Source Limitation Affects Partitioning To Grain and Stem Sugar In a Photoperiod Insensitive Sweet Sorghum.

Sweet sorghum (Sorghum bicolor L.) is a multipurpose crop that produces both grain and accumulates soluble sugars in the stem. Despite recent attention, the partitioning of photosynthate to grain formation or sugar accumulation in the stalk following anthesis is still not well understood. The objective of this study was to evaluate partitioning between grain and stem carbohydrates in ‘M81-E’ sweet sorghum with mechanically-induced source limitation. The sorghum crop was planted in early May and grown in the field near Gainesville, FL during the 2012 growing season. At the onset of anthesis, defoliation treatments were implemented. Treatments included control, defoliation of flag leaf only, partial defoliation (1/3 of leaf area below flag leaf), severe defoliation (2/3 of leaf area below flag leaf), and full defoliation (except flag leaf). When the control reached early hard dough stage all treatments were harvested and data were collected on grain and stem yield and carbohydrate composition. The control treatment yielded a total of approx. 6800 kg ha^-1 of glucose equivalents (GEQ), distributed as approx. 76% stem sugars, 21% grain starch, and 3% stem starch.  We observed about a 35% or 1700 kg ha^-1 increase in total GEQ from anthesis to harvest in the control treatment. Interestingly this gain occurred primarily in the form of grain starch (i.e., over 80% of the gain in GEQ). The gain in total GEQ declined with defoliation treatments to a point where a loss in total GEQ was seen at full defoliation, although an increase in grain GEQ was seen for all treatments from anthesis to flowering. In the presence of source limitation, sweet sorghum retranslocated carbohydrates from the stem to the grain, resulting in a reduction in stem GEQ at harvest. Thus, partitioning between grain and stem sugars is complex, but these results indicated that grain was a greater sink for source photosynthate compared to stem sugar accumulation under source limitation.