ISSN:
1573-5079
Keywords:
sucrose
;
irradiance intensity
;
papillose cells
;
epidermis
;
14CO2 fixation
;
CO2 compensation point
;
starch partitioning
;
C3 photosynthesis
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
Abstract The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C4-type photosynthesis in cassava leaves. 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%, incorporation of 14CO2 into C4 organic acids in short time periods, 10 s, and the lack of 14C transfer from C4 acids to other compounds in 12CO2, ii) by the lack of C4 enzyme activity changes during leaf development and the inability to detect C4 acid decarboxylases, and iii) by leaf CO2 compensation values between 49 and 65 μl of CO2 1−1 and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C3 pathway 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 nearly 45% of the label in sucrose in about one min of 14CO2 photosynthesis, contrasting with 34% in soybean (C3) and 25% in pigweed (C4). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 μmol m−2 s−1. Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form ‘fence-like’ arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00015062
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