3.3. Effects of callus age
Callus rapidly grew across the three tested lines within twoweeks of
incubation (Fig. 1b and c), with a significant increase in callus fresh
weight (Table 4). However, variation in callus growth was observed
among the tested lines; SA281 callus grew faster than the other two.
The most rapid callus growth occurred between one and two weeks
as demonstrated by weekly callus growth ratio (the fresh weight of
current week divided by the fresh weight of previous week). Both
regeneration rate and regenerability (shoots per IE) peaked at two
weeks across these three lines (Table 4).
Unlike rice embryogenic callus which can maintain regenerability
over 40 weeks (Nabors et al., 1983), sorghum embryogenic callus does
not maintain a useful level of regenerability over fourweeks. The reason
for this phenomenon is, as yet, unknown. Our data from three tested
lines unequivocally demonstrated that the optimal callus age was
two-weeks old. When callus was four-weeks old, the regeneration
rate and regenerability dropped markedly. For instance, SA281 could
produce 59 shoots per IE when callus was two-weeks old; however, it
only generated 3 shoots per IE when callus was four-weeks old
(Table 4). Therefore, callus age plays a critical role on regenerability,