Context or problem: For the maize (Zea mays L.) crop, tillering is promoted in resource -abundant seasons (e.g., rainfalls, nutrient availability) at low plant densities. For this crop husbandry, farmers rarely use nitrogen (N) fertilization. Particularly, the impact of tillering on N economy processes and the effects of N availability on grain yield generation of tillered maize crops have yet to be studied. Objective or research question: (i) To compare the evolution of biomass and N uptake for tillered and de-tillered (manually removed) maize crops, and (ii) to evaluate the impact of tillers on nitrogen internal efficiency for biomass (NIEB), grain yield (NIEG), harvest index (HI) and N harvest index (NHI). Methods: Two irrigated field experiments in different cropping seasons (2017-2018 and 2018-2019) with a tillering-prone maize hybrid were conducted in Buenos Aires, Argentina. Treatments were a combination of plant density [3 levels, 4 pl m- 2 = D4; 2 pl m- 2 = D2, including an additional tiller removal treatment during the whole tillering window at D2, D2( -T)] and two soil N availabilities [unfertilized= 60 kg N ha -1 of native soil N (N-) and fertilized= 220 kg N ha -1 of native soil + applied N (N+)]. Biomass accumulation, N uptake, HI, NHI, grain N concentration (GrainN%), NIEB and NIEG were analyzed at the shoot (main shoot and tillers) and crop levels. Results: At D2 with N+, tillered crops had greater biomass and N uptake than non-tillered crops but did not compensate for the density reduction compared to D4. NIEB of tillers was similar to that of main shoots, but HI, NIEG and NHI of tillers were lower than those of main shoots. A negative relationship between crop NIEG and GrainN% was sustained for main shoots, but not for tillers due to the low and variable HI of these secondary shoots. Conclusions: In low -density maize management scenarios, tillers increase resource capture in resource -abundant seasons, generating greater crop biomass through a sustained NIEB. However, an intrinsic lower HI of tillers (and NIEG) versus main shoots decreases NIEG of tillered crops. Implications or significance: Knowledge gaps regarding the impact of tillers on N economy of maize crops were answered for the first time. New issues arise: to compare the N dilution curves for main shoots and tillers and to quantify post -flowering N uptake of tillers and N remobilization from tillers to main shoots.

In maize (Zea mays, L.) changes in the partitioning of biomass (W) and nitrogen (N) between organs linked to photosynthetically active tissues, such as leaves (with high N concentration, %N) and organs related to supporting functions, such as stems (with low %N) at silking stage, could modify the diagnosis of crop N status (i.e., N nutrition index, NNI) for fertilizer N recommendations. To test this hypothesis we used a database of maize measurements performed at silking stage of crops cultivated under different N fertilization rates, plant densities, hybrids, and sowing dates, to investigate i) the allometric relationships between %N and W of leaves (%NL and WL) and stems (%NS and WS), ii) changes in N uptake in vegetative organs for crops with low (<6700 kg ha−1) and high (>6700 kg ha−1) crop W and contrasting NNIs, iii) the responses of crop W at silking stage to N fertilization relative to responses of WL, WS, %NL and %NS, and iv) the inclusion of the %NL/%NS ratio in a predictive model of relative grain yield based on crop NNI at silking. Variations of WS (from 2143 to 10,067 kg ha−1) were greater than those of WL (from 1010 to 4107 kg ha−1), whereas the values of WL/WS ratio (for low W crops) and leaf/stem ratio for N uptake (for both high and low W crops) were lower for NNI> 1.1 than for NNI< 0.9, reflecting the key role of stems as N storage organs when N is non-limiting. Changes in WS largely modulate the response of crop W to N fertilization, while changes in %NL largely modulated the response of crop %N to N fertilization for high W crops. Optimal crop N status (i.e., NNI=∼1) was attained with a greater %NL/%NS ratio for high (2.0) than for low (1.2) W crops, suggesting that this allometric ratio should be taken into account at this crop growth stage, particularly for low crop W. Accordingly, the prediction of relative grain yield based on NNI at silking was improved by the inclusion of %NL/%NS. Therefore, our results are useful to better understand the response of maize crop W, grain yield, and crop %N (for leaves and stems) to late N fertilization, adding valuable insights to improve current NNI-based N diagnostic tools.

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 Paran ' a (31 degrees 48' S, 60 degrees 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 < 0.001). Prolificacy was expressed for plants with SPAD units greater than 50 and residuals of KNP vs. PGR relationship decreased with increases in SPAD units only in prolific plants of early-sown crops (R2 = 0.55, P < 0.10). The analysis of KNP vs. PGR and KNP vs. rPGR allowed us to elucidate the direct effect of plant N status on KSE under a wide range of environmental conditions, especially those promoting by sowing date x plant density x N rate.

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 degrees 44' S 60 degrees 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.

Late sowing dates of maize are widely adopted in the Pampas region of Argentina, stabilising grain yields due to a more favourable water balance around flowering. However, late-sown crops are exposed to high soil N availabilities (N-av), high temperatures during the pre-flowering period and declining photo-thermal conditions during grain filling, which may affect nitrogen use efficiency (NUE, kg of grain per kg of N-av). These effects could be exerted through nitrogen uptake efficiency (NupE, kg of N uptake per kg of N-av) and/or nitrogen utilisation efficiency (NutE, kg of grain per kg of N uptake). Environmental conditions could affect i) pre (N(upt)pre) and/or post-flowering N uptake (N(upt)post) and, consequently, NupE and ii) the determinants of NutE, such as N harvest index (NHI) and N source per grain. Early- and late-sown maize were cropped in order to analyse i) grain yield, N-av and NUE and ii) relationships among NUE and related-N efficiencies. The experiments were carried out in Parana (31 degrees 48' S 60 degrees 32' W), Argentina, during 2014-2015 and 2015-2016. Treatments were combinations of two sowing dates (early and late), three N rates (0, 90, and 270 kg N ha(-1)) and two genotypes (DK 70.10 VT3P and DK 73-10 VT3P). NUE decreased in late-sown crops (ca. 32 to 26 kg grain kg N-av(-1)), mediated by lower grain yields (ca. 8564 kg ha(-1) and 7832 kg ha(-1) in early- and late-sown crops, respectively) and higher N-av (ca. 267-312 kg N-av ha(-1)). DK 73-10 VT3P exhibited the highest NUE (ca. 31 kg grain kg N-av(-1)) and NutE (ca. 63 kg grain kg N-upt(-1)). N rate affected more strongly N-av than grain yield; and there was a greater association between NUE and NupE (P < 0.0001, R-2 = 0.72) relative to NutE (P < 0.01, R-2 = 0.65). In both sowing dates, N(upt)pre had a positive impact on NupE, which strongly declined with N rate especially in late-sown crops. The lower NutE of late-sown crops (66 vs. 52 kg grain kg N-upt(-1) in early and late sowing dates, respectively) was related to the highest post-flowering N source per grain (2.5 vs. 3.5 mg N grain(-1)). Thus, our study highlights the components of N economy of late-sown crops with the highest impact on NUE, i.e., N(upt)pre and NutE. Therefore, nutritional management of late-sown maize crops should be focused on these NUE components. High plant densities could be useful to increase N(upt)pre. Finally, the choice of a genotype with high NutE appears as a valid strategy to mitigate NUE reductions, promoted by the high N-av typical of late sowing dates.

Maize (Zea mays. L) traditional breeding presents limitations when selection is performed by adaptation to suboptimal conditions. One strategy to achieve greater efficiency in these environments is to include measurements of secondary traits related to the main grain yield component, i.e. kernel number per plant (KNP), which is associated to other three secondary traits: plant growth rate during the critical period (PGRcp) for kernel setting, biomass partitioning index to the ear during this period (PIcp) and the efficiency of the ear to set kernels or reproductive efficiency (RE). Phenotypic variations in these KNP related traits were evaluated considering: i) plant densities, and ii) simple and staked transgenic maize hybrids. The objective of this study was to identify the contribution of these secondary traits to KNP in six modern single-cross transgenic hybrids: the simple and staked transgenic versions of DK747 and DK190 (Bt: 747MG and 190MG; RR: 747RR and 190RR; and Bt-RR: 747MGRR and 190MGRR) cultivated at low and high plant densities. For this purpose, KNP and secondary traits were recorded at the individual plant level and data set was analyzed using multivariate analysis techniques. At low plant density, PGRcp was the critical trait for kernel setting in all hybrids but PIcp or RE also contributed to KNP of the DK747 group and the D190 group, respectively. At high plant density, a differential contribution of secondary traits to KNP was recorded among hybrids. For DK747MG and DK190MG, RE mainly determined KNP, but for the DK747RR and DK747MGRR higher PIcp had positive impact on KNP. These results highlight the importance of including secondary traits associated to KNP in breeding programs.

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

The available evidence suggests that the current increasing trend in global surface temperatures will continue during this century, which will be accompanied by a greater frequency of extreme events. The IPCC has projected that higher temperatures may outscore the known optimal and maximum temperatures for maize. The purpose of this study was to improve the ability of the maize model CSM-IXIM to simulate crop development, growth, and yield under hot conditions, especially with regards to the impact of above-optimal temperatures around anthesis. Field and greenhouse experiments that were performed over three years (2014-2016) using the same short-season hybrid, PR37N01 (FAO 300), provided the data for this work. Maize was sown at a target population density of 5 plants M-2 on two sowing dates in 2014 and 2015 and on one in 2016 at three locations in Spain (northern, central, and southern Spain) with a well-defined thermal gradient. The same hybrid was also sown in two greenhouse chambers with daytime target temperatures of approximately 25 and above 35 degrees C. During the nighttime, the temperature in both chambers was allowed to equilibrate with the outside temperature. The greenhouse treatments consisted of moving 18 plants at selected phenological stages (V4, V9, anthesis, lag phase, early grain filling) from the cool chamber to the hot chamber over a week and then returning the plants back to the cool chamber. An additional control treatment remained in the cool chamber all season, and in 2015 and 2016, one treatment remained permanently in the hot chamber. Two maize models in the Decision Support System for Agrotechnology Transfer (DSSAT) V4.6 were compared, namely CERES and IXIM. The HUM version included additional components that were previously developed to improve the crop N simulation and to incorporate the anthesis-silking interval (ASI). A new thermal time calculation, a heat stress index, the impact of pollen-sterilizing temperatures, and the explicit simulation of male and female flowering as affected by the daily heat conditions were added to IXIM. The phenology simulation in field experiments by IXIM improved substantially. The RMSE for silking and maturity in CERES were 7.9 and 13.7 days, decreasing in DCIM to 2.8 and 7.3 days, respectively. Similarly, the estimated kernel numbers, kernel weight, grain yield and final biomass were always closer to the measurements in HUM than in CERES. The worst simulations were for kernel weight, and for that reason, the differences in grain yield between the models were small (the RMSE in CERES was 1219 kg ha(-1) vs. 1082 kg ha(-1) in IXIM). The greenhouse results also supported the improved estimations of crop development by IXIM (RMSE of 2.6 days) relative to CERES (7.4 days). The impact of the heat treatments on grain yield was consistently overestimated by CERES, while HUM captured the general trend. The new HUM model improved the CERES simulations when elevated temperatures were included in the evaluation data. Additional model testing with measurements from a wider latitudinal range and relevant heat conditions are required.

En maíz (Zea mays L.), la competencia intraespecífica por recursos abióticos afecta atributos morfofisiológicos claves como el tamaño y duración del área foliar verde. En este estudio, se calibraron y validaron cuatro ecuaciones (Ec.1 a Ec.4) de estimación no destructiva del área foliar por planta en floración (AFp) en cuatro genotipos creciendo en canopeos con distinta disponibilidad de N (0 y 400 kg de N ha-1) y tres densidades de siembra (2, 9 y 16 pl m-2). El genotipo y la densidad produjeron las mayores variaciones del perfil vertical del área foliar. Las ecuaciones exhibieron distinta bondad de ajuste (Ec.4 > Ec.1=Ec.2> Ec.3). La Ec.4, que utiliza un parámetro asociado con la senescencia foliar, superó al resto en su capacidad predictiva para estimar AFp en un amplio rango de crecimiento, particularmente en los individuos más suprimidos de la población. Dada la simplicidad de su aplicación por sus parámetros de fácil y rápida medición, la Ec.4 sería más apropiada para la estimación del AFp en estudios poblacionales que valoran la habilidad competitiva de individuos que crecen en canopeos con distinta presión de competencia por recursos.

Rainfed maize (Zea mays, L.) crops in temperate semi-arid regions with high inter-annual variation of summer precipitation, are commonly cultivated at low population densities. During seasons with favorable conditions for plant growth (e.g., summer rainfalls above normal records), the number of kernels of sub-apical ear can contribute significantly to total kernel number per plant (TKN). However, there is no information of the determinant traits of kernel setting at sub-apical ear, or the effects of breeding on these traits. We used a crop physiology model with an individual plant approach in attempt to describe genotypic differences in those traits related to kernel setting at the apical (KNE1) and the sub-apical ear (KNE2) of older and newer Argentinean maize hybrids. Four representative hybrids of the decades of 80', 90', 00' and 10' were cultivated during two growing seasons at three densities (4, 8 and 12 pl m(-2)). Non-destructive techniques were used to estimate the growth rate of individual plant (PGR(PC)), apical (E(1)GR(CP)) and sub-apical (E(2)GR(CP)) ear during the period bracketing silking (critical period), biomass partitioning to reproductive sinks (E(1)GR(CP) PGR(CP)(-1) and E(2)GR(CP) PGR(CP)(-1)) and the efficiency to set kernels of the plant (TKN PGR(CP)(-1)) and the ears (KNE1 E(1)GR(CP)(-1) and KNE2 E(2)GR(CP)(-1)). Differences among tested hybrids in KNE1 (ca. 400-600 k ear(-1)) were mainly determined by KNE1 E(1)GR(CP)(-1) (ca. 230-280k d(-1) g(-1)) while newer hybrids had the highest E(1)GR(CP) PGR(CP)(-1) (>0.50). By contrast, hybrids had similar KNE2 (ca. 124 k ear(-1)) due to their similar E(2)GR(CP) PGR(CP)(-1) (ca. 0.08) and KNE2 E(2)GR(CP)(-1) (ca. 93 k d(-1) g(-1)). These results suggest that breeding effect on TKN was mainly determined by KNE1 E(1)GR(CP)(-1) and for the newest hybrid this trait did not interact with plant density. Hence, the newest hybrid exhibited a positive linear response of TKN to PGR(PC) (r(2) = 0.82) reaching the highest TKN among hybrids at low, mid and high density. A better performance of maize crops at low densities could be obtained by breeding for E(2)GR(CP) PGR(CP)(-1). 2016 Published by Elsevier B.V.

Interactions between nitrogen (Ns) and water stress (Ws) effects on annual crops productivity have been widely investigated in Mediterranean-type regions, but not in the humid temperate ones as the central Pampas of Argentina, where early-sown maize crops are usually exposed to severe yield penalties due to the mentioned stresses. Additionally, the recommendation of increasing plant populations promoted by seed companies may push crops to high density stress (HDs), which has usually a multiplicative (i.e. less intensive) and not an additive (i.e. more intensive) effect when combined with other constraints. In current research we re-examined multiple stress effects (HDs, Ns, Ws and the interaction of HDs with either Ns or Ws) on the determinants of grain yield of two maize hybrids, and used relative plant biomass (SI = 1 - stress/reference) as an integrative seasonal index for describing their intensities. Field experiments included two maize hybrids of contrasting tolerance to stress (high for AX820 and low for AX877), grown under different combinations of stand densities (9 and 12 plants m(-2)) and N offer (0 and 200 kg N ha(-1)) or water regime (well-watered and water deficit). The SI of combined stresses was always larger than the SI of any individual stress. For the tolerant hybrid, effects of combined abiotic stresses on SI were always multiplicative, whereas for the intolerant one the response intensified (i.e., turned additive or even synergic) under Ws x HDs. For both hybrids, a single model described the sensitivity of certain traits (number of complete and total florets, number of exposed silks) to the wide range of evaluated SIs, whereas independent models were necessary to accommodate the variation observed in the anthesis-silking interval associated with Ws and Ns. The second pattern was also observed in the case of kernel number per plant and plant grain yield of the intolerant but not of the tolerant hybrid. The former was more sensitive to Ws than the latter. Our results confirm a different sensitivity of the analyzed traits according to the origin of stress and the genotypic variability in these responses. (C) 2016 Elsevier B.V. All rights reserved.

La produccion de macollos puede otorgar estabilidad al rendimiento del maiz en ambientes con oferta variable de recursos, o compensar fallas en el establecimiento de plantulas. El objetivo de este trabajo fue analizar la fenologia de los estadios vegetativos y reproductivos de vastagos principales y macollos y su relacion con el crecimiento en etapas tempranas del cultivo, en escenarios de alta oferta de recursos bajo densidades de siembra contrastantes. Se realizo un experimento a campo en la FAUBA durante 2014-2015, con dos hibridos dentados (ARV2194HXRR y ARV2183MGRR) y un hibrido pisingallo (Argenpop 141) sembrados en dos densidades (3 y 6 plantas m -2 ) sin limitaciones hidrico-nutricionales. Desde etapas tempranas del ciclo, la proporcion de plantas con macollos presento diferencias (0,005< P <0.1) entre densidades (mayor en baja densidad). En ARV2194HXRR y Argenpop141, esta proporcion respondio positiva y linealmente a la tasa de crecimiento del vastago principal durante los primeros 42 dias desde emergencia. Para similares tasas, el ARV2183MGRR nunca presento macollos. Debido a la mayor produccion de macollos en baja densidad, ARV2194HXRR y Argenpop141 presentaron estabilidad en el numero de vastagos m -2 (ca. 7,6 y 8,3 vastagos m -2 para ARV2194HXRR y Argenpop141; respectivamente) entre densidades. A pesar del menor numero de hojas (ca. 13,8 vs 22 y 10,4 vs 18 hojas para macollos y vastagos principales de ARV2194HXRR y Argenpop141; respectivamente) y similar filocrono (ca. 44,6 y 56,5 °Cd hoja -1 para ARV2194HXRR y Argenpop141; respectivamente), los macollos florecieron mas tardiamente (ca. 120 y 70 °Cd para ARV2194HXRR y Argenpop141; respectivamente) y presentaron una mayor asincronia floral (ca. -123 y -71°Cd para ARV2194HXRR y Argenpop141; respectivamente) que el vastago principal (ca. 18 y 7°Cd para ARV2194HXRR y Argenpop141; respectivamente), probablemente debido a la demora en el inicio del macollaje (aprox. en 8-9 hojas aparecidas del vastago principal) y en la iniciacion floral de sus apices.

Previous studies have documented that transgene introduction may alter the phenotypic expression of several traits (e.g., biomass production, grain yield). We hypothesized that genetic diversity could influence the phenotypic variation among hybrids of a same genetic background and also among plants of a hybrid. The objectives of this preliminary study were: (i) to quantify the genetic diversity between the non-transgenic (DK747) and transgenic versions (DK747MG, DK747RR and DK747MGRR) of a single-cross maize hybrid and among plants of each version, (ii) to observe the distribution of genetic diversity along the genome and (iii) to explore relationships between phenotypic variability and genetic diversity. Hybrids were cultivated at field conditions during two growing seasons and plants of each hybrid with high, intermediate and low biomass at physiological maturity were selected to perform a study of single nucleotide polymorphisms (SNP). Genetic diversity among plants of each version was greater than among versions and both sources of variation were significant (Phi(ST) = 0.45, P < 0.01). Genetic diversity of the non-transgenic DK747 was higher than those of the transgenic versions, probably reflecting the conventional breeding history of these iso-hybrids. Similarity coefficients indicate that the most homogeneous group was that composed by plants of DK747MGRR. A Fisher's exact test together with a principal component analysis identified certain SNPs related to the contrasting plant biomass of DK747, 747MG and DK747RR. Caution should be taken with these results, because of the small sample size for SNPs study and the narrow set of tested hybrids. (C) 2015 Elsevier B.V. All rights reserved.

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.

The use of transgenic maize (Zea mays L.) hybrids (Bt, RR, Bt-RR) has simplified crop husbandry, mainly due to a more effective control of pests and weeds. The effects of transgenes insertion on phenotypic traits of maize hybrids are not fully documented, especially without the incidence of pests and weeds. The objectives of this work were (i) to establish phenotypic differences in terms of phenology, growth and yield among a non-transgenic maize hybrid and their transgenic versions (Bt hybrid, RR hybrid and Bt-RR hybrid) and among the transgenic versions of other genetic background and (ii) to analyze the impact of crowding stress on inter-plant variability of the different traits under study. Field experiments were conducted in Buenos Aires (34 degrees 36S, 58 degrees 26'W), Argentina during 2008-2009 (Exp 1), 2009-2010 (Exp 2) and 2010-2011 (Exp 3). Genotypes were cultivated at contrasting plant densities (6 and 12 pl m(-2) in Exp 1 and Exp 2) and with contrasting inter-plant spaces within the row (Exp 3), irrigated, without nutrient limitations, and with chemical and mechanical controls of weed and animal pests. At both plant densities, phenotypic variability of several traits among versions within each group was recorded. For example, the RR version of DK747 group had the shortest thermal time to flowering. The stacked transgenic hybrids DK747MGRR and DK190MGRR, exhibited the highest ear growth rate (EGR(CP)) during the critical period for kernel set. However, both genotypes had the lowest reproductive efficiency (i.e. kernel number per unit of EGR(CP)), that counterbalanced their higher EGR(CP). Only the Bt-RR version of DK747 showed a higher inter-plant variability (CV) of EGR(CP) at low mean EGR(CP) values a symptom of the intolerance of this genotype to crowing stress. Consequently, the different versions of each group of hybrids had a similar kernel number per plant and grain yield. We conclude that the introduction of transgenes may alter in some way, the behavior of plants in different eco-physiological aspects in the absence of pests and weeds. (C) 2012 Elsevier B.V. All rights reserved.

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<P<0.10). The CVs of PGRCP, PI, EGRCP and KNP augmented (0.001<P<0.10) at the highest stand density. The CVs of PGRER, PGRCP, PI and KNP were larger for hybrid AX877 than for hybrid AX820 (0.001<P<0.10). N fertilization smoothed the initial plant-to-plant variability, but the extent of this benefit in a maize crop is genotype dependent; it was much larger in the hybrid tolerant to crowding stress than in the intolerant one. For the latter, the variability held during the critical period around silking and produced a high CV of KNP.

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 ar development eproductive efficiency stress 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.