Alkaloids in White Lupin and Their Effects on Symbiotic N Fixation

Abstract

Seeds of white lupin (Lupinus albus L.), known to potentially fix 150 to 200 kg/ha N via Symbiotic N Fixation (SNF), are classified as sweet or bitter based on their alkaloid contents: sweet lupins contain very low whereas bitter lupins contain high contents of alkaloids. However, precise information about effects of alkaloid content on SNF is not very well unknown. Experiments were conducted to determine if alkaloid content in the seed is related to that in other plant tissue such as leaves and pod shells, characterization of relationship between alkaloid concentration and SNF, and to study variation among 126 white lupin accessions for concentration of alkaloids. In the first experiment, the Dragendorff test, a colorimetric test, was used to categorize field-grown lupin lines into 6 categories: 0, 1, 2, 3, 4, and 5 with 0 representing sweet and 5 representing extreme bitterness with bitterness increasing from 0 to 5. In the second experiment, ten lupin lines differing in their Dragendorff score from 0 to four were inoculated with seven bradyrhizobial strains and studied for SNF in a greenhouse. A numerical scale from 0 to 4 with 1 representing absence of nodules and 4 representing many functional nodules was used as a measure of SNF. In the third experiment, concentrations of alkaloids (Dragendorff scores), oil, and protein were studied in 126 accessions.Results indicated that all progenies with Dragendorff score of 1 for the seeds also had the same score for leaves and pod shells. However, this was not true for other progenies. In the case of progenies with Dragendorff scores of 0, 2, 3, or 4 in the seed, the Dragendorff scores for leaves and pod shells were above and below the seed scores indicating existence of variation for alkaloids in various tissues of the lupin plant. Alkaloid content also had significant effects on root nodulation. The interaction between alkaloid categories and bradyrhizobial strains was non-significant. The root nodulation score for lupin lines in alkaloid categories 0, 3, and 4 were similar, indicating that the same bradyrhizobial strain could be used to effectively inoculate sweet and bitter white lupin lines. The root nodulation induced by the seven bradyrhizobial strains were statistically different. It was observed that S96-A15, S96-A19, and S96-B9 were more efficient bradyrhizobial strains whereas S96-A5 was observed to be less efficient for root nodulation. Significant variation existed among 126 white lupin accessions for Dragendorff score and concentrations of oil and protein. The alkaloid content did not affect oil content in the seed, however, alkaloid content significantly affected protein content. The results indicated that bitter seeds had higher protein content.