The origin of the exudate from cut maize roots

The exudate that wells out of the cut surface of detached rents is commonly thought to flow from the xylem and to be forced up the vessels by root pre ssure. Direct monitoring of the cut surface of maize (Zea mays L.) roots by optical microscopy and cryomicroscopy of fast-frozen material showed that the liquid originated not only from cut vessels but was also exuded from li ving tissues and intercellular spaces over the entire cut surface. Indeed, exuded liquid usually appeared last over the vessels. This pattern of exuda tion was seen also from partially submerged segments: of mature roots in wh ich the lower end had been sealed. Exudation also occurred from similarly p repared root sleeves from which the stele had been removed. Measurements of rates of exudation often showed a periodicity of about 1 h. These observat ions add to earlier published descriptions of the phenomenon, particularly extensive 19th century studies, which remains unexplained by current unders tanding of root water relations. (C) 2000 Editions scientifiques et medical es Elsevier SAS.