The influence of light regime on growth and differentiation of cultured tomato explants.
Growth of cultured tomato (Lycopersicon esculentum Mill.) leaf explants was assessed
in response to quantitative variation in the light regime. Exposure to a range of photon
flux densities (PFD) resulted in distinct PFD optima for callus, shoot and root initiation.
Incubation of explants under a range of temperatures also resulted in a wide range of
growth responses both in terms of tissue type and dry matter production. It was
observed that manipulation of PFD by varying the separation between culture and light
source was accompanied by a non-uniform temperature regime between PFD treatments.
Therefore irradiance-dependent growth responses cannot be accurately interpreted with
this arrangement unless additional compensatory heating is employed to maintain a
uniform temperature regime throughout PFD treatments.
Culture of explants on media shielded from illumination resulted in a distinct
organogenesis response from explants exposed to full illumination when the media
contained indole-3-acetic acid (IAA). A comparison of the morphogenic response to
PFD with that produced under a matrix of exogenous auxin and cytokinin concentration
combinations indicated that morphogenesis may be influenced by PFD-dependent
depletion of IAA. A high performance liquid chromatography study of auxin depletion
from basal medium showed that IAA was rapidly removed when illuminated and that
the rate of removal was proportional to the prevailing PFD and the concentration of
The inclusion of the photosynthetic inhibitors 3-(3,4-dichlorophenyl)I,I-dimethylurea
and 3-amino-l,2,4-triazole in the basal medium resulted in inhibition of explant growth
and elimination of the sequential change in the culture vessel headspace CO2
concentration apparent during the light/dark cycle in noninhibited controls. This
indicates that in vitro photosynthesis made a significant contribution to PFD-dependent
While culture of explants under a range of photoperiods of uniform PFD produced a
range of growth responses, a uniform growth response occurred when explants were
exposed to 16 hour and 24 hour photoperiods of uniform photon dose achieved through
PFD variation. This demonstrates that growth was enhanced as a function of the
increase in total incident radiation rather than through an effect of the photoperiod per
se. Dark or low PFD incubation of explants for the first 4 days of culture significantly
enhanced growth as a result of reduced photodestruction of IAA during a period of
optimal tissue sensitivity to exogenous auxin.