Lowland Tropical Maize Research Articles

Genetic analysis of adaptation differences between highland and lowland tropical maize using molecular markers

Molecular-marker loci were used to investigate the adaptation differences between highland and lowland tropical maize. An F2 population from the cross of two inbred lines independently derived from highland and lowland maize germplasm was developed, and extracted F3:4 lines were phenotype in replicated field trials at four thermally diverse tropical testing sites, ranging from lowland to extreme highland (mean growing season temperature range 13.2–24.6°C). Traits closely related with adaptation, such as biomass and grain yield, yield components, days from sowing to male and female flowering, total leaf number, plant height and number of primary tassel branches (TBN), were analyzed. A large line × environment interaction was observed for most traits. The genetic basis of this interaction was reflected by significant, but systematic, changes from lowland to highland sites in the correlation between the trait value and genomic composition (designated by the proportion of marker alleles with the same origin). Joint analysis of quantitative trait loci (QTLs) over sites detected 5–8 QTLs for each trait (except disease scores, with data only from one site). With the exception of one QTL for TBN, none of these accounted for more than 15% of the total phenotypic variation. In total, detected QTLs accounted for 24–61% of the variation at each site on average. For yield, yield components and disease scores, alleles generally favored the site of origin. Highland-derived alleles had little effect at lowland sites, while lowland-derived alleles showed relatively broader adaptation. Gradual changes in the estimated QTL effects with increasing mean site temperature were observed, and paralleled the observed patterns of adaptation in highland and lowland germplasm. Several clusters of QTLs for different traits reflected the relative importance in the adaptation differences between the two germplasm types, and pleiotropy is suggested as the main cause for the clustering. Breeding for broad thermal adaptation should be possible by pooling genes showing adaptation to specific thermal regimes, though perhaps at the expense of reduced progress for adaptation to a specific site. Molecular marker-assisted selection would be an ideal tool for this task, since it could greatly reduce the linkage drag caused by the unintentional transfer of undesirable traits.

Selection Improves Drought Tolerance in Tropical Maize Populations: II. Direct and Correlated Responses among Secondary Traits

Recurrent selection for drought tolerance for three to eight cycles has increased grain yield (GY) under drought at flowering by 30 to 50% in three lowland tropical maize (Zea mays L.) populations. The relationships among secondary traits as a result of this selection have not been determined, however. The objectives of this study were to measure direct and correlated changes due to selection in secondary traits by evaluating cycles of selection and appropriate check entries at five water‐stressed (mean yield 2.35, range 1.01–4.48 Mg ha−1) and five well‐watered environments (mean yield 7.96, range 5.81–10.40 Mg ha−1). Under drought, changes per cycle with recurrent S1 selection (P < 0.05) were as follows: GY 12.6%, fertile ears per plant (EPP) 8.9%, grains per fertile ear (GPE) 6.3%, grain number per square meter 12.2%, 1000 grain weight no change, anthesis‐silking interval (ASI) −22.0%, days from sowing to 50% anthesis −0.7%, plant height −2.0%, primary tassel branch number −5.9%, and senesced leaf area 2.7%. Responses under well‐watered conditions were smaller but generally of the same sign. Grain yield was strongly associated with grain number per square meter in both water‐stressed and well‐watered environments Grain yield, EPP, and GPE were strongly correlated with ASI across entries under drought , though not when water was plentiful. The use of managed stress environments that consistently reveal genetic variation for these traits at specific times during crop development is endorsed for selection purposes.

Interpreting genotype × environment interaction in tropical maize using linked molecular markers and environmental covariables

An understanding of the genetic and environmental basis of genotype×environment interaction (GEI) is of fundamental importance in plant breeding. In mapping quantitative trait loci (QTLs), suitable genetic populations are grown in different environments causing QTLs×environment interaction (QEI). The main objective of the present study is to show how Partial Least Squares (PLS) regression and Factorial Regression (FR) models using genetic markers and environmental covariables can be used for studying QEI related to GEI. Biomass data were analyzed from a multi-environment trial consisting of 161 lines from a F(3:4) maize segregating population originally created with the purpose of mapping QTLs loci and investigating adaptation differences between highland and lowland tropical maize. PLS and FR methods detected 30 genetic markers (out of 86) that explained a sizeable proportion of the interaction of maize lines over four contrasting environments involving two low-altitude sites, one intermediate-altitude site, and one high-altitude site for biomass production. Based on a previous study, most of the 30 markers were associated with QTLs for biomass and exhibited significant QEI. It was found that marker loci in lines with positive GEI for the highland environments contained more highland alleles, whereas marker loci in lines with positive GEI for intermediate and lowland environments contained more lowland alleles. In addition, PLS and FR models identified maximum temperature as the most-important environmental covariable for GEI. Using a stepwise variable selection procedure, a FR model was constructed for GEI and QEI that exclusively included cross products between genetic markers and environmental covariables. Higher maximum temperature in low- and intermediate-altitude sites affected the expression of some QTLs, while minimum temperature affected the expression of other QTLs.

Efficiency of High‐Nitrogen Selection Environments for Improving Maize for Low‐Nitrogen Target Environments

Most maize (Zea mays L.) in the tropics is grown under low‐nitrogen (N) conditions, raising the need to assess efficient breeding strategies for such conditions. This study assesses the value of low‐N vs. high‐N selection environments for improving lowland tropical maize for low‐N target environments. Fourteen replicated trials grown under low (no N applied) and high (200 kg N Ha−1 applied) N at CIMMYT, México, between 1986 and 1995 were analyzed for broad‐sense heritability of grain yield, genetic correlation between grain yields under low and high N, and predicted response of grain yield under low N to selection under either low or high N. Broad‐sense heritabilities for grain yield under low N were on average 29% sm ller than under high N because of lower genotypic variances under low N. Error variances were similar at low and highN. Genetic correlations between grain yields under low and high N were generally positive. They decreased with increasing relative yield reduction under low N, indicating that specific adaptation to either low or high N became more important the more low‐N and high‐N experiments differed in grain yield. Selection under high N for performance under low N was predicted significantly less efficient than selection under low N when relative yield reduction due to N stress exceeded 43%. Maize breeding programs targeting low‐N environments in the tropics should include low‐N selection environments to maximize selection gains.

Efficiency of Secondary Traits for Improving Maize for Low‐Nitrogen Target Environments

Consideration of secondary traits could improve selection efficiency under stress conditions. This study assesses the value of secondary traits for improving lowland tropical maize (Zea mays L.) for low‐N target environments. Nineteen experiments grown with no N applied at CIMMYT, México, between 1986 and 1995 were analyzed for grain yield, anthesis‐silking intervai (ASI), number of ears per plant, leaf chlorophyll concentration, and an estimate of leaf senescence. Broad‐sense heritabilities of traits, genetic correlations between secondary traits and grain yield, and predicted response of grain yield to selection for single or multiple traits were calculated. Multiple traits were combined using an unrestricted selection index (Smith‐Hazel index). Broad‐sense heritabilities averaged 0.46 for grain yield, 0.52 for ASI, 0.44 for ears per plant, 0.35 for leaf chlorophyll concentration, and 0.60 for leaf senescence. Genetic correlations of secondary traits with grain yield averaged −0.47 for ASI, 0.78 for ears per plant, 0.24 for leaf chlorophyll concentration, and 0.42 for leaf senescence. Direct selection for grain yield was superior to selection for single secondary traits in most experiments. When information on all traits was combined in a Smith‐Hazel index, selection efficiency improved by 14% on average over selection for grain yield alone. Among secondary traits, ears per plant and leaf senescence discriminated high‐yielding genotypes the best. Leaf chlorophyll concentration, and in some instances ASI, provided information on environmental variation within experiments. We conclude that secondary traits can increase the efficiency of selection for grain yield in maize breeding programs targeting low‐N environments.

Nutritional, growth, and reproductive responses of maize ( Zea mays L.) to arbuscular mycorrhizal inoculation during and after drought stress at tasselling

The effects of root colonization by the arbuscular mycorrhizal (AM) fungus Glomus intraradices Schenck & Smith on nutritional, growth, and reproductive attributes of two tropical maize cultivars with different sensitivities to drought were studied. Freshly regenerated seeds of selection cycles 0 (cv. C0, drought-sensitive) and 8 (cv. C8, drought-resistant) of the lowland tropical maize population "Tuxpeno sequia" were used in this greenhouse experiment. Maize plants were subjected to drought stress for 3 weeks following tasselling (75–95 days after sowing) and rewatered for the subsequent 5 weeks until harvest. Mycorrhizal (M+) plants had significantly higher uptake of N, P, K, Mg, Mn, and Zn into grain than non-mycorrhizal (M–) plants under drought conditions. AM inoculation also produced significantly greater shoot masses in C0 and C8 regardless of the drought-stress treatment. In the sensitive cultivar C0, drought stress reduced the shoot mass and grain yield by 23% and 55%, respectively, when roots were not colonized, while the reductions were only 12% and 31%, respectively, with mycorrhizal association. In addition, the emergence of tassels and silks was earlier in M+ plants than in M– plants under drought conditions. Mycorrhizal response was more pronounced under both well-watered and drought conditions in C0 than in the C8 cultivar. The overall results suggest that AM inoculation affects host plant nutritional status and growth and thereby alters the reproductive behaviour of maize under drought conditions.

Stress Tolerance in Tropical Maize is Linked to Constitutive Changes in Ear Growth Characteristics

The tolerance of maize (Zea mays L.) grain yields to abiotic stresses is largely determined by events that occur at or shortly after flowering. Our objective was to evaluate ear growth after silking under unstressed conditions or low N environments in two late lowland tropical maize populations, previously selected for tolerance to drought or to low N. Selection under low N or drought reduced florets per ear by 12 and 11 per cycle when evaluated under unstressed conditions. The lag period between pollination and the onset of linear grain growth increased by 1.7 d cycle−1 in the low N tolerant population grown under unstressed conditions and by 0.6 d cycle−1 in the drought tolerant population under low N. Selection for tolerance to low N increased biomass per kernel at the end of the lag and linear grain filling periods by 40 and 7% per cycle and by 10 and 4% per cycle in the drought tolerant population, across N environments. The duration of linear grain filling declined with selection in the low N tolerant population when grown under unstressed conditions. Selection for tolerance to either stress did not significantly increase grain yield under unstressed conditions, though improvement for drought tolerance increased yield by 7.5% per cycle (P < 0.05) under low N. Selection for tolerance to both stresses increased kernels per plant across environments. Apparently, selection for tolerance to low N or drought results in more kernels achieving the minimum biomass needed to prevent kernel abortion at the onset of the linear growth phase, resulting in greater grain sink strength under stressed and unstressed environments.

Selection for resistance to aflatoxin formation in maize through seed inoculation.

The efficiency of selection for resistance to aflatoxin development caused by Aspergillus flavus L. ex Fries, in maize (Zea mays L.) grain was studied in three CIMMYT lowland tropical maize populations. Seed samples of each population were inoculated with a spore suspension of cultures of Aspergillus spp. known to produce aflatoxins. Inoculated seeds were maintained at 85% RH in plastic boxes kept at room temperature. At 50% germination, seed samples were planted in the field. At flowering time, selected plants were bulk pollinated. After three cycles of selection, gains for tolerance to aflatoxin formation and improvement for agronomic characters were evaluated. The results indicate there are differences in aflatoxin accumulation between different genotypes and cycles of selection. Populations 22 (DMR) and 28 (DMR) were the most damaged and Population 31 DMR suffered the lowest infection. The study suggests that there are differences in aflatoxin production between the maize genotypes tested. However, our results failed to show that tolerance to aflatoxin development can be accumulated through a recurrent selection breeding scheme.

Improvement for tolerance to low soil nitrogen in tropical maize III. Variation in yield across environments

Abstract When improving grain yield of maize (Zea mays L.) grown under low soil N by recurrent selection, it is desirable to maintain or increase the stability of performance across sites differing in N status. The objective of this study was to identify the possible causes of increase variation in grain yield with selection for tolerance to low soil N in four cycles of selection (C0 to C3) of the lowland tropical maize population, Across 8328 BN, when grown in 12 irrigated environments characterized by different radiation and N availabilities. Yield components and patterns of biomass and N partitioning were examined. Correlation analysis of the data collected indicated that some relationships among traits thought to be important to performance differed in magnitude and sign among environments. For example, a positive correlation was observed between grain yield and preflowering N accumulation in the high-N environments, but no association was observed among these traits at low N. Correlations also differed significantly among seasons within an N level for some traits. For example, the correlation between final grain N concentration and N floret−1 at flowering under low N was 0.97 (P An analysis of yield components and N concentrations in plant parts indicated that yield variation in C1 (the selection cycle which had the greatest variation among environments) was associated with an over-investment in preflowering vegetative biomass, without a concomitant increase in post-flowering grain sink size. By C3, the number of kernels plant−1 had been increased, but yield apparently was limited by inadequate source capacity in the latter stages of grain-filling. Cycle 3 also produced 25% more root biomass than C0 when grown with adequate N supply in pots, while aboveground biomass increased by a smaller proportion (9%). Changes in biomass partitioning to roots may be an important adaptation to low-N environments. These results indicate that seasonal differences in the timing and magnitude of N and C supply play a large role in determining the adaptive value of a given trait, even when environments may be similar in the amount of N applied to, or absorbed by, the under nonlimiting N levels, and extensive testing in environments differing in C and N abundance seems essential to establishing progress from such breeding schemes. In addition, more effort to quantify the extent and timing of N stress in each season appears justified.

Causes for Silk Delay in a Lowland Tropical Maize Population

A shortened anthesis‐to‐silking interval (ASI) in maize (Zea mays L.) is associated with tolerance of stresses which occur around flowering. It is not known if this reflects differences in tassel and ear initiation dates, or in rates of development and growth. We examined these characteristics in selection cycles 0, 2, 4, 6, and 8 (designated Cx, where the subscript indicates the cycle number) of the lowland tropical maize population ‘Tuxpeño Sequía’, which was selected recurrently for drought tolerance and whose selection cycles differ for ASL Plants were grown at high plant density and under mild and severe drought stresses that occurred at flowering. Silking was delayed significantly, especially in C0 and C2 under severe drought. Stress treatments did not affect days to 50% tassel initiation, ear initiation, or anthesis. The interval between 50% tassel and ear initiation, the rate of spikelet initiation, and crop growth rate near flowering were unaffected by selection. Per cycle changes were −0.21 d in duration of ear spikelet initiation and −16.9 for spikelet number per ear at 50% anthesis. Selection changed relative growth rates at 50% anthesis by 0.005 d−1 cycle−1 for ears, 0.007 d−1 cycle−1 for spikelets and −0.009 d−1 cycle−1 for tassels. These changes were unaffected by stress level. Eight cycles of selection increased the mean ear and spikelet weights at 50% anthesis by 127 and 173%. The ASI increased rapidly when mean spikelet biomass at 50% anthesis was less than 0.8 rag. Results indicate that a decreased ASI arising from selection in this population is due largely to an increased rate of biomass accumulation per spikelet, and development of fewer spikelets per ear.

Eight cycles of selection for drought tolerance in lowland tropical maize. III. Responses in drought-adaptive physiological and morphological traits

Selection for grain yield under severe drought stress has often been considered inefficient because the estimate of heritability of grain yield has been observed to decline as yields fall. Under these conditions secondary traits may increase selection efficiency, provided they have adaptive value, high heritability, and are easy to measure. Increased relative stem and leaf elongation rate (RLE), delayed foliar senescence, reduced canopy temperatures and reduced anthesis-silking interval (ASI) were used to augment efficiency of selection for grain yield under drought during eight cycles of recurrent fullsib selection in the lowland tropical maize ( Zea mays L.) population, ‘Tuxpeño Sequía’. Six cultivars comprising Cycles 0, 2, 4, 6 and 8 of Tuxpeño Sequía, and a check cultivar were grown for two consecutive years at Tlaltizapán, Mexico, under three moisture regimes that provided a well-watered control, a severe moisture stress during flowering, and a severe stress during grain-filling. Previous reports have documented significant improvements in grain yield and ASI in this population. When observed under drought, no significant differences were detected among cultivars in RLE or canopy-air temperature differentials, nor in chlorophyll per unit leaf area during grain-filling (an indicator of foliar senescence). Cultivars did not differ in seasonal pre-dawn or diurnal courses of leaf water potential, in leaf osmotic potential, in capacity to adjust osmotically, nor in their seasonal profiles of soil water content with depth to 140 cm. Selection significantly altered final plant height, total leaf number, and tassel primary branch number by −0.9%, −0.5% and −2.6% cycle −1, respectively. Observations on root growth in 2-m deep pots showed that eight cycles of selection had reduced root biomass in the upper 50 cm by 33%, consistent with a significant change of −1.2% cycle −1 in vertical root-pulling resistance. The lack of direct and correlated changes in traits related to plant water status due to selection suggests that in this population heritabilities of such traits are low, or that the traits are only weakly associated with grain yield under severe moisture stress. The present study indicates that improved drought tolerance in Tuxpeño Sequía was due to increased partitioning of biomass towards the developing ear during a severe drought stress that coincided with flowering, rather than to a change in plant water status.

Eight cycles of selection for drought tolerance in lowland tropical maize. II. Responses in reproductive behavior

Drought stress can greatly reduce grain yields of maize ( Zea mays L.) if it coincides with flowering. Eight cycles of full-sib recurrent selection in the population ‘Tuxpeño Sequía’ were carried out with the objective of improving performance under moisture stress targeted to coincide with flowering and grain-filling. Selection was based on an index comprising increased grain yield, the maintenance of a constant number of days to 50% anthesis, reduced anthesis-silking interval (ASI, or days from 50% anthesis to 50% silking) and other drought-adaptive traits. The objectives of this study were to evaluate the direct and correlated selection responses in days to flower, ASI, and biomass of reproductive organs at anthesis. During two consecutive winter seasons, Cycles 0, 2, 4, 6 and 8 of Tuxpeño Sequía and a check representing six cycles of international multilocation testing in the same population, were evaluated under three moisture regimes at Tlaltizapán, Mexico. Selection resulted in significant per cycle responses in days to 50% anthesis (−0.4 days or −0.5%), time to 50% silking (−3.4 days or −3.2%) and in ASI (−3.0 days or −16.1%) under drought. Comparable figures under well-watered conditions were −0.4 days or −0.5%, −0.8 days or −0.9%, and −0.4 days or −23.0%, respectively. Grain yield and its components, especially kernel number per plant, showed a strong statistical dependence upon ASI. The regression of best fit between grain yield and ASI across cultivars and moisture regimes showed that 76% of the variation in grain yield was accounted for by variation in ASI. Grain yield declined by an average of 8.7% day −1 increase in ASI up to 10 days. Selection resulted in a significant increase in ear biomass at 50% anthesis (EB) of 0.004 Mg ha −1 (14.7%) cycle −1, and a significant reduction in tassel biomass at 50% anthesis of −0.008 Mg ha −1 (−1.7%) cycle −1 under drought. Comparable figures under well-watered conditions, also significant, were 0.007 Mg ha −1 (14.7%) and −0.014 Mg ha −1 (−2.6%) cycle −1, respectively. ASI and the percentage biomass distributed to the tassel and ear at anthesis were not significantly affected by selection in the check entry. These data suggest that selection for reduced ASI under carefully managed moisture stress imposed at flowering provides an effective and rapid route to higher and more stable grain yield in lowland tropical maize.

Eight cycles of selection for drought tolerance in lowland tropical maize. I. Responses in grain yield, biomass, and radiation utilization

Drought is a major source of grain yield instability in maize ( Zea mays L.) grown in the lowland tropics, and use of cultivars with improved drought tolerance may be the only affordable option for many small-scale farmers. Eight cycles of full-sib recurrent selection were carried out in the population ‘Tuxpeño Sequía’ during the rain-free winter season at Tlaltizapán, Mexico, under controlled moisture stress timed to coincide either with flowering or grain-filling. Selection was based on an index comprising grain yield and physiological and morphological traits with presumed adaptive value under drought. The objectives of this study were to evaluate direct and correlated responses to selection in grain yield and its components, total biomass, and radiation-use efficiency (RUE). Cycles 0, 2, 4, 6 and 8 of Tuxpeño Sequía, and a check cultivar representing the results of six cycles of selection based primarily on multilocation testing, were evaluated under three moisture regimes at Tlaltizapán during two consecutive winter seasons. Grain yield (GY) increased at 108 kg ha −1 cycle −1 across 12 yield environments ranging in yield potential from 1 to 8 Mg ha −1, with no significant interaction between gains and moisture environments. Yield gains resulted from an increase of 0.03 ears per plant (EPP) cycle −1 under drought, and small but significant increases in EPP, kernel number per ear and kernel weight in well-watered environments. Selection had no effect on biomass production, so yield increases were due to a gain in harvest index (HI) of 0.0058 to 0.0067 cycle −1 in either wet or dry environments. Seasonal readiation-use efficiency averaged 1.48 g MJ −1 PAR under well-watered conditions, a value lower than expected for maize. Although selection slightly reduced radiation interception and slightly increased RUE during the pre-anthesis phase, both changes were relatively unimportant. The regression of GY on biomass (B) ( GY = −1.85 + 0.47 B; R 2 = 0.94 ∗∗ ) predicted zero GY at biomass yields of less than 4 Mg ha −1. The check entry showed only limited progress in drought tolerance, EPP, HI, and GY. These results suggest that drought stress, when managed to coincide with flowering, can be an effective selection environment for increasing HI, yield stability, and GY of lowland tropical maize across a wide range of moisture environments.

Value of Selection for Osmotic Potential in Tropical Maize

AbstractOsmotic adjustment, the lowering of osmotic potential (ΨS) under drought, is considered a desirable component of drought tolerance since it has been shown to maintain turgor, growth, and photosynthesis at lower water potentials. This study attemped to determine the relative merits of selecting for osmotic adjustment and low ΨS under drought in lowland tropical maize (Zea mays L.). During 3 yr of trials in Mexico that examined 204 maize landacres, elite open pollinated populations (OPPs), experimental synthetics (ESs), and hybrids, osmotic adjustment averaged 0.10 MPa and ranged from 0.54 to −0.23 MPa. Only 7% of genotypes showed a capacity to adjust by more than 0.40 MPa, a level considered minimum for them to serve as sources for the trait. Variation for ΨS, under drought for 222 to 225 S1 progeny of three maize populations ranged from −1.04 to −1.79 MPa. Divergent selection within each population for high and low ΨS under drought resulted in the formation of ESs which differed in ΨS by an average of 0.17 MPa when grown under a line source moisture gradient. Realized heritability for ΨS averaged 0.46. The ESs selected for high and low ΨS did not yield differently from the cycle bulk, suggesting little adaptive value for ΨS Correlations between several traits indicative of performance under drought and osmotic adjustment or ΨS under drought generally were weak, inconsistent, and non‐significant. These studies suggest that selecting for increased capacity to adjust osmotically or low ΨS under drought does not appear advantageous in lowland tropical maize.

Recurrent Selection for Reduced Plant Height in Lowland Tropical Maize1

Reduced plant height is considered a desirable attribute in most maize (Zea mays L.) breeding programs. This study was conducted to evaluate direct and correlated responses to selection for reduced plant height in tropical maize. Fifteen cycles of visual, full‐sib recurrent selection for reduced plant height were conducted in one lowland tropical maize population, ‘Tuxpeño Crema I’. Cycles 0, 6, 9,12, and 15 were evaluated for correlated responses in plant morphology, grain yield, dry matter production and distribution, leaf area index, and duration and rate of kernel growth at three plant densities at two or three locations in Mexico over a period of 2 yrs. Selection resulted in a linear reduction in plant height, from 282 to 179 cm (2.4% per cycle). The estimate of broad‐sense heritability for plant height was 0.84. Optimum plant density for yield increased from 48 000 to 64 000 plants ha−1, an increase of 2.1% per cycle. As height was reduced, yield, when determined at the optimum plant density for yield for each generation, increased by 4.4% per cycle. Later cycles showed reduced total lodging, reduced barrenness, earlier flowering, and fewer leaves per plant. When grown at or near their optimum densities for grain yield, leaf area index, and total dry matter per unit area at harvest for selected cycles were similar. Yield improvement was associated with a linear increase in harvest index from 0.30 to 0.45 from the original to the 15th cycle. At 50% silking, total aboveground dry weight and its proportion as stem were reduced; kernel number per unit area at harvest increased from 1592 to 2667 kernels m −2 when grown at or near optimum density, and there was an increase in crop dry weight production from anthesis to black layer formation. Selection for reduced plant height had little effect on the effective filling period or kernel weight.

Growth and yield of lowland tropical maize in Mexico

SUMMARYThe growth and yield of three tropical varieties of maize were studied at two elevations in Mexico: Poza Rica (60m) and Tlaltizapan (940m). Grain yields were between 3·5 and 8·5 t/ha. The growing period was longer and the crop produced more dry weight and yield at Tlaltizapan than at Poza Rica. Crop growth rates (C) increased to a maximum of about 35 g/m2/day at both sites and then declined. Grain growth rates (maximum 35 g/m2/day) exceeded current C during most of the grain filling period. After silking when C exceeded grain growth, dry matter accumulated in the stem. Later as grain growth increased and exceeded C, some of the accumulated material was incorporated into the grain and stem weight decreased. The dry weight increase after flowering was similar at the two sites, but the grain yield at Tlaltiapan was larger because a larger proportion of the dry weight increase was incorporated into the grain than at Poza Rica. The results indicate that at both sites grain ‘sink’ capacity was limiting yield.