Effect of duration of light:dark cycles on in vitro shoot regeneration of tomato

Cotyledonary explants of the Red Coat cultivar were exposed to light:dark cycles in regeneration medium (MS basal+15 µM zeatin) for four weeks, with the view of elucidating the role of light in direct shoot regeneration. Light has not been found essential for shoot regeneration in tomato. However, maximum shoot regeneration response (60%) could only occur in the explants exposed to 16 h light and 8 h dark. The response declined at 2 h dark (47%) or 24 h light (40%). Comparable number of shoots was produced in the 16 h light (3.3) and 24 h dark (3.2) treatments, but in 24 h light treatment the number of shoots produced per explant was less (2.7). Shoots with maximum height were observed in 24 h light (6.3 mm) followed by 16 h light (4.5 mm) and 24 h dark (3.9 mm). The shoots regenerated in the dark were not etiolated. Mimicking of light regulated processes by exogenous application of high concentration of cytokinin can be the possible reason for the non-etiolated behaviour of the dark regenerated shoots. Although light was not essential for shoot regeneration, appropriate light:dark cycles were required for proper chlorophyll development.