Maize damage has been observed from time to time in many parts of South Africa where registered herbicides were applied. Differential cultivar tolerance to certain herbicides was identified as an important factor in many cases where herbicide selectivity was inadequate. A study was done to investigate the existence of differential tolerance of maize genotypes to selected herbicides. Several maize genotypes were screened for tolerance to selected herbicides, both in the greenhouse and in the field. Maize genotypes demonstrated significant differences in their tolerance to all herbicides. Some genotypes were severely injured by certain herbicides while others were not affected at all by the same herbicide. This suggests that maize genotypes should be screened for tolerance to herbicides in order to reduce crop injury by using only appropriate ones for specific genotypes. Generally, hybrids were more tolerant to herbicides than inbreds, indicating that screening may be most necessary in seed production, where inbreds are used, than in commercial production where only hybrids are used. Tolerance of maize to herbicides was more variable to metazachlor than to other herbicides, and metazachlor also injured more genotypes than other herbicides. This was probably due to differences in herbicide mechanism of action. Shoot or root dry mass reduction of some of the maize genotypes occurred without visual injury symptoms, thus suggesting that visual injury may not reliably indicate susceptibility or tolerance to herbicides. The degree of correspondence of herbicide effects on maize in the greenhouse and the field was determined. Comparison of results from the greenhouse and the field showed that there is positive correlation between herbicide effects in the greenhouse and in the field. There was generally good correspondence of major parameters, such as shoot dry mass and injury symptoms, in the greenhouse and in the field. Similarly these parameters were positively correlated with the grain yield obtained from the field. It appears that shoot dry mass and visual injury symptoms could be good predictors of the yield. This indicates that reliable data could be generated through quicker screening at greenhouse level. A total of four herbicides, metazachlor, dimethenamid, acetochlor and the combination atrazine / metolachlor / terbuthylazine, had significant correlations while only two, flufenacet and acetochlor + atrazine/sulcotrione, had no significant correlations for major parameters with the yield. This indicates that the correlation of data was herbicide-dependent. The influence of temperature on maize tolerance to alachlor, metazachlor and metolachlor was investigated. Results showed that low temperatures reduce the tolerance of maize to these herbicides. This could mean that low temperature may reduce the selectivity of these herbicides. Fluctuating temperature conditions of 10°C at night and 35°C during the day, which are found in some maize producing areas, did not affect maize tolerance to the herbicides. The possibility of improving metazachlor tolerance in maize was also investigated. Evidence provided for possible gene effects on the tolerance of metazachlor indicates that maize tolerance to the herbicide could be improved by crossing tolerant parents. The results suggest that it may be possible to improve metazachlor tolerance by crossing appropriate parent lines with dominant genes for tolerance to metazachlor. Ultrastructural changes in the maize seedling root and shoot cells caused by metazachlor were investigated. In susceptible genotypes root cell nucleoli were found to be abnormally large, empty and more abundant than those in untreated control plants. In susceptible plants the chromatids appeared disorganised in cell nucleoli, and both the nuclear and plasma membranes showed signs of disintegrating. There were more and larger vacuoles in the herbicide-susceptible plants. Leaf cells from the susceptible plants had more empty vacuoles and more chloroplasts with generally disorganised content. The bundle sheath chloroplast membranes were dilated in susceptible plants, and the orientation of the grana was disrupted. In the herbicide-tolerant plants, the ultrastructure was not different from that of all the untreated plants. The established differential tolerance of maize to herbicides necessitates the screening of all genotypes to all registered herbicides in order to recommend specific herbicides for certain maize genotypes. Due to the large number of genotypes that would require screening, techniques that yield reliable data quickly have obvious merit. Pot experiments under controlled conditions, which could be selected to promote herbicide bioactivity, are likely to provide data with which the best possible predictions on the risk of herbicide damage in the field could be made. Based on this requirement, environmental factors that should be considered for greenhouse work are: soil with low adsorptive capacity, soil water content close to the field capacity level, and cool temperatures. When screening for herbicide tolerance, the use of herbicide rates in excess of the recommended rate could obviate the need for special environmental conditions, since all the aforementioned factors basically promote the accumulation of higher than usual amounts of herbicide at the site of action in the plant. Therefore, the use of at least a 2X-herbicide rate in screening experiments is advised.
Thesis (PhD (Plant Production: Weed Science))--University of Pretoria, 2006.