POLLEN-PISTIL INTERACTIONS IN POPULUS - BETA-GALACTOSIDASE ACTIVITY ASSOCIATED WITH POLLEN-TUBE GROWTH IN THE CROSSES POPULUS-NIGRA X POPULUS-NIGRA AND POPULUS-NIGRA X POPULUS-ALBA

In order to understand the nature of interspecific barriers in Populus , we have explored pollen/pistil interactions in intra- and interspeci fic crosses Populus nigra x P. nigra and P. nigra x P. alba. The kinet ics of pollen-tube growth demonstrated that P. nigra and P. alba polle n tubes have distinct behavioral patterns inside P. nigra pistils. P. alba pollen tubes exhibit an unique S-shaped growth curve and an arres ted growth site near the sylodium. P. nigra pollen tubes exhibit two g rowth phases, in the stigmatic tissues and in the ovarian cavity respe ctively. P. nigra and P. alba curves diverge 5 h after controlled poll ination and could be related to a change in the physiology of the P. n igra pollen tube, which shifts from an autotrophic to a heterotrophic type of nutrition. Protein analysis of pollinated stigmatic extracts ( 0,6 and 20 h after pollination) revealed qualitative and quantitative differences that are related to the presence of either P. nigra or P. alba pollen tubes inside the stigmatic tissues. Increasing numbers of protein bands were detectable from 0 to 20 h after pollination only in intraspecific cross. Glycoproteins were detected, and the differences observed were dependent of the cross. Beta-Galactosidase activity was found in pollinated stigmas, but an increase in its activity (one iso zyme of pHi 4.2) between 6 h and 20 h after pollination was detected o nly in the intraspecific cross. This enzyme could play a role in heter otrophic pollen-tube nitrition, and its activity could be the final re sult of a series of interactions started by the initial pollen-stigma dialog.