| In this paper, we studied the cellular localization of Rubisco and Rubisco activase (RCA) in rice (Oryza sativa subsp. indica cv. Zhenong 952) leaf with immunogold-labeled electron microscope techniques on the basis of determining the diurnal changes of net photosynthetic rate (Pn), Rubisco and RCA activities, and quantifying two enzyme contents in the leaf with immuno-diffusion method in order to understand why Rubisco and RCA activities decreased in the midday when their contents were high. Meanwhile, we investigated the Pn and dynamic changes of Rubisco and RCA of flag leaf during the senescence, surveyed the physiological and biochemical characteristics of the different leaves in a whole rice plant, the 11th leaf and flag leaf from their extending to dying, respectively. Moreover, we analyzed the physiological relationships between the kinetic changes of 11th leaf or flag leaf during that time and the positional changes of the leaves for the whole plant at 10-leaf stage. These studies can help us to understand further the physiological and biochemical characteristics or the regulatory mechanisms of senescence in rice leaves.Our results showed that peaks of Pn, initial Rubisco carboxylation activities (IRA) and RCA activities appeared around 8:00 and 14:00, respectively, and the midday-depressions occurred around 13:00. Comparing the changes of Pn and initial Rubisco activity, the highest peak of RCA was around 14:00 rather than around 8:00. The results of quantification of Rubisco and RCA using immuno-diffusion method showed a trend that contents of Rubisco and RCA were not abruptly fluctuated during a day compared with their activities. Rubisco content increased from 6:00 to 8:00, and it kept rather constant thereafter. Immunogold-labeled electron microscope techniques indicated that Rubisco mainly located in chloroplast, and RCA were found both in chloroplasts and mitochondria. The decline of Rubisco in chloroplast as well as Rubisco activity at noon can be one of good reasons to explain the photosynthetic midday depression in leaf. The density of RCA in chloroplasts reached the maximum at 14:00 and a valley at 11:00. The result was much coincided with the activity of RCA in leaf. In mitochondria, the density of RCA changed abruptly in one day with the highest at 13:00, and itcan well elucidate why the activities of Rubisco declined at noon when its amount was increasing. Therefore the cellular localization and/or distribution of Rubisco and RCA during a day is much important for Pn, Rubisco and RCA activity.The decreasing of Pn in flag leaf can be obviously divided into two phases, one was a slow decline occurred during 7-14d after heading, the other was a rapid one initiated at 15d after heading. The falling pattern of Pn corresponded with those of the initial Rubisco and RCA activities as well as RCA content.In different leaves of the whole rice plant, the chlorophyll content, Hill reaction activities, soluble protein contents and SOD activities increased first and then declined, while the insoluble protein contents kept steady, and the malondialdehyde (MDA) contents increased continually as lowering of leaf position. The decreasing of those physiological and biochemical characteristics was also found in the 11th leaf or flag leaf during their senescence.The analysis of the relationship showed a positive correlation between the senescence patterns for all characteristics except for the contents of chlorophyll and insoluble protein in the 11th leaf or flag leaf and the positional leaves of a whole plant at 10-leaf stage. The R2 for the Hill reaction activities was the highest (0.78-0.8695), the R2 for MDA contents was second (0.6368-0.6936), and the following were soluble protein contents (0.558-0.6188). For SOD activities, there was no positive correlation between the flag leaf and the positional leaves, but the determinant coefficient between the 11th leaf and the positional leaves of a whole plant was high (0.6603).It intimated that the senescence-related changes of different leaves...
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