Shade‐Sensitive Interval of Kernel Number of Maize1

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Abstract
There have been many investigations into the time interval when maize (Zea maysL.) yield is most susceptible to stress. Knowledge of this interval is important for understanding yield potential, modeling, irrigation scheduling, and the study of the partitioning of assimilate into grain. The objective of this experiment was to determine the growth stage when shading stress affected number of kernels per ear. Three commercial maize hybrids were grown on a Houston black clay (fine montmorillonitic, thermic Udic Pellustert) under irrigation near Temple, TX in 1982 and 1983. Plants were stressed using horizontal shade panels that reduced the light by 79% for approximately 13‐day intervals. In order to define when number of kernels was affected, the panels were moved three times each week to provide an overlapping series of treatments. The number of kernels of each treated plant was measured and treatment means were compared with unshaded controls. The growth stage when shading decreased the final kernel number was closely associated with early kernel development, occurring near the end of the lag period of grain filling. Duration of the sensitive period varied from 6 to 23 days. It was speculated that a stress applied in the first 1 to 2 weeks after pollination caused a temporary shortage of assimilate for the ear, severely limiting endosperm cell number of some tip kernels. These kernels would not fill later, even if stress was relieved.