Leaf physiological aspects of nitrogen-use efficiency in Brassica campestris L.: quantitative genetic variation across nutrient treatments.

Quantitative genetic parameters for leaf physiological and whole-plant aspects of nitrogen-use efficiency in Brassica camprestris L. were estimated in three nutrient treatments in the greenhouse. Narrow-sense heritabilities and genetic correlations varied across treatments for some traits. Sire effects were significant for leaf nitrogen content in near-optimal and super-optimal, but not in suboptimal nutrient treatments. Additive genetic variation for two estimates of leaf physiological nitrogen-use efficiency (nitrogen-based photosynthetic capacity and leaf carbon: nitrogen ratio) was significant only in the suboptimal nutrient treatment. Area-based photosynthetic capacity, on the other hand, exhibited no heritable variation in any nutrient treatment. Heritability estimates of aboveground biomass and flower production were greatest in sub- and super-optimal treatments, respectively. Negative genetic correlations between leaf nitrogen content and both estimates of leaf nitrogen-use efficiency were evident in the super-optimal treatment. Aboveground biomass and leaf nitrogen-use efficiency were positively correlated in the suboptimal treatment, suggesting that growth differences were due in part to the efficiency with which nitrogen was utilized in physiological processes. Although implications for breeding may differ for different sources of germ plasm or different measures of performance or yield, selection for improved whole-plant performance through increased nitrogen-use efficiency should proceed best in suboptimal nutrient treatments.