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Population Density and Low Nitrogen Affects Yield-Associated Traits in Tropical Maize

^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)

	ABSTRACT

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Worldwide, tropical maize (Zea mays L.) is commonly exposed to low N conditions. Identification of low N tolerance-related traits would help to develop indirect selection for yield and marker assisted selection under stress. Tolerance to high plant population density has been proposed as an alternative breeding strategy to improve stress tolerance in maize. A better understanding of mechanisms underlying tolerance to high plant population density and low N is, however, needed. For this purpose, elite CIMMYT open-pollinated varieties (OPVs), inbred lines, and hybrids were grown under optimal, high plant population density and low N conditions. Yield, yield components, and a set of morpho-physiological traits (secondary traits) were assessed in the different treatments and germplasm types. Emphasis was placed on anthesis-silking interval and traits related to senescence, dry matter partitioning, and ovule and grain number. Association was observed under low N conditions between grain yield and anthesis-silking interval, delayed senescence as expressed by either chlorophyll concentration or the number of green leaves above the ear, and ear/tassel weight ratio. Under optimal, high-plant population density and low N conditions, final grain number depended more on abortion rate than on the total number of ovules at anthesis. Under low N stress, grain yield was significantly negatively correlated with abortion rate. Under high plant population density, a positive association was noted between ovule number and abortion rate, suggesting a source limitation for C products. The effect of stress on yield components and the strength of association between secondary traits and yield varied greatly according to germplasm type.

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

	INTRODUCTION

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OF THE 100 MILLION HA of maize planted in developing countries, about 50 million ha are in the lowland tropics (Pingali, 2001). Most maize in developing countries is also produced under low N conditions (McCown et al., 1992; Oikeh and Horst, 2001) because of low N status of tropical soils, low N use efficiency in drought-prone environments, high price ratios between fertilizer and grain, limited availability of fertilizer, and low purchasing power of farmers (Bänziger et al., 1997). Breeding for tolerance to low N is therefore a major focus of CIMMYT's maize program.

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.

	MATERIALS AND METHODS

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Plant Materials and Experimental Conditions
A total of 24 populations and synthetics (hereafter referred to as open pollinated varieties, OPVs), 30 inbred lines, and 25 hybrids (Table 1) were grown under optimal, high plant population density and low N conditions during the summer rainy season (June–November) of 2001. All this material was tropical late white type. Lines were advanced generation inbred lines selected at CIMMYT experimental stations and improved for the main diseases and pests prevalent in the tropics and for abiotic stresses including high plant population density and low N stresses. The OPVs and hybrids were chosen on the basis of their superior performance in CIMMYT's international testing trials. Experiments were performed at the CIMMYT Experimental Station in Tlaltizapán, Morelos, Mexico (18°41' N lat, 99°08' W long, 940 m elev.). The soil is a calcareous vertisol (Isothermic Udic Pellustert) 1.3 to 1.8 m in depth, with a pH of 7.6. Average daily values for photosynthetically active radiation (PAR) were 10.2 MJ m^–2 d^–1, 30.3°C for maximum temperature, 17.0°C for minimum temperature, and 94.4 and 39.6% for maximum and minimum relative humidity. Cumulative rainfall was 574.7 mm.

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.

	RESULTS

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Yield, Yield Components, and Light Interception
Significant genotype (G) and treatment (T) effects were found for all traits in OPVs, lines, and hybrids (Table 2). The G x T interaction was low but significant for most traits. Average reduction in grain yield of 8.4 and 65.3% was noted in OPVs under high plant population density and low N conditions. In inbred lines, high plant population density planting enhanced yield 24.5% while large yield reductions were noted under low N. Hybrid yield losses under high plant population density and low N were 5.6 and 67.4%. Yield reduction per plant under high plant population density planting was 54.2, 37.7, and 52.8% in OPVs, inbred lines and hybrids (Table 3).

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.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Relationship between ear/tassel weight ratio and grain yield within tropical maize OPVs, inbred lines, and hybrids under low N conditions.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Relationship between tassel/total aboveground biomass weight ratio and grain yield within tropical maize OPVs, inbred lines, and hybrids under low N conditions.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Relationship between number of green leaves below the primary ear and grain yield within tropical maize OPVs and hybrids under low N conditions, 6 and 8 wk after male flowering.

View larger version (15K): [in this window] [in a new window]   Fig. 4. Evolution of grain number after female flowering within tropical maize OPVs, inbred lines, and hybrids under optimal, high plant population density, low N, and drought conditions. Bars are standard deviation of mean; the horizontal dotted lines indicate for each type of germplasm the potential grain number (ovule number under optimal condition).

View larger version (16K): [in this window] [in a new window]   Fig. 5. Evolution of ear dry weight after female flowering within OPVs, inbred lines, and hybrids under optimal, high plant population density, low N, and drought conditions. Bars are standard deviation of mean; M = maturity; EGR = ear growth rate in g ear–1 d–1.

View larger version (17K): [in this window] [in a new window]   Fig. 6. Evolution of grain dry weight after female flowering within OPVs, inbred lines, and hybrids under optimal, high plant population density, low N, and drought conditions. Bars are standard deviation of mean; M = physiological maturity; GGR = grain growth rate in g kernel–1 d–1.

	DISCUSSION

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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.

	ACKNOWLEDGMENTS

 
Thanks are due to Ing. Jose-Luis Barrios (Natural Resources Program, CIMMYT) for providing weather data. The authors also acknowledge Dr. D.L. Beck (Tropical Ecosystems Program, CIMMYT) for helpful comments on the manuscript.

Received for publication March 25, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• Bänziger, M., F.J. Betrán, and H.R. Lafitte. 1997. Efficiency of high-nitrogen selection environments for improving maize for low-nitrogen target environments. Crop Sci. 30:556–561.
• Bänziger, M., G.O. Edmeades, D. Beck, and M. Bellon. 2000. Breeding for drought and nitrogen stress tolerance in maize: From theory to practice. CIMMYT, Mexico.
• Bänziger, M., G.O. Edmeades, and H.R. Lafitte. 1999. Selection for drought tolerance increases maize yields across a range of nitrogen levels. Crop Sci. 39:1035–1040.[Abstract/Free Full Text]
• Bänziger, M., and H.R. Lafitte. 1997. Efficiency of secondary traits for improving maize for low-nitrogen target environments. Crop Sci. 37:1110–1117.[Abstract/Free Full Text]
• Below, F.E. 1997. Growth and productivity of maize under nitrogen stress. p. 235–240 In G.O. Edmeades et al. (ed.) Developing drought and low N-tolerant maize. CIMMYT, Mexico.
• Binford, G.D., and A.M. Blackmer. 1993. Visually rating the nitrogen status of corn. J. Prod. Agric. 6:41–46.
• Blackmer, A.M., and J.S. Schepers. 1995. Use of a chlorophyll meter to monitor nitrogen status and schedule fertigation for corn. J. Prod. Agric. 8:56–60.
• Bolaños, J., and G.O. Edmeades. 1993. Eight cycles of selection for drought tolerance in tropical maize: II. Response in reproductive behaviour. Field Crops Res. 31:253–268.
• Bolaños, J., and G.O. Edmeades. 1996. The importance of the anthesis-silking interval in breeding for drought tolerance in tropical maize. Field Crops Res. 48:269–286.
• Boyle, M.G., J.S. Boyer, and P.W. Morgan. 1991. Stem infusion of liquid culture medium prevents reproductive failure of maize at low water potential. Crop Sci. 31:1246–1252.[Abstract/Free Full Text]
• Bruce, W.B., G.O. Edmeades, and T.C. Barker. 2002. Molecular and physiological approaches to maize improvement for drought tolerance. J. Exp. Bot. 53:13–25.[Abstract/Free Full Text]
• Buren, L.L., J.J. Mock, and I.C. Anderson. 1974. Morphological and physiological traits in maize associated with tolerance to high plant density. Crop Sci. 14:426–429.[Abstract/Free Full Text]
• Christensen, L.E., F.E. Below, and R.H. Hageman. 1981. The effect of ear removal on senescence and metabolism of maize. Plant Physiol. 68:1180–1185.[Abstract/Free Full Text]
• Dow, E.W., T.B. Daynard, J.F. Muldoon, D.J. Major, and G.W. Thurtell. 1984. Resistance to drought and density stress in Canadian and European maize (Zea mays L.) hybrids. Can. J. Plant Sci. 64:575–585.
• Duvick, D.N. 1997. What is yield? p. 332–335. In G.O. Edmeades et al. (ed.) Developing drought and low N-tolerant maize. CIMMYT, Mexico.
• Dwyer, L.M., A.M. Anderson, B.L. Ma, D.W. Stewart, M. Tollenaar, and E. Gregorich. 1995. Quantifying the non-linearity and chlorophyll meter response to corn leaf nitrogen concentration. Can. J. Plant Sci. 75:179–182.
• Dwyer, L.M., M. Tollenaar, and L. Houwing. 1991. A non-destructive method to monitor leaf greenness in corn. Can. J. Plant Sci. 71:505–509.
• Edmeades, G.O., M. Bänziger, M. Cortes, and A. Ortega. 1997. From stress-tolerant populations to hybrids: The role of source germplasm. p. 263–273. In G.O. Edmeades et al. (ed.) Developing drought and low N-tolerant maize. CIMMYT, Mexico.
• Edmeades, G.O., J. Bolaños, M. Hernández, and S. Bello. 1993. Causes for silk delay in a lowland tropical maize population. Crop Sci. 33:1029–1035.[Abstract/Free Full Text]
• Fischer, K.S., E.C. Johnson, and G.O. Edmeades. 1987. Recurrent selection for reduced tassel branch number and reduced leaf area above the ear in tropical maize populations. Crop Sci. 27:1150–1156.[Abstract/Free Full Text]
• Fox, R.H., W.P. Piekielek, and K.E. Macneal. 2001. Comparison of late-season diagnostic tests for predicting nitrogen status of corn. Agron. J. 93:590–597.[Abstract/Free Full Text]
• Jacobs, B.C., and C.J. Pearson. 1991. Potential yield of maize, determined by rates of growth and development of ears. Field Crops Res. 27:281–298.
• Johnson, D.R., and J.W. Tanner. 1972. Calculation of the rate and duration of grain filling in corn (Zea mays L.). Crop Sci. 12:485–486.[Abstract/Free Full Text]
• Lafitte, H.R., and G.O. Edmeades. 1994. Improvement for tolerance to low soil nitrogen in tropical maize: I. Selection criteria. Field Crops Res. 39:1–14.
• Lafitte, H.R., and G.O. Edmeades. 1995. Stress tolerance in tropical maize is linked to constitutive changes in ear growth characteristics. Crop Sci. 35:820–826.[Abstract/Free Full Text]
• Lemcoff, J.H., and R.S. Loomis. 1986. Nitrogen influence on yield determination in maize. Crop Sci. 26:1017–1022.[Abstract/Free Full Text]
• McCown, R.L., B.A. Keating, M.E. Probert, and R.K. Jones. 1992. Strategies for sustainable crop production in semi-arid Africa. Outlook Agric. 21:21–31.
• McCullough, D.E., P. Girardin, M. Mihajlovic, A. Aguilera, and M. Tollenaar. 1994. Influence of N supply on development and dry matter accumulation of an old and new maize hybrid. Can. J. Plant Sci. 74:471–477.
• Modarres, A.M., R.I. Hamilton, M. Dijak, L.M. Dwyer, D.W. Stewart, D.E. Mather, and D.L. Smith. 1998. Plant population density effects on maize inbred lines grown in short-season environments. Crop Sci. 38:104–108.[Abstract/Free Full Text]
• Moll, R.H., W.A. Jackson, and R.L. Mikkelsen. 1994. Recurrent selection for maize grain yield: Dry matter and nitrogen accumulation and partitioning changes. Crop Sci. 34:874–881.[Abstract/Free Full Text]
• Oikeh, S.O., and W.J. Horst. 2001. Agro-physiological responses of tropical maize cultivars to nitrogen fertilization in the moist savanna of West Africa. p. 804–805 In W.J. Horst et al. (ed.) Plant nutrition: Food security and sustainability of agro-ecosystems. Kluwer Academic Publ., Dordrecht, the Netherlands.
• Pearson, C.J., and B.C. Jacob. 1987. Yield components and nitrogen partitioning of maize in response to nitrogen before and after anthesis. Aust. J. Agric. Res. 38:1001–1009.
• Pingali, P.L. 2001. Meeting world maize needs: Technological opportunities and priorities for the public sector. CIMMYT, Mexico.
• Poneleit, C.G., and D.B. Egli. 1979. Kernel growth rate and duration in maize as affected by plant density and genotype. Crop Sci. 19:385–388.[Abstract/Free Full Text]
• Rajcan, I., L.M. Dwyer, and M. Tollenaar. 1999. Note on relationship between leaf soluble carbohydrate and chlorophyll concentrations in maize during leaf senescence. Field Crops Res. 63:13–17.
• Russell, W.A. 1968. Testcross of one- and two-ears types of Corn Belt maize inbreds: I. Performance at four plant stand densities. Crop Sci. 8:244–247.
• Rutger, J.N., and L.V. Crowder. 1967. Effect of high plant density on silage and grain yields of six corn hybrids. Crop Sci. 7:182–184.[Abstract/Free Full Text]
• SAS Institute. 1987. SAS/STAT user's guide. Version 6. SAS Inst., Cary, NC.
• Schussler, J.R., and M.E. Westgate. 1991. Maize kernel set at low water potential: II. Sensitivity to reduced assimilates at pollination. Crop Sci. 31:1196–1203.[Abstract/Free Full Text]
• Tollenaar, M. 1977. Sink-source relationships during reproductive development in maize. A review. Maydica 22:49–75.
• Tolley-Henry, L., and C.D. Raper. 1991. Soluble carbohydrate allocation to roots, photosynthetic rate of leaves, and nitrate assimilation as affected by nitrogen stress and irradiance. Bot. Gaz. 152:23–33.[Medline]
• Uhart, S.A., and F.H. Andrade. 1995a. Nitrogen deficiency in maize: I. Effects on crop growth, development, dry matter partitioning, and kernel set. Crop Sci. 35:1376–1383.[Abstract/Free Full Text]
• Uhart, S.A., and F.H. Andrade. 1995b. Nitrogen deficiency in maize: II. Carbon–nitrogen interaction effects on kernel number and grain yield. Crop Sci. 35:1384–1389.[Abstract/Free Full Text]
• Vasal, S.K., H. Cordova, D.L. Beck, and G.O. Edmeades. 1997. Choices among breeding procedures and strategies for developing stress tolerant maize germplasm. p. 336–347. In G.O. Edmeades et al. (ed.) Developing drought and low N-tolerant maize. CIMMYT, Mexico.
• Westgate, M.E. 1997. Physiology of flowering in maize: Identifying avenues to improve kernel set during drought. p. 136–141. In G.O. Edmeades et al. (ed.) Developing drought and low N-tolerant maize. CIMMYT, Mexico.
• Westgate, M.E., and J.S. Boyer. 1986. Reproduction at low silk and pollen water potentials in maize. Crop Sci. 26:951–956.[Abstract/Free Full Text]

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