"Harmomegathy": un problema aperto e misconosciuto

« Harmomegathy »: a still open problem not yet well-known. - Wodehouse used the term « Harmomegathy » to indicate the form and volume change in pollen grains, due to water loss or assumption. Roundish and tricolporate grains in the anther become oval and decrease in volume during the dehydration preceeding anthesis. Viceversa during rehydration on the stigma they undergo the opposite process. Pollen water content at an thesis varies from species to species and it usually ranges between 10 and 20%; nevertheless in some families as Zingiberaceae and Araceae it is higher. Anther and pollen dehydrations are affected by environmental factors, such as humidity of air and temperature, before the anthesis and when pollen is suspended in air. Besides, to avoid the further loss of water during pollen flow, an oily layer derived from tapetum degeneration is deposited outside the pollen grains. During dehydration and rehydration pollen grains undergo a stress due to shape and volume changes expecially in the colpus and/or pore areas. Pollen walls are structured in order to avoid their own damage, which could be followed by protoplast death and/or irregularities in grain shape. Intine, formed by pectocellulosic filaments, has elastic properties, as sporopollenin, the component of exine. Stigma can be dry or wet, namely covered or not by stigmatic exudate giving place to different hydration patterns. The structure of intine and exine has to match that of the stigma, in fact pollen will find the best place and way to proper hydration only on stigmas of its own species. Grains structure also must be sterically conform to stigma: this is well-shown in species as Linum, Lytrum and Primula which have an heteromorphic autoincompatibility. The right keeping of shape and structure of grains, despite the hydration and dehydration, is therefore essential to the physical-morphological recognition between pollen and stigma, which preceedes the biochemical recognition.