Some melliferous aspects of cohabiting lamiaceae species competing for efficient pollinators

Abstract

Some flower morphophysiological and pollinator-luring features as well as co-adaptive traits, including nectar production, nectaries and pollen of five Lamiaceae species, namely Hyssopus officinalis, Lavandula officinalis, Melissa officinalis, Salvia officinalis and Salvia sclarea, were studied, with reference to the pollinator visits, especially considering honey bee forage preferences. Although these species are cultivated in plantation for medicinal purposes, at the same time they represent a significant source of pollen and nectar for honey production, therefore the research aimed to estimate their melliferous potential, attractiveness and significance for bee pasture. The pollen grains are isopolar, radially symmetric, predominantly hexacolpate in all species, but often mixed together with tetracolpate and octacolpate in S. sclarea. They are mostly medium-sized to large. The smallest grain was found in L. officinalis and the largest one in Salvia sclarea. The exine sculpturing is bireticulate (H. officinalis, M. officinalis, S. officinalis), reticulate (to foveolate) (S. sclarea) and foveolate (to perforate) (L. officinalis). The shape of pollen grains in equatorial view varies among different species, and within the same species. In the majority of studied species, shape is most frequently prolate (L. officinalis - type a, M. officinalis, S. officinalis and S. sclarea -Class I), except in H. officinalis and S. sclarea - Class II, in which they are sub-prolate and L. officinalis - type b in which they are oblatespheroidal Floral nectaries are located on the receptacle, surrounding the ovary base in the shape of an asymmetrical annular disc with four lobes. Unevenly distributed modified stomata were observed on the upper part or the entire surface of the functional, and generally, the most developed nectary lobe facing the lower corolla lip. The nectariferous tissue is composed of the uniseriate epidermis overlying several layers of polygonal cells forming subepidermal secretory tissue. Epidermal cells are lined with very thin cuticle. Xylem elements alone, separating from the vasculature of the flower receptacle, were noticeable nearby or inside the nectariferous tissue. The highest total daily and summary diurnal nectar amount per flower, as well as the nectar secretion rate were found in S. sclarea, and the lowest in the M. officinalis respectively. Pattern of diurnal dynamics of nectar secretion with one secretion peak was observed: in the morning, in both Salvia species, H. officinalis and L. officinalis, and early in the afternoon in M officinalis. Pollinator observation indicated the visitation of seven species of Hymenoptera, three species belonging to Diptera and two species of daily-active Lepidoptera. The honey bee (Apis mellifera) was the most abundant pollinator. The major elements of attractiveness were flower scent and colour, followed by the shape and size of the corolla and to a lesser extent the nectar amount. According to all analyzed flower characteristics and observation of pollinator behaviour and visits, the most attractive plants were H. officinalis, S. officinalis and L. officinalis, whereas M. officinalis was the least attractive. S. sclarea was completely unattractive to all recorded pollinators, except Xylocopa violacea.