1. Acid soils in the world 4 48 developing countries with 1.7 billion ha 4 43% worlds tropical land area 4 38% of tropical Asia Indonesia, Thailand, Malaysia, India, China, and the Philippines. 4 27% of tropical Africa Ivory Coast, Zaire, Zambia, Tanzania, Uganda and Zimbabwe. 4 10% of Central America, Caribbean and Mexico
1. Acid soils in the world 4 In South America 80% of agricultural area Brazil, Colombia, Ecuador, Peru and Venezuela Brazil, Colombia, Ecuador, Peru and Venezuela 4 Colombia Llanos Orientales 17 millions ha. 17 millions ha. 4 Brazil acid savannas 205 million ha of which 112 are suitable for agricultural Ref: Sanchez, 1977; Torres et al., 1997
2. What are the acid soils? 4 Aluminum ( Al) and Manganese (Mn) toxicity 4 pH < Deficiency Ca, Mg, P, Mo, and Fe 4 Al saturation > 35% 4 P < 16 p.p.m Ref: Granados et al., 1993; Duque-Vargas et al., 1994.
Environmental characteristics for some acid soils Country Colombia Brazil India Indonesia Site Carimagua Sete lagoas Meghalaya Sitiung pH Al P %mg kg -1 Source: Granados et al., 1993; Pandey et al., 1994.
3. The problem 4 Between 8-20 million ha are planted 4 Maize is more susceptible than rice, wheat, sorghum, cotton and soybean. 4 Maize produces fewer and smaller roots 4 Reduces survival and function of micro- organisms in the soil => organic matter => availability of N, P, S 4 A survey of 48 developing countries only five are conducting research Ref: TropSoils, 1991; Pandey and Gardner, 1992; Tan, 1993.
4. Some solution approaches 4.1 Liming 4 Lime is a reliable soil amendment 4 For poor farmers is not an economic option 4 At the sub-soil level is difficult 4 Temporal solution 4 Incompatible with conservation tillage Ref: Pandey and Gardner, 1992; Pandey et al., 1994.
4.2 Genetics of tolerance to soil acidity 4 Acid tolerant varieties are reliable, permanent, economical, and environmental clean solutions 4 Several authors have reported genetic variation for tolerance to acid soils in maize. Quantitative inheritance: Magnavaca et al., 1987; Pandey et al., 1994; Borrero et al., 1995; Salazar et al., Qualitative inheritance: Rhue et al., 1978; Miranda et al., 1984.
4.2 Genetics of tolerance to soil acidity 4 Additive genetic variance is generally most important in yield: Magnavaca et al., 1987; Pandey et al., Dominance genetic variance has been also reported: Duque-Vargas et al., 1994; Borrero et al., 1995.
4.2 Genetics of tolerance to soil acidity 4 CIMMYT has conducted several field-based studies to determine inheritance of yield: GCA ** accounting for 89% of the genotypic variation and SCA n.s for yield. Heritability, estimated using half-sib family mean averaged 38% for yield. They suggested that recurrent selection would be effective. Ref: Duque-Vargas et al., 1994; Pandey et al., 1994, and Borrero et al., 1995.
4.3 Physiology of tolerance 4 Aluminum (Al) toxicity is the most severe limiting factor: Foy, Al resistance is prerequisite for adaptation to acid soils. There are some evidences in wheat and barley. 4 P and water deficiencies are correlated with Al toxic effect. 4 Root elongation of seedlings, in nutritive solutions + Al, can be measured: Horst et al., 1992.
4.4 Populations: Recurrent Selection 4 Several authors have reported good results using recurrent selection to improve maize yield on acidic soils. Magnavaca et al., (1987), in Composto Amplo after four cycles half-sib selection, reported significant yield improvement Granados et al., (1993) in SA3 after 14 cycles MER and two FS reported yield improvement of 2% and 14% per cycle, respectively.
4.4 Populations: Recurrent Selection Ceballos et al., (1995) in five populations and two acidic and one no-acidic environments reported 4.72% per cycle. 4 CIMMYT-NARS has developed six maize populations: SA3, SA4, SA5, SA6, SA7 and SA8 Narro et al., (1997) diallel study conclude: (SA3 + SA5 ) x SA4 and (SA7 + SA8) x SA6 ETO x Tuxpeño was used as heterotic pattern
4.4 Populations: Recurrent Selection Granados et al., (1994) compared SA3 and Tuxpeño across 20 sites, with a wide range of acidities, SA3 range from 96 to 1500% of Tuxpeño. SA3 was released in Colombia as ICA SIKUANI V-110 Brazil has released : 3 hybrids and some varieties Indonesia: One variety
4.5 Inbred lines 4 CIMMYT-NARS and CNPMS/ENBRAPA are developing maize inbred lines. These lines are being used in: are developing maize inbred lines. These lines are being used in: - Hybrids - Hybrids - Inheritance and physiological studies - Molecular markers.
4.6 Molecular markers 4 At this level only a few reports are available in maize for tolerance to tropical acid soils Torres et al., (1997) worked F 2 population derived from L53 x L1327 (CNMPS/EMBRAPA) with bulked segregant analysis (BSA) and RFLP markers. They concluded that there is a region on Chromosome 8 related to aluminum tolerance
4.6 Molecular markers Arias et al., (1997) In tolerant and susceptible S 6 lines from SA3, SA4, SA5, and Tuxpeño Sequia C8 populations AFLPs were applied. They did not find molecular differences between susceptible and tolerant lines. They recommended to include new lines, new probes and increase endogamy level at the lines.
5. Summary 4 Acid soils with 1.7 billion ha cover a significant part of a least 48 countries. 4 Maize is one of the most susceptible crop. 4 Lime is one reliable but expensive and temporal solution. 4 Acid tolerant varieties are one reliable, permanent, economical and clean solution.
5. Summary 4 Both quantitative and qualitative genetic variation have been reported. 4 Additive genetic variance is more important but dominance is present. 4 Aluminum (Al) toxicity is the most severe limiting factor 4 Recurrent selection has been effective to improve maize yield on acidic soils.
5. Summary 4 Maize inbred lines are being development in order to use them in: Hybrids, inheritance / physiological studies, and molecular markers. 4 There is a region on Chromosome 8 possibly related to aluminum tolerance
6. Future Research 4 In general trials in acid soils have: high experimental error, which reduces heritability and gains from selection. 4 Soil acidity involves: H +, Al, Mn toxicities and deficiencies of P, Ca, Mg, and OM. 4 More multi-environmental field testing are needed. 4 Mechanisms for tolerance and efficient screening techniques must be research.
6. Future Research 4 Isogenic and near-isogenic lines must be use in molecular studies with RFLP, RAPD and SSR. 4 Physiological mechanisms of Al tolerance and P uptake and utilization must be studied Field data must be supplemented with Molecular and Physiological information Field data must be supplemented with Molecular and Physiological information
4 Extension- Expansion- Overlap of the networks working in the tropical acid soils. Hannover University: Germany, France, Spain, Brazil, Guadeloupe, and Camerun CIMMYT- NARS - NGOs: Brazil, Colombia, Peru, Venezuela, Malawi, Ivory Coast, Indonesia, The Philippines, Thailand, Vietnam 4 Consortium on Developing Maize Cultivars for Sustainable Production System in Acid Soils: 5 years and ~ 4 U$ million
ACKNOWLEDGMENTS CIMMYT Dr Shivaji Pandey Dr Carlos De Leon Dr Luis Narro Mr. Juan C Perez ISU Dr Arnel R. Hallauer Dr Michael Lee CORPOICA MV. Sony Reza Mr. Jose G. Ospina Mr. Guillermo Torres CNPMS/ EMBRAPA: Mr. Sidney N. Parentoni NARS of CIMMYTs network for acid soils University of Hannover: Dr. W. J. Horst