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Genetic and physiological studies on potassium and nitrogen uptake and utilization in wheat Woodend, John J.


Experiments were undertaken to examine the extent of variation for potassium and nitrogen uptake and utilization in wheat and also to address some issues of relevance to the improvement of these traits. These issues included the inheritance of these traits and the difficulties that could arise due to (1) the methodology that is used to measure ion fluxes and utilization, (2) ontogenetic variation in the expression of these traits, and (3) the growth stage at which nutrient utilization is evaluated. To compare varieties developed during different periods in the history of wheat breeding, the varieties were assigned to five groups on the basis of height and origin. Nutrient fluxes were measured either as average net fluxes or short-term net fluxes. Nutrient utilization was expressed as shoot fresh weight per plant, efficiency ratio or utilization efficiency. Substantial variation was observed for all traits except potassium and nitrogen efficiency ratios. Although short-term net potassium fluxes were negatively correlated with root potassium concentration, some of the differences in flux were not associated with differences in root potassium concentration. These differences must therefore be heritable. Due to the complexity of the regulation of nitrate uptake, genotypic differences in short-term net nitrate flux were not examined in relation to root nitrate concentration. Therefore, some of the variation in nitrate flux could be due to differences in root nitrate concentration or some other factor(s) which regulates nitrate uptake. Significant differences between groups were also observed. The tall varieties had the highest potassium and nitrate fluxes but were not significantly different from the triple dwarfs. The double dwarfs were the poorest performers for both nutrient uptake and utilization. In general, the tall traditional varieties were more vigorous and hence showed the highest shoot weight per plant and utilization efficiencies. These findings are examined in relation to the contention that plant breeding under high fertility conditions may have resulted in a decline in the ability of plants to acquire and utilize mineral nutrients. The inheritance of short-term net potassium flux, shoot weight per plant, potassium efficiency ratio and potassium utilization efficiency was studied in four crosses. Complex modes of inheritance were observed for all the traits. For one of the crosses significant reciprocal effects were observed for shoot weight per plant, efficiency ratio and utilization efficiency. Narrow sense heritabilities for the two traits most likely to be selected for, namely short-term net potassium flux and shoot weight per plant, indicated that selection for these traits should be carried out amongst families rather than amongst single plants. Diallel analysis for nitrate uptake and utilization indicated that both additive and dominance gene effects are important in the determination of these traits. The effect of developmental changes in potassium uptake and utilization on varietal comparisons and genetic studies was investigated by comparing the performance of six varieties at different stages of growth over a five-week period. The rankings of the varieties for short-term net potassium flux and shoot weight per plant were found to be fairly consistent. Correlations between average net fluxes for different time periods as well between short-term and average net fluxes were poor. These findings indicate that selection for differences in uptake should be based on fluxes obtained from solutions identical in concentration to the growth solution rather than on perturbation fluxes obtained by depletion of a solution much more concentrated than the growth solution. All measures of potassium utilization based on vegetative growth were poorly correlated with performance at the adult stage. Significant negative rank correlations between shoot fresh weight per plant and grain weight per plant were obtained most likely due to differences in harvest index. This finding casts some doubt on the usefulness of vegetative measures of nutrient utilization as indicators of nutrient-use efficiency for a crop in which the economic product consists of grain.

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