The effect of the rootstock on the acropetal and basipetal transport of photoassimilates and hormones was studied in the 'Kiran' (Ki) melon cultivar grafted onto pumpkin rootstocks with different degrees of compatibility. A complementary experiment was performed to compare the incompatible combination (as evidenced by plant collapse at the fruit ripening stage), designated Ki/r53, with self-grafted r53/r53 as a model compatible combination. Both experiments showed the accumulation of a number of amino acids, sugars, and sugar alcohols in the scion of the incompatible Ki/r53 grafts. Additionally, they showed a marked reduction of trans-zeatin-type cytokinins and an elevated content of cis-zeatin-type cytokinins in the rootstock, and the opposite pattern in the scion, hinting at the possible involvement of a hormonal signal for graft compatibility. There was no direct evidence of a blockage at the graft union, since hormone acropetal and basipetal trafficking was demonstrated for all combinations. Dye uptake experiments did not show xylem flow impairment. A possibly significant finding in the incompatible combination was the deposition of undifferentiated cells in the hollow space that replaces the pith region in melon and pumpkin. The link between the above findings and the collapse of the plants of the incompatible combination remains unclear.

Arava Research and Development Yair Experimental Station M P Arava 86825 Israel

French Associates Institute for Agriculture and Biotechnology of Drylands Blaustein Institutes for Desert Research Sede Boqer Campus Ben Gurion University of the Negev P O B 653 Beer Sheva 84104 Israel

Jacob Blaustein Center for Scientific Cooperation The Jacob Blaustein Institutes for Desert Research Ben Gurion University of the Negev Sede Boqer Campus 8499000 Israel

Laboratory of Growth Regulators Faculty of Science Palacký University and Institute of Experimental Botany The Czech Academy of Sciences Šlechtitelů 27 CZ 783 71 Olomouc Czech Republic

Laboratory of Growth Regulators Faculty of Science Palacký University and Institute of Experimental Botany The Czech Academy of Sciences Šlechtitelů 27 CZ 783 71 Olomouc Czech Republic; Department of Chemical Biology and Genetics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

The Albert Katz International School for Desert Studies The Jacob Blaustein Institutes for Desert Research Ben Gurion University of the Negev Sede Boqer Campus 8499000 Israel

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Impairment of root auxin-cytokinins homeostasis induces collapse of incompatible melon grafts during fruit ripening