Abstract:
Breeding maize (Zea mays L.) for tolerance to acidic soils could improve maize yields. The current study aims to identify maize genotypes with tolerance to acidic soils, as well as identifying secondary traits associated with the tolerance to soil acidity. Ten maize varieties were screened for tolerance to aluminium (Al) toxicity under glasshouse, laboratory and field conditions. In the glasshouse, two soil acidity levels (limed and unlimed soil) were used and the experiment was set up in a complete randomised design (CRD) with three replications. The experiment lasted for 10 days and measurements were taken on plant height (PH), leaf area, stem diameter and dry matter. In the laboratory, a haematoxylin staining (HS) experiment was conducted to determine the response of 10 maize varieties to Al toxicity. Two Al concentrations (0 and 222 μM) were used and the experiment was set up in a completely randomized design with three replications. After 7 days, shoot length, was recorded. Five stress tolerance indices were estimated to determine the resilience of each genotype. A root growth stress tolerance index was also computed for both experimental procedures. In the field, two trials were established at two sites, namely Mbinja and Mpumaze. Limed and unlimed plots were used, and the trial was set up in a randomized complete block design with three replications. Maize kernel yield and other standard field parameters were recorded. Selection of tolerant genotypes from the field screening was also done using three indices, namely harmonic mean (HM), stress tolerance index (STI) and stress susceptibility index (SSI).
Both the glasshouse and laboratory assays identified similar genotypes of maize as being tolerant. These tolerant genotypes were Ngoyi, PANBG3492 BT, PAN 6Q408 and PHB 3442 based on the root growth stress tolerance index (RGSTI). It was therefore demonstrated that these two assays produced the same level of efficiency in identifying tolerant genotypes using this index. Based on ranking of seedling vigour index under soil acidity stress, the top three genotypes at Mpumaze were PHB32W71, PAN6616 and Sahara while at Mbinja, the top three were PAN6616, PAN6Q408 CB and PAN6P110. The genotypes PANBG3492 BT, PAN6Q408 and PHB3442 were also found to be tolerant to acidic soils at seedling stage. These genotypes are recommended for further evaluation in more sites to confirm their tolerance and yield potential under acidic soils.
The study also revealed that plant height, leaf area and stem diameter could be used for indirect selection for tolerance to Al toxicity under glasshouse conditions. The seedling vigour index was also effective in identifying tolerant genotypes under glasshouse conditions. On the other hand, shoot length stress tolerance index and the haematoxylin score were useful for indirect selection for tolerance to Al toxicity in the laboratory. In the field, it was observed that ear length, leaf area and ear diameter can be useful in identifying genotypes that are tolerant to soil acidity. They can therefore be useful as indirect selection criteria under field conditions. Additionally, the best selection indices for identifying soil acidity tolerant genotypes under field conditions were the HM and the STI. It is recommended that varieties that were identified as tolerant be further evaluated in several soil acidity hot spots to confirm their tolerance and stability of performance under field conditions.
