This review examines and whenever appropriate, reanalyses published literature related to two general and independent hypotheses having the underlying assumption that phytoactive secondary compounds produced by plants evolved primarily as plant defences against competitor plant species. The first hypothesis is that production and the main way of release of phytoactive compounds reflect an adaptive response to climate conditions. Thus, higher phytoactivity by volatile-compounds should prevail in plants of hot, dry environments whereas higher phytoactivity by water- solubles should be preponderant in plants from wetter environments. The second hypothesis is that the synergy between phytoactive compounds of plants should be widespread while antagonism or absence of interaction of effects should be rare because of the higher efficiency of energy and use of resources provided by synergy. Published literature does not support either hypotheses. We found no pattern of association between higher phytoactivity in volatile compounds in plants from drier environments or in water-soluble compounds in plants from wetter environments. Neither did we found evidences for the predominance of synergy. On the contrary, antagonism or no interaction of effects among allelopathic compounds largely prevailed
