Growth and yield in Lupinus angustifolius are depressed by early transient nitrogen deficiency

Abstract

Yield and harvest index of narrow-leafed lupin (Lupinus angustifolius L.) are variable, and factors affecting their reliability have not been clearly identified. In this study, plants were grown in sand culture and were non-nodulated and supplied with mineral nitrogen (N) or acquired N through symbiotic N2 fixation. Transient N deficiency was imposed a number of times during development in nodulated plants by flushing pure O2 to the roots to suppress N2 fixation and in non-nodulated plants by changing the rate of N addition. Low N supply (0·4 mM) before floral initiation or for 2 weeks during floral initiation caused a reduction in seed yield. Transient N deficiency induced by O2 flush during early growth (Weeks 6 and 7 after sowing) had a marked effect on vegetative growth, the number of main stem flowers, pod set, and seed yield. The early N deficiency also affected shoot and root N concentrations and total cytokinin concentrations in root exudate. Compared with N2-fixing plants, those supplied with adequate mineral N had a greater flower number and greater branch growth and biomass, but not higher pod set and seed yield, resulting in lower harvest index. Seed N concentrations were also decreased by transient N deficiencies at early (floral initiation), mid (flowering), and late (grain filling) stages. These findings indicate that any field conditions which transiently reduce N2 fixation (e.g. temporary waterlogging or drought) are likely to result in lower grain yield and quality of lupin crops.