Maize grain yield is closely associated with kernel number at harvest. Kernel number is a
function of the physiological condition of the plant at a critical period bracketing silking. It is
necessary, however, to evaluate the effect of stress during this critical period in current hybrids
with high kernel number at harvest. In addition, increased ear demand during the effective grain
filling stage in these hybrids would have extended the critical period for yield determination. The
objectives of this study were to analyze the effect of short (5 days) and intense (75 % reduction
of incident radiation) stresses imposed from early vegetative stages to physiological maturity on
grain yield components in a recently released maize hybrid. The potential grain number per ear
was no affected by the treatments. Grain yield was affected by resource availability from 250°Cd
before silking to 699°Cd after silking (base temperature=8°C). Grain yield reductions were
similar and averaged 16.0 % throughout this period. Grain number was reduced when the 5 day
shading treatments were applied during a 700 °Cd period centered on silking, but no differences
in kernel number response within this period were observed. When treatments were applied
from 424 °Cd to 699 °Cd after silking (interval included in the effective grain filling period)
individual kernel weight decrease was responsible for yield reduction, in spite of the short
duration of the imposed stress treatment. These results indicate that source capacity during the
effective grain filling period would have become more limiting for yield determination for this
recently released maize hybrid.

Key words: Short stress, kernel number, kernel weight, source, sink.