C-4 ACID METABOLISM AND DARK CO2 FIXATION IN A SUBMERSED AQUATIC MACROPHYTE (HYDRILLA-VERTICILLATA)
C-4 ACID METABOLISM AND DARK CO2 FIXATION IN A SUBMERSED AQUATIC MACROPHYTE (HYDRILLA-VERTICILLATA)
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DOI：
10.1104/pp.65.2.331
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发表时间：
1980-01-01
期刊：
影响因子：
7.4
通讯作者：
BOWES, G
中科院分区：
文献类型：
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作者：
HOLADAY, AS;BOWES, G
沉水水生大型植物 H. verticillata 的 CO2 补偿点从高值（高于 50 μl/l）到低值（10-25 μl/l）变化，具体取决于生长条件。冬季或在 11 摄氏度下孵化后从湖中采摘的植物。 C/9小时光周期具有较高的值，而夏季植物或在27℃下培育的植物则具有较高的值。 C/14 h 光周期和低值。具有低CO2补偿点的植物表现出暗14CO2固定率高达光固定率的30%。这种固定减少了呼吸二氧化碳的损失，但并没有导致夜间二氧化碳的净吸收。低补偿点植物还表现出可滴定酸的昼夜波动，例如景天科酸代谢植物中发生的情况。具有高 CO2 补偿点的黑藻植物的暗固定和昼夜酸波动可以忽略不计。将低补偿点植物暴露于20μM 14CO2导致60％的14C被掺入苹果酸和天冬氨酸，只有16％进入糖磷酸盐。在高 CO2 水平下，C4 酸标记降低。脉冲追踪研究表明，除了天门冬氨酸之外，苹果酸中的 14C 在长时间（270 秒）的追踪期后会下降；因此，C4 酸的周转比 C4 植物慢得多。在具有低补偿点的植物中，与核酮糖二磷酸羧化酶(20-25)相比，磷酸烯醇丙酮酸羧化酶活性高(330μmol/mg叶绿素每小时)。这些植物的叶子中还具有每小时41μmol/mg叶绿素的丙酮酸、Pi二激酶活性，这表明它们不是C3植物。 NAD-和NADP+-苹果酸脱氢酶活性分别为每小时6136μmol/mg叶绿素和24.5μmol/mg叶绿素。在所测定的3种脱羧酶中，NAD-和NADP-苹果酸酶的活性分别为104.2和23.7μmol/mg叶绿素每小时，而磷酸烯醇丙酮酸羧激酶仅为0.2。低补偿点黑藻植物将一些二氧化碳固定为 C4 酸，这些酸可以脱羧以供以后重新固定，大概进入卡尔文循环。在夏季湖泊环境中，二氧化碳浓度在夜间较高但白天较低，重新固定将是有利的。黑藻不适合任何现有的光合作用类别，可能必须与其他具有环境可变二氧化碳补偿点的沉水大型植物一起归入一个新组。
The CO2 compensation point of the submersed aquatic macrophyte H. verticillata varied from high (above 50 .mu.l/l) to low (10-25 .mu.l/l) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11.degree. C/9 h photoperiod had high values, whereas summer plants or those incubated in a 27.degree. C/14 h photoperiod and low values. The plants with low CO2 compensation points exhibited dark 14CO2 fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO2 loss, but did not result in a net uptake of CO2 at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. Dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO2 compensation points. Exposure of the low compensation point plants to 20 .mu.M 14CO2 resulted in 60% of the 14C being incorporated into malate and aspartate, with only 16% in sugar phosphates. At a high CO2 level, the C4 acid label was decreased. A pulse-chase study indicated that the 14C in malate, but for asparate, decreased after a long (270 s) chase period; thus, the C4 acid turnover was much slower than in C4 plants. Phosphoenolpyruvate carboxylase activity was high (330 .mu.mol/mg chlorophyll per h), as compared to ribulose bisphosphate carboxylase (20-25), in the plants with low compensation points. These plants also had a pyruvate, Pi dikinase activity in the leaves of 41 .mu.mol/mg chlorophyll per h, which suggests they are not C3 plants. NAD- and NADP+-malate dehydrogenase activities were 6136 and 24.5 .mu.mol/mg chlorophyll per h, respectively. Of the 3 decarboxylating enzymes assayed, the activities of NAD- and NADP-malic enzyme were 104.2 and 23.7 .mu.mol/mg chlorophyll per h, while phosphoenolpyruvate carboxykinase was only 0.2. Low compensation point Hydrilla plants fix some CO2 into C4 acids, which can be decarboxylated for later refixation, presumably into the Calvin cycle. Refixation would be advantageous in summer lake environments where the CO2 levels are high at night but low during the day. Hydrilla does not fit any of the present photosynthetic categories, and may have to be placed into a new group, together with other submersed aquatic macrophytes that have environmentally variable CO2 compensation points.