La importancia del intervalo de la floración y el índice en el mejoramiento para la resistencia a sequía en Maíz tropical

The length of the Interval between anthesis and silking (ASI) is increased by drought which coincides with flowering. Four elite CIMMYT lowland tropical populations are undergoing recurrent selection (S1 or full-sib) for improved grain yield and several other traits under drought and well watered conditions. Data collected from more than 200 families per population grown In single row plots under three water stress levels (pre and post-flowering stress; post-flowering stress; normal irrigation, all in the absence of rain) showed weak or no correlation between grain yield and traits related to plant water status, such as leaf rolling and senescence, photooxidation, leaf chlorophyll concentration, shoot elongation rate, canopy temperature and predawn water potential. Yield under all levels of stress was significantly negatively correlated with AST, and as AST increased due to drought, kernels and ears per plant were significantly reduced. In all populations yield decreased by approximately 10 % per day increase in AST up to 8 days. In several stress situations broad-sense heritability of AST was greater than that of grain yield and the genetic correlation between grain yield and AST approached -1.00. Synthetics formed from one population following bidirectional selection and tested under drought showed adaptive advantage of cool canopy temperature, delayed leaf senescence, reduced AST and erect leaves, especially when all were combined with grain yield in a single index during selection. Selection for reduced AST and high grain yield under drought can be an effective means of improving drought tolerance in tropical maize.La longitud de intervalo entre la aparición de estigmas y antesis se incrementa cuando la sequía coincide con la época de floración del maíz (Zea mays L.). Cuatro poblaciones élite de maíz tropical del CIMMYT están siendo mejoradas para resistencia a sequía por esquemas de selección recurrente (S1 o hermanos completos) para rendimiento de grano y varias otras características, tanto bajo estrés de sequía, como bajo condiciones de buena humedad. Los datos recolectados de más de 2,000 famillas por población evaluadas en parcelas de un solo surco, bajo tres niveles de estrés de humedad de suelo (1. estrés severo durante el período de pre y postfloración; 2. estrés intermedio durante el llenado de grano; y 3. irrigación normal, todos en ausencia de lluvia), mostraron correlación débil o ausencia de ésta entre el rendimiento de grano y otras características relacionadas al balance hídrico de la planta, tales como: enrollamiento foliar y senescencia, foto-oxidación, concentración foliar de clorofila, tasa de elongación vegetativa, temperatura foliar y potencial hídrico matutino. El rendimiento bajo todos los niveles de estrés fue correlacionado negativamente con el intervalo de floración y el intervalo de floración se incrementó debido a la sequía. Asimismo, los granos y mazorcas por plantase redujeron significativamente. En todas las poblaciones el rendimiento disminuyó aproximadamente 10% por día de incremento en el intervalo de floración y hasta 8 días. En varias situaciones de estrés, la heredabilidad de sentido amplio para el intervalo de floración fue mayor que aquella del rendimiento de grano y la correlación genética entre rendimiento de grano y el intervalo de floración aproximadamente -1.00. Los sintéticos formados a partir de una de las poblaciones, seguidos de selección bidireccional y evaluados bajo sequía, demostraron ventajas adaptativas como baja temperatura foliar, baja senescencia foliar, un intervalo de floración reducido y hojas erectas, especialmente, cuando todas estas características fueron combinadas en un índice de selección. La selección por intervalo de floración corto y alto rendimiento de grano puede ser un medio eficaz de mejorar la tolerancia a la sequía en maíz tropical

Morphology and growth of maize: IITA research guide, No. 9

Maize is one of the important food crops worldwide. It has a remarkable productive potential. However, considerable variation exists among varieties in morphology and growth habit. Management of a maize crop with respect to interaction of genotype and environment requires specific knowledge of maize growth and development

Selection for the improvement of maize yield under moisture deficits

Throughout the lowland humid tropics, unpredictable periods of non-protracted drought are responsible for significant reductions in maize (Zea mays L.) yield, and losses may be disastrously large if drought coincides with the period around flowering. This study was conducted to develop and evaluate a selection procedure to improve the drought resistance of maize populations grown under limited moisture supply, particularly around flowering. Eighty-five full-sib progenies of the tropical lowland population Tuxpeno were grown under three soil moisture-deficit treatments at a site in Mexico without rainfall during the season. The treatments were; mild (normal irrigation); medium (irrigation to field capacity soon after emergence and again 10 days after flowering); and severe (irrigation to field capacity soon after emergence only) soil moisture-deficits. Mean yields were 6120, 4330 and 1560 kg ha−1, respectively, under the three treatments. There was significant genotype (progeny) × soil moisture-deficit interaction for grain-yield. Yield under the severemoisture-deficit was significantly correlated with a measure of the rate of leaf and stem extension (). Tnterval between male and female flowering () and rate of foliar senescence (). These indices were used along with grain-yield in a selection index. To test its usefulness, experimental varieties formed from progenies selected for yield per se unde the different soil moisture-deficit treatments and for a divergence (index-tolerant and index-susceptible) of performance using the selection index were grown under conditions similar to those of the initial progeny evaluation. The drought-tolerant variety (selected for yield and favourable adaptive traits) outyieldeall others by 500 kg ha−1 under the severe moisture-deficit but not at the expense of yield under the well-watered conditions. Recurrent selection for these traits under similar moisture-deficits was practised among 250 full-sib progeny in this population for three cycles. Canopy temperature, determined by infrared thermometry prior to flowering, was highly correlated () with yield under severe moisture-deficits, and was included in the selection index after the second cycle. After three cycles of improvement with a 33% progeny-selection intensity, evaluation under the same soil moisture-deficits showed that grain-yield increased by 1.8, 7.8 and 21.6%, or 320, 420 and 410 kg ha−1 cycle−1 under the mild, medium and severe moisture-deficits, respectively. There were significant changes in the drought-adaptive traits, but there was no significant change in days-to-flowering under the well-watered conditions

Flowering of diverse maize cultivars in relation to temperature and photoperiod in multilocation field trials

International audienc

Tropical maize (Zea mays L.).

From its origins in Mexico, maize has spread throughout the cropping world and now has the largest annual production of any cereal crop. Maize adapted to the tropics has a shorter breeding history than its temperate equivalent, and maize yields in the tropics average 46% those of temperate regions. Diverse microclimates in its center of origin in Mexico resulted in a great diversity of landraces, and early improvement focused on identifying and compositing the most productive of these into genetically diverse populations that have subsequently formed the basis of modern inbred line extraction and pedigree breeding. The International Maize and Wheat Improvement Center (CIMMYT) has been the focus of much of this research for the past 50 years, and they, along with multinational seed companies, have been largely responsible for the major movements of elite tropical maize germplasm to Africa and Asia. Crop improvement has focused primarily on changing biomass partitioning by reducing plant height, increasing ear growth, reducing barrenness, and consistently focusing on increasing biotic and abiotic stress tolerances. Photoperiod sensitivity and tassel size, however, remain high and harvest index and tolerance to plant density low relative to temperate maize. Tropical maize breeding programs have shown genetic gains of around 100 kg/ha/year under optimal conditions, though less under abiotic stress. Genetic rates of gain have averaged 1–2% annually despite the high incidence of stresses in target environments, and today farm yields in tropical environments are increasing at the same rate (70–80 kg/ha/year) as in temperate production areas. Although open-pollinated varieties (OPVs) have been largely superseded by hybrids, there are niches in low-yielding environments or where commercial seed companies are dysfunctional where OPVs have a role. Tropical maize breeding programs should maintain their focus on yield stability in stressed environments, increased yield potential by further changes in plant morphology and partitioning, precise high-throughput phenotyping, and the systematic adoption of real-time marker-assisted selection (MAS). Considerable improvements in technologies that shorten the selection cycle have been made, e.g., the combination of doubled haploid inbred line production and MAS, especially genomic selection. Effective management of the deluge of genotypic and phenotypic data is a continuing priority. Impact will however only be achieved by increasing the rate of varietal turnover at the farm level so the challenges of global warming can be effectively met by the new generation of stress-tolerant maize cultivars. This can only come about through effective private-public partnerships in the seed sector and in a continued investment in well-trained motivated plant breeders and agronomists committed to quality fieldwork with the widest possible array of useful genetic variation

Drought tolerance improvement in tropical maize source populations : evidence of progress

International audienceThe objectives of this study were to evaluate direct and correlated responses to recurrent selection for drought tolerance in two CIMMYT maize (Zea mays L.) source germplasm populations, ‘DTP1’ and ‘DTP2’, adapted to the lowland and mid-altitude tropics. Selection was primarily based on grain yield, ears per plant, anthesis-silking interval, and leaf senescence under drought. Cycles C0, C3, and C6 of DTP1 and C0, C3, C5 and C9 of DTP2 were evaluated under drought, low N, and optimal conditions. In both populations, significant yield gains were observed under drought conditions, associated with a significant increase in numbers of ears per plant and grains per ear, and significant reductions in anthesis-silking interval, ovule number and abortion rate during grain filling. Abortion rate was positively correlated with the number of ovules at silking and with anthesis-silking interval. In DTP1, recurrent selection under drought was associated with a derease of tassel and stem dry weight and with an increase of ear dry weight and harvest index. This study confirms the effectiveness of recurrent selection under drought as a means of improving tropical maize source populations for performance under water deficits and to a lesser extent under low N. The primary mechanism underlying these changes appears to be improved partitioning of assimilates to the ear at flowering, at the expense of tassel and stem growt