Carlos De León

Low-P tolerance by maize (Zea mays L.) genotypes: Significance of root growth, and organic acids and acid phosphatase root exudation

We investigated some mechanisms, which allow maize genotypes to adapt to soils which are low in available P. Dry matter production, root/shoot-ratio, root length and root exudation of organic acids and acid phosphatase were investigated in four maize genotypes grown under P-deficient and P-sufficient conditions in sterile hydroponic culture. A low-P tolerant, an acid-tolerant and a low-P susceptible genotype of maize were compared with a Swiss commercial cultivar. The study found increased root development and increased exudation of acid phosphatase under P-deficient conditions in all maize genotypes, except for the Swiss cultivar. Effects on root formation and acid phosphatase were greater for the low-P tolerant than for the low-P susceptible, and the acid soil tolerant genotypes. Organic acid contents in root tissues were increased under P deficiency and related to increased PEPC activity. However, the increase in contents was associated with an increase in exudation for the low-P tolerant genotype only. The low-P susceptible genotype was characterized by high organic acid content in roots and low organic acid exudation. The organic acids content in the phloem exudates of shoots was related to root exudation under different P supply, to the difference between lines in organic acids root content, but not to the low-P tolerance or susceptibility of maize genotypes.

Assessment of aluminum sensitivity of maize cultivars using roots of intact plants and excised root tips

Maize cultivars (Zea mays L.) were evaluated for their aluminum (Al) sensitivity using intact plants and excised root tips exposed to 25 μM Al in nutrient solution of low ionic strength and pH 4.3. Aluminum supply increased callose formation and Al concentrations in root tips of intact plants as well as in excised root tips. Using intact plants, differences in Al sensitivity among cultivars could be characterized by Al-induced callose formation, Al-induced inhibition of root elongation, as well as Al contents in root tips as parameters. Significant correlations between Al-induced callose formation and Al contents in root tips (r2 = 0.64**) and inhibition of root elongation (r2 = 0.80***) were found. Excised root tips did not show a significant Al-induced inhibition of root elongation. While average Al-induced callose formation was similar for root tips of intact plants and excised root tips, mean Al contents in excised root tips were up to 1.5-fold higher than in root tips of intact plants after 24 h of Al treatment. Aluminum-induced callose formation as found in excised root tips did neither correspond to Al-induced callose formation nor to inhibition of root elongation of intact plants. The addition of 10 mM glucose to the incubation medium led to a significant increase in the elongation of excised root tips and a 2-3-fold increase in Al-induced callose formation. Staining with triphenyl-tetrazolium-chloride (TTC) revealed increased viability of these root segments. However, these effects of glucose supply did not improve the characterization of the cultivars for Al resistance. The results presented suggest that Al exclusion mechanisms expressed in root tips of intact plants might be non-operational in excised root tips. Therefore, the characterization of maize germplasm for Al resistance using excised root tips appears not to be reliable. Charakterisierung der Aluminium-Empfindlichkeit von Mais-Genotypen an intakten Pflanzen im Vergleich zu abgeschnittenen Wurzelspitzen Die Aluminium (Al)-Empfindlichkeit von Mais-Genotypen (Zea mays L.) wurde an Wurzeln intakter Pflanzen im Vergleich zu abgeschnittenen Wurzelspitzen durch Angebot von 25 μM Al in Nahrlosung mit geringer Ionenstarke und pH 4,3 charakterisiert. Aluminium-Angebot fuhrte sowohl bei Wurzeln intakter Pflanzen als auch bei abgeschnittenen Wurzeln zu vermehrter Kallose-Bildung und erhohten Al-Gehalten. Bei Wurzeln intakter Pflanzen konnten ubereinstimmende Unterschiede in der Al-Empfindlichkeit sowohl durch das Ausmas der Al-induzierten Kallose-Bildung, der Al-Gehalte in den Wurzelspitzen als auch der Hemmung des Wurzellangenwachstums festgestellt werden. Die Kallose-Gehalte waren mit den Al-Gehalten (r2 = 0,64**) und mit der Hemmung des Wurzellangenwachstums (r2 = 0,80***) signifikant positiv korreliert. Bei abgeschnittenen Wurzelspitzen hemmte Al-Angebot das generell geringe Wurzelwachstum nicht. Wahrend die Al-induzierte Kallose-Bildung vergleichbar zu intakten Wurzeln war, waren die Al-Gehalte nach 24 h Al-Angebot um den Faktor 1,5 erhoht. Aluminium-induzierte Kallose-Bildung in abgeschnittenen Wurzeln war nicht korreliert mit der Kallose-Bildung und der Hemmung des Wurzellangenwachstums bei intakten Pflanzen. Zusatz von 10 mM Glukose zum Inkubationsmedium fuhrte zu einer signifikanten Forderung des Langenwachstums und einer 2-3fach erhohten Kallose-Bildung bei abgeschnittenen Wurzeln. Anfarbung der Wurzelspitzen mit Triphenyl-Tetrazolium-Chlorid (TTC) zeigte eine stark verminderte Vitalitat 24 Stunden nach dem Abschneiden. Der Glukosezusatz verbesserte jedoch nicht die Klas-sifikation der Genotypen nach Al-Empfindlichkeit. Die Ergebnisse weisen darauf hin, dass der fur Al-Resistenz in-takter Pflanzen verantwortliche Mechanismus der Diskriminierung von Al von der Aufnahme in die Wurzelspitzen in abgeschnittenen Wurzeln beeintrachtigt ist. Daher ist ein Screening von Mais-Geno-typen auf Al-Resistenz mit abgeschnittenen Wurzelspitzen nicht zuverlassig moglich.

Implications of Soil-Acidity Tolerant Maize Cultivars to Increase Production in Developing Countries

About 3950 million ha (30%) of the total ice-free land area in the world is under acid soils. Acid soils are characterized by low fertility caused by high levels of Al, Mn, and Fe, and deficiencies of P, Ca, Mg, K, S, and Zn. Of these, Al toxicity and P deficiency seem to be the most important causes for low maize yields (about 400 kg/ha for land race cultivars) in these soils. Maize is a staple for millions of people in developing countries where imports are growing up by 1.5 million tons (7%) per year. There are at least two alternatives to increasing maize production in acid soils. The first is to use amendments (lime, gypsum) to correct soil acidity. This is expensive and not available for small farmers. Another disadvantage is that only the upper 30 cm of soil is corrected making maize roots to be concentrated in that layer and not growing beyond. The second approach is to develop tolerant cultivars. This solution is relatively inexpensive, environmentally clean, permanent, and energy conserving. International Center for the Improvement of Wheat and Maize (CIMMYT), in collaboration with several National Agriculture Research NARS all over the world, has been developing soil-acidity tolerant maize cultivars to increase maize production. For this purpose, acid soil tolerant maize populations were formed and recurrent selection was used to improve these populations for grain yield under both acid and normal soils. Cultivars developed from these populations show a consistent increase in grain yield in both acid and non-acidic soils. Under acid soils, the average grain yield of genotypes used to form the base population in 1977, was below 0.4 t/ha. The average grain yield of acid soil tolerant open pollinated varieties (OPVs) developed in 1993 and evaluated across 13 acid soil environments was 3.2 t/ha, while that of non conventional hybrids developed in 1995 and evaluated across six acid soil environments was 3.84 t/ha. In acid soils, high parent heterosis of up to 42.5% has been observed in inter-variety crosses and up to 261% in single crosses. Also, although superiority of hybrids compared to OPVs has been reported, OPVs will continue to be more important than hybrids for the acid soils over the next years, due to the poor economic conditions of farmers cultivating them. From our agronomic research, response to P of Sikuani, an acid soil tolerant cultivar, is higher than that of Tuxpeno (a susceptible cultivar) both in acidic and non-acidic soils. Grain yield of maize was higher using amendments with Ca, Mg, and S than with Ca and Mg only, although there were no differences among methods of application of amendments. Studies of microelements in acid soils showed a highly significant response to Zn. Planting maize in association with pastures in savannas could be a profitable alternative for farmers allowing them to raise more animals/ha with an associated gain of weight/animal. We are also looking for morphological and biochemical traits that may improve grain yield. Molecular and physiological studies are being continued to improve the efficiency of our breeding program. These activities are being developed in collaboration with prominent universities and research institutions worldwide.

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.

Genetic effects for maize traits in acid and non-acid soils

Breeding programs for acid-soil tolerance are desirable as a relatively inexpensive and permanent way for increasing maize (Zea mays L.) yield on these soils. Our objective was to compare the genetic effects controlling the expression of maize traits in acid and non-acid soils. Seven related and one unrelated inbred lines, with different levels of tolerance to acid soil, and their F 1 ,F 2 ,B C 1 , and BC 2 generations were evaluated in four acid and two non-acid soils. Estimates of additive, dominance, and epistatic effects were computed for grain yield, plant height, days to mid-silk, and prolificacy, using the generation means analysis procedure. For all traits the major part of the variation was accounted for by additive and dominance effects, with dominance effects being more important than additive and epistatic effects for both acid and non-acid soils. Epistatic effects were significant for some crosses only, being more pronounced for plant height than for the other traits. Furthermore, epistatic effects were randomly distributed among the crosses and were not related to the grain yield of the single-crosses (F1’s) and to the genetic relationships of the inbreds in either type of soil. The results suggest that similar pooled gene effects control the expression of the traits assessed in both acid and non-acid soils.