Genotypic differences in phosphorus use efficiency and yield of wetland rice of southern Viet Nam

This study was conducted during 1992 to 1994 with the objective of assessing genotypic differences in phosphorus use efficiency of wetland rice (Oriza sativa L.) and to select genotypes which grow well on soils low in available P or requiring only small amounts of added phosphorus fertilizer. In the first experiment, 24 genotypes of rice were screened under field conditions at low P (0 P added) and high P (90 kg P2O5/ha) on a gray soil of South Viet Nam. Significant genotypic differences were found in straw and grain yield, total P uptake, plant height and phosphorus use efficiency. Grain yield was highly correlated with phosphorus use efficiency. The effect of P supply was generally small. On average P supply decreased straw yield and increased grain yield. In the second experiment, four genotypes selected from the first experiment were grown under greenhouse conditions for four weeks supplied with 0, 30 and 90 ppm. Dry weight of shoots did not respond to P fertilization, but dry weight of roots increased with increasing P supply. The two efficient genotypes, G6 and G22, showed a higher root/shoot ratio than the two inefficient genotypes, G11 and G19. This indicates a higher potential for P acquisition of G6 and G22. In the third experiment, the same contrasting genotypes, and G18 with intermediate efficiency, were grown under field conditions at 0, 30, 60, 90 and 120 kg P2O5/ha. Of the two inefficient genotypes, G11 gave the highest grain yield at 60 and G19 at 90 kg P2O5/ha; G18 gave a clear response up to 60 kg P2O5/ha. Based on the information obtained in this study we conclude that G6 and G22 are efficiently and well adapted rice genotypes to gray soils of South Viet Nam, low in available P. At low P supply G6 may be slightly better than G22. Further, the 15N tracer study showed that for G6, a small P supply (30 kg P2O5/ha) can increase the % N recover from urea applied at 120 kg N/ha. (author). 12 refs, 6 figs, 3 tabs