Stimulatory and Inhibitory Effects of Sucrose Concentration on Xylogenesis in Lettuce Pith Explants; Possible Mediation by Ethylene Biosynthesis

Numbers of tracheary elements differentiating in lettuce pithexplants rose with increase in concentration of sucrose in themedium up to an optimal concentration of 0·2%, and fellwith further increase in concentration to about one-tenth maximalat 3% sucrose. Although a few tracheary elements formed withoutexogenous sucrose, a very low concentration of sucrose (0·001%)was sufficient to stimulate additional xylogenesis. Pretreatmentof explants with 3% sucrose caused a persisting inhibition ofxylogenesis, especially in tissue that had been near the siteof sucrose application (sandwich technique). The requirementfor adequate, but not inhibitory, concentrations of sucrosefor xylogenesis may underlie the development of xylem alongsidethe sucrose-rich phloem in normal apical morphogenesis. For callus growth the response to sucrose was different: theoptimal concentration was 3%, with a broad plateau from 1 to4% sucrose. Sucrose concentrations of 2 to 3%, used in manytissue culture media, are thus roughly optimal for callus growth,but ten times the optimum for xylogenesis in lettuce pith explants. It is surprising that 0·001% (0·03 mM) sucrose,applied exogenously, can stimulate xylogenesis: endogenous sugarconcentrations are normally higher. Perhaps the stimulationis mediated by ethylene biosynthesis, which is known to be xylogenic.Rates of ethylene production per explant rose with increasingsucrose concentration from about 0·1 nl h^-1 at 0% sucroseto a slightly (significantly) higher level at 0·004%sucrose and to about 0·5 nl h^-1 at 3% sucrose. D -glucoseresembled sucrose in its effects on xylogenesis and ethyleneproduction, but L-glucose yielded no xylogenesis and littlestimulation of ethylene biosynthesis.Copyright 1994, 1999 AcademicPress Lactuca sativa, Coleus blumei, Nicotiana tabacum, lettuce pith explants, tracheary element differentiation, sucrose, glucose, ethylene