333 EM (ECOLE DE MONTPELLIER) is a Cabernet Sauvignon x V. berlandieri hybrid used as a rootstock. It was created in 1883 by Professor Gustave Foëx of Montpellier. Dr Richard Smart (2015, p628) points out that it is is one of the few rootstocks with a V. vinifera parent (Fercal and 41 B are others) and despite this 333 EM is sufficiently phylloxera tolerant. 333 EM is slightly more lime tolerant than 41 B (another V. vinifera x V. berlandieri hybrid), and is also drought tolerant, Dr Smart adds.
333 EM is well adapted to calcareous soils, having excellent active lime resistance (upto 40%) and thus good resistance to chlorosis. In Bordeaux 333 EM is hampered by its Cabernet parentage which precludes the early ripening that growers on the right bank require. It is sensitive to saline soils.
Dr Richard Smart in the Oxford Companion to Wine 4th edition ed. Jancis Robinson MW and Julia Harding MW (Oxford University Press, 2015, p628.