Analysis of Root Formation in Leaf Discs of Lycopersicon esculentum Mill. Cultures in vitro

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Abstract
A root regeneration system was developed which utilized 5 mm diameter tomato leaf discs cultured in vitroon defined nutrient media. Root regeneration was totally dependent on such external factors as organic and inorganic nutrients and hormones of the auxin class. Furthermore, root primordia could originate from two distinct types of tissues simultaneously, i.e. type I primordia from the phloem parenchyma of the primary and secondary veins, and type ll primordia from the sheath parenchyma of the minor veins. Multiple roles of GA3 in the inhibition of rooting were evident. (1) An early inhibitory effect (i.e. during the initial 48 h of culture) which could be related to a GA3-induced failure to rapidly accumulate starch within the leaf tissue; (2) a late inhibitory effect which could be related to a GA3-mediated destruction of late-forming primordia by a ‘parenchymatization’ process which had its origin in an earlier GA3 effect on growth stimulation of the adjacent cortical parenchyma; (3) an inhibition of meristemoid formation from sheath parenchyma cell derivatives surrounding the minor veins. This inhibition would insure an absence of type II primordia formation.