G.O. Edmeades

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.

Improvement for tolerance to low soil nitrogen in tropical maize I. Selection criteria

Abstract Inadequate nitrogen supply limits maize production in much of the tropics because inorganic fertilizer is unavailable or is often costly relative to the expected returns. The objective of this study was to evaluate potential selection criteria for improving the tolerance of maize cultivars to low soil N supply. Relationships among primary (grain yield) and secondary traits were examined at two N levels among full-sib families forming part of two selection cycles (C 0 and C 2 ) of a recurrent selection scheme in the tropical maize population Across 8328 BN. This population was undergoing improvement for grain yield under low soil N while maintaining yield potential under fertile conditions. The phenotypic correlations ( r p ) between grain yields at +N and −N, among full-sib progenies, were weak ( r p =0.11 to 0.38, with 224 to 251 df), though the genetic correlation ( r g ) was stronger ( r g = 0.51). Significant values of r p between grain yield under low N and ear-leaf chlorophyll concentration, ear-leaf area, plant height, the anthesis-silking interval and senescence rate were detected under low soil N (−N). These associations were less strong when traits were measured under high soil N (+N). Genetic and phenotypic correlations were generally similar in sign and magnitude, except in the case of ear-leaf chlorophyll concentration, which showed no genetic correlation with grain yield −N, even though the value of r p ranged from 0.45 to 0.74. Divergent full-sib selection was performed for grain yield and for correlated secondary traits in C 0 and C 2 of Across 8328 BN. When divergent selections from both cycles were evaluated under two N levels, the largest grain yields under − N were obtained from direct selection for that trait. Simultaneous selection for yield and secondary traits in C 0 resulted in increased biomass production at both N levels. The realized heritability ( h 2 ) for grain yield at − N was: 0.32 to 0.58; grain yield at + N: 0.20 to 0.46; grain yield across N levels: 0.16 to 0.27; ear-leaf area at − N: 0.18 to 0.74; ear-leaf chlorophyll concentration at − N: 0 to 0.21; and green leaf number below the ear at − N: 0.13 to 0.23. These results indicate that selection for yield and correlated traits under low N should result in improved maize performance in the low-N target environment, with modest increases in yield potential in fertile environments.

Improvement for tolerance to low soil nitrogen in tropical maize II. Grain yield, biomass production, and N accumulation

A program of full-sib recurrent selection to improve maize grain yield under conditions of low soil N, while maintaining grain yields under high soil N, was conducted for three cycles in the lowland tropical population, Across 8328. Superior families were identified from an index of traits comprising high grain yields under high (200 kg N ha−1) and low N (zero applied), and, under low N, high chlorophyll concentration per unit ear-leaf area, slow leaf senescence, and increased plant height. An attempt was made to keep plant height and time to flower under high N unchanged. The objective of this study was to evaluate changes which resulted from this selection program. Cycles 0, 1, 2, and 3 were evaluated under two N levels (0 and 200 kg N ha−1) in four seasons. The per cycle linear increase in grain yield under low N was 2.8% (0.075 Mg ha−1) (P <0.10), and under high N was 2.3% (0.137 Mg ha−1) (P<0.01), indicating that improved performance at low N is not incompatible with yield gains under high N. Increased grain yields were associated with significant linear increases per cycle, measured across N levels, in kernels ear−1 (4.6 kernels ear−1; 1.6%), plant height (8 cm; 4.4%), days to anthesis (0.3 d; 0.4%), aboveground biomass at silking (0.182 Mg ha−1; 3.5%) and at maturity (0.205 Mg ha−1; 1.9%), and N loss from vegetative parts during grain-filling (0.14 g m−2; 4.8%). Across N levels, there was a significant decrease in the number of florets formed per ear (−8 florets ear−1; −1.5%), but an increase in the proportion of florets that formed kernels (0.016 cycle−1; 3.1%). Leaf senescence rate decreased with selection. Selection cycles differed significantly in total N uptake and patterns of N and biomass accumulation with time, but no consistent trends were observed for those traits. Selection for performance under low N in elite maize germplasm appears to improve the efficiency with which N is utilized to produce biomass and grain.

Selection for the improvement of maize yield under moisture-deficits

Abstract 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 ( r=-0.39 ∗∗ ). Tnterval between male and female flowering ( r-−0.73 ∗∗ ) and rate of foliar senescence ( r=−0.39 ∗∗ ). 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 ( r=−0.73 ∗∗ ) 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.

ADAPTIVE STRATEGIES IDENTIFIED AMONG TROPICAL MAIZE LANDRACES FOR NITROGEN-LIMITED ENVIRONMENTS

Abstract Landraces of maize ( Zea mays L.) may serve as components of source populations for traits with adaptive value for nitrogen-deficit environments because traditionally they have been managed at more restricted levels of soil fertility than those used during the development of improved cultivars. Grain yields, N uptake, and N partitioning patterns of 38 landrace accessions from CIMMYTs germplasm bank and 26 improved tropical cultivars were compared under adequate and limited levels of soil N in the winter season at Poza Rica, Mexico. The improved cultivars generally outyielded the landraces at both N levels by an average of 56%. Improved cultivars were not consistently superior, however, in total N recovery, in aboveground biomass or in the fraction of N partitioned to the grain under limited N. The landraces had greater grain N concentrations at both N levels. It is concluded that landraces can contribute useful traits for stable production in N-limited environments, but that selection on the basis of grain yield alone may be insufficient. As a part of routine screening activity, a further 171 accessions were evaluated at the same two N levels, 112 in the summer, and 59 in the winter season. A preliminary principal components analysis was used with landrace data from each season to identify key traits related to N uptake and allocation patterns. Based on these traits, cluster analysis identified six groups within each cropping season. Each cluster was divided into early and late-flowering groups, and principal components analysis was repeated within each subgroup on these variables. Some clusters performed well under adequate N but not under limited N, while others showed the opposite response. This indicated specific adaptation to N environments, in which total N accumulation, N partitioned to the grain, and grain N concentration varied independently. In some cases, the clusters revealed an association with geographical areas where accessions were collected, but no relationship was observed between precipitation at the collection sites for a cluster and crop performance. When breeding maize for large and stable grain yield under N-limited conditions a reasonable strategy would be to develop early and late-maturing source populations from landraces that exhibit large N uptake, partition a large proportion of dry matter and N to the grain, and maintain a large grain N concentration under limited N supply.

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.

Recent advances in breeding for drought tolerance in maize

Drought is an important cause of instability of national maize grain yields and of the food supply and economy of small-scale maize-based farming systems in the tropics. Water shortages affect maize yields throughout the crop cycle, but most severely at flowering and to a lesser degree at establishment. Maize is often grown in environments thought to be better suited to sorghum and millet, yet because farmers in these areas persist with maize, researchers have an obligation to help stabilize maize production. Recurrent selection, which focused mainly on increasing tolerance to drought during flowering and grainfilling, has successfully increased grain yield over a wide range of moisture regimes. Gains ranged from 80-108 (mean 94) kg ha−1 yr−1 when selecting among full-sib (FS) families to 73–144 (mean 111) kg ha−1 yr−1 when selecting among S, families, and were observed when selections were evaluated at a drought-induced yield level of 1.5–2.4 ton ha−1. This represents annual gains in severely droughted environments of >5%. Under well-watered conditions where yields ranged from 5.6-8.0tonha−1, gains from the same two selection schemes ranged from 38–108 (mean 73)kgha−1 yr−1 and from 27–89 (mean 59)kgha−1 yr−1, respectively. Gains in yield were mainly due to increased numbers of grains per plant, and were associated with a reduced anthesis-silking interval (ASI) and an increased harvest index. These changes are consistent with increased assimilate partitioning to the ear at flowering, a phenomenon that will need to be addressed by maize models that focus on genotypic responses to stress. Little change was observed in plant water status or foliar senescence rates. Drought-tolerant varieties have shown similar gains when evaluated under low N conditions, suggesting that partitioning of N to the ear has also been improved. The challenge now is to develop drought-tolerant hybrids. Conventionally improved populations have provided a lower frequency of drought-tolerant hybrids than their counterparts with a history of improvement for drought tolerance. Molecular marker-based linkage maps of ASI, an easily observed indicator of drought tolerance at flowering, suggest that this trait could be efficiently transferred to elite inbred lines using marker-assisted backcrossing techniques. Yields of drought-tolerant varieties, lines and hybrids, when grown under well-watered conditions, have shown clearly that drought tolerance does not come at the cost of yield potential. The key to efficient improvement for drought-tolerance lies in the choice of elite adapted germplasm, use of carefully managed drought stress, and selection based on a minimum data set comprising anthesis date, ASI, ears per plant and shelled grain yields. This results in stabilized and improved yield for the maize component of maize-based cropping systems exposed to mid-season and terminal drought in highly variable rainfall environments.