Abstract:

Photosynthetic characteristics are considered to be significant in nitrogen use efficiency (NUE) in view of their close associations with dry matter production and grain yield in cereal crops. Photosynthetic nitrogen use efficiency (PNUE) and photosynthetic characteristics were investigated using six tropical rice (Oryza sativa L.) genotypes including two indica inbreds, two indica hybrids, and two NPT (new plant type) lines under field-grown conditions. The objectives of this study were to compare genotypic variations in agronomic efficiency (AE), PNUE, and related leaf parameters and their correlations among six rice genotypes. The results indicated that net photosynthetic rate (P_n) was higher for NPT lines than for indica inbreds, while it was mediate for indica hybrids. Variations were observed in stomatal conductance (G_s), inter CO₂ concentration (C_i), transpiration rate (T_r), and the ratio of C_i to G_s among all rice genotypes. Differences in leaf nitrogen concentration (N%) and specific leaf weight (SLW) were also found among all genotypes. The P_n per Chlorophyll unit (P_n/Chl) was lower in indica inbreds than in NPT lines while indica hybrids appeared to be middle types. There was a significantly positive correlation between P_n and leaf N content per unit leaf area (Na). Generally, both partial factor productivity of applied N (PFP) and AE of indica hybrids were higher than those of both NPT lines and indica inbreds. However, the lowest PFP and AE were observed in PSBRc18 and IR72903-121-2-1-2, respectively. PNUE was positively and significantly correlated with AE in all genotypes, suggesting that the PNUE at reproductive growth stage of rice might be a good indicator for NUE and could be utilized in evaluating genotypes with high NUE in rice.

Key words: Net photosynthetic rate (P_n), Photosynthetic nitrogen use efficiency (PNUE), Tropical rice