Exogenous Cytokinin and Nitrogen Do Not Increase Grain Yield in Narrow-Leafed Lupins

Application of exogenous cytokinin N6-benzylaminopurine (BAP) to flowers at anthesis increases pod set but not grain yield in lupins (Lupinus angustifolius L). This study aimed to examine whether supplemental nitrogen supply could enhance the fulfilment of enlarged sink capacity induced by BAP application and thus increase seed yield. An abscission-minus lupin mutant (cv. Danja) was grown in pots under glasshouse conditions. Application of BAP to all flowers produced more pods on the main stem, but not on the lateral branches, with four to 13 more total pods set on the BAP-treated plants than on the control plants. When BAP was applied to main stem flowers only, the number of pods on the main stem increased from four to 18, but pods on the lateral branches decreased from 19 to 11. In contrast, when BAP was applied to lateral flowers only, the number of pods on the lateral branches increased. However, the increase in pod set induced by BAP caused a decrease in the number of seeds per pod and average seed weight, and as a consequence, plant seed yield was not affected by BAP. Urea fertilizer caused much more shoot N accumulation than did symbiotic N2 fixation, but this shoot N enrichment did not result in an increase in seed yield or seed N content. These findings indicate that there is a limit to grain yield and N harvest index in lupins. Although exogenous application of cytokinin can increase sink strength and cause a redistribution of yield components between the main stem and lateral branches, it did not increase the grain yield of the whole plant under these experimental conditions.