Photosynthesis, chlorophyll content and ribulose diphosphate carboxylase activity in relation to yield in wheat genotypes

SummaryPhotosynthetic rates, chlorophyll content and activity of ribulose 1,5-diphosphate carboxylase (RuDPC) were determined for 18 genotypes of wheat (Triticum aestivum L.) in pot culture. Measurements were made at various growth stages. Correlation studies between physiological characteristics and seed yield were made at each stage separately.Genotypic variations were observed at different growth and developmental stages in photosynthetic rates (1·8–2·7-fold), chlorophyll contents (1·2-fold) and enzyme activities (2-fold). Chlorophyll content and enzymic activity increased in plant leaves with advancing age while apparent rates of photosynthesis decreased.RuDPC activity in flag leaf at anthesis showed significant positive correlation with grain yield (r = 0·55). Other attributes did not show significant association with grain yield.

Properties of Ribulose Diphosphate Carboxylase/Oxygenase in the Tobacco

Photorespiration, Ribulose 1,5-diphosphate-Carboxylase/Oxygenase, Gene Composition The activity of ribulose 1,5-diphosphate (RuDP)-carboxylase and RuDP-oxygenase was measured in crude leaf extracts of the tobacco (N . tabacu m) phenotypes which differed with respect to their gene constitution and with respect to their photosynthetic and photorespiratory activity. The green wild type (JWB) which carried the discussed two nuclear factors su and aur in the condition su/su Aur/Aur and su/su Aur/aur exhibits normal photosynthetic activity and low photorespiratory activity. A yellow-green chlorophyll-deficient phenotype (Su/su) carrying Su/su Aur/Aur has high photosynthetic activity on a chlorophyll basis but also high photorespiratory activity. A new yellow phenotype (Su/su var. Aurea) carrying both nuclear factors in a heterozygous condition Su/su Aur/aur has high photosynthetic activity on the basis of chlorophyll and low photorespiratory activity. The comparison of the RuDP-carboxylase/oxygenase activity in these three phenotypes shows that in the yellow-green phenotype Su/su the affinity of the RuDP-oxygenase towards oxygen is higher than in the green phenotype JWB and in the yellow phenotype Su/su var. Aurea. The Km ((O2) values for the RuDP-oxygenase activity are 890 /um for JWB, 630 μм for Su/su and 940 μм for Su/su var. Aurea and the corresponding Ki(CO2) values are 7.4 μм for JWB, 14.9 μм for Su/su and 5.8 μм for Su/su var. Aurea at pH 8.34. On the other hand, the affinity of the carboxylating activity of the enzyme towards CO2 shows no difference between JWB and Su/su var. Aurea, but a lower affinity in Su/su. This is expressed by the K m (C 02) values which are 107 μм for JWB, 143 μм for Su/su and 96 μм for Su/su var. Aurea at pH 7.8. However, the affinity of the oxygenase function of the enzyme towards RuDP seems to be unchanged in all three tobaccos and is found to be around Km (RuDP) 27 μм. From this result it appears that the nuclear factor su decreases in the condition Su/su Aur/Aur the affinity of the RuDP-carboxylase towards CO2 and increases the affinity of the RuDP-oxygenase towards oxygen. On the other hand, the factor aur seems to suppress this gene expression in the condition Su/su Aur/aur whereas both factors do not affect the binding of RuDP onto the enzyme.