Novel photosynthetic characteristics of cassava, Manihot esculenta Crantz, a reputed C3-C4 intermediate crop species

The biochemical pathway of CO2 fixation in cassava leaves was reported initially as C3 in work from Canada. But cassava is a tropical root crop and later work from Colombia in South America reported it was a C3/C4 intermediate photosynthesis plant. More recently it was again reported to be a C3-type plant from Washington State. It has been claimed that these differences are because cassava is a tropical plant. Because of its importance as a productive crop on low fertility soils, in subsistence-type agriculture, we have investigated its leaf carbon metabolism further by working with plants grown both in Athens, GA, and Cali, Colombia. Our data show that whole leaf photosynthesis values can be quite high in air, indeed comparable to C4 plants. But other gas exchange data such as leaf CO2 compensation values and several photorespiration measurements, show C3 values. The products of (14)CO2 leaf labeling experiments and their turnover in (12)CO2 air show C3 patterns with no hing of C4 organic acids. However, cassava exhibits a rapid and high labeling of sucrose which is novel for C3 photosynthesis and its exhibits some high values for certain C3 carbon cycle enzymes. Thus, cassava leaf photosynthesis, in both Athens and the tropics, is C3; however, the leaves exhibit vigorous photosynthesis and have a very active sugar metabolism which likely contributes to the high productivity of the cassava crop in subsistence tropical agriculture.