Metabolism of Pollen Tubes
Pollen grains contain auxins, and gibberellins which are known to be involved in post- pollination enlargement of the ovary and the development of the fruit. Pollen from unrelated species, nonviable pollen or even pollen extracts can prevent abscission and cause swelling of the ovary and formation of near-normal but seedless fruits.
The initial and small amounts of auxins such as IAA or other auxin-like substances and GA supplied by the germinating pollen to the pistil serve to initiate some minimal growth and metabolic processes as a result of which enzymes liberate additional amounts of auxin from the tissue of the style and the ovary, e.g., Nicotiana tabacum. The auxin released initiates growth also in the fertilized ovules which later produce appreciable amounts of auxins and gibberellins in the endosperm, and auxins and cytokinins in the developing embryo. Thus, the initial supply of auxins and gibberellins from germinating pollen not only triggers the development of the fruit but is also responsible for the subsequent release and production of additional amounts of plant growth regulators in the pistil tissues.