Molecular-cellular basis of pollen allelopathy, related to possible chemosensory mechanisms has been discussed. The phenomenon consists of a series of events, viz., (I). excretion of signalling and regulatory substances from donor cell (pollens, pistil stigma), (II). recognition of specific signal-stimulus from plant excretions by acceptor cell (pollen or pistil stigma), (III). transmission of chemical information within the acceptor cell (pollen) and (IV). development of characteristic response in acceptor cell. If it is allelopathy, the changes occur in growth, development and normal fertilization. In the first stage of interactions, allelochemicals are excreted, which act as chemical signals, growth regulators and modulators of cellular metabolism, etc. The allelochemicals, acting on fertilization may be, nitrogen-containing substances (acetylcholine, histamine, serotonin, dopamine, noradrenaline), phenols [(flavonoids : quercetin, kaempferol, rutin) aromatic acids (benzoic, gallic, vanillic)], terpenoids (monoterpenes : citral, linalool, cymol), sesquiterpene lactones : azulene and proazulenes (desacetylinulicine, inulicine, ledol, artemisinine, grosshemine, gaillardine and austricine) and polyacetylenes (capilline) found in flower excretions and were tested in vitro and in vivo on pollen germination of Hippeastrum hybridum. Nitrogenous compounds stimulate the growth of pollen tube, whereas, their antagonists blocked normal fertilization and thus fruits or seeds did not form. Terpenoids act on pollen germination and their stimulatory and inhibitory effects (block fruit formation) depend on their concentration. These effects of terpenoids on pollen germination are through chemosignalling and possible steps are (I). spreading of information in pollen secretions e.g. in olfactory slime, (II). binding with special sensors or receptors in plasmalemma and (III). transfer of stimulus within the pollen cell to nucleus, where spermia appear and a pollen tube starts to grow. Moving from donor cell, allelochemicals penetrate the wall of acceptor cell either (I). directly (without any changes in protoplasmic membrane) or (II) after conversions [interaction with foreign substance of low or high-molecular weight (enzymes and protectory proteins) secreted from donor cells. or compounds of acceptor cell]. Often the second case includes free radical processes. The transmission of information within cell is third stage which includes participation of secondary messengers (cyclic AMP and GMP, inositol triphosphate, Ca ions) and some related enzymatic systems. The final transmission occurs in membranes. of cellular organelles, which respond to information received through changes in enzymatic activity and metabolism. At cellular level, in pollen and pistil it may be active excretion, changes in the autofluorescence and membrane permeability, regulation of alternative pathways in respiration and photosynthesis and switching on free radical processes.
