Comparison Of Photosynthetic Characteristics For C₃ And C₄ Plants

Terrestrial plants are classified into three major photosynthetic types, namely, C3, C4 and Crassulacean acid metabolism (CAM) plants, according to the mechanism of their photosynthetic carbon assimilation. C4 plants have CO2 concentrating mechanism and higher photosynthetic efficiency than C3 plants, especially under high light intensity, high temperature, high oxygen partial pressure and drought conditions. Since the discovery of the C4 pathway, it has been postulated that the transfer of C4 traits to C3 plants should improve the photosynthetic performance of C3 plants. Initially, conventional hybridization between C3 and C4 plants was carried out. This approach was available only in several plant genera and most C3-C4 hybrids were infertile. Another approach that has been adopted in the last ten years involves the use of recombinant DNA technology.Our objective was to provide the mechanism of high photosynthetic efficiency of C4 plants. We studied on net photosynthesis, key enzyme activities, stomata conductance and chlorophyll fluorescence parameters in C3 plants, C4 plants and rice-Echinochloa rusgalli hybrids. The major results were as follows:1.Compared with C3 plants, C4 plants have higher photosynthetic rate. In different periods, photosynthetic rate is higher than C3 plants and the advantage decreases with the plants maturing. In rapid growing period C4 plants' advantage of photosynthesis is over 30%. C4 plants are a family, so their photosynthetic rate is different such as Echinochloa rusgalli has highest photosynthetic rate than any other plants. But some C4 plants even tower than some C3 plants in the same environmental condition. And as the environment is changed, the photosynthesis are also changed.2.Phosphoewo/pyruvate carboxylase (PEPC) is the key enzyme of photosynthetic carbon assimilation in C4 plants and C3 plants also have PEPC. But PEPC activity is lower in C3 plant. C4 plants concentrate CO2 around Rubisco through the use of dual carboxylation pathways, first by CO2 fixation with PEPCase and secondarily by Rubisco. Because C4 plants, such as maize, are capable of concentrating CO2 at the Rubisco active site, they have many desirable agronomic traits. And the RuBPCase activity in C4 plants is higher than in C3 plants because ofconcentrating CO2 at the site. The value of PEPCase/RuBPCase dedicates the C4 photosynthetic capacity and is higher in C4 plants than in C3 plants.3. Stomata characteristic is very important to improve photosynthetic capacity. The stomata of C4 plants such as Echinochloa rusgalli, maize, are larger than C3 plants such as rice and stomata density of C4 plants is less than C3 plants. Stomata of rice and its progenies of hybrids are smaller than C4 plants, but the stomata of progenies of hybrids (YFGL) are less larger than their mother. Stomata density of progenies of hybrids (YFGL) is higher than their parents, so it is possible to select plants of high photosynthetic capacity in progenies of YFGL. Using conventional hybridization or transgenic technology to improve photosynthetic capacity ofrice in our country, it is important to choose correct C4 plants which have high photosynthetic capacityand water use efficiency such as Echinochloa rusgalli and sorghum.