Field screening and genetic evaluation of divers genetic material on acid tropical soils using standardised experimental conditions at several field sites in the tropics, carefully characterized for soil characteristics. Further improvement of quick laboratory screening techniques for Al resistance (root elongation, callose formation, hematoxylin staining) and correlation to field screening.
Long-term field experiments are established at the tropical sites on acid soils. Factorial treatments comprise 2 lime rates, 2 P rates, 3 organic manure rates, and 3 maize cultivars differing in adaptation to soil acidity. Measurements to be taken each year are: analysis of soil chemical and biological characteristics, shoot and root growth, yield, nutrient status and uptake of maize. In addition, in field and laboratory studies, different factors affecting seedling viability in an acid soil environment such as localised lime application and N amount and source will be studied in more detail.
A mathematical model based on two variable charge exchangers (soil, root) linked by the soil solution will be further developed and validated under controlled conditions and evaluated by comparison with data measured under field conditions. Possible interacting effects of N source, Mg deficiency, Mn toxicity, and organic soil compounds on Al toxicity will be taken into consideration.
The fundamental role of organic ligands and polyamines and the possible role of polypeptides and proteins in Al toxicity and resistance of maize cultivars is studied under controlled conditions. A technique will be developed which allows the rapid characterization of the P efficiency of maize cultivars, and possible physiological mechanisms of P efficiency in relation to Al resistance will be studied.
It is expected that the project contributes to the development of maize cultivars and accompanying agronomic technologies for natural recource-friendly, sustainable, and economic cultivation of maize on the acid soils of the world with special emphasis on South America.
Germplasm with improved adaptation to soil acidity, both Al-resistant and P-efficient, was identified and genetic progress for adaptation to soil acidity assessed.
In close cooperation between physiologists and breeders germplasm was characterised for Al resistance using different quick screening methods The complex long-term field experiments in Colombia and Cameroon clearly showed that adapted cultivars and organic amendments can at least partly substitute for lime application, but high productivity and sustainability can only be achieved by both genetic and agronomic strategies. The developed soil ionic exchange model highlights the importance of the processes of dissolution and diffusion of Al in the rhizosphere. The physiological studies revealed that Al excretion of organic ligands, low Al-binding capacity of root-tip cell-walls, and sequestration of Al in vacuoles are involved in Al resistance.
In conclusion, the better understanding of the inheritance and of the physiological mechanism leading to acid soil tolerance in general and Al resistance and P efficiency as principal requirements for adaptation to such soils, specifically, lead to the development of quick screening techniques and allowed the better focus of molecular approaches thus facilitating and enhancing the progress made by breeding. Consequently, new varieties with improved yields on acid soils have been released in Brasil and Cameroon. The particular role of adapted germplasm in addition to agronomic management strategies such as organic manuring, liming and nutrient application have been demonstrated.