Branch development in Lupinus angustifolius L. I. Not all branches have the same potential growth rate

Abstract

Although the basal and uppermost lateral branches of Lupinus angustifolius L. frequently grow and contribute to yield, buds formed in the axils of leaves 6–12 (referred to as middle buds) rarely grow. This may be due to an inherent limitation of these buds, or some form of apical dominance or competition imposed by the plant. The hypothesis that middle buds have the full capacity to grow, but remain suppressed on intact plants was tested. The main stem apex and buds from the axils of leaves 1 and 8 (bud 1 and bud 8) were excised and cultured on sterile agar. The buds were removed from culture and weighed every 2–3 d for 21 d. The growth rate of apices from the main stem was approximately 5.8 mg d−1, compared to 2.4 mg d−1 for bud 1 and 0.9 mg d−1 for bud 8. Buds in the axils of leaves 6–10 on intact plants were painted six times with a synthetic cytokinin, benzylaminopurine, from 40 d after sowing. This promoted rapid elongation and thickening of these buds, visible as early as 5 d after painting began. The rapid growth of these branches was associated with a reduction in the length of the remaining branches on the plant. However, excision of lower branches did not increase the growth of the middle buds. It is concluded that buds 6–12 of Lupinus angustifolius L. have a partial potential to grow. This potential appears to be limited by innate factors in the bud, and may be structural and/or hormonal. The limitation appears to develop very early in the plant, and potential growth is not modified by subsequent nutrition of the plant.