Floral biology of the dioecious species Laurus nobilis L. (Lauraceae)

The present study examines the cytological, physiological, chemical and ecological characteristics of pollen and nectar offered by male and female flowers of the dioecious plant Laurus nobilis. The various phases of floral phenology and the insect pollinators were observed. We used cytological methods to determine anther, pollen and nectary structure. Nectar sugar composition was evaluated by HPLC. Pollen viability in time was compared with cytoplasmic and intine water content. Pollen presentation was found to be reversible by opening and closing of anther valves, determined by hydration of the mechanical layer of the anther. Pollen, covered by pollenkitt, was presented for dispersal for 3 consecutive days and during this time the intine and cytoplasm lost water and pollen viability diminished. At germination exine ruptured together with the outermost layer of the intine. Nectaries of male flowers were observed on the anther filament and on staminodes of female flowers. The nectar consisted almost entirely of sucrose and was more concentrated in male flowers. Secreted through stomata, nectar was presented in a thin layer. In the study area, the main pollinators (about half the total number of all visits) were hymenopterans. Pollen is of the recalcitrant type due to its high water content (>30%) but its viability is long-lasting because the intine is thick and stores water, keeping the cytoplasm of the vegetative cell hydrated and viable, and because anther valves may close under adverse conditions, protecting the pollen. Insects are attracted by male and female flowers similarly, males offer nectar and pollen, whilst females only nectar.