Physiological efficiency under different nitrogen nutrition conditions and relationship with photosynthetic N-use efficiency in Myanmar local rice varieties (Oryza sativa L.)

Rice genotypes with improved N-use efficiency are becoming a prerequisite for optimizing grain yield, reducing production cost, and developing environmentally friendly cropping systems. The physiological efficiency of grain (PE-grain) and biomass (PE-biomass), photosynthetic N-use efficiency (PNUE), and in vivo carboxylation efficiency of Rubisco (Pn/Rubisco) under varying N nutrition conditions were investigated in two Myanmar varieties (Bawgyi and Sawbwa), an indica variety (IR24), and a japonica control variety (Nipponbare). Nipponbare and IR24 had significantly higher grain yield and PE-grain at both soil N and standard N supply, Bawgyi exhibited moderately high grain yield and PE-grain, and Sawbwa had a low grain yield and PE-grain at both N levels. Nipponbare and IR24 had significantly higher PE-biomass at all N levels; while Bawgyi had lower PE-biomass than Nipponbare and IR24 at all N levels, its PE-biomass was significantly higher than that of Sawbwa at the low N level. Net photosynthesis (Pn) of Bawgyi was not significantly different from that of Nipponbare and IR24, but was higher than that of Sawbwa, even with no significant difference in leaf N content. Nipponbare, IR24, and Bawgyi showed no significant difference in PNUE, but had significantly higher PNUE than Sawbwa, especially at low and standard N levels. There was no significant difference in Pn/Rubisco among Bawgyi, Nipponbare, and IR24, but Pn/Rubisco was significantly higher in these varieties than in Sawbwa at low and standard N levels. PE-biomass corresponded well to PE-grain and showed significant positive correlations with PNUE and Pn/Rubisco, indicating that rice genotypes with high physiological efficiency have high Pn with low leaf N and low Rubisco content with high Rubisco efficiency. These results suggest that, of the two Myanmar rice varieties, Bawgyi is the more desirable genotype, with considerably higher physiological efficiency and high photosynthetic activity through the highly efficient use of Rubisco.