Raven John A
Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, UK.
New Phytol. 1985 Sep;101(1):25-77. doi: 10.1111/j.1469-8137.1985.tb02816.x.
The benefits which this paper addresses are those of maintaining the intracellular acid-base balance during growth, and of generating osmolarity related to regulation of turgor in environments of low water potential. These benefits may incur costs in terms of the quantity of potentially growth-limiting resources (photons, water, nitrogen) which are needed to produce unit quantity of 'baseline' plant biomass. The direction (excess H or excess OH ) and magnitude of acid-base perturbation during growth depends on the nature of the N-source (NH , N or NO ), so that the costing of pH homoiostasis involves consideration of the costs of overall N-assimilation for comparison with the other costs of growth of a terrestrial C plant. Photon costs for the various biochemical and transport processes involved in overall growth, N-assimilation, pH regulation and osmolarity generation are computed using known stoichiometries of coupled reactions. Water costs are deduced from the C-requirements for the various processes (including C lost in associated decarboxylations) by assuming a constant value of water lost in transpiration per unit net C fixed in an illuminated shoot. Nitrogen costs are deduced from the N-content of the plants or compounds under consideration. The computed costs for N-assimilation and the generation of osmolarity are referred to the costs of 'baseline' plant synthesis using the cheapest mechanisms (NH as source for N-assimilation; inorganic ions as the basis for osmolarity generation) so that the increment of cost related to assimilation of N or NO , or of osmolarity generation using an organic compatible solute, can be presented. Photon costs of growth with N fixation and the processes associated with regulation of pH are (granted the assumptions made as to stoichiometries and plant composition) 9 % higher than are those of growth with NH as N˜ source. The predicted cost of growth with NO as N source depends on the location of NO reduction and the mechanism of OH disposal, and ranges from 5 to 12% more than that for growth with NH as N source. H O (transpiration) costs follow a similar pattern, with growth on N as N source costing 12% more, and growth on NO costing to 1-2 to 167 % more, than growth with NH as N source. The extra costs in photons of using compatible solutes (sorbitol, proline or glycine betaine) to generate an osmolarity of 500 osmol m in all of the non-apoplastic water of the plant add 21·5 to 26·1 % to the total costs of growth, while use of compatible solutes to generate osmolarity in 'N' phases (i.e. cytosol, plastid stroma, mitochondrial matrix) alone would add 5·2 to 6·2% The costs of growth in terms of transpirational water are increased 7·9 to 98 % by the use of compatible solutes for osmolarity generation in the 'N' phases only. The increments for the N-containing solutes are higher when NO is the N-source rather than NH . The N-cost of growth with N-containing compatible solutes generating 500 osmol m in 'N' phases increases the N cost of growth by 33%. These predicted costs are under-estimates of 'real' costs which take into account the occurrence of alternate oxidase activity under some growth conditions and the production of additional organic acid anions with N as opposed to NH as N source. Nevertheless, the predicted minimum costs of attaining the benefits of pH regulation and of turgor generation are of use in suggesting where selectively significant (i.e. low requirement for a scarce resource) alternative mechanisms may occur. Examples include a possible photon saving by using NH rather than N or NO where all three are available; a possible water saving by use of photoreduction of NO in leaves in arid environments; and a possible N saving by use of non-N-containing compatible solutes (polyols) in environments of low water potential. Proof of these suggestions involves comparisons of inclusive fitness of genotypes possessing the trait under consideration with that of genotypes lacking the trait. CONTENTS Summary 26 I. Introduction 27 II. pH Regulation and Osmolarity Generation 27 III. Photon Costs of Various Syntheses Related to pH Regulation and Osmolarity Generation 31 IV. Conclusions on Energy Costs of pH Regulation During Nitrogen Assimilation and Growth 56 V. Conclusions on Energy Costs of Osmolarity Generation 60 VI. Water Costs of pH Regulation and Nitrogen Assimilation 61 VII. Water Costs of Osmolarity Generation 67 VIII. Nitrogen Costs of Osmolarity Generation 69 IX. Conclusions 70 Acknowledgements 72 References 73.
本文探讨的益处包括在生长过程中维持细胞内酸碱平衡，以及在低水势环境中产生与膨压调节相关的渗透压。这些益处可能会带来成本，即生产单位数量的“基准”植物生物量所需的潜在生长限制资源（光子、水、氮）的数量。生长过程中酸碱扰动的方向（过量的H或过量的OH）和幅度取决于氮源（NH₄⁺、N₂或NO₃⁻）的性质，因此pH稳态的成本核算涉及考虑整体氮同化的成本，以便与陆地C植物生长的其他成本进行比较。利用已知的偶联反应化学计量关系，计算了整体生长、氮同化、pH调节和渗透压产生过程中各种生化和运输过程的光子成本。通过假设在光照下的枝条中每固定单位净碳时蒸腾损失的水的恒定值，从各种过程（包括相关脱羧过程中损失的碳）的碳需求中推导出水成本。氮成本从所考虑的植物或化合物的氮含量中推导得出。将计算得出的氮同化和渗透压产生的成本与使用最便宜机制（NH₄⁺作为氮同化源；无机离子作为渗透压产生的基础）进行“基准”植物合成的成本进行比较，以便能够呈现与同化NH₄⁺或NO₃⁻相关的成本增加，或使用有机相容性溶质产生渗透压的成本增加。在假设化学计量关系和植物组成的情况下，固氮生长以及与pH调节相关过程的光子成本比以NH₄⁺作为氮源的生长高9%。以NO₃⁻作为氮源的生长预测成本取决于NO₃⁻还原的位置和OH处置的机制，比以NH₄⁺作为氮源的生长高5%至12%。H₂O（蒸腾）成本遵循类似模式，以N₂作为氮源的生长成本比以NH₄⁺作为氮源的生长高12%，以NO₃⁻作为氮源的生长成本比以NH₄⁺作为氮源的生长高12%至167%。在植物所有非质外体水中使用相容性溶质（山梨醇、脯氨酸或甘氨酸甜菜碱）产生500 osmol m⁻³的渗透压，光子额外成本使总生长成本增加21.5%至26.1%，而仅在“N”相（即细胞质、质体基质、线粒体基质）中使用相容性溶质产生渗透压将增加5.2%至6.2%。仅在“N”相中使用相容性溶质产生渗透压，蒸腾水方面的生长成本增加7.9%至98%。当NO₃⁻作为氮源而非NH₄⁺时，含氮溶质的增量更高。在“N”相中产生500 osmol m⁻³的含氮相容性溶质的生长氮成本使生长氮成本增加33%。这些预测成本是对“实际”成本的低估，实际成本考虑了某些生长条件下交替氧化酶活性的发生以及与以NH₄⁺作为氮源相比，以NO₃⁻作为氮源时额外有机酸阴离子的产生。然而，预测实现pH调节和膨压产生益处的最低成本有助于指出可能存在选择性显著（即对稀缺资源需求低）替代机制的地方。示例包括在三种氮源都可用时，使用NH₄⁺而非N₂或NO₃⁻可能节省光子；在干旱环境中，通过叶片中NO₃⁻的光还原可能节省水；在低水势环境中，使用不含氮的相容性溶质（多元醇）可能节省氮。对这些建议的验证涉及比较具有所考虑性状的基因型与缺乏该性状的基因型的综合适应性。目录摘要26一、引言27二、pH调节和渗透压产生27三、与pH调节和渗透压产生相关的各种合成的光子成本31四、氮同化和生长过程中pH调节能量成本的结论56五、渗透压产生能量成本的结论60六、pH调节和氮同化的水成本61七．渗透压产生的水成本67八、渗透压产生的氮成本69九、结论70致谢72参考文献73
