REPRODUCTIVE LABILITY IN PRE-DISPERSAL BIOLOGY OF RHAMNUS-ALATERNUS L(RHAMNACEAE)

Rhamnus alaternus L. is a dioecious, fleshy-fruited shrub, typical of Mediterranean vegetation, which has been recorded from the region sinc e the tropical Tertiary. Reproductive biology of this species has been studied in Southern Italy over a period of 4 years with both field an d laboratory experiments. In all the studied populations male plants p redominate over the females, have a higher flower density and flower r egularly every year while females are usually alternate-bearing. A str ong negative association between fruit formation and vegetative growth has been found indicating that the presence of fruits on a branch red uces the growth of new shoots. This phenomenon, together with alternat e-bearing, is evidence of a strong competition between vegetative grow th and reproductive activity and indicates resource limitation. Flower morphology and quantity of pollen produced are typical of insect poll inated species, and flies of the Empididae and Muscidae were trapped b earing pollen at female plants but pollen is also dispersed by wind. F ield experiments on pollination success showed that the exclusion of i nsects as pollen vectors produces a significant decrease in fruit deve lopment. Excess pollen provided by hand-pollination increases fruit se t over open-pollination and reproductive success in the field is there fore pollen limited. The variability of sex expression, male fitness, fruit production, regrowth and pollination presented here suggests tha t these predispersal characteristics of R. alaternus have been subject to evolutionary change since the tropical Tertiary climate was replac ed by a Mediterranean one. This points to the possibility that reprodu ctive, as well as morphological and physiological characteristics, may be equally selected for in extreme environments such as the Mediterra nean.