Raven John A, Farquhar Graham D
Department of Biological Sciences, University of Dundee, Dundee DD1 4HN, U.K.
Plant Environmental Biology Group, Research School of Biological Sciences, Australian National University, G.P.O. Box 475, Canberra, ACT 2601, Australia.
New Phytol. 1990 Nov;116(3):505-529. doi: 10.1111/j.1469-8137.1990.tb00536.x.
This paper relates the C/ C ratio of C plant material relative to that of source CO to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The C/ C ratio is quantified as Δ, defined as (δ C substrate -δ C product)/(1+δ C product), where δ C values of substrate or product (i.e. the samples) are defined as [ C/ C] ]/[( C/ C) ]-1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO ). Application of this approach to a 'typical' C land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0-24 %, for NH assimilation (which always occurs in the roots) to 2-80%, for NO assimilation in shoots with the organic acid salt which results from acid-base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N , or where NO assimilation in root or shoot is accompanied by some acid-base regulation via OH- loss to the root medium. Comparison with published data on the difference in Δ of Ricinus communis cultured with NH or NO shows that the measured influence of nitrogen source is in the right direction (NO grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non-Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH -grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.
本文阐述了C植物材料相对于源CO的C/C比与生长的氮源、植物的有机氮含量以及有机酸合成程度之间的关系。C/C比被量化为Δ，定义为(δC底物 - δC产物)/(1 + δC产物)，其中底物或产物（即样品）的δC值定义为[C/C] ]/[(C/C) ] - 1。通过将植物组成差异作为氮营养和酸合成的函数与通过Rubisco和其他羧化酶获取的植物碳分数相关联来进行计算。然后，利用这些羧化酶表现出的已知同位素分馏，在一个模型中将不同羧化酶对碳固定的分数贡献与预测植物最终Δ（相对于大气CO）相关联。将这种方法应用于“典型”的C陆地植物，得出相对于假设所有碳都通过Rubisco固定的情况，Δ下降的预测值。预测的下降范围从0 - 24%（对于NH同化，其总是在根部发生）到2 - 80%（对于在地上部同化NO并伴有酸碱平衡产生的有机酸盐，加上任何额外的有机酸盐以及游离酸，对于有机物质中基础C:N摩尔比为15的植物）。对于与N共生生长的情况，或者在根部或地上部同化NO并伴有通过向根部介质损失OH-进行一些酸碱调节的情况下，预测为中间值。与已发表的关于用NH或NO培养的蓖麻Δ差异的数据比较表明，测量到的氮源影响方向正确（用NO生长的植物Δ较小，即与不存在非Rubisco羧化时预测的值偏差更大），可以用观察到的组成差异以及因此各种羧化酶的碳分数贡献来解释。然而，氮源对Δ的影响大于模型预测的影响，即下降2.1%而不是0.10%。两种氮源上生长的植物δC差异的主要原因可能是叶片光合作用中运输和生化传导率的比率变化。这种变化在数量上与用NH生长的植物较低的水分利用效率一致。预测的以及观察到的Δ随氮源的变化幅度与在不同温度或光子通量密度下生长的C陆地物种，或在通过改变根部供水相对于地上部蒸发潜力而产生不同水分利用效率的环境中发现的变化幅度相同。氮源变化应添加到可能改变田间生长植物δ的因素中。
