Hematoxylin staining as a phenotypic index for aluminum tolerance selection in tropical maize (Zea mays L.)

Hematoxylin staining is an early indicator of Aluminum (Al) toxicity effect s on the apices of young, developing roots grown in nutrient solution. In t his work, the potential of this technique as a reliable and reproducible ph enotypic index for Al tolerance in tropical maize genotypes was assessed, w ith its performance systematically compared to two other parameters widely used in breeding programs - relative seminal-root length (RSRL) and net sem inal-root length (NSRL). Seeding roots from contrasting genotypes for Al se nsitivity stained remarkably different after 24- and 48-h and 7-day exposur es to 222 mu M Al in nutrient solution, with the Al-dye complex being detec ted in both the outer (epidermis) and inner (cortex) portions of the roots from the sensitive cultivar. Hematoxylin staining was compared to the RSRL and NSRL parameters using 20 families from the third generation of selfing (S3) following the cross between two contrasting inbred lines that had been previously classified by the RSRL index in an independent procedure. The c oloration technique showed the highest capacity to discriminate among toler ant and sensitive genotypes and displayed significant correlation coefficie nts to the other two indexes. Evaluation of the results from diallel crosse s involving nine inbred lines proved that hematoxylin staining was also par ticularly adequate for identifying expressive hybrid vigor, as demonstrated by the general (GCA) and specific (SCA) combining ability estimates obtain ed by using the three indexes simultaneously. Hence, hematoxylin staining o f Al-stressed root apices appears to be a powerful tool to assist in Al-tol erance selection in tropical maize breeding programs.