Biochemistry Department, University of Georgia, 30602-7229, Athens, GA, USA.
Photosynth Res. 1993 Oct;38(1):61-72. doi: 10.1007/BF00015062.
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 (14)CO2 into C4 organic acids in short time periods, 10 s, and the lack of (14)C transfer from C4 acids to other compounds in (12)CO2, 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 (14)CO2 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.
木薯植物（Manihot esculenta）在低肥力和干旱环境下生长得特别好，但允许这种生长的机制尚不清楚。早期的、有时相互矛盾的工作推测木薯叶片中存在 C4 型光合作用。在本工作中，我们没有发现成熟附着木薯叶片中有 C4 代谢的证据，这表明：i）在短时间内（10 秒），（14）CO2 仅少量（2-8%）掺入 C4 有机酸，且（12）CO2 中 C4 酸无（14）C 转移至其他化合物；ii）叶片发育过程中 C4 酶活性无变化，且无法检测到 C4 酸脱羧酶；iii）叶片 CO2 补偿值在 49 至 65 μl CO2 1(-1)之间，以及其他红外气体交换光合测量。因此，木薯叶片的生物化学遵循 C3 光合作用途径，没有 C3-C4 机制的迹象。然而，木薯叶片表现出几种新的特征。附着的叶片具有将碳有效地分配到蔗糖中的能力，约在 1 分钟的（14）CO2 光合作用中，约有 45%的标记物在蔗糖中，而大豆（C3）为 34%，猪草（C4）为 25%。木薯叶片表现出强烈地偏向于合成蔗糖而不是淀粉。在田间生长的木薯叶片中，光合作用速率较高，对增加的阳光辐照度呈曲线响应，仅在高辐照度（高于 1500 μmol m(-2) s(-1)）下才趋于饱和。形态上，木薯叶片的下叶肉表面有具乳突的表皮细胞，形成环绕保卫细胞的“栅栏状”排列。据推测，糖的主动合成在木薯植物中具有渗透功能，而乳突状表皮细胞的功能是在各种环境中维持健康的叶片水分状态。
