Nitrogen partitioning among proteins in chloroplasts and mitochondria was examined in pea (Pisum sativum L.) and wheat (Triticum aestivum L.) grown hydroponically with different nitrogen concentrations. In pea leaves, chloroplast nitrogen accounted for 75 to 80% of total leaf nitrogen. We routinely found that 8% of total ribulose-1,5-bisphosphate carboxylase/oxygenase adhered to thylakoids during preparation and could be removed with Triton X-100. With this precaution, the ratio of stroma nitrogen increased from 53 to 61% of total leaf nitrogen in response to the nitrogen supply, but thylakoid nitrogen remained almost constant around 20% of total. The changes in the activities of the stromal enzymes and electron transport in response to the nitrogen supply reflected the nitrogen partitioning into stroma and thylakoids. On the other hand, nitrogen partitioning into mitochondria was appreciably smaller than that in chloroplasts, and the ratio of nitrogen allocated to mitochondria decreased with increasing leaf-nitrogen content, ranging from 7 to 4% of total leaf nitrogen. The ratio of mitochondrial respiratory enzyme activities to leafnitrogen content also decreased with increasing leaf-nitrogen content. These differences in nitrogen partitioning between chloroplasts and mitochondria were reflected in differences in the rates of photosynthesis and dark respiration in wheat leaves measured with an open gas-exchange system. The response of photosynthesis to nitrogen supply was much greater than that of dark respiration, and the CO2 compensation point decreased with increasing leaf-nitrogen content.Nitrogen is the most important element for higher plants, and plant productivity is to a large extent determined by nitrogen nutrition. Photosynthesis and respiration are two major physiological processes in plants that determine plant productivity, but there are few studies of the balance between photosynthesis and respiration in relation to nitrogen nutrition. Much attention has been paid to the relationship between photosynthesis, nitrogen nutrition, and the role of Rubisco in nitrogen use efficiency (7,15,30 thylakoids (24). We have taken precautions against this problem. Nitrogen allocation to mitochondria has not been examined in previous studies, even though enzymes of photorespiratory decarboxylation are major protein components of these organelles (6, 10, 28). We anticipate that even though mitochondrial nitrogen is probably a quantitatively smaller component of total leaf nitrogen, it might be responsive to factors associated with photorespiratory activities, such as Rubisco amount and activity, and therefore to nitrogen nutrition and CO2 concentration.In this study, we grew pea and wheat hydroponically with different nitrogen concentrations at ambient CO, and examined the effects on the nitrogen distribution in chloroplasts and mitochondria. Several enzyme activities located in these organelles were also measured, and the results were related to observations of photosynthesis and respiration measured by...

The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chi), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chi was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or Chi in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C3 plants examined, which had 10-to 30-fold higher carbonic anhydrase activity than wheat at any leafnitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO2-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport. ach, clarified the relation between nitrogen nutrition and nitrogen partitioning into the various photosynthetic components and activities. They found that although nitrogen supply increased the ratio of Rubisco activity to electron transport activity, ATPase, Chl, or total leaf nitrogen, the balance between the in vivo activities of Rubisco and electron transport remained constant. They concluded that this difference was compensated for by the presence of a C02-transfer resistance between intercellular air spaces and the carboxylation sites. As a result of this resistance, the in vivo Rubisco specific activity was reduced progressively with increasing amount of enzyme because the partial pressure of CO2 at the carboxylation sites was reduced and kept in a constant balance with electron transport activity. The increase in the ratio of Rubisco to total leaf nitrogen or Chl with nitrogen supply is frequently found for other C3 species, such as tobacco (1), cotton (32), Solanum (11), bean (26), and pea (18).However, in spite of the existence of significant C02-transfer resistance in wheat (5,8,23, 30), the ratio of Rubisco to total leaf nitrogen or Chl in fully expanded young leaves seems to be independent of nitrogen nutrition (5,17,18 Plant Physiol. Vol. 100, 1992 CO2 diffusion to the carboxylation sites remains uncertain. In addition, the response of CA activity to changing nitrogen content is not known.In this study, we used fully expanded, youn...

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Scaling is a naturally iterative and bi-directional component of problem solving in ecology and in climate science. Ecosystems and climate systems are unquestionably the sum of all their parts, to the smallest imaginable scale, in genomic processes or in the laws of fluid dynamics. However, in the process of scaling-up, for practical purposes the whole usually has to be construed as a good deal less than this. This essay demonstrates how controlled large-scale experiments can be used to deduce key mechanisms and thereby reduce much of the detail needed for the process of scaling-up. Collection of the relevant experimental evidence depends on controlling the environment and complexity of experiments, and on applications of technologies that report on, and integrate, small-scale processes. As the role of biological feedbacks in the behavior of climate systems is better appreciated, so the need grows for experimentally based understanding of ecosystem processes.We argue that we cannot continue as we are doing, simply observing the progress of the greenhouse gas-driven experiment in global change, and modeling its future outcomes. We have to change the way we think about climate system and ecosystem science, and in the process move to experimental modes at larger scales than previously thought achievable.