Ecophysiology of Desiccation/Rehydration Cycles in Mosses and Lichens
Ecophysiology of Desiccation/Rehydration Cycles in Mosses and Lichens
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DOI：
10.1007/978-3-642-19106-0_6
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发表时间：
2011-01-01
期刊：
影响因子：
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通讯作者：
Pintado, Ana
中科院分区：
文献类型：
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作者：
Allan Green, T. G.;Sancho, Leopoldo G.;Pintado, Ana
虽然地衣和苔藓都是杂多酸的，但这两个类群的行为似乎非常不同。苔藓植物也表现出明显的偏爱较潮湿的地区，这似乎是有机体结构不同的结果。地衣是悬浮在真菌生物体中的藻类(或蓝藻)，过量的水分通常会对光合作用产生负面影响。相比之下，苔藓植物是真正的多细胞植物，可以构建能够有效分离其光合作用和储水功能的光合作用组织。在干燥的大气条件下，地衣和苔藓会干燥到低水分，并进入休眠状态。耐受干燥的能力在群体之间和群体内都有很大的差异。有些令人惊讶的是，地衣似乎表现出比苔藓甚至一些维管束植物更不耐受长时间干燥的能力。在苔藓植物中对干燥的实际机制进行了最好的研究，似乎是结构性的，重新水化时不需要蛋白质合成来启动新陈代谢，必要的保护似乎总是存在的。例如，据报道，苔藓植物的蔗糖水平一直很高。细胞结构在干燥时通常保持不变。从干燥中恢复的情况在不同的组之间也不同，苔藓植物的水合通常较慢，但存在很大的物种差异。一般来说，回收率可能与水化活动期的长度有关，水化后迅速干燥的物种，如在岩石表面，恢复得很快。栖息地中的物种一旦水分充足，就会长时间保持潮湿，从干燥中恢复的速度似乎会更慢。除了对光和温度的光合作用反应外，地衣和苔藓植物也对藻体的水分含量有光合作用反应。从干燥的菌体开始，加水既可以增加菌体的水分含量，也可以开始光合作用和呼吸作用。在低水含量下，随着水化程度的加深，这两个过程几乎是线性增加的。光合作用在最适菌体水分含量(WCopt)时达到最大值，这与物种有很强的相关性。在这两组植物中，这种光合作用的最佳状态代表了整个细胞的膨胀。当水分含量超过这一最适表面或外部水时，会干扰二氧化碳的吸收，并会严重限制光合作用速率，特别是在地衣中。当藻体水分含量归一化为WCopt= 1时，地钱、苔藓和高等植物在WCopt以下的净光合作用(NP)响应曲线非常相似，表明了控制净光合作用的共同机制。推测这可能是一种抑制Rubisco活性的物质。与维管植物相比，这两组植物都可以在较低的、次优的叶状体水分含量和非常低的水势下进行光合作用，但这对总碳收支的贡献似乎是两组之间的主要差异。苔藓植物在干燥和补水数十分钟通常都足够的情况下，似乎能迅速通过这一含水率范围。相比之下，现在很明显，地衣通常在较低的菌体水分含量时活跃。它们不仅可以单独从潮湿的空气中水合，或者从露水和雾气中水合，而且可以非常有效地利用这些水源，通常可以实现它们每年碳增量的主要部分。关于地衣和苔藓何时真正活跃的信息最近才开始出现，但这两个类群似乎又一次不同。苔藓植物强烈喜欢较潮湿的栖息地，可以长期活跃和充分水分，似乎有…
Although both lichens and bryophytes are all poikilohydric the groups seem to behave very differently. Bryophytes also show a clear preference for wetter areas and this seems to be a result of the different structures of the organisms. A lichen is algae (or cyanobacteria) suspended in a mycobiont with excess water often having a negative effect on photosynthesis. Bryophytes, in contrast, are true multicellular plants and can construct photosynthetic tissues that can effectively separate their photosynthetic and water storage functions. Under dry atmospheric conditions lichens and bryophytes will desiccate to low water contents and they become dormant. Ability to tolerate desiccation varies considerably both between and within the groups. Somewhat surprisingly, lichens appear to show less ability to tolerate long periods of desiccation than bryophytes, and even some vascular plants. Actual mechanisms of desiccation have been best studied in bryophytes and appear to be constitutive, no protein synthesis is required on rehydration to enable the commencement of metabolism and the necessary protection appears to be always present. Consistently high sucrose levels, for instance are reported from bryophytes. Cellular structure is often maintained when desiccated. Recovery from dryness also differs between the groups with bryophytes generally hydrating more slowly but there are large species differences. In general, rate of recovery may be related to the length of the hydrated activity period, species that hydrate and then dry rapidly, as on rock surfaces, recover rapidly. Species in habitats that remain wet for long periods once hydrated appear to recover more slowly from dryness. In addition to a photosynthetic response to light and temperature, the poikilohydric lichens and bryophytes also have a photosynthetic response to thallus water content. Starting with a dry thallus, addition of water will both increase the thallus water content and also allow photosynthesis and respiration to commence. Both processes increase almost linearly with further hydration at low water contents. Photosynthesis reaches a maximum at an optimal thallus water content (WCopt) that is strongly species dependant. In both groups this photosynthetic optimum represents full cellular turgor. At water contents above this optimum surface or external water can interfere with carbon dioxide uptake and can severely limit photosynthetic rates, especially in lichens. When thallus water contents are normalised to WCopt= 1, then the net photosynthesis (NP) response curves at water contents below WCoptare very similar for liverworts, mosses and higher plants, suggesting a common mechanism in controlling NP. It is suggested that this might be an inhibitor acting on Rubisco activity. In contrast to vascular plants both groups can carry out photosynthesis at lower, suboptimal thallus water contents and very low water potentials but the contribution that this makes to total carbon budget appears to be a major difference between the groups. Bryophytes seem to pass rapidly through this water content range when both drying and hydrating for tens of minutes are often enough. In contrast, it is now apparent that lichens are often active at low thallus water contents. They can not only hydrate from humid air alone, or from dew and fog, but can use these water sources very effectively, often achieving a major part of their annual carbon gain. Information on when the lichens and bryophytes are actually active is only recently starting to appear but, again, the groups seem to differ. Bryophytes strongly prefer wetter habitats and can be active and fully hydrated for long periods and seem to have …