Seed diversity and germination behavior in Nerium oleander L.

Nerium oleander var. ‘Villa Romaine’, commonly called oleander, is an ornamental shrub that exists in Tunisia in the wild and cultivated form. This plant is frequently propagated and produced in nurseries and used in urban plantings and on roadsides. The effects of nature of the plants (spontaneous or cultivated), climatic conditions (localities), and altitude (from 6 to 447 m) on seed shape, size, and germination of Tunisian populations, were investigated. Seeds were harvested from 23 locations belonging to six bioclimatic stages and distributed from North to South of Tunisia. The seed parameters studied were area, roundness, J index, and germination rate. A significant variation was obtained between populations, categories (cultivated and spontaneous), bioclimatic zones and altitudes. Seeds of cultivated plants were the smaller and showed the higher germination rates, compared with spontaneous populations, which had large seeds and a low germination rate. Climatic conditions and geographical location had an effect on seed morphology; seeds in lower humid were small for both cultivated and spontaneous types, they also showed a low J index value and a high germination rate. On the other hand, in upper Saharan, seeds were large and roundness and germination rate were the lowest. Also, seeds from plants in higher altitudes were the smaller and presented the lower values of the J index compared with those collected from low-altitude sites.