Delayed pollination and low availability of assimilates are major factors causing maize kernel abortion.

Selective seed abortion is a survival strategy adopted by many species that sacrifices some seeds to allow the remaining ones to set. While in evolutionary terms this is a successful approach, it causes huge losses to crop yields. A pollination time gap (PTG) has been suggested to be associated with position-related grain abortion. To test this hypothesis, we developed a novel approach to alter the natural pattern of maize (Zea mays L.) pollination and to examine the impact of PTGs on kernel growth and the underlying physiological basis. When apical and basal kernels were synchronously pollinated, the basal kernels set and matured but the apical kernels were aborted at an early stage. Delaying pollination to the basal ovaries suppressed their development and reduced invertase activity and sugar levels, which allowed the apical kernels to set and grow normally. In situ localization revealed normal cell wall invertase activity in apical and basal kernels under synchronous pollination but reduced activity in the delayed-pollinated kernels independent of their position. Starch, which was abundant in basal kernel areas, was absent in the apical kernel regions under synchronous pollination but apparent with delayed pollination. Our analyses identified PTG-related sink strength and a low level of local assimilates as the main causes of grain abortion.