Degree Type


Date of Award


Degree Name

Doctor of Philosophy




Two studies were conducted in an effort to elucidate the short- and long-term effects of alteration in the source-sink ratio of selected maize genotypes;Experiment 1 was designed to determine if, and when, ear removal and/or 50 percent defoliation of maize hybrids would alter carbon dioxide exchange rates (CER) and total nonstructural carbohydrates (TNC). It was conducted in the field during the 1978 and 1979 growing seasons. Leaf CER, stalk TNC, and ear leaf TNC (1979 only) were measured every one to three days during the nine- to 15-day period after ear removal. The results indicate that leaf position had a greater effect on leaf CER after ear removal than did genotype. The CER of the second leaf decreased via feedback inhibition after five to 12 days elapsed, whereas the decrease in ear leaf CER was not as apparent or significant as with the second leaf. The CER of both leaf positions was intermediate between control plants and plants without ears when ears plus 50 percent of the leaves were removed. A large, alternative sink in the stalk was indicated by the data, since the TNC of the ear leaf and the stalk at, and above, the ear leaf was not changed significantly;Experiment 2 sought to determine the long-term effects of defoliation and/or ear removal on field-grown maize genotypes differing in the relative source-sink ratio for two growing seasons. Leaf CER, stalk TNC, and the dry weights of the leaves, sheaths, stalk, and ears were measured at 10-day intervals following source and/or sink manipulation. Grain and cob weights, 100-kernel weight, and the days to physiological maturity were also measured with defoliated plants;The CER of both leaf positions was altered by 10 days after treatment. The relative source-sink ratio of the genotypes was the most important factor in determining how the imposed treatments affected leaf CER. If the genotype had a large source-sink ratio, as with BSSS 56 and BSSS 133, then leaf CER increased after defoliation. When ears were removed, leaf CER decreased via feedback inhibition only if the relative source-sink ratio of the genotype was smaller. All genotypes, except BSSS 133, had CER decrease. For plants without ears plus 50 percent of their leaves, leaf CER decreased only in genotypes which had large, estimated sink size. However, the decline in CER was not as much when compared to plants without ears, possibly because the increased level of assimilate stored in the stalk was still insufficient to produce the full effect of feedback inhibition on CER. None of the treatments delayed leaf senescence. In 1979, leaves senesced about 10 days earlier than 1978;The other measured parameters also changed following defoliation and/or ear removal. Following defoliation, the weights of the stalk, leaves, sheaths, and ears, as well as stalk TNC, decreased. Ear removal increased these parameters, except for the weight of leaves and stalk TNC. Ear removal plus 50 percent defoliation decreased the weight of leaves and sheaths, increased the stalk weight and TNC of the top three nodes, and did not alter the TNC of the ear node. The data indicate that the translocation of assimilate out of leaves was not affected by ear removal, possibly because of alternative sinks in the stalk. Of the measured parameters, the stalk weight had the best correlation with leaf CER;The cob and grain weights, 100-kernel weight, and the days to physiological maturity were decreased by defoliation, especially the 75 percent level, elucidating how maize responds to a stress. Carbohydrates stored in various plant parts were remobilized to the ear, but were not capable of maintaining grain weight, even if leaf CER was increased. Grain weight and days to physiological maturity were increased in 1979. A greater than normal remobilization of stalk TNC may have occurred, since leaves had senesced and the grain-filling period was extended.



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Kenneth Hardy Barnett



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100 pages