Horticulture and Product Physiology, Department of Plant Sciences, Wageningen University and Research, Wageningen, The Netherlands.
Plant Lighting B.V., Bunnik, The Netherlands.
Physiol Plant. 2021 May;172(1):134-145. doi: 10.1111/ppl.13312. Epub 2020 Dec 22.
Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO uptake, via phosphoenolpyruvate carboxylase (PEPC, C carboxylation), from the diurnal refixation by Rubisco (C carboxylation). At the end of the day (CAM-Phase IV), when nocturnally stored CO has depleted, stomata reopen and allow additional CO uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO uptake via C and C carboxylation in phase IV in the CAM species Phalaenopsis "Sacramento" and Kalanchoe blossfeldiana "Saja." Short blackout periods during phase IV caused a sharp drop in CO uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.
景天酸代谢（CAM）是一种光合作用途径，它将夜间的 CO 吸收（通过磷酸烯醇丙酮酸羧化酶（PEPC，C 羧化））与白天的 Rubisco 再固定（C 羧化）暂时分开。在一天结束时（CAM 阶段 IV），当夜间储存的 CO 耗尽时，气孔重新打开并允许额外的 CO 吸收，这些 CO 可以被 Rubisco 或 PEPC 固定。本研究检查了 CAM 物种蝴蝶兰“萨克拉门托”和长寿花“萨贾”在阶段 IV 中通过 C 和 C 羧化的 CO 吸收。在阶段 IV 期间进行短暂的停电会导致长寿花的 CO 吸收急剧下降，但蝴蝶兰不会，这表明只有长寿花具有很强的 Rubisco 活性。叶绿素荧光显示，蝴蝶兰的 ΦPSII 逐渐下降，暗示 Rubisco 活性下降，而在长寿花的阶段 IV 中，ΦPSII 保持不变。然而，短时间切换到 2%的 O 表明两种物种在整个阶段 IV 都存在光呼吸和 Rubisco 活性。最后，在蝴蝶兰中，在阶段 IV 积累了淀粉。这些结果表明，在蝴蝶兰中，PEPC 是阶段 IV 的主要羧化酶，尽管 Rubisco 在整个阶段都保持活性。这将导致双重羧化（无效循环），但可能有助于避免光抑制。
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