modify the onset of the hierarchical growth among plants, but did affect ( P −2 (ca. 0.12 g/g per 100 °C day) than at 6 plants m −2 (ca. 0.07 g/g per 100 °C day). For all treatments, the largest difference in estimated shoot biomass between plant types took place between 350 (V 7 ) and 750 °C day (V 13 ) from sowing, and remained constant from V 13 onwards. Dominant plants always had more kernels per plant ( P P −2 . Our research confirmed the significant ( P P 3 ) reflected the variation in KNP ( r 2 ≥0.62), and was significantly ( P 13 ). This response suggested that the physiological state of each plant at the beginning of the critical period had conditioned its reproductive fate. This early effect of plant type on final KNP seemed to be exerted through current assimilate partitioning during the critical period.

Maize crop management involves decision making on several cultural practices aimed to maximize grain yield, like plant population and row spacing. These practices affect the light environment perceived by plants and the post-flowering source–sink ratio, but there is scarce information on the way they influence plant leaf senescence. The objectives of our research were to: (i) characterize the development of leaf area senescence for contrasting canopy architectures (i.e. plant population×row spacing), and (ii) analyze the response of leaf senescence to changes in the light environment and the post-flowering source–sink ratio. Field experiments were conducted in Argentina between 1997/1998 and 2000/2001. Four hybrids were grown at a wide range of plant populations (3, 9, 10 and 12 plants m−2), row spacings (0.35, 0.7 and 1 m) and pollination treatments (natural and restricted pollination). Senescence development was well described (r2=0.61–0.99,P

Light attenuation within a row crop such as maize is influenced by canopy architecture, which has to be defined in terms of the size, shape and orientation of shoot components. Cultural practices that improve the efficiency of light interception affect canopy architecture by modifying such components. Our objectives were to: (i) determine the nature and timing of leaf growth responses to plant population and row spacing; (ii) analyze light attenuation within fully developed maize canopies. Field experiments were conducted at Pergamino (33°56′S, 60°34′W) and Salto (34°33′S, 60°33′W), Argentina, during 1996/1997 and 1997/1998 on silty clay loam soils (Typic Argiudoll) that were well watered and fertilized. Four maize hybrids of contrasting plant type were grown at three plant populations (3, 9 and 12 plants m −2 ) and two row spacings (0.35 and 0.70 m). Plant population promoted larger changes in shoot organs than did row spacing. As from early stages of crop growth, leaf growth (V 6 –V 8 ) and azimuthal orientation (V 10 –V 11 ) were markedly affected by treatments. Modifications in shoot size and leaf orientation suggest shade avoidance reactions, probably triggered by a reduction in the red:far-red ratio of light within the canopy. An interaction between hybrid and plant rectangularity on leaf azimuthal distribution was determined, with one hybrid displaying a random azimuthal leaf distribution under most conditions. This type of hybrid was defined as rigid . The other hybrids showed modified azimuthal distribution of leaves in response to plant rectangularity, even at very low plant populations. These hybrids were defined as plastic . Once maximum leaf area index (LAI) was attained light attenuation did not vary among hybrids and row spacing for plant populations ≥9 plants m −2 ( k coefficient: 0.55 and 0.65 for 9 and 12 plants m −2 , respectively). A more uniform plant distribution increased light attenuation ( k coefficient: 0.37–0.49) only when crop canopies did not reach the critical LAI.

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 ( 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.

Maize production in Argentina is obtained mostly under dryland conditions and grain yield is highly related to water availability around silking. Drought, and differences in soil quality caused by intensive land use, sometimes limit water availability for the crop. Field experiments were performed on silty clay loam soils in the temperate humid region of Argentina (32° to 35°S, 58° to 62°W), to (i) validate a methodology to determine maize water stress (Crop water stress index, CWSI), (ii) describe the behavior of CWSI along the crop cycle of three maize hybrids growing in environments with different soil quality, and (iii) establish the effect of soil quality and hybrids on the relationship between CWSI and available soil water. The CWSI methodology was satisfactorily validated. Crop water stress increased along the growing season in all environments and hybrids. No interactions between soil quality and hybrid on CWSI were found. The effect of soil quality on CWSI did not present a single and clear trend. Differences between hybrids in CWSI were detected. A relationship between CWSI and available soil water was fitted for all hybrids and environments (r2 = 0.52, n = 51, P < 0.001). The threshold below which crops presented symptoms of water stress was 60% of available soil water.

Agronomic efficiency (kg grain yield kg−1 N applied) is conditioned by environmental factors and nitrogen availability during the growing period. Hence, a fertilization model that considers environmental factors affecting wheat crop growth and effective N supply should be based on crop N demand. In this work, a simple model based on N balance during the growing season is used as the frame to simulate both the demand and the availability of N, and to determine grain yield. Fertilization experiments were conducted under different environments (50 sites, 8 y) of the Southern Pampa of Argentina. Nitrogen fertilization rates ranged between 25 and 125 kg N ha−1. Soil initial conditions and water balance during the crop cycle were found to modify both N demand and soil N supply. The amount of N taken up by crops, water balance during the crop growth period and mean maximum temperature during grain filling, all affected grain yield components. The proposed model provided a good agreement between observed independent data sets and simulated values of grain yield (root mean square error = 9% of the mean value). Model operation was performed for one site within the region using climatic records to estimate annual grain yield variability under three levels of N availability.

Few models used to estimate the fraction of photosynthetically active radiation intercepted by maize crops (fIPAR) as a function of leaf area index (LAI) account for genotype differences and ontogenetic stage. In this study, the development of the fIPAR/GLAI relationship of three maize hybrids having contrasting plant type and grown in different environmental conditions was characterized. Three field experiments were conducted at Rojas (34°08′S), Argentina, on a silty clay loam soil, without nutrient restriction. The effects of sowing date and water regime were tested. Plant density was always 7 plants m−2. The fIPAR was calculated from measurements above and below the canopy, before and after maximum GLAI was attained. Hybrids differed significantly (P 0.77; n ≥ 15) the fIPAR/GLAI relationship for pre-maximum GLAI data of all hybrids. Nevertheless, maximum fIPAR was always below values quoted in the literature (< 0.90), and differences were detected among hybrids in the attenuation coefficient (k) that were probably related to leaf angle and leaf area. For hybrids with a similar leaf angle at all leaf positions no difference existed in the fIPAR/GLAI relationship along the cycle (pre- and post-maximum GLAI). The relationship changed after tasselling for the hybrid with erect upper leaves. Ignoring such differences could be misleading when fIPAR/GLAI models are used to estimate canopy photosynthesis and hence biomass production, radiation-use efficiency, and kernel set of specific hybrids and growth stages.