The time course and direction of 14 C‐labeled photosynthate export from upper and lower leaf positions in a corn ( Zea mays L.) canopy were compared at five different stages from silking to kernel maturity. Export of 14 C‐assimilates was slower from the lower than from the upper position. Some of the difference was due to more accumulation of 14 C in leaf starch at the lower position. The direction of export from a given leaf position changed as development progressed, indicating that sink demand strongly influenced the direction photosynthate moved. Transport of 14 C from upper leaves was predominantly downward, whereas direction of export from lower leaves changed from downward to upward as the ear became the dominant sink.
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Tripathy et al. (1972) studied this question.