European Journal of Scientific Research - EuroJournals

413 Ahmed Malkawi
plants is reported to have a promoting effect on stolon elongation (Smith and Rappaport, 1969; Kumar
and Wareing, 1972) and an inhibitory effect on tuber induction (Hussey and Stacey 1984; Fernie and
Willmitzer, 2001; Xu et al. 1998a). Furthermore, treatments with inhibitors of gibberellins biosynthesis
caused tuberization of solanum tuberosum plants under non-inducing conditions (Jackson and Prat
1996; Hussey and Stacey 1984; Menzel 1980; Vreugdenhil et al. 1994). In addition, suppression of GA
20-oxidase, a key regulatory enzyme in the biosynthesis of gibberellins, was found to be correlated
with tuber induction in potato (Carrera et al. 1999).
Transgenic plants over-expressing this enzyme gave taller plants that tuberized later than
control plants (Carrera et al. 2000). This observation resembles the effect of exogenous application of
GA3 on ordinary (untransformed) plants. The observation that taller plants were observed is consistent
with the promoting effect of GA3 on vegetative growth of plants concurrent with the inhibitory effect
on tuber induction.
Interrupting a tuber inducing long night with a red light treatment (which activates
phytochrome photoreceptors) for few minutes had an inhibitory effect on tuberization (Batutis and
Ewing 1982; Amador et al. 2001). This effect was similar to that of lengthening the photoperiod. It is
also reported (Amador et al. 2001) that transgenic plants suppressed with phytochrome B levels
produced tubers when grown under long nights supplemented with a night-break red light. The red
light effect was observable 2 days after the short-day treatment started and preceded tuber initiation.
Our interpretation of such observation is that red light activates phytochrome B to initiate the
biosynthesis of GA3; the 2-day interval was sufficient to bring GA3 back to the critical inhibitory level.
In our experiment, we found that 2 days after initiating the inducing conditions the level of GA3
declined to below inhibitory level required for tuber induction. It is reported that gibberellin activity,
determined by bioassays, declined in leaves exposed to inducing conditions and as few as 2 days in
inducing conditions caused a decrease in the gibberllin activity of andigena leaves (Railton and
Wareing 1973). Gibberellin activity has also been reported by other researchers to be lower in plants
grown in tuber-inducing conditions compared with non-inducing conditions (Smith and Rappaport
1969; Kumar and Wareing 1974; Krauss and Marschner 1982; Machachova et al. 1998). In fact, small
visible tubers were observed 8 days after initiating tuber-inducing conditions in the present experiment.
Transgenic S. tuberosum ssp. andigena plants inhibited in phytochrome B expression lost their
photoperiodic sensitivity and consequently tuberized equally well under both non-inducing and
inducing conditions (Jackson and Prat, 1996; Jackson et al. 1996; Jackson et al. 2000). These
transgenic plants suppressed with phytochrome B suggest that phytochrome B either regulates the
production of a graft transmissible inhibitor of tuberization (Jackson et al. 1998; Suarez-Lopez 2005)
or is required for phloem loading of this inhibitor in the leaves (Suarez-Lopez 2005). When this finding
is combined with the present result, we find it reasonable to suggest that Phytochrome B is involved in
the production of the inhibitor GA3 in non-inducing conditions. This suggestion is in line with
transgenic potato plants enhanced with phytochrome B level which showed increased tuber yield in
experiments performed both in greenhouse (Jackson et al. 1996; Thiele et al. 1999) and under field
conditions (Boccalandro et al. 2003). We, therefore, propose that enhanced levels of phytochrome B
cause the production of more GA3, which in turn promotes stolon elongation (Xu et al. 1998a) that is a
pre-requisite for tuber induction thus increasing the potential sites for tuberization.
Conclusion
Supportive evidence has been obtained for the effect of photoperiod and temperature on GA3 level in
potato plants (Solanum tuberosum cultivar Russet Burbank) grown in a controlled environment plant
system. Our study has established a strong correlation between lower GA3 level and tuber induction.
GA3 was more prevalent in tissues grown in non-inducing conditions and its level declined 3 folds
upon transferring the plants to inducing conditions. These results demonstrate that the sustained
decrease of GA3 is required for tuber induction and that its involvement is restricted to the phase
preceding tuber induction, which is stolon elongation.