Patterns of Tracheary Differentiation in Lettuce Pith Explants: Positional Control and Temperature Effects

When lettuce pith explants were cultured for 14 d on a xylogenicmedium, tracheary elements differentiated in greatest numbersbetween 25 and 30 °C. Numbers were depressed at lower temperaturesby slower development and at higher temperatures by adverseprocesses. The data did not support previous suggestions ofa great stimulation of xylogenesis above 30 °C and of aspecial sensitivity to low temperatures. Tracheary elements differentiated in various spatial patterns:as clumps in the peripheral callus, as strands which extendedradially and longitudinally from some of these clumps, as individuallarge tracheids especially at the more extreme temperatures,and as short strands associated with nodules and roots thatformed at favourable temperatures. We suggest that indoleacetic acid (IAA) has various roles inthe positional control of these tracheary patterns: (1) IAAdestruction at the explant surface leads to concentration gradientsthat inhibit tracheary induction close to the surface; (2) IAAtransport from the source in the culture medium to sinks especiallyat the explant surface, coupled with autocatalytic flow facilitation,leads to canalization along pathways that become meristematicand then trachcary strands; (3) the IAA flux (and associatedproton flux) along these pathways tend to orient cortical microtubulesat right angles to the flow, by some mechanism as yet unknown,and hence to control the orientation of tracheary element elongationand secondary wall banding. These suggestions, supported bymorphometric studies of tracheary element dimensions and orientations,and by experiments with localized IAA application, lead to ageneral interpretation of the development of polarity in plants. IAA, Lactuca sativa, lettuce, pith explants, positional control, temperature effects, tracheary element differentiation