Does temperature stress induce nectar secretion in Mediterranean plants ?

We studied the effect of temperature on nectar secretion in Thymus capitatus, a typical labiate of phrygana (i.e. the garrigue of the East Mediterranean Basin). Experiments were carried out at controlled temperatures in a climatic chamber. We measured the nectar standing crop of the flowers at the end of the first day of their anthesis. All nectar values (i.e. volume per flower, sugar content and concentration) increased with temperature up to 38 °C, as long as the plants were not water stressed. However, in the open and under normal temperate conditions (i.e. at relatively low temperatures) nectar secretion depended more on changes in solar irradiance than on temperature. Under the same conditions, nectar secretion in Ballota acetabulosa, a species that is sympatric and coflowering with T. capitatus, was affected neither by light nor temperature. Since in the wild these two species are found in different microhabitats (full sun and shade, for T. capitatus and B. acetabulosa, respectively), we attribute the differences we observed to the differential natural adaptation of the plants to these factors. We conclude that T. capitatus is very well adapted to the Mediterranean conditions. By performing optimally as a nectar source at high temperatures, it can support a very large insect fauna that visits its flowers for nectar. It is in fact a temperature-induced cornucopian species.