Photosynthesis, Ribulose-1,5-bisphosphate Carboxylase, Electron Transport, and Ribulose 1,5-Bisphosphate of Virescent and Normal Green Wheat Leaves

Characterization and mapping of novel chlorophyll deficient mutant genes in durum wheat

The yellow-green leaf mutant has a non-lethal chlorophyll-deficient mutation that can be exploited in photosynthesis and plant development research. A novel yellow-green mutant derived from Triticum durum var. Cappelli displays a yellow-green leaf color from the seedling stage to the mature stage. Examination of the mutant chloroplasts with transmission electron microscopy revealed that the shape of chloroplast changed, grana stacks in the stroma were highly variable in size and disorganized. The pigment content, including chlorophyll a, chlorophyll b, total chlorophyll and carotene, was decreased in the mutant. In contrast, the chla/chlb ratio of the mutants was increased in comparison with the normal green leaves. We also found a reduction in the photosynthetic rate, fluorescence kinetic parameters and yield-related agronomic traits of the mutant. A genetic analysis revealed that two nuclear recessive genes controlled the expression of this trait. The genes were designated ygld1 and ygld2. Two molecular markers co-segregated with these genes. ygld 1 co-segregated with the SSR marker wmc110 on chromosome 5AL and ygld 2 co-segregated with the SSR marker wmc28 on chromosome 5BL. These results will contribute to the gene cloning and the understanding of the mechanisms underlying chlorophyll metabolism and chloroplast development in wheat.

Photosynthetic properties of leaves of a yellow green mutant of wheat compared to its wild type

We investigated several photosynthetic parameters of a virescent mutant of durum wheat and of its wild-type. Electron transport rate to ferricyanide was the same in the two genotypes when expressed on leaf area basis while O2 evolution of the leaf tissue in saturating light and CO2 was slightly higher in the yellow genotype. RuBPCase was also slightly higher. Quantum yield per absorbed light was similar in the two genotypes. P700 and Cyt f were less concentrated in the mutant while PS II was only marginally lower. The light response curve of CO2 assimilation indicated higher level of photosynthesis of the mutant in high light, which corresponded to a lower non-photochemical quenching compared to the wild-type. It is concluded that the reaction centres, cyt f and chlorophyll are not limiting factors of electron transport in wheat seedlings and that electron transport capacity is in excess with respect to that needed for driving photosynthesis. Since the differences in photosynthesis reflect differences in RuBPCase activity, it is suggested that this enzyme limits photosynthesis in wheat seedlings also at high light intensities.

Differences between wheat and rice in the enzymic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase and the relationship to photosynthetic gas exchange

The kinetic parameters of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (EC 4.1.1.39) in wheat (Triticum aestivum L.) and rice (Oryza sativa L.) were determined by rapidly assaying the leaf extracts. The respective K m and V max values for carboxylase and oxygenase activities were significantly higher for wheat than for rice. In particular, the differences in the V max values between the two species were greater. When the net activity of CO2 exchange was calculated at the physiological CO2-O2 concentration from these kinetic parameters, it was 22% greater in wheat than in rice. This difference in the in-vitro RuBP-carboxylase/oxygenase activity between the two species reflected a difference in the CO2-assimilation rate per unit of RuBP-carboxylase protein. However, there was no apparent difference in the CO2-assimilation rate for a given leaf-nitrogen content between the two species. When the RuBP-carboxylase/oxygenase activity was estimated at the intercellular CO2 pressure from the enzyme content and kinetic parameters, these estimated enzyme activities in wheat and rice were similar to each other for the same rate of CO2 assimilation. These results indicate that the difference in the kinetic parameters of RuBP carboxylase between the two species was offset by the differences in RuBP-carboxylase content and conductance for a given leaf-nitrogen content.