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a ENSA Montpellier, France and CIMMYT Maize Program, P.O. Box 6-641, 06600 Mexico D.F., Mexico
b Indian Agricultural Research Inst., New Delhi, 110 012, India
c CIMMYT Maize Program, P.O. Box 6-641, 06600 Mexico D.F., Mexico
* Corresponding author (p.monneveux{at}cgiar.org)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: ASI, anthesis–silking interval • CIMMYT, Centro Internacional de Mejoramiento de Maíz y Trigo • DA, days to anthesis • DS, days to silking • EGR, ear growth rate • EH, ear height • ETWR, ear to tassel weight ratio • GGR, grain growth rate • GLN, green leaf number • OPV, open pollinated variety • PH, plant height • RSBWR, reproductive sink to total aboveground biomass weight ratio
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Variation in N supply affects both growth and development of maize plants (McCullough et al., 1994). Uhart and Andrade (1995a) reported that N deprivation reduced leaf area index, leaf area duration, radiation interception, and radiation use efficiency. Low N also increases the anthesis-silking interval (Jacobs and Pearson, 1991). Lack of N enhances kernel abortion (Pearson and Jacob, 1987) and reduces final grain number (Lemcoff and Loomis, 1986; Uhart and Andrade, 1995b). Delayed senescence (or stay-green) was proposed as indirect selection criteria for low N tolerance (Moll et al., 1994). Anthesis-silking interval and senescence related traits have been proposed by Bänziger and Lafitte (1997) and Bänziger et al. (1999)(2000) as secondary traits for improving maize for low N target environments.
Because a number of lines with improved tolerance to low N were obtained by selecting and inbreeding under high plant density, tolerance to high plant population density was suggested as an alternative breeding strategy to improve tolerance to diverse abiotic stresses including drought and low N (Vasal et al., 1997). Plant density is an efficient management tool for maximizing grain yield by increasing the capture of solar radiation within the canopy. Efficiency of conversion of intercepted solar radiation into economic yield is, however, limited by mutual shading and competition of plants (Buren et al., 1974). An association was reported between ASI and yield under high plant population density (Edmeades et al., 1993).
Although ASI has proved to be an effective predictor of grain yield under high plant population density and low N conditions (Bolaños and Edmeades, 1993), additional secondary traits are needed to improve selection efficiency under stress. One of the main reasons today for evaluating secondary traits is to improve the precision of our search for candidate genes and key processes. The objectives of the present study were (i) to describe and compare the effects of high plant density and low N stress on yield and its components; (ii) to identify morpho-physiological traits associated with yield under each specific stress; and (iii) to elucidate the impact of grain abortion, senescence, and dry matter partitioning on grain yield.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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All trials were irrigated by sprinkler before soil preparation (for germination of volunteer seeds) and after sowing (for homogeneous emergence). Trials were furrow-irrigated throughout their life cycle at intervals of approximately 2 wk. A total of 75 kg N ha–1 was applied at two times (before sowing and at V6 stage) as ammonium sulfate (NH4)2SO4, except in the low N trial. Low N treatment was established according to Bänziger et al. (1999). No N was applied for a 5-yr period and biomass was removed in the trial where the low N evaluation was sown. In all trials 60 kg P2O5 (triple superphosphate with 46% P2O5) was applied before sowing. The experiments were kept free from weeds, insects, and diseases. Seeds were treated before sowing with a mixture of one insecticide (thiodicarb,3,7,9,13-tetramethyl-5,11-dioxa-2,8,14-trithia-4,7,9,12-tetraazapentadeca-3,12-diene-6,10-dione) and two fungicides (fludioxonil, 4-(2,2-difluoro-1,3-benzodioxol-4-yl)-1H-pyrrole-3-carbonitrile and metalaxyl, methyl-N-(2,6-dimethylphenyl)-N-(2-methoxyacetyl)-DL-alaninate). An herbicide treatment was applied at preemergence (2.24 kg ha–1 atrazine, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine + 1.74 kg ha–1 s-metolachlor, C15H22ClNO2). Plants were also treated against fall armyworm (Spodoptera frugiperda) using permethrin (C21H20Cl2O3) granules.
Measurements
All nondestructive measurements were made in the central two rows of the plots in each trial. Days to anthesis (DA) and days to silking (DS) were recorded from a well-bordered group of 50 plants in each plot. A plant was considered as having reached anthesis or silking if at least one extruded anther or one silk was visible. A plot was considered as having reached anthesis or silking when at least 50% of the plants reached these stages. Anthesis-silking interval (ASI) was calculated as DS – DA. After completion of male flowering, plant height (PH) was recorded as the distance between the ground surface and the node bearing the flag leaf and ear height (EH) as the distance between the ground surface and the insertion of upper ear. The PH and EH values were recorded on 10 plants per plot and averaged. In all trials, in vivo chlorophyll concentration of the ear leaf was assessed 2 wk after male flowering and each following 2 wk, on 10 plants, using a portable chlorophyll meter (SPAD-502, Minolta, Tokyo, Japan) and was expressed in arbitrary absorbance (or SPAD) values (Dwyer et al., 1991). Since chlorophyll content in a leaf is closely correlated with leaf N concentration (Blackmer and Schepers, 1995), the measurement of chlorophyll provides an indirect assessment of leaf N status. After silking, the number of green leaves (<25% yellowed) below the primary ear was noted on the same plants, according to Binford and Blackmer (1993) and referred to as green leaf number (GLN). Fraction of incident solar radiation intercepted by the crop canopy was measured between 1100 and 1400 h in all the trials, each 3 wk starting from 4 wk after sowing, using a bar sensor 0.9 m long (Ceptometer, Decagon Device, Pullman, WA) placed across the inter-row space at the ground level.
One day after DA eight well-bordered plants were harvested and divided into tassels, ears, and aboveground vegetative biomass (husks, leaves, and stems). The number of ovules per upper ear was determined on the sampled ears as the product of ovules number per row (averaged from two ear rows) and number of rows on each ear. Abortion rate during a given period after DA was calculated as the relative decrease of grain number during this period. Tassels, ears, husks, leaves, and stems were then oven-dried at 80°C for 3 d to a constant weight. The weights of individual plant parts were combined to obtain the total aboveground biomass dry weight. Several partitioning indices were calculated, such as the tassel/total aboveground biomass weight ratio (TSBWR) and the ear/tassel weight ratio (ETWR). In two border rows of each plot four areas of six plants each were marked for observation on ear and kernel growth. Ears were harvested 20, 30, 40, and 50 d after female flowering. Grain number per ear was determined using the same procedure as for ovules. Ears and 200-grain samples were oven-dried at 80°C for 3 d, and weighed. Ear growth rate (EGR) and grain growth rate (GGR) were calculated for each treatment using a construct suggested for grain growth by Johnson and Tanner (1972). A linear regression of ear weight and grain weight on days from female flowering was fitted using data collected at the first three harvests. At physiological maturity, plant aboveground biomass, ear number per plant, and grain yield were determined, and grains per ear were calculated from grain yield and thousand kernel weight.
Statistical Analysis
Data from each experiment were subjected to analysis of variance (ANOVA), considering entries as fixed and replicates, plots, and incomplete blocks within replicates as random factors. Data were analyzed using SAS, version 8.1. (SAS Inst., 1987). Phenotypic correlations were used to determine the relationships among traits, within each environment.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Fraction of incident solar radiation intercepted by the crop canopy was maximal in all trials 7 wk after sowing. Maximum light interception under optimal, high plant population density and low N conditions was 89.7, 95.4, and 70.8% in OPVs; 62.0, 75.9, and 52.9% in inbred lines; and 93.2, 97.7, and 68.4% in hybrids.
Morpho-Physiological Traits
In all germplasm types, silking was significantly delayed by low N and high plant population density stress (Table 4). Under optimal conditions, ASI was <1 d. Under low N conditions, it increased to >3 d and was significantly negatively correlated with grain yield in OPVs and hybrids. No correlation was found between ASI and grain yield in lines. The ASI was larger under low N stress than under high plant population density and correlations with yield were noted for all types of germplasm. In hybrids, a significant negative correlation was found between ASI and days to anthesis.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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High plant population density increased plant and ear height in all germplasm types. This is consistent with Rutger and Crowder (1967), and is likely to be a response to lower light level and greater competition for light (Modarres et al., 1998). Contrasting results were obtained for yield response to high plant population density between OPVs, inbred lines, and hybrids. Yield increased under high plant population density in inbred lines but decreased in OPVs and hybrids. Lines yielded more under high plant population density, probably because of lower vigor and lower competition between plants (Modarres et al., 1998). Even under high plant population density, radiation interception in inbreds was approximately 20% lower than in OPVs and hybrids. No relationship was found, whatever the type of germplasm, between tolerance to high plant population density and plant height (data not shown). Modarres et al. (1998) found an association between plant height and density tolerance, but in inbred backgrounds involving compact (ct1) and reduced-stature (rd1) mutants and consequently differing greatly for plant height.
High plant population density and low N differed for their stress intensity as shown by the respective yield reduction induced by each treatment. The magnitude of relationships between yield and its components also differed with growing conditions. Under high population density grain yield was highly correlated with ears per plant, confirming data from Russell (1968). Yield was, however, more strongly associated with grain number per ear than with ears per plant. Under low N, yield was significantly correlated with grain number per ear and ears per plant, as observed by Lemcoff and Loomis (1986). Variation of ears per plant explained a larger portion of yield variation when yield loss and stress intensity was greater, as postulated by Bolaños and Edmeades (1996). The observed differences are probably attributable to different severities and timing of stress.
Anthesis–Silking Interval
Anthesis–silking interval significantly increased under high plant population density, as reported by Buren et al. (1974) and Jacobs and Pearson (1991). A significant negative relationship was observed between ASI and grain yield in OPVs and hybrids, confirming data from Edmeades et al. (1993). Grain yield was significantly negatively correlated with ASI under low N, as already reported by Lafitte and Edmeades (1994).
Senescence
The onset of grain filling is considered by Christensen et al. (1981) as a critical phase for N supply within the maize plant. As a consequence of grain filling, transport of carbohydrates to the roots is reduced and N uptake decreases. When N supply is limiting, leaves become the main source of remobilized N to the ear (Below, 1997). Chlorophyll concentration reduction and leaf yellowing are good indicators of N remobilization (Dwyer et al., 1995). Nitrogen deficiency accelerates senescence as revealed in the present study by the strong decrease in chlorophyll concentration (53.0%) under low N as compared with nonstressed conditions. Leaf N decrease in turn is expected to have a direct effect on canopy photosynthesis, resulting in greater kernel abortion and lower grain number (Moll et al., 1994; Uhart and Andrade, 1995a). The strong association between chlorophyll concentration and soluble carbohydrates reported by Rajcan et al. (1999) confirms this hypothesis and suggests that maintaining N and chlorophyll concentration of leaves during grain filling may lead to maintenance of leaf photosynthesis resulting in better grain filling. Similarly to Lafitte and Edmeades (1994), a significant correlation was noted in the present study between SPAD values (4 wk after anthesis) and grain yield of OPVs and hybrids under low N conditions (Table 2). Similarly, Moll et al. (1994) reported an association between longer green leaf area duration (delayed senescence) and yield in maize hybrids and populations. In OPVs and hybrids, an association was also found between the number of green leaves below the primary ear (GLN) and yield at 2 and 4 wk after fertilization. This simple and inexpensive visual criterion is a reasonably good predictor of yield under low N (Lafitte and Edmeades, 1994). According to Binford and Blackmer (1993) and Fox et al. (2001), its accuracy could be improved by normalizing readings with high N reference plots.
Dry Matter Partitioning
Ear weight at anthesis was correlated with yield under low N conditions. Selecting for a larger ear at anthesis (together with lower ASI) was proposed by Edmeades et al. (1997) and Westgate (1997) as a practical approach to increase biomass partitioning to the ear sink, thus increasing grain number.
Under low N, a significant negative correlation was noted between tassel weight and yield in all types of germplasm. An association between reduced tassel size and improved partitioning toward the ear has been suggested by Fischer et al. (1987). Selection for higher yield under stress (Duvick, 1997) and lower ASI (Bolaños and Edmeades, 1993) generally led to a decline in tassel size. Reduced tassel size thus appears to be a relevant breeding objective, particularly under low N. This trait can be easily altered by selection (Fischer et al., 1987; Duvick, 1997) and is highly heritable (Bolaños and Edmeades, 1996). Under low N, grain yield was strongly associated with ETWR (Fig. 1). The mechanisms underlying this competition between tassel and ear are still not elucidated. The negative association between tassel/vegetative aboveground biomass weight ratio and grain yield noted under low N conditions indicates that development of tassels, even to the detriment of vegetative biomass, is associated with delayed ASI and lower yield. The lack of correlation between yield and aboveground biomass at anthesis, however, indicates an increase in vegetative organ reserves does not necessarily increase stress tolerance.
Ovule Number, Grains Abortion, and Grain Weight
The number of ovules (potential kernels) is established in maize when spikelet formation ceases a few days before silking (Tollenaar, 1977). According to Westgate and Boyer (1986), stress-induced failure of spikelet primordia may occur at three stages: (i) before anthesis when megasporogenesis is affected by severe stress, (ii) at pollination when silks emerge after pollen is shed, and (iii) after pollination, by abortion of fertilized spikelets. Grain abortion leads to malformed (nubbin) ears (Boyle et al., 1991) or barrenness. In the present study, loss of grain was mainly due to grain abortion through the first 2 wk after silking, in agreement with Westgate and Boyer (1986) and Schussler and Westgate (1991). As reported by Uhart and Andrade (1995b), stress affects kernel number primarily by increasing kernel abortion and secondarily by affecting the number of ovules. Under low N around 85% of the abortion occurred during the 20 d after female flowering, and 15% during the 30 following days. These results are in good agreement with data from Below (1997) obtained from in-vitro culture. According to this author, an abrupt increase of grain abortion occurs under N stress just before the linear phase of grain filling (i.e., the first week after fertilization) and is closely related with a lack of post-flowering N accumulation by whole plants. Ovule number generally had a limited impact on final yield in our study. Weak but significant negative correlations were even found between ovule number and grain yield under low N. Conversely, a significant negative correlation was found between yield and abortion rate in all germplasm types. These results confirm that grain abortion during the 2 wk following silking is the main factor controlling final grain number and grain yield under stress, as suggested previously by Westgate and Boyer (1986).
Under high plant population density, abortion rate was positively associated with ovule number, indicating a compensation effect for C products and strong source-limitation of final grain number, and suggesting that a reduction in the number of spikelet formed could facilitate overall seed set by reducing C constraints (the main limiting factor in this case) per spikelet (Edmeades et al., 1993; Bruce et al., 2002). Under low N, a positive association was also found between abortion rate and ovule number in inbred lines and hybrids, suggesting that C constraints were also present.
High plant population density and low N had a greater effect on grain filling duration as compared with grain growth rate. This result suggests that competition for assimilates among grains mainly occurred during the last stages of grain filling, in good agreement with studies by Poneleit and Egli (1979) under high plant population density and Lemcoff and Loomis (1986) under low N. Moreover, there was a negative correlation between grain weight and number of grains per ear in OPVs and hybrids (r = –0.506, P < 0.01, and r = –0.504, P < 0.01, respectively) confirming a source limitation for grain filling in this type of germplasm.
Indirect Selection for High Plant Population Density and Low Nitrogen Tolerance in Different Germplasm Types
In the present study, an association was found between ASI and grain yield in all germplasm types under low N conditions, and for OPVs and hybrids under high plant population density. Selection for reduced ASI has proved to be effective in improving tolerance to both low N and drought (Bolaños and Edmeades, 1993). Selecting for low ASI or even protogyny (negative ASI) under optimal conditions, as proposed by Westgate (1997), should avoid the delicate application of stress, but is likely to be ineffective because of the strong G x T effects and absence of relationship between yield under optimal and stressed treatments. Senescence related traits (i.e., chlorophyll concentration and number of green leaves below the primary ear used as senescence score) appeared to be particularly useful as secondary traits under low N. According to Edmeades et al. (1997), chlorophyll concentration could be of lower utility than senescence score, because of its lower heritability. Ear and tassel weight near flowering can also be practically used as secondary traits to select for yield under low N conditions. Associations between grain yield and number of grains per ear and abortion rate, taken together, indicate that development of initiated spikelets mainly determines yield and closely relates to the assimilate supply to developing grains. In turn, the relationship found between yield and ear and tassel weight suggests that current photoassimilate flux to the spikelet depends on the competing reproductive organs, with better partitioning of biomass to the developing ear resulting in greater spikelet dry matter accumulation.
Relationships have been suggested between low N and drought stress (Lafitte and Edmeades, 1995; Bänziger et al., 1999), high plant population density and drought stress (Bruce et al., 2002) and high plant population density, low N and drought stress (Dow et al., 1984; Vasal et al., 1997). The hypothesis of association between low N and drought tolerance is supported by the dependence of N flux on the C demand by the sink (Lemcoff and Loomis, 1986). As a consequence, N shortage may affect grain number mainly through C assimilation. This effect of low N on kernel set through C reduction may explain the above-mentioned association between abortion rate and ovule number. Also, the reduced transport of carbohydrates to the roots at anthesis, due to an increased demand by reproductive organs, was found to strongly affect N uptake (Tolley-Henry and Raper, 1991). The significant G x T interactions found for most traits (including yield) in the present study, however, reflect the limited relationship between high plant population density and low N tolerance. Under high plant population density, the relationship found between grain yield and tassel weight by Buren et al. (1974) and grain yield and ASI by Bolaños and Edmeades (1993) were not confirmed in our conditions. Thus, relationships between these stresses could depend on the germplasm tested and environmental conditions.
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ACKNOWLEDGMENTS |
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Received for publication March 25, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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