IN-VITRO NITRATE REDUCTASE-ACTIVITY OF LABORATORY-GROWN SEEDLINGS AS AN INDIRECT SELECTION CRITERION FOR MAIZE

This study examines whether the predictive value of in vitro nitrate reductase (NR) activity of laboratory-grown seedlings for grain yield, biomass yield, and N-related traits (N uptake, grain N yield, grain N concentration, N harvest index) of field-grown maize cultivars can be improved by changing the screening conditions. Seedlings of 12 tropical and 12 European cultivars were grown in growth cabinets under two temperature regimes (14 and 30 degrees C) and three levels of N supply (0, 0.128, and 0.384 g N kg(-1) substrate, applied as NH4NO3) until the third leaf of 50% of the plants was fully expanded. The second leaf was assayed for in vitro NR activity and nitrate concentration. There were significant (P < 0.05) temperature x origin of cultivar interactions for both traits. At 14 degrees C, tropical cultivars had higher NR activity and lower nitrate concentration than the European cultivars, whereas at 30 degrees C, the situation was reversed. Field experiments were conducted with the tropical cultivars in Thailand and with the European cultivars in Austria and Germany. The NR activities of seedlings grown at 30 degrees C in the growth cabinet and grain yield of plants grown in the field in Thailand and Austria were correlated at r < +0.72 (P < 0.01) and +0.23, respectively. For plants harvested at silage maturity in Germany, the correlation between NR activity and biomass was r < +0.73 (P < 0.01). The correlations between seedling NR activity and N uptake were r = +0.66 (P < 0.05), +0.56 (P < 0.10), and +0.83 (P = 0.01) for Thailand, Austria and Germany, respectively. These results indicate that selection among laboratory-grown seedlings for NR activity might accelerate progress in breeding for grain yield and acquisition of N.