Apical dominance maximizes reproductive strategies in Lilium longiflorum

Abstract

Axillary meristems are the regenerating insurance of most plant species. They are initiated while the shoot apical meristem (SAM) produces leaves and usually remain inactive until a specific physiological change occurs. This common growth regulation is named “apical dominance”. In bulbous plants, axillary meristems initiate storage organs, which usually contain their own apical meristem. In Lilium longiflorum, an ornamental geophyte, the axillary meristems produce the bulbs of the following season. Large bulbs must get cold in order to flower, while small ones can flower without cold exposure under long-day conditions. Our aim was to clarify the foundation of the different flowering pathways regulated by bulb size, in view of the apical dominance mechanism, and identify its horticultural consequences. We monitored the development of L. longiflorum's bulbs produced from axillary meristems under an array of environmental and physiological alterations, including natural conditions and agrotechniques. The growth rate of L. longiflorum bulbs was highly affected by the developmental stage of the SAM, as well as by SAM decapitation and auxin application. Leaf biomass contributed as well to the final bulb size. Cold exposure quantitatively hastened flowering time and induced bud abortion. Bulb size and floral induction pathway (cold exposure or long-day conditions) affected flowering phenotype of the plants. We suggest a model by which apical dominance plays a major role in the reproductive strategy of L. longiflorum by regulating its prospective flowering pathways: either vernalization or photoperiod. This new concept enlightens the benefits of apical dominance for the plant growth-cycle under changing environments and disturbed habitats. Based on these results, we propose a novel horticultural protocol for lily forcing, taking into account the bulb size and the flowering pathway.