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Results

Significant differences were observed in the net photosynthetic rates among the seven genotypes of triticale (Table 1). 6A-1093 consistently maintained highest photosynthetic rates (25.8 micromole m^-2s^-l) at an irradiance of 1500 micro-moI m^-2s^-1 followed by 801/1208 (23.2 micromole m^-2s^-1).
The activity of RuBP carboxylase in leaf extracts was significantly correlated with net photosynthetic rates. The leaves of 6A-1093 recorded highest enzyme activity (298.6 micromole mg^-1 chl hr^-1) while the lowest activity was noticed in 6A-854 (Table 1). However there was no significant difference in the leaf chlorophyll content among seven triticale genotypes (Table 1). The kinetic characteristics of RuBP carboxylase in leaf extracts were compared in Table 2. The Km (CO₂) values of the enzyme were significantly low (8.36 microM) with corresponding high Vmax values (28.95 micromole kg^-1 prot s^-1) in the leaves of 6A-1093. The highest Km (CO₂) and lowest Vmax values were obtained in the leaves of 6A-854 (Table 2).


Discussion

The present findings provide a strong correlation between the net CO₂ assimilation rates and RuBP carboxylase activity (Table 1). The results clearly show that the differences in the CO₂ assimilation rates were due to the differences in the levels of RuBP carboxylase in leaves. The rates of net photosynthesis and RuBP carboxylase activity were highly correlated (r=0.93, P< 0.001) and the linear regression equation of net photosynthesis on RuBP carboxylase activity was Y=9.05+8.45X. The positive correlation between net photosynthetic rates and RuBP carboxylase activity has been previously reported (Seeman and Berry 1982; Evans and Seeman 1984; Ramachandra Reddy and Das 1986). It is suggested that the differences in the in vivo activity of RuBP carboxylase in the leaves of triticale may also affect CO₂ assimilation.

The superior photosynthetic rates in 6A-1093 among the genotypes used in this study were presumed to be due to its efficient carboxylation capacities as evidenced by lower Km (CO₂) and high Vmax values of RuBP corboxylase in leaf extracts. The Km (CO₂) values reported for triticale are comparable to those reported for certain other C₃ plants (Bird et al. 1982; Jordan and Ogren 1983; Makino et al. 1985; Ramachandra Reddy and Das 1986; Paul and Yeoh 1988). It is plausible that the genotypes with superior photosynthetic performance would be highly useful in selecting photosynthetically efficient triticale cultivars for obtaining greater photosynthetic productivity. The in vitro kinetic characteristics of RuBP carboxylase can be one of the useful targets to obtain greater CO₂ assimilation capacity in triticale. The significant differences in the catalytic properties of RuBP carboxylase also suggest that it might be possible to improve the photosynthetic performance of certain triticale genotypes through genetic manipulation of this enzyme.


Acknowledgements

Grateful thanks are due to Dr. J.P. Gustafson, University of Missouri, Columbia, U.S.A. and Dr. T. Wolski, Triticale Breeding Station, Warsaw, Poland for the supply of genetic stocks.

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