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A Comparative Analysis of Gibberellic Acid Content with Respect to Tuber Induction in Potato Plants Grown Under Differential Photoperiod and Temperature 412 biosynthesis of GA3 in potato tissues. Level of GA3 was 3 times higher in tissues grown under noninducing conditions (10 h photoperiod / 14 h darkness, 24/12ºC day/night) than in those grown under inducing conditions (18 h photoperiod / 6 h darkness, 30/26ºC day/night) throughout the time-course experiment. Following transfer to inducing conditions, GA3 production was transiently reduced with the highest rates of decline observed immediately after transfer. It is worth mentioning that gibberellins in other plant species have been reported to be under photoperiodic and thermal control. For instance, photoperiod affects certain steps in the GA biosynthetic pathway in various species such as spinach (Talon et al. 1991), willow (Olsen et al. 1995), and tulip (Aung et al. 1969). Comparing the tissue types, GA3 was more abundant in the aerial than in the underground tissues. Our results correlate a decreased GA3 content with tuber induction in potato plants. Table 2: Levels of gibberellic acid (GA3) in potato tissues under tuber inducing and non-inducing conditions Amount (ng GA3/g dry tissue) a,b Induced Tissue Non-induced Tissue Harvest Days Aerial Underground Aerial Underground 2 312.3 ± 5.0 a 210.8 ± 3.4 a 668.0 ± 4.0 a 470.2 ± 3.1 a 4 250.8 ± 4.1 b 150.6 ± 1.7 b 630.2 ± 3.8 a 467.3 ± 2.9 a 6 210.0 ± 3.9 c 110.2 ± 1.4 c 640.0 ± 3.1 a 460.2 ± 3.0 a 8 195.4 ± 2.4 c 108.0 ± 1.6 c 645.4 ± 3.4 a 454.5 ± 3.2 a 10 190.2 ± 1.9 c 106.4 ± 1.4 c 654.6 ± 3.0 a 455.7 ± 2.8 a Meanc 231.7 y 137.2 z 647.6 w 461.6 x a The data are presented as the mean ± standard deviation of three replicates (two independent determinations per replicate) b Mean values followed by the same letter within a column are not significantly different by Duncan’s multiple-range test at P < 0.05 c Mean values in the last row with different letters (wxyz) are significantly different by Duncan’s multiple-range test at P < 0.05 II. Physiological significance of the present findings The content of GA3 was relatively high, but did not significantly change during the sampling period under non-inducing conditions, which promote stolon elongation (Smith and Rappaport, 1969; Kumar and Wareing, 1972). However, GA3 level declined very rapidly to a relatively low level after transfer to inducing conditions. Clearly, this decline suggests a regulating (inhibitory) role for GA3 in tuber induction. It appears that induction of tubers might occur only when the amount of endogenous GA3 (and/or sensitivity of responsive tissues) in the apical part of the stolons has dropped below a certain threshold/critical value. This minimum concentration (sensitivity) model is in line with the observation that gibberellins are found to inhibit cytokinin-mediated tuberization of Solanum tuberosum (Hussey and Stacey 1984). Furthermore, the inhibitory effect of GA3 was relieved by the addition of equal amount of abscisic acid to the culture medium (Xu et al. 1998a). The action of GA3 appears to take place through modulating its level and sensitivity by the environmental conditions. For example, a decrease in GA3 in below ground tissues has been associated with tuber induction. At the same time, the relatively higher levels in above ground tissues might support the initial increase in stem elongation; a short-term adaptive response of potato plants to tuberinducing conditions (Martinez-Garcia et al. 2002). Our findings reinforce earlier reports in the literature and provide supportive evidence for the inhibitory effect of GA3 on tuber induction. For example, exogenous application of GA3 to potato
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.
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