NOVEL CHARACTERISTICS OF CASSAVA, MANIHOT-ESCULENTA CRANTZ, A REPUTEDC-3-C-4 INTERMEDIATE PHOTOSYNTHESIS SPECIES

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allo w this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C4-type photosynthesis in cassava l eaves. In the present work we found no evidence for a C4 metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, i ncorporation of (CO2)-C-14 into C4 organic acids in short time periods , 10 s, and the lack of C-14 transfer from C4 acids to other compounds in (CO2)-C-12, ii) by the lack of C4 enzyme activity changes during l eaf development and the inability to detect C4 acid decarboxylases, an d iii) by leaf CO2 compensation values between 49 and 65 mul of CO2 l- 1 and by other infrared gas exchange photosynthetic measurements. It i s concluded that the leaf biochemistry of cassava follows the C3 pathw ay of photosynthesis with no indication of a C3-C4 mechanism. However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with ne arly 45% of the label in sucrose in about one min of (CO2)-C-14 Photos ynthesis, contrasting with 34% in soybean (C3) and 25% in pigweed (C4) . Cassava leaves displayed a strong preference for the synthesis of su crose versus starch. Field grown cassava leaves exhibited high rates o f photosynthesis and curvilinear responses to increasing sunlight irra diances with a tendency to saturate only at high irradiances, above 15 00 mumol m-2 s-1. Morphologically, the cassava leaf has papillose epid ermal cells on its lower mesophyll surface that form 'fence-like' arra ngements encircling guard cells. It is proposed that the active synthe sis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water st atus in various environments.