Genetic analysis of maize  (Zea mays l.) tolerance to aluminium toxicity and low-phosphorus stress and development of synthetics for use in acid soils of Western Kenya

Dickson, Otieno Ligeyo (BSc, MSc)

Doctor of philosophy in Genetics and Plant breeding of the Department of Biological Sciences Moi University 2007

Abstract

Soil acidity is a major constraint to maize (Zea mays L.) production on tropical soils due to toxic levels of aluminium (Al) and the concomitant phosphorus (P) deficiency that hinder plant growth.  In Kenya acid soils cover 13% of the main maize growing areas. The present study was undertaken to: (i) screen and identify Kenyan maize germplasm for tolerance to Al-toxicity and improved P uptake efficiency under low P limiting conditions in the field; (ii) transfer tolerance to Al toxicity or P uptake efficiency to adapted Kenyan varieties; (iii) study the genetics of inheritance of tolerance to Al-toxicity among Kenyan maize germplasm and (iv) determine the effect of P and/or lime application on maize yield on acid soils of Western Kenya. Tolerance to aluminium among local and exotic genotypes was studied using intact roots of maize seedlings grown in nutrient solution containing 222µM Al at a controlled pH (4.2) and temperature (26°C).  It was found that landrace 203B and inbred lines K4, and K17 had least reduction in root length when grown in a 222µM Al solution and they were comparable in tolerance to the most tolerant Brazilian check (CAT Al 237/67). One inbred line from KARI (Reg 00114) and (L11) and the Brazilian Al sensitive check showed the highest reduction. Screening of several Brazilian, CIMMYT and Kenyan materials led to classification of the Kenyan materials into highly tolerant (203B, K4, K17), moderately tolerant (5A, K15, 1x1, E15, 2A1) and sensitive (Reg 00114, E10, E16) aluminium toxicity phenotypic groups. In addition, the genetics of Al-tolerance studied using solution culture with a 4×4 diallel cross involving two tolerant and two sensitive genotypes indicate that tolerance in K4 and K17 is quantitatively inherited and additive gene effects were more predominant than dominance effects and contributed up to 82% of the total genetic variance, hence its importance over dominance genetic variance. Dominance genetic effects were preponderant where liming was done and the manifestation of additive genetic interacton was not consistent for most of the crosses under the four treatment (No Phosphorus nor lime, +Phosphorus, +Lime and Phosphorus, +Lime) conditions.  Most of the crosses exhibited significant additive × additive genetic effects where lime or lime together with phosphorus was applied.   The results also indicated that aluminium tolerance among the Kenyan maize gerplasm is controlled by about 4-8 genes. Field experiments were done at two sites (Bumala and Kuinet) characterized with low pH and low P. Lime (L) and P was applied at the rates of 4 tons and 36 kgP/ha respectively.  The results showed that addition of phosphorus (P) and phosphorus together with lime (LP) increased yield by 28% and improved other agrobotanical traits such as root growth, plant dry matter and days to anthesis.   The identified Al-tolerant materials from Kenyan maize germplasm are useful for the maize yield improvement in acid soils of Kenya. The detected inheritance of Al tolerance in Kenyan germplasm will be useful in transferring these genes into adapted maize germplasm. This study also that P and lime limits yield of maize in acid soils by 28% and 16% respectively. Generated germplasm populations between Kenyan and Brazilian topcrosses, useful land races and inbred lines will form part of useful maize germplsm for development of Al-tolerant/P-efficient varieties for acid soils. These could also be useful in crosses for other maize breeding programs.