Effect of nitrate limitation on the photosynthetically active pools of aspartate and malate in maize, a NADP malic enzyme C4 plant |
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Authors: | Saadi Khamis Thierry Lamaze Jack Farineau |
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Affiliation: | Laboratoire du Métabolisme et de la Nutrition des Plantes, INRA, F-78026 Versailles Cedex, France;Section de Bioénergétique, Dépt de Biologie cellulaire et moléculaire, CEN Saclay, F-91191 Gif-sur-Yvette Cedex, France. |
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Abstract: | After two weeks of moderate N restriction, growth of 3-week-old Zea mays L. plants was less than half that of the control and aspartate and malate levels in the leaves were severely suppressed (45 and 65% decrease, respectively). Since in NADP malic enzyme type C4 plants, such as maize, malate and aspartate are intermediates in the C4 photosynthetic pathway, the operation of the latter was investigated. Moderate nitrogen deficiency had only a small effect on the rate of photosynthesis (20% decrease) measured under 1000 umol m?2 s?1 irradiance. 14CO2 pulse-12CO2 chase experiments combined with measurements of in vitro photosynthetic enzyme activities demonstrated the operation of a typical C4 photosynthetic pathway in N-restricted plants. The turnover rates of malate and aspartate molecules involved in the C4 cycle were determined by the loss of label in the carbon 4 moiety of these molecules during the chase period. It is shown that N restriction did not alter the turnover of malate but greatly accelerated that of aspartate. The amounts of malate and aspartate moving through photosynthetically active pools were estimated using a kinetic model. For malate, the size of this pool appeared to be only slightly diminished whereas for aspartate the size of the corresponding pool decreased by a factor of 3. It is proposed that under moderate NO3? deficiency, despite deviations in malate metabolism leading to a pronounced decrease in the size of its cellular pool, a large amount of malate remained in the operation of the C4 pathway. By contrast, the participation of aspartate in the operation of the C4 pathway was greatly reduced. |
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Keywords: | Aspartate C4 photosynthesis maize malate nitrate limitation nitrogen deficiency photosynthetic intermediates Zea mays. |
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