Solar radiation interception and canopy expansion of sweet corn in response to phosphorus

Insufficient phosphorus (P) availability decreases the yield of Zea mays, particularly for sweet corn crops grown in cool environments. This research examined the mechanisms of yield reductions with initial emphasis on canopy expansion processes that affect the interception of solar radiation. Experiments in two consecutive seasons (2001/2002 and 2002/2003) were grown at a low P site (Olsen P = 6 μg ml⁻¹) at Lincoln, New Zealand. Each experiment contained five rates of P application. In 2001/2002 rates of 0, 50, 100, 150, or 200 kg P ha⁻¹ were applied. In 2002/2003 an additional 0, 0, 10, 20 or 40 kg P ha⁻¹ was applied to the same plots producing total P treatments of 0, 50, 110, 170 or 240 kg P ha⁻¹ summed over the two seasons.When P availability was limited (0 or 50 kg P ha⁻¹) the rates of leaf tip and fully expanded leaf appearance were slower in both seasons. Phyllochrons (°Cd leaf tip⁻¹) were 5 °Cd longer in crops that received 0 kg P ha⁻¹ than those fertilised with ≥100 kg P ha⁻¹. The area of individual leaves was also reduced by low P inputs but the ranking of leaf area by main stem leaf position was conservative. The leaf area of the largest leaf of the unfertilised crops was at least 22% less than the maximum measured leaf area in both seasons. In contrast, P fertiliser application had no effect on leaf senescence.The rate of leaf appearance per plant, individual leaf area and plant population were integrated to calculate green leaf area index (GLAI) and to estimate accumulated radiation interception (RI_cum) for these crops. The total RI_cum throughout the season in the unfertilised crops was 12–28% less than for those crops that received ≥100 kg P ha⁻¹ in both seasons. This difference partly explained the differences in crop biomass production in response to P availability. A sensitivity analysis showed that RI_cum was equally sensitive to changes of the rate of leaf appearance and the area of individual leaves in response to P supply. Both processes need to be incorporated in mechanistic models of P effects on Z. mays which can be used to design efficient P fertiliser strategies.     

Sugar characteristics of sweet corn populations from a sugary enhancer breeding program

Summary Endosperm sugars and sorbitol were determined on 18 sweet corn (Zea mays L.) populations from a breeding program undertaken to transfer the sugary enhacer (se) gene into diverse sugary (su) backgrounds. Three subpopulations of IL677a, the source of se, and six su Se inbreds were included for comparison. Mature dry kernels of between two and four successive generations of these populations were analyzed. Immature kernels at two developmental stages (21 and 35 days after pollination) were also analyzed for one generation.Eleven of the breeding populations closely resemble IL677a, with high sucrose at the edible stage (21 days after pollination) and maltose accumulating as the seeds matured. In most instances, the distinctive sugar profiles were observed consistently in dry seeds of various generations. Hence, it appears that the se trait can be transferred successfully into su cultivars having various genetic backgrounds. The remaining seven breeding populations possessed elevated sucrose and glucose compared to most of the standard su Se inbreds, but maltose did not accumulate during seed maturation. It is not clear whether se was transferred to these populations and its expression modified or whether another trait was responsible. There were also marked differences in glucose levels among the su Se populations examined.Supported by the National Science Foundation (Grant No. 7922 686) and the Illinois Agriculture Experiment Station (Project No. 65-0330).     

Variation in sweet corn kernel characteristics associated with stand establishment and eating quality

Summary A better understanding of the relationships between kernel characteristics associated with eating quality and stand establishment could be helpful in selection of superior genotypes by the sweet corn industry. A set of sweet corn (Zea mays L.) inbred lines with different endosperm mutations (su1, su1 se1 and sh2) were evaluated for field emergence and seedling growth rate at two locations over two years. Kernel characteristics associated with eating quality (kernel moisture concentration, kernel tenderness, sugars, phytoglycogen and dimethyl sulfide (DMS) concentrations were determined for the same inbreds by laboratory analysis from ears harvested at 18 and 22 days after pollination (DAP). Amounts of sugars, phytoglycogen and starch were also measured in mature dry kernel samples of the same inbreds. Extensive genetic variability was found among endosperm mutations and among genetic backgrounds within the different endosperm groups for most of the characteristics under study. Most of the kernel attributes associated with eating quality were uncorrelated indicating that selection to improve specific eating quality characteristics can be conducted simultaneously. A negative correlation between field emergence and sugar concentrations in immature kernels suggests that in breeding programs designed to develop germplasm with improved germination and stand establishment, concurrent attention must be given to the fresh quality of the harvested product. This information is of value to breeders and commercial growers for selection of sh2 and su1 se1 lines with superior field emergence and eating quality.Abbreviations (DAP) days after pollination - (DMS) dimethyl sulfide     

Light interception and radiation use efficiency in temperate quinoa (Chenopodium quinoa Willd.) cultivars

Sea level quinoas are grown at low altitudes in Central and Southern Chile. Both sensitivity to photoperiod and response to temperature largely determine quinoa adaptation, but crop biomass production must be quantified to evaluate agronomic performance. The objectives of this work are: (i) to characterize development effects on leaf area evolution for genotypes of sea level quinoa differing in cycle length, (ii) to quantify the extinction coefficient (k) for photosynthetically active radiation (PAR) and radiation use efficiency (RUE) from emergence up to the beginning of grain filling and (iii) to identify which crop attributes related to canopy architecture should be considered to improve biomass production. Four cultivars (NL-6, RU-5, CO-407 and Faro) were cropped in Pergamino (33°56′S, 60°35′W, 65 m a.s.l.), Argentina, at three densities (from 22 to 66 plants m⁻²) in two consecutive years under field conditions with adequate water and nutrient supply. Thermal time to first anthesis and maximum leaf number on the main stem were linearly correlated (r² = 0.87; p < 0.0001). Leaf area continued to increase during the flowering phase, notably in NL-6, the earliest genotype. There were significant differences in maximum plant leaf area between cultivars. Increasing density reduced plant leaf area but effects were comparatively small. Estimated k was 0.59 ± 0.02 across genotypes and was higher (p < 0.05) for 66 plants m⁻². Values for RUE changed as cumulative intercepted PAR (IPAR) increased; at initial stages of development RUE was 1.25 ± 0.09 g MJ IPAR⁻¹, but if cumulative IPAR was higher than 107.5 ± 10.4 MJ IPAR m⁻², RUE was 2.68 ± 0.15 g MJ IPAR⁻¹. That change occurred when leaf area index (LAI) and fraction of PAR intercepted were still low and ranged from 0.61 to 1.38 and from 0.33 to 0.51, respectively. No significant association was found with any developmental stage. Our results agreed to the notion that RUE variation during pre-anthesis phases is largely determined by LAI through its effect on radiation distribution within the canopy. Biomass production could be improved if periods of interception below 50% of incoming PAR were reduced to ensure high RUE. This seems to be possible in temperate areas both by the use of late genotypes with a higher number of leaves on the main stem and by early genotypes provided adequate plant density is chosen. Early increment in LAI and overlapping of the leaf area increase period with the flowering phase are desirable strategies for earliest genotypes to maximize yield.     

Heterosis for leaf photosynthesis,grain yield and yield components in maize

Summary One of the primary avenues of improving the biological efficiency of crop plants is through the improvement of the leaf and canopy photosynthetic rates. However, the question whether the superiority of hybrids in respect of productivity potential could be traced, in retrospective fashion, to the photosynthetic parameters should be answered first. Once established and standardized, such indices could be streamlined in innovative breeding to predict the heterotic combinations for final yield formation. To answer this question, the photosynthetic rate and other components of photosynthetic efficiency were monitored among eight inbred stocks of maize (selected for variable photosynthetic rate from a previous study) and their all possible one-way crosses. The results demonstrated extensive heterosis in respect of photosynthetic rate and other photosynthetic indices which, in turn, was also realized in terms of higher biomass productivity and yield. Nonetheless, high leaf photosynthesis alone did not result in higher grain yield. On the contrary, component interaction among the photosynthetic indices like photosynthetic rate, leaf area/plant, number of leaves and chlorophyll content on one hand and the complementary gene action on the other, could be held responsible for higher yields in hybrids. Since the components of photosynthetic efficiency can predict heterosis for biomass and grain yield upto a reasonable extent, judicious incorporation of such indices in selection parameters for applied genetic protocols can add yet another dimension to the strategies for future yield improvements.     

Genetic mapping for resistance to gray leaf spot in maize

The molecular marker technology has been used on mapping of quantitative trait loci (QTLs) associated with plant resistance. The objectives of this research were to estimate genetic parameters and to map genomic regions involved in the resistance to gray leaf spot in maize. Ninety F₃ families from the BS03 (susceptible) and BS04 (resistant) cross were used. Field trials were performed using a 10 × 10 square lattice design with three replications. Data from 62 SSR markers were used for linkage analysis. The locations of the QTLs on the linkage groups were determined by composite interval mapping method and the phenotypic variance explained by each marker was determined by regression analysis. Several QTLs associated to disease resistance were identified in the population BS03 × BS04. Some QTLs showed significant effects over the different environments studied. The existence of significant QTLs in common among different environments indicates these genomic regions as possible new tools for marker-assisted selection in maize breeding programs.     

Identification of RAPD and SCAR markers linked to northern leaf blight resistance in waxy corn (Zea mays var. ceratina)

Exserohilum turcicum causes northern corn leaf blight (NCLB), an important disease occurring in maize producing areas throughout the world. Currently, the development of cultivars resistant to E. turcicum seems to be the most efficient method to control NCLB damage. Marker-assisted selection (MAS) enables breeders to improve selection efficiency. The objective of this work was to identify random amplified polymorphic DNA (RAPD) and sequence characterized amplified region (SCAR) markers associated with NCLB resistance. Bulked segregant analysis (BSA) was used to search for RAPD markers linked to NCLB resistance genes, using F₂ segregating population obtained by crossing a susceptible inbred ‘209W’ line with a resistant inbred ‘241W’ line. Two hundred and twenty-two decamer primers were screened to identify four RAPD markers: OPA07₅₂₁, OPA16₄₅₇, OPB09₅₂₀, and OPE20₅₃₆ linked to NCLB resistance phenotype. These markers were converted into dominant SCAR markers: SCA07₄₉₆, SCA16₄₂₀, SCB09₄₆₄, and SCE20₄₂₉, respectively. The RAPD and SCAR markers were developed successfully to identify NCLB resistant genotypes in segregating progenies carrying NCLB resistant traits. Thus, the markers identified in this study should be applicable for MAS for the NCLB resistance in waxy corn breeding programs.     

Variation and inheritance of response to acetochlor among maize inbred lines and hybrids

Summary The variation of response to acetochlor was studied in a two-year experiment carried out by subjecting 18 maize (Zea mays L.) inbred lines to three herbicide rates (0, 2.5 and 5 l a.i./ha). In both years some inbred lines consistently exhibited an evident susceptibility, with symptoms consisting of the seedling curling up below the soil surface and causing impaired field emergence. The results were poor plant density and lower grain yield in comparison to control. In contrast, other lines showed a satisfactory level of tolerance.Then, to gather data on the inheritance of response to acetochlor, four tolerant inbreds (T) and four susceptible inbreds (S) were crossed to obtain four T×T, four S×S, four S×T and the corresponding four T×S two-way hybrids. These hybrids were studied together with parental lines by applying the same herbicide rates used in the previous trial. The S×S hybrids showed susceptibility to the herbicide and the T×T were tolerant, whereas the S×T and the T× S hybrids showed a tolerance very close to that of the T×T hybrids. No difference was found between S×T and the corresponding T×S hybrids as to herbicide response. On average, the 16 hybrids exhibited greater tolerance than the eight parental lines, with each hybrid group being more tolerant than its parental line group. These results indicate that tolerance to acetochlor is prevailingly dominant, that action of extranuclear genes should be ruled out, and that the level of plant vigour can affect herbicide reactiveness.     

Inheritance of photosynthesis in a diallel among eight maize inbred lines from Iowa Stiff Stalk Synthetic

Summary Photosynthesis is a trait that should be improved in a selection program for yield potential of maize (Zea mays L.). We measured leaf CO₂-exchange rate (CER), an estimate of photosynthetic efficiency, of a complete diallel (parents, F₁ crosses, and their reciprocals) among eight inbred lines (4 with low and 4 with high CER) from the Iowa Stiff Stalk Synthetic maize population. Measurements were made during vegetative (CER 1) and grain filling (CER 2) stages of growth, and the experiment was conducted two years at one location. We measured large differences among crosses and significant heterosis for high CER at CER 1 (0.0 to 25.1%) and CER 2 (0.0 to 53.8%). Several crosses exhibited overdominant phenotypes for high CER at both growth stages, and one cross showed significant overdominance for low CER at CER 1. General combining ability effects (gca) were the largest components of among-cross variation at both CER 1 and CER 2. Specific combining ability (sca) also was significant at both growth stages, but gca effects were 9.4 and 4.8 times larger than sca effects at CER 1 and CER 2, respectively. Furthermore, high CER lines showed positive gca effects, and low CER lines showed negative gca effects at each stage. Maternal and reciprocal effects were not significant; thus, CER in these crosses was controlled largely by additive effects of nuclear genes. A high positive genotypic correlation (r=0.74) between CER 1 and CER 2 suggested that selection at either growth stage would improve CER throughout the growing season.Journal Paper No. J-9023 of the Iowa Agriculture and Home Economics Exp. Stn., Ames, Iowa. Project No. 1990.     

Leaf thickness and response of leaf photosynthesis to water stress in soybean varieties

Summary A comparative study of photosynthetic response to soil moisture was conducted with soybean varieties (Harosoy, Norin No. 1 and Hogyoku). When the plants were grown in May before the rainy season under a high radiation level, per unit leaf area the photosynthetic rate of Harosoy leaves was significantly higher throughout the entire range of soil water potentials and leaf water potentials than that of Norin No. 1 leaves. The high photosynthetic rate of Harosoy under non-stress conditionsaand mild water stress was associated with both the high specific leaf weight and the high leaf areal nitrogen content. The photosynthetic response of the Harosoy leaves developed during rainy season under a low radiation level, however, was similar to that of Norin No. 1; no significant difference was observed between these two varieties in the specific leaf weight and in the leaf areal nitrogen content.On the other hand, Hogyoku cultivated especially along the footpaths between the paddy field was very sensitive to water stress, closing the stomata at higher soil water potentials. Under non-stress conditions, as compared with per unit leaf nitrogen content, however, Hogyoku performed higher leaf photosynthesis even with a thinner leaf than Harosoy.     

Radiation use efficiency and biomass partitioning of lucerne (Medicago sativa) in a temperate climate

This research quantifies the influence of seasonal variations in solar radiation, temperature and biomass partitioning on lucerne production in a temperate climate. Above ground biomass (shoot) production of fully irrigated ‘Kaituna’ lucerne was measured in the field over 5 years and 33 regrowth cycles in Canterbury, New Zealand. Shoot production increased linearly (R² of 0.93 ± 0.07) with intercepted total radiation within each regrowth cycle but radiation use efficiency (RUE_shoot, in g DM/MJ total radiation) ranged from 0.29 to 1.09 g DM/MJ. Covariate analysis showed season and temperature both influenced RUE_shoot with temperature adjusted decreasing from 1.01 g DM/MJ in September to 0.77 g DM/MJ between October and February, decreasing again to 0.47 g DM/MJ in March and April before increasing back to 0.99 g DM/MJ in May. A second fully irrigated experiment with lucerne plants grown in plastic columns under near field conditions investigated the seasonality of biomass partitioning between shoots and perennial biomass (roots and crowns). The proportion of total biomass partitioned to shoots (P_shoot) was 0.90 in September, 0.67 from October to February but only 0.35 in March and could be related to photoperiod. These P_shoot values were closely correlated with showing seasonal changes in RUE_shoot were partly caused by changes in biomass partitioning. Field data for RUE_shoot were divided by P_shoot to estimate the RUE for total biomass production (RUE_total). The RUE_total increased linearly from 0.60 to 1.60 g DM/MJ as mean air temperatures increased from 6 to 18 °C. These results quantified the effects of solar radiation and mean temperature on total lucerne biomass production and its seasonal partitioning between shoots and perennial biomass. The influence of regrowth duration on this partitioning was also investigated.     

Yield response to honeycomb selection in maize

Summary This research evaluated the response to three cycles of mass honeycomb selection based on the grain yields of widely-spaced plants from the F₂ generation of a maize (Zea mays L.) single cross hybrid. Frequency distributions of individual plant yields in the selection plots showed strong positive skewness. A significant linear yield increase of 11.23% per cycle compared to the check was obtained. Correlated responses were the significant reduction in days to mid-silking, increases in ear and plant heights and in prolificacy. The selected population was similar to the original hybrid in days to mid-silking, ear and plant heights, but had significantly more ears per plant. It is concluded that mass honeycomb selection at wide spacings was effective in improving grain yield and prolificacy but may produce correlated responses in some other agronomic traits similar to those obtained with other selection schemes.     

Mid-parent heterosis and combining ability of European corn borer resistance in maize

Success in breeding maize resistant to the European corn borer has been limited, with the exception of leaf feeding resistance. The inheritance of resistance to leaf, sheath-collar and ear damage in four maize germplasms and their six F₁ crosses was evaluated by diallel analysis. Plants in a completely randomized design were artificially infested at the whorl, anthesis or full silk stage of plant development and were evaluated in the field for insect damage. A damage index based on size, number and location of lesions was calculated for each stage. Stowell's Evergreen (susceptible) had a mean damage index three to six times that of Maiz Amargo (resistant) at the whorl stage and the progeny plants were more resistant than the susceptible parent. Maiz Amargo and its crosses had significantly lower mean indices than Stowell's Evergreen for sheath-collar damage in Year 1 but not Year 2. Zapalote Chico, Maiz Amargo and their cross were significantly less damaged than other genotypes at the full silk stage. Heterosis values indicated an increase in resistance of crosses over the midparent average at all three stages of development. General combining ability (GCA) was highly significant for all types of damage, but specific combining ability was significant only for leaf damage. Based on estimates of GCA, Maiz Amargo was the best source of resistance to leaf and sheath-collar damage and both Zapalote Chico and Maiz Amargo would be good parents for ear damage resistance. Results suggest that resistance at different plant development stages can be combined.     

Genotype x environment interactions in a 10 × 10 diallel cross of quality protein maize (Zea mays L.)

Summary A 10×10 diallel cross experiment involving white modified opaque-2 maize inbred lines was grown at four sites. A stability analysis, based on both a combining ability and a heterotic pattern model, was developed. The stability analysis provided valuable information on the genotype x environment interaction properties of the 10 inbred lines. The Gail & Simon (1985) test for qualitative interactions provided a means of determining the nature of these interactions. The inbred, SO507W(M), is shown to have the best potential for use in a hybrid breeding programme, in terms of having the highest weighted general combining ability and line heterosis, and the best general adaptability to all four sites. The single cross, SO713W(P) x PO558W(F), has been identified as one of the genotypes to be used in a recurrent selection programme that favours specific combining ability.     

Expression of mature plant resistance to Striga hermonthica in maize

Twelve maize genotypes with different degrees of resistance were evaluated in plots inoculated with seeds of the phytoparasite Striga hermonthica (Del.) Benth. and in Striga-free (control) plots for three seasons between 1991 and 1993. Resistant genotypes, although showing similar levels of underground infection as susceptible 9 weeks after maize planting (WAP), had significantly fewer emerged parasitic plants and sustained lower damage. Host damage was not determined by the severity of infection. Percent height and dry matter reductions increased from 3 to 6 WAP for resistant and susceptible genotypes. While percent height and dry matter reductions for resistant genotypes declined at 9 and 12 WAP, susceptible genotypes either maintained or increased their levels of damage. Correlation between the severity of height and dry matter reductions at the vegetative (3 and 6 WAP) and reproductive (9 and 12 WAP) stages of maize growth were not significant, indicating that damage at early stages of growth cannot be used to predict mature plant response. Maize damage score (1–9) at 8 WAP, a non destructive and composite shoot indicator of host performance under Striga infestation, was significantly correlated (r = 0.88, r = 0.82; p < 0.01) to the level of shoot reduction at mature plant stages. Emerged Striga count at 8 WAP was significantly correlated (r = 0.98, p < 0.01) to the count at 10 WAP, the time when parasite emergence was highest. Under high and uniform infestation, mature plant resistance can be detected at 8 WAP, the onset of flowering in maize. This revised version was published online in July 2006 with corrections to the Cover Date.     

Taxonomy of Mexican landraces of maize,Zea mays,based on their resistance to European corn borer,Ostrinia nubilalis

Summary The resistance to the European corn borer, Ostrinia nubilalis (Hubner), of thirty-seven indigenous landraces of Mexican maize was examined. The relationship of resistance and existing taxonomy of maize according to Wellhausen et al., (1952), was subjected to numerical analyses. Variables examined were: seedling DIM-BOA content, the extent of leaf feeding damage by early instar larvae both in the field and in the laboratory, the extent of plant breakage and stalk tunneling by late instar larvae, plant height, and the extent of fungal damage by Gibberella zeae and Ustilago maydis. Significant differences in resistance among the major taxonomic groupings were reflected in the existing taxonomy of maize (Wellhausen et al., 1952). The most resistant landrace grouping was Wellhausen et al.'s Prehistoric Mestizos. Eighty-five percent of a series of modern inbred lines, pools, and Argentine landraces were found to have affinities with one of the more susceptible groupings, the Ancient Indigenous Races, based upon analysis of the resistance data.     

Six years of maize selection in ‘Kitale synthetic II’, ‘Ecuador 573’, and ‘kitale composite a’ using methods of the comprehensive breeding system

Summary Evaluation of the results of six years of selection in the Kitale maize breeding methods study proved reciprocal recurrent selection to be an effective interpopulation improvement method. Ear-to-row selection was effective in improving Kitale Composite A (KCA). Data from a diallel of the cycle-6 ear-to-row substrains showed that where less than 10 lines were selected, inbreeding appeared to depress gain, but differences among the various ear-to-row experiments were not significant. Intererossing the substrains was predicted to reduce the effect of inbreeding. Yield gains in reciprocal recurrent selection and ear-to-row selection were associated with increases in ears per 100 plants. The reciprocal recurrent selection variety cross yield gain was estimated at 3.5% per year (7%/cycle) and that of the best ear-to-row procedure at 2.6% per year. The variety cross syn-2 from reciprocal recurrent selection was predicted to improve at the same rate as the best ear-to-row procedure (E7) with one long growing season per year. Under a cropping system with two similar seasons per year, however, ear-to-row selection should be more effective than reciprocal recurrent selection or its derived syn-2. Mass selection at 10% selection intensity produced significant improvement, but not at 2% selection intensity.S₁ and three half-sib selection methods were ineffective in KCA. Inbreeding and the lack of linkage equilibrium in KCA were discussed as contributing. Theoretical substrain compositing required 30 to 40 lines selected to equal the effectiveness of ear-to-row selection with 10 lines. Full-sib selection was effective in improving yield at 1.2% per year. All the recurrent selection methods showed a significant increase in ears per 100 plants. Population size was shown to be important in reducing losses due to inbreeding.If a hybrid or variety cross is potentially useable, reciprocal recurrent selection provides the best population approach. Initially, the variety cross syn-2 could be released with no additional breeding effort. Transition to variety crosses or traditional hybrids from extracted lines could be done as farmer husbandry improves and seed production facilities develop. Otherwise, open-pollinated varieties superior to existing maizes can be effectively developed using ear-to-row selection. The other methods may be effective in breeding populations when larger effective population sizes are used, but they were not effective in KCA when only 10 lines were recombined each cycle.Contribution from the East African Agriculture and Forestry Research Organization of the East African Community, the U.S. Agency for International Development (Project 618-11-110-657, East African Food Crop Research), the Kenya Ministry of Agriculture, and the Agricultural Research Service of the U.S. Department of Agriculture.     

Genetic variation of the rate of leaf appearance in maize: Possible yield prediction at the early stage

Summary Evidence of genetic variation for early vigour is presented using maize line × tester crosses. The leaf appearance rate and associated variance components are affected by a physiological stress attributable to the transition to autotrophic nutrition by the plant. At this stage, specific combining ability plays an important role in the total genetic variation. It is concluded that differing genetic controls exist in early and late material, and that this difference is also manifested in leaf initiation and elongation rates. The ground coverage rate, as a component of plant development, is genetically correlated to total dry matter yield. The genetic correlations vary according to the growth stage.     

Populations of maize with more rapid and reliable seedling emergence than cornbelt dents at low temperatures

Summary Selected races, populations and genotypes of maize (Zea mays L.) from regions where maize is sown under cool conditions were evaluated in controlled-environment rooms for time to seedling emergence and percentage emergence at low temperatures. The objective of the study was to identify populations with the ability to emerge more rapidly and more reliably than Cornbelt Dent, the race most widely used to produce cultivars for temperate regions.Several populations emerged markedly faster and more reliably than Cornbelt Dent populations. All of these populations contained germplasm of highland Mexican origin, except for Gaspé Flint, and the fastest population in each experiment always contained a high proportion of germplasm from the Cónico race. The advantage of populations containing Cónico germplasm was greatest in environments where emergence was slowest.     

Response of maize inbred lines to a defoliation treatment inducing tolerance to cold at germination

Defoliation during maize (Zea mays L.) kernel development has been observed to induce tolerance to cold of germinating seeds in responsive genotypes. The objectives of this study were to evaluate the response to defoliation of immature embryo and mature seed germinability at cold and to verify if the response was influenced by the developmental stage at which the treatment was applied. In three environments, six inbred lines (B73, IABO78, Lo1016, Lo964, Mo17, Os420) were defoliated (D) approximately 20 days after pollination (DAP) or not defoliated (ND). Immature embryos were excised three days after defoliation and germinated in vitro at 9 or 25 ^∘C. At maturation, kernel germination was tested at the same temperatures. Defoliation improved cold tolerance and mean time to germination (MTG) at 9 ^∘C of both embryos and kernels of Lo1016. To study the effect of kernel developmental stage on response to defoliation, plants of B73, Lo1016 and Lo964 were defoliated at 15, 18, 21, 24, 27, 30, 33, 36, and 39 DAP, or not defoliated. At the same DAP, immature grains were analyzed for dry weight, water and abscisic acid (ABA) content. In Lo1016, low amounts of kernel ABA were detected at all stages, while in Lo964 and B73 ABA increased during development. Lo1016 mature kernels showed an improvement of cold tolerance due to defoliation at all times, while the other genotypes did not. In conclusion inbred lines showed variability for mature seed and immature embryo tolerance to cold at germination and for the ability to acquire tolerance after defoliation.