The authors thank Tatiana Slosman, Yael Abramovich and Adi Lavan for their technicalassistance.
This research was supported by the Israeli Science Foundation (Grant no. 418/99-1) and by research grant no.
US-2664-95 from BARD, The United States–Israel Binational Agricultural Research and Development Fund.
While performing this research E.M.H. was supported by the Fulbright Senior Scholar Program, Virginia Polytechnic Institute and State University, and the US Department of Agriculture Hatch Program.