CARYOPSIS WEIGHT PATTERNS WITHIN THE SORGHUM PANICLE

Understanding the intrapanicle pattern of caryopsis weights commonly f ound in sorghum [Sorghum bicolor (L.) Moench] fields and the factors t hat result in changes in this pattern would help in determining limita tions on caryopsis weight. Our objectives were to determine the patter n of caryopsis weights found in the sorghum panicle, the effect of ass imilate supply on the weight pattern, and whether this pattern resulte d from the rate of grain fill or length of the effective fill period ( EFP). Two dryland field studies were conducted at Manhattan, KS, in 19 89 and 1990 on a Reading silt loam (fine-silty, mixed, mesic Typic Arg iudoll; 0-1% slope). In 1989, panicles of DeKalb 'DK 46' and Pioneer ' 8500' tagged at anthesis were harvested at regular intervals. In 1990, seven population-light treatments were applied to Pioneer 8500 either at anthesis or 7 d after anthesis. Panicles from both experiments wer e separated into four sections. Caryopses from these sections were use d to determine differences in caryopsis weights, rate of grain fill, a nd length of the EFP. In 1989, caryopsis weights within the sorghum pa nicle increased from 20.2 mg caryopsis-1 at the base to 22.8 mg caryop sis-1 at the apex. In 1990, this same intrapanicle pattern was observe d in all treatments that had medium or high plant populations during t he EFP. This pattern was caused by increases in the rate of grain fill from the base to apex of the panicle. In the low population treatment s in 1990, caryopses in the middle of the panicle were as heavy or hea vier than those in the apex. This was the result of intrapanicle grain -fill rates that were more uniform and an increase in the length of th e EFP from the apex to the base of the panicle.