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Discussion
Rubiscoo is the key enzyme for the photosynthetic C02
assimilation (Johal et al. 1980; Lorimer 1981). It has been known
that Rubisco activity is the rate-limiting factor under
light-saturated and low C02 environments (Stitt and
SchuIze 1994). We compared in vitro Rubisco activity and in
vivo C02 assimilation rate in 15 alloplasmic
lines of CS with cytoplasms of Triticum and Aegilops
species. All alloplasmic lines showed either equivalent or higher
levels of Rubisco activity on both fresh weight and
chlorophyll bases as compared with CS. Significantly higher Rubisco
activity was observed in 5 lines (Fig. 2).
Although the line with T. boeoticum cytoplasm showed a
markedly higher Rubisco activity and the highest C02
assimilation rate, the line exhibits highly depressed plant vigor.
The high Rubisco activity, therefore, does not result in good plant
vigor in this line. Two alloplasmic lines with H-type large
subunit of Ae. speltoides and T. timopheevi showed
higher Rubisco activities than CS, but those in two other lines with
Ae. sharonensis and Ae. bicornis subunit were
equivalent to that of CS. Moreover, no correlation was found between
the Rubisco activity and the photosynthetic C02
assimilation rate.
A question remains how one can approach towards solving this
inconsistent result, i.e., high Rulbisco activity does not
necessarily associate with high C02 assimilation rate.
Apparently some other factor(s) is involved in the regulation of
in vivo photosynthetic C02 assimilation. Somerville
et al. (1982) found a mutant of Arabidopsis that could survive
only under high (5 %) C02. The mutant was later shown to
lack an enzyme that activates Rubisco; thus it was named Rubisco
activase (Salvucci et al. 1985). Rubisco activase encoded by nuclear
gene(s) interacts with the catalytic site of the large, subunit of
Rubisco (Yokota and Tsujimoto 1992). It is now well
known that Rubisco and Rubisco activase play central roles
in the regulation of photosynthetic C02
assimilation, thus suggesting the necessity of studying the
interaction of Rubisco with Rubisco activase. in wheat and related
species. A study on other factors, particularly the quantity and
quality of chlorophyll-protein complexes in photosystem II (Watanabe
et al. 1991) might also help understand the photosynthetic property
of wheat.
Acknowledgments
This research was supported in part by Grant-in-Aid for JSPS
Fellow (to KK). The authors would like to thank Prof. Tsunewaki for
providing us with the alloplasmic lines used in this study.
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