Yiotis C, Evans-Fitz Gerald C, McElwain J C
Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Ireland.
School of Biology and Environmental Science, University College Dublin, Belfield, Ireland.
Ann Bot. 2017 Jun 1;119(8):1385-1395. doi: 10.1093/aob/mcx018.
Fluctuations in [CO 2 ] have been widely studied as a potential driver of plant evolution; however, the role of a fluctuating [O 2 ]:[CO 2 ] ratio is often overlooked. The present study aimed to investigate the inherent physiological plasticity of early diverging, extant species following acclimation to an atmosphere similar to that across the Triassic-Jurassic mass extinction interval (TJB, approx. 200 Mya), a time of major ecological change.
Mature plants from two angiosperm ( Drimys winteri and Chloranthus oldhamii ), two monilophyte ( Osmunda claytoniana and Cyathea australis ) and one gymnosperm ( Ginkgo biloba ) species were grown for 2 months in replicated walk-in Conviron BDW40 chambers running at TJB treatment conditions of 16 % [O 2 ]-1900 ppm [CO 2 ] and ambient conditions of 21 % [O 2 ]-400 ppm [CO 2 ], and their physiological plasticity was assessed using gas exchange and chlorophyll fluorescence methods.
TJB acclimation caused significant reductions in the maximum rate of carboxylation ( V Cmax ) and the maximum electron flow supporting ribulose-1,5-bisphosphate regeneration ( J max ) in all species, yet this downregulation had little effect on their light-saturated photosynthetic rate ( A sat ). Ginkgo was found to photorespire heavily under ambient conditions, while growth in low [O 2 ]:[CO 2 ] resulted in increased heat dissipation per reaction centre ( DI o / RC ), severe photodamage, as revealed by the species' decreased maximum efficiency of primary photochemistry ( F v / F m ) and decreased in situ photosynthetic electron flow ( Jsitu ).
It is argued that the observed photodamage reflects the inability of Ginkgo to divert excess photosynthetic electron flow to sinks other than the downregulated C 3 and the diminished C 2 cycles under low [O 2 ]:[CO 2 ]. This finding, coupled with the remarkable physiological plasticity of the ferns, provides insights into the underlying mechanism of Ginkgoales' near extinction and ferns' proliferation as atmospheric [CO 2 ] increased to maximum levels across the TJB.
二氧化碳浓度的波动作为植物进化的潜在驱动因素已得到广泛研究；然而，氧气与二氧化碳比例波动的作用却常常被忽视。本研究旨在探究早期分化的现存物种在适应类似于三叠纪 - 侏罗纪大灭绝时期（约2亿年前的TJB）的大气环境（这是一个主要生态变化时期）后的内在生理可塑性。
将两种被子植物（冬叶盖裂木和老鸦瓣）、两种蕨类植物（阴地蕨和澳洲桫椤）以及一种裸子植物（银杏）的成熟植株，在模拟TJB处理条件（16%氧气 - 1900 ppm二氧化碳）和环境条件（21%氧气 - 400 ppm二氧化碳）的Conviron BDW40步入式气候箱中重复培养2个月，并用气体交换和叶绿素荧光方法评估它们的生理可塑性。
适应TJB环境导致所有物种的最大羧化速率（V Cmax）和支持核酮糖 - 1,5 - 二磷酸再生的最大电子流（J max）显著降低，但这种下调对它们的光饱和光合速率（A sat）影响不大。研究发现，银杏在环境条件下光呼吸强烈，而在低氧气与二氧化碳比例环境中生长会导致每个反应中心的热耗散增加（DI o / RC），出现严重光损伤，这表现为该物种的初始光化学最大效率（F v / F m）降低以及原位光合电子流（Jsitu）减少。
有人认为，观察到的光损伤反映了银杏在低氧气与二氧化碳比例环境下，无法将过量的光合电子流转移到除下调的C3和减少的C2循环之外的其他途径。这一发现，再加上蕨类植物显著的生理可塑性，为银杏目近乎灭绝以及蕨类植物在TJB期间随着大气二氧化碳浓度增加到最高水平而大量繁殖的潜在机制提供了见解。
