Genetic grain yield improvement of maize (Zea mays L.) in Ontario during the past three decades can be attributed, in part, to increased tolerance to environmental stresses. We have observed a differential response of field-grown old and new hybrids after application of the photosystem II inhibiting herbicide bromoxynil (4-hydroxy-3,5-dibromobenzonitrile). Studies were conducted to test whether tolerance to bromoxynil is associated with stress tolerance and grain yield in maize hybrids representing three decades of yield improvement in Ontario. Experiments were carried out with seedlings of eight maize hybrids grown in pots in controlled-environment growth cabinets and, in one experiment, with seedlings grown in pots outside during the months of July and August at Guelph, Ontario. Bromoxynil was applied to the youngest fully-expanded leaf of plants at the 6- to 8-leaf stage and chlorophyll fluorescence of the treated leaves was measured in the 2- to 24-h period after bomoxynil application. Results showed that the chlorophyll fluorescence ratio Fv/Fm, an indicator of photosynthetic efficiency, declined after bromoxynil application until 4 h after application and, subsequently, recovered slightly during the next 20 h. Large differences in the Fv/Fm ratio were apparent among hybrids in the response to bromoxynil. The Fv/Fm ratio was significantly higher for hybrids released in the 1980s than for hybrids released in the 1970s, and the Fv/Fm ratio of the latter group was significantly higher than that of the four oldest hybrids. Chlorophyll fluorescence ratios Fv/Fm after bromoxynil application in the seedling phase were highly correlated with grain yield of the hybrids in field experiments conducted during 1987 and 1988 (r = 0.91). Results of this study support the contention that maize genotypes that are less sensitive to bromoxynil are less susceptible to environmental stresses. Key words: Zea mays L., stress tolerance, bromoxynil, chlorophyll fluorescence, Fv/Fm ratio, detoxifying agents
Evaluation of genetic potential for various traits in maize hybrids (Zea mays L.) extracted from crosses with CIMMYT germplasm