Tracheary Element Differentiation Induced in Isolated Cylinders of Lettuce Pith: a Bipolar Gradient Technique

To study the influence of morphogenetic gradients on vasculardifferentiation patterns, a new technique was developed whichallows different substances to be applied at opposite ends ofa tissue block. It yielded information on the mobility of particularmorphogens and on the dependence of callus formation and trachearyelement differentiation on their presence. Application of indol-3ylacetic acid (1AA) (10 mg l^–1), zeatin (0.1 mg l^–1)and sucrose (3 per cent, w/v) in various combinations to theends of cylindrical explants of lettuce pith (Lactuca sativaL.) showed that (a) callus formation was stimulated by IAA,whereas induction of tracheary elements required both IAA andzeatin; (b) callus was confined to a few millimetres at theends of the explants, and tracheary elements occurred mainlywithin the callus; (c) sucrose or its metabolic products diffusedthe 10 mm length of the explants, while IAA and zeatin wereeffective only close to the application site; and (d) some callusand tracheary elements formed when no sucrose was applied, butboth increased with sucrose application, though inhibition oftracheary elements formation occurred with high sucrose concentrations. differentiation, pith explant, tissue culture, xylogenesis, indol-3yl acetic acid, sucrose, zeatin, lettuce, Lactuca sativa