Jorge Bolaños

THE IMPORTANCE OF THE ANTHESIS-SILKING INTERVAL IN BREEDING FOR DROUGHT TOLERANCE IN TROPICAL MAIZE

Selection for improved performance under drought based on grain yield alone has often been considered inefficient, but the use of secondary traits of adaptive value whose genetic variability increases under drought can increase selection efficiency. In the course of recurrent selection for drought tolerance in six tropical maize (Zea mays L.) populations, a total of 3509 inbred progenies (S1 to S3 level) were evaluated in 50 separate yield trials under two or three water regimes during the dry winter seasons of 1986–1990 at Tlaltizapan, Mexico. In over 90% of the trials, ears plant−1, kernels plant−1, weight kernel−1, anthesis-silking interval (ASI), tassel branch number and visual scores for leaf angle, leaf rolling and leaf senescence were determined. Low scores indicated erect, unrolled or green leaves. Canopy temperature, leaf chlorophyll concentration and stem-leaf extension rate were measured in 20–50% of the trials. Across all trials, linear phenotypic correlations (P < 0.01) between grain yield under drought and these traits, in order listed, were 0.77, 0.90, 0.46, −0.53, −0.16, 0.06NS, −0.18, −0.11, −0.27, 0.17 and 0.10. Genetic correlations were generally similar in size and sign. None of physiological or morphological traits indicative of improved water status correlated with grain yield under drought, although some had relatively high heritabilities. Genetic variances for grain yield, kernels ear−1, kernels plant−1 and weight kernel−1 decreased with increasing drought, but those for ASI and ears plant−1 increased. Broad-sense heritability for grain yield averaged around 0.6, but fell to values near 0.4 at very low grain yield levels. Genetic correlations between grain yield and ASI or ears plant−1 were weak under well-watered conditions, but approached −0.6 and 0.9, respectively, under severe moisture stress. These results show that secondary traits are not lacking genetic variability within elite maize populations. Their low correlation with grain yield may indicate that variation in grain yield under moisture stress is dominated by variation in ear-setting processes related to biomass partitioning at flowering, and much less by factors putatively linked to crop water status. Field-based selection programs for drought tolerance should consider these results.

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

Abstract 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 ‘Tuxpeno Sequia’ during the rain-free winter season at Tlaltizapan, 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 Tuxpeno Sequia, and a check cultivar representing the results of six cycles of selection based primarily on multilocation testing, were evaluated under three moisture regimes at Tlaltizapan 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.

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

Abstract 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 ‘Tuxpeno Sequia’ 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 Tuxpeno Sequia and a check representing six cycles of international multilocation testing in the same population, were evaluated under three moisture regimes at Tlaltizapan, 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. III. Responses in drought-adaptive physiological and morphological traits

Abstract 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, ‘Tuxpeno Sequia’. Six cultivars comprising Cycles 0, 2, 4, 6 and 8 of Tuxpeno Sequia, and a check cultivar were grown for two consecutive years at Tlaltizapan, 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 Tuxpeno Sequia 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.

Physiological bases for yield differences in selected maize cultivars from Central America

Abstract For the past 30 years, maize ( Zea mays L.) improvement has been the most important activity of a collaborative network of national maize researchers from Central America and the Caribbean. Emphasis has been placed on development of hybrids for areas with high yield potential and open-pollinated cultivars (OPCs) for environments limited by biotic or abiotic stresses. This study was conducted to determine the physiological, phenological and morphological bases for yield differences in a group of representative hybrids and OPCs available in the region. Nine cultivars (five hybrids and four OPCs) were evaluated in 11 diverse environments of the region ranging in mean grain yield from 0.5 to 8.0 Mg ha −1 . Hybrids consistently outyielded OPCs by 1.0 to 1.5 Mg ha −1 across the range of environments tested. Differences in grain yield between hybrids and OPCs were mostly due to greater biomass production, higher harvest index and a larger daily ear growth rate (2.2 g d −1 ear −1 for hybrids versus 1.8 g d −1 ear −1 for OPCs). Within hybrids or OPCs, however, differences in yield depended on the duration of grain filling and not on ear growth rate. For example, grain filling in higher-yielding hybrids CB-HS7 and HB-85 required about 7 more days than the lower-yielding hybrids P-8916 and H-5. A similar situation existed within the OPCs. A strong negative relationship was detected between grain-filling length and maturity, indicating that earliness and smaller leaf number were associated with longer grain-filling duration and yield. Grain moisture was strongly related to grain phenological development and was independent of cultivar. These results reveal that an effective mechanism to increase maize yield is to select for extended grain filling. Breeding schemes in use should consider these selection criteria.

Increasing the drought tolerance in tropical maize - from research on plant-water relations to impacts in farmers' fields.

This presentation gives an overview of CIMMYT’s research on improving the drought tolerance of tropical maize germplasm. Strategic research was initiated more than twenty years ago and has resulted in a breeding methodology that recently produced considerable yield increases under drought in southern Africa. Yield gains mainly originate from increases in partitioning toward harvestable organs.