Kernel set and flower synchrony within the ear of maize .2. Plant population effects

Kernel number (KN) per plant declines with increasing plant population CPP) in maize (Zea mays L.) and controversy exists about the critical period for spikelet abortion. In this work, the effect of PP (5 and 9 plants m(-2)) on floral differentiation and kernel set was analyzed for two genotypes of contrasting prolificacy at Grignon, France, (48 degrees 51'N). Kernel set variability was tested at Balcarce (37 degrees 45'S), Argentina, with three PP (2, 8, and 16 plants m(-2) and a thinning treatment (from 16 to 2 plants m(-2)) at silking. Plants were grown with no water or nutrient restrictions. At 9 plants m(-2), floral differentiation was delayed in apical ears but almost all spikelets started silk elongation. In subapical ears, 17% of the spikelets of the nonprolific hybrid and 19% of the prolific hybrid had not completed development at silking. At 5 plants m(-2) these proportions were reduced to 6%. Significant (P < 0.01) differences were detected in apical ear length after the onset of rapid ear growth (silking - 20 d), resulting in reduced ear biomass at silking at high PP. Barrenness occurred at 16 plants m(-2) (0.5 ears plant(-1)), but spikelet abortion took place in all apical ears after silking, except at 2 plants m(-2) (control). Kernel number per plant was not related to the degree of floral differentiation reached at silking, but to the amount of intercepted photosynthetically active radiation (KN = 149.5 + 204.6 MJ plant(-1) d(-1), r(2) = 0.92) when the silking - 15-d period was included in the calculations. Presilking limitations to ear growth appear to reduce ear sink strength.