| Abstract: | Changes in the nonstructural carbohydrate (NSC) levels in grain sorghum, Sorghum bicolor (L.) Moench, due to greenbug, Schizaphis graminum (Rondani) feeding were examined. Plants in the seedling, vegetative and reproductive growth stages were grown in laboratory, greenhouse and field conditions. Concentrations of fructose, glucose, sucrose, fructan, and starch in greenbug-infested and uninfested sorghum plant samples were determined using high performance liquid chromatography. Greenbug feeding altered concentrations of NSCs in sorghum plants for each plant growth stage. For seedling sorghum plants grown in the laboratory, greenbug infestations reduced shoot length, shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight (df = 3, P [less than or equal to] 0.05). Greenbugs feeding on these plants excreted honeydew consisting of 58% polysaccharides (DP [greater than or equal to] 3), 18% fructose, 12% sucrose and 12% glucose. Honeydew accounted for much of the decrease in plant growth amounting to 18% of the shoot dry weight difference and 78% of the total NSC difference between greenbug-infested and uninfested plants. Infested seedlings had greater concentrations of NSCs in shoots and reduced concentrations in roots. Sucrose increased in seedling shoots with increasing greenbug density (slope = 5.78 μg per greenbug, P = 0.012, R^2 = 0.98) and starch decreased in the roots (slope = -7.15 μg per greenbug, P = 0.013, R^2 = 0.98). Greenbug-infested vegetative sorghum had a greater concentration of total NSC in roots compared with uninfested plants. Increasing greenbug densities caused deficiencies in the shoots before depleting NSCs from the roots. For reproductive plants, greenbugs were confined to the third leaf below the head. Greenbug-infested plants had greater concentrations of total NSCs in the infested leaf while uninfested leaves above and below and infested leaf, peduncle and root samples had reduced concentrations of NSCs compared with uninfested plants. Results among the various experiments suggest that greenbug feeding initially triggers a plant response that increases the concentration of NSCs, then creates a nutrient sink near the feeding site that draws NSCs from roots and other leaves. Additional feeding depletes NSCs from the infested plants and may explain why greenbug damaged plants exhibit delayed recovery after the aphids are removed. |
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