In order to study the effects of various plant densities and nitrogen rates on phenology of two medium - maturity com (Zae mays L.) hybrids an experiment was
conducted in 1995 at the experimental farm of College of Agriculture, University of
Tehran (Karaj), using split - split plot design with three replicates. Main plots were, com hybrids (KSC 604 & KSC 601), three plant densities 6, 7 and 8 plants/m2 as subplots and urea fertilizer rates: 0, 300, 400 & 500 Kg/ha (respectively equal to 0, 138, 184 & 230 Kg/ha N) as sub - sub plots. Some important factors related to grain yield including phenological stages, such as tassel, pollen and silk emergence, physiological maturity dates, duration of grain filling on the basis of days after planting, cummulative growing degree days, thermal unit, were measured. Also grain yield per hectare, grain yield per plant and grain yield components such as ear number per plant, kernel row number, number of kernels in each row, kernel number per ear and one thousand kernel weight were found following ear harvest. Results showed that hybrid, plant densities and nitrogen levels significantly affected duration of vegetative and reproductive period. Duration of these stages were increased with increase in plant density and nitrogen rates. Plant density significantly affected duration of pollination, flowering and flowering concidence period. Flowering concidence period' duration decreased with increase in plant density and increased with nitrogen rate increase. Pollination period duration and its GDD were affected by hybrid xplant density and hybrid x nitrogen interactions. KSC 604 in comparison with KSC 601 had a longer vegetative, reproductive growth and grain filling period as well as a shorter pollination period. By increase in nitrogen rate and plant density, vegetative and reproductive growth period GDD's increased. Grain yield per hectare showed with vegetative and reproductive growth period duration respectively. Under the conditions of this experiment, KSC 604 grain yield per plant was higher than that in KSC 601 and grain yield per hectare in KSC 601 was higher than that in KSC 604.
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