Interspecific incompatibility in Populus: inhibition of tube growth and behaviour of the male germ unit inP. deltoides×P. alba cross

Abstract

In order to better understand the cellular events controlling interspecific incompatibility in the genus Populus, the incompatible cross betweenP. deltoides andP. alba has been investigated both at the light and electron microscopic levels. Stained in decolourized aniline blue and observed by epifluorescence microscopy, most incompatible pollen grains are seen to germinate at the stigma surface. Numerous incompatible pollen tubes reach the base of the style where they are arrested 19 h after pollination. Ultrastructural observations on in vivo growing incompatible pollen tubes confirm these data. Very few cytoplasmic modifications are seen within living pollen tubes reaching the lower end of the style or within arrested ones, except the presence of polymorphic plastids. In this predominantly tricellular system, the male germ unit (MGU) is apparently initiated at pollen maturity as an association between the vegetative nucleus and sperm cells. It is maintained during pollen tube growth within the style and persists within arrested incompatible pollen tubes. The unique observation of an association between a dividing generative cell at metaphase and the vegetative nucleus is also reported. Arrested pollen tubes are characterized by apical deformations and accumulation of callose within their thickened cell walls. These cytological data provide additional information on the cellular events associated with interspecific incompatibility in Populus.