The present study aims to investigate the effects of osmotic water potential, (Ψs), temperature (T) and zinc (Zn) addition to the seed incubation medium to evaluate the effects of their interactions on the germination processes of seeds in three plant species of different ecological affiliation. These species were namely: Senna alexandrina Mill, Senna italica Mill (native to hot deserts) and Senna occidentalis (L.)Link (a wild mesophytic plant). Conversions of potentially water soluble (storage) carbon metabolites in storage organs of seeds into soluble forms and allocation into the embryonic axis which contribute to osmotic potential adjustment were studied. Addition of zinc improved the adjustment of radicles to water deficiency conditions through increasing the allocation of water soluble metabolites into the radicles and hence increasing the osmotic potentials of radicles. Also, zinc induced additional adaptation of the plants to extreme temperatures through increasing total osmotically active metabolic fractions in the radicle. The statistical analysis indicated that, the trifactorial interaction (Ψs x T x Zn) exhibited the major effect on the metabolic conversions in the three species.