The relationship between ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase (Rca) levels was studied using transgenic rice overexpressing maize Rca (OX-mRca) and knockdown transgenic rice expressing antisense Rca (KD-Rca). The ratio of Rubisco to total soluble protein was lower in OX-mRca, whereas it was higher in KD-Rca than in WT, indicating that Rca expression was negatively correlated with Rubisco content. The expressions of other Calvin-Benson-Bassham cycle enzymes such as sedoheptulose-1,7-bisphosphatase and phosphoribulokinase analyzed by immunoblotting did not show such a negative correlation with Rca, suggesting that the effect of Rca on protein expression may be specific for Rubisco. Although Rubisco content was decreased in OX-mRca, the transcript levels of the Rubisco large subunit (OsRbcL) and the Rubisco small subunit mostly increased in OX-mRca as well as in KD-Rca. Additionally, polysome loading of OsRbcL was slightly higher in OX-mRca than it was in WT, suggesting that the OsRbcL translation activity was likely stimulated by overexpression of Rca. S-methionine labeling experiments demonstrated that there was no significant difference in the stability of newly synthesized Rubisco among genotypes. However,S-methionine-labeled Rubisco was marginally decreased in OX-mRca and increased in KD-Rca compared to the WT. These results suggest that Rca negatively affects the Rubisco content, possibly in the synthesis step.
Relationships of six vegetation indexes to the aboveground biomass and nitrogen content at tillering and stem elongation stages were compared in a wheat cultivar 'Satonosora'; normalized difference vegetation index (NDVI) measured with the portable crop growth sensor GreenSeeker, NDVI, green NDVI (GNDVI), simple ratio (SR) and chlorophyll index green(CI green ) measured with a spectroscope, and plant canopy coverage ratio (PCC) calculated from digital images. The aboveground biomass was highly related to PCC, followed by GNDVI and CI green . These relationships were stronger than that to NDVI. The values of PCC, GNDVI and CI green fitted with nitrogen content better than NDVI especially at the stem elongation stage. Unlike the other indexes, CI green was not saturated even during the stem elongation stage, suggesting that it is suitable for estimating the nitrogen content at this stage. While PCC increased consistently throughout the survey period, the other vegetation indexes decreased temporarily under a dry soil condition at an early developmental stage. Therefore, for the indexes obtained with a spectroscope, an improved measurement method that is resistant to environmental fluctuation including soil moisture would better explain biomass and nitrogen content in wheat.
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